CA1106696A - Gear pump - Google Patents
Gear pumpInfo
- Publication number
- CA1106696A CA1106696A CA275,715A CA275715A CA1106696A CA 1106696 A CA1106696 A CA 1106696A CA 275715 A CA275715 A CA 275715A CA 1106696 A CA1106696 A CA 1106696A
- Authority
- CA
- Canada
- Prior art keywords
- pump
- gears
- gear
- housing
- outlet
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
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- Extrusion Moulding Of Plastics Or The Like (AREA)
- Rotary Pumps (AREA)
- Details And Applications Of Rotary Liquid Pumps (AREA)
Abstract
ABSTRACT OF THE INVENTION
Plastics recovery, compounding and fabricating systems are disclosed utilizing a gear pump specially designed to include a media free space between the gear face and side walls with minimum sealing to improve volumetric pump efficiencies and provide pumping capacity insensitivity to viscosity over a wide range. A method of monitoring melt index and production rate for reactor control is also disclosed.
Plastics recovery, compounding and fabricating systems are disclosed utilizing a gear pump specially designed to include a media free space between the gear face and side walls with minimum sealing to improve volumetric pump efficiencies and provide pumping capacity insensitivity to viscosity over a wide range. A method of monitoring melt index and production rate for reactor control is also disclosed.
Description
The present in~ention relates to systems for handling polymers and more particularly to systems for handling compositions of polymers and volatile constitu-ents, such as polyethylene and ethylene, and means for increasing the efficiency of compounding systems for these compositions.
As employed herein, the term "polymer(s)" is ~derstood to mean organic homopolymers, copolymers, and polymeric mixtures either alone or containing additives of the type normally encountered in the plastics field, e.g., stabilizers, fillers, processing aids, colorants and other additives (such as anti-block, anti-oxidation, anti-static and the like).
Background In the typical production of low density poly-ethylene, a reactor discharges a ~tream which i~ a mixture of polymer and unreac~ed ma~erials to a product reeeiver.
The product receiver operates at a pressure substantially below the reactor pressur~ and flow of the reactor dis-~ 20 charge is controlled by the product valve. In the product I receiver, the m~jor portio~ of the unreacted materials are removed due to flashing which results from the trop in pressure experienced by ~che mixture. The flashed material, commonly referred o as the return gas, is sub-sequen~ly returned to the reac~or. The remaining polymer-lzed material se~tles in the product receiver and still contains some unreacted m~teria~ which are removed in the remainder of ~he polymer recovery system.
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The polymer discharged from the product receiver is fed to an extruder through a polymer flow control system. The extruder performs two functions in this system: final devolatilization to remove the remaining unreacted material; and pumping of the polymer through a screen pack, if one is being used, and a pelletizer die plate.
The material enters the side of the extruder and the unreacted materials flash and form a foam having a very low density. Therefore, an extruder having a very large volumetric conveying capacity in the feed section is necessary ~o handle the material as final devolatili-zation ls occurri~g. Normally, an ex~ruder having a two~diameter screw or an oversize single-diameter screw ; is required to obtain the con~eyi~g capacity to handle - the material entering the extruder. In some installation~, a portion of the flashed material is removed from the extruder through a vent stack.
As the production rate o~ single low density polyethylene (LDPE) reactors are increased, larger and larger extruders, which become prohibitively expensive, are needed. In an effort to eliminate the u~e of two-diameter extruders or oversized extruders, some existing units have been modified to include a secondary e~hylene separation (flashing) operation up~tream of the extruder inlet subseq~ent to the primary product receiver ethylene separation ~flashing) oper~tion.
is ~ystem differs from the side-fed extruder, 3.
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top mounted vent stack type in that the m~erial is fed into the top of the vent stack and essentially all of the remaining unreacted ma~erials are released before the polymer stream enters the extruder. This provides a material to the extruder which has a much greater density and eliminates the need for two-diameter extruders or large single-diameter extruders. The devolatilization an~
pumping functions of the original, two-diameter extruder system have now been separated, i.e. the final devolatil-lzation is performed in the vent stack and only the polymer pumping is performed by the extruder. However, extruders pump polymer by developing viscous drag, and are very ine~ficient pumps.
ummary of the Invention The present inven~ion has utility in a plasticsrecovery system in which polymer produced in a reactor is partially devolatilized in a product receiver, trans-ferred to the top of a vent stack for further devolatili-zation and flood feed to a low ne~ positive suction head gear pump.
: The use of the specially dPsigned gear pump of the preseDt invention ha~ing a free filling space surround-ing substantially the entire periphery of the gears allows an interaction of the material being pumped caused by the rotating gears and the pump body above the sealing zones to generate pressure above ~che pump inlet pressure, similAr to the pressure generation tha~ occurs in a hydromatic housing, ~o facili~ate filling of the gear 4.
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tooth cavities. Additionally, the shear areas and energy dissipation in shear is an order of magnitude lower than comparable positive displacement pumps.
Specifically, the gear pump includes a pair of intermeshing, rotatable gears surrounded by a filling chamber increasing in volume ~oward the pump inlet.
Horizontal gear axes orientation is preferred, but not essential. The gears seal with the housing at a minimum seal area sized to seal substantially the distance between adjacent teeth across ~he full axial length near the outlet of the pump, said outlet having a cross-sectional area from 0.2 to 2.0 times the product of the gear tooth height and ~he ~ear face width.
The outlet, which is sized to interface with the gear pump, is preferably rectangular in shape with the major dimension parallel to the axes of the gears. The outlet may, however, be circular, oval or of any other cross-sectional shape desired from the operating standpoint.
In some polymer processing systems wh~re the polymer is discharged in the form of a rope, a pair of rollers is provided directly above the gears. The rollers pull the polymer rope in~o a charging area between -the rollers and gears to fil} the gear cavities and also ;~
densify the foamed polymer.
Suitable instrumentation is disclosed for ;~ monitoring pump parameters ~o provide on line ~iscosity and production rate control.
Thu8, the features to be described in greater detail ~elow include n comprehensive low density polymer 5.
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recovery system which includes a novel gear pump.
Another feature is a novel gear pump structure for handling a broad range of production rates.
A fur~her feature is a gear pump having roller charging capabilities for pumping polymer material produced in the form of strand or rope.
Yet another fea~ure is the provision of on line additive injec~ion at the pump of a recovery system and modular construc~ion of the pump body, feed gears and rollers.
These and still other features will be readily apparent from ~he drawings and the following disclosure.
Brief Description of the Drawings Fig. 1 is a schematic view of a low energy ~
recovery system in accordance with ~he present invention;
Fig. 2 is an enlarged cross-sectional view of a gear pump for use in the low energy recovery system of Pig, l;
Fig. 3 is a eross-sectional view with certain portions broken away for clarity taken generally alsng line 3-3 in Fig. 2;
Fig. 4 is a top plane view taken generally long line 4-4 of the pump in Fig. 2;
~` Fig. 5 is a cross-sectional view of a gear pump - with feed~rollers for handling feed in rope form;
Fig. 6 is a sectional view taken along line 6-6 i~ Fig~ 5;
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Fig. 7 is a top plane Yiew of the pump of Fig. 5;
Figs. 8A and 8~ are schematic views illustrating compounding sys~ems equipped with the pump of Figs.2 and 5, respectively;
Fi~. 9 is a schematic view illustrating the use of a gear pu~p of the present invention in combination with an extrusion die ~r mold in a fabrication processi Fig. 10 is a cross-sectional view of a modular gear pum~ with feed rollers wherein the entire inlet/gear cylinder section is replaceable;
Flg. ll ls a sectional view taken alung line ll-ll in Fig. 10;
Fig. 12 is a cross-sectional view of a modular gear pump wherein the body is formed by two halves for increasing t~e gear width by modular additions;
Fig. 13 is a sectional view taken along line 13-13 in Fig. 12;
Fig. 14 is a top plane view of a modular gear pump with feed rollers wherein the gears and rollers are : 20 modular for increasing the capacity of th2 pump;
Fig. lS is a cross-sectional view taken generally along line 15-15 in Fig. 14;
Fig. 16 ~s a cross-sectional view taken generally along line 16 16 ~n Fig. 15;
Fig. 16A is a cross-sectional view similar to Fig. 16, illustra~ing an external drive for the feed rollers;
Fig. 17 is a side elevation view with certain portlcns broken away in cr~ee-sectl~n illustrsting a 7.
completely modular 8ear pump embodiment; and Fig. 18 is a ront Plevation view of the pump of Fig, 17 wi~h certain portions broken away in cross-section.
Detailed Description of a Preferred Embodiment While this invention is susceptible of embodiment in many differene forms, there is shown ~n the drawings ; and will hereina~ter be described in detail a preferred embodimert of the invention, and modifications thereto, with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is n~t intended to limit the ihvention to the embodiments illustrated.
The ~ecovery 5vstem Fig. 1 illustrates an integrated system for the production of low density polyethylene in accordance with ~-the prese~t invention. A composition of liquid polyethylene :
and entrained ethylene is produced in a reactor (not ~hown), as is well known in the art. The composition is ~; 20 discharged from he reartor through line 10 into a product receiver 12. The mixture flow to produet receiver 12 is controlled by a product valve 14 and a pressure relief valve 16 in line~10.~ The bulk of ~he entrained e~hylene gas i removed from the mixture by flashing in product recei~er 12 and ~he thus removed ethylene gas is condensed , ;~ ~ and recycled to the reactor through line 12A for fur~her use. The partially devolatilized polye~hylene collects at the lower portion of produe~ reeeiver 12 and is transferred through line 20 o the i~let of ven~ s~ack 22.
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: , ~ ~ ~ 6 ~ ~ 6 Transfer of the liquid polyethylene to the vent stack is accomplished by a pressure gradient which exists between product receiver 12 ant vent stack 22. The product receiver operates at pressures in the order of 1000 p.s.i.
or higher and the vent stack is operated at pressures at or slightly above or below a~mospheric pressure. The fl~w rate of polyethylene to vent stack 22 is controlled by a feed control valve ~24 in line 20, and a Rufficient le~el of liquid polyethylene is main~ained in the pr~duct receiver to prevent blow through of ethylene directly to the vent stack.
As the polyethylene enters vent stack 22, sub-stantially all of the remaining entrained ethylene flashes from the mixture due to ~he pressure-equilibrium relation-~hip at the pressure of the vent stack. The low pressure in the vent stack and removal of the ethylene is accomp-lished through a return line 26 which includes a pumping means 28, such as a venturi nozzl~ or vacuum line. The ~: return fluid in line 26 is collected for further process-ing in the reactor.
Polyethylene melt P ~e tles in the vent stack and 18 sll~wet ~o achie~e a l~quid level L. The l~wer portion of ~ent stack 22 is i~ fluid communication with a gear pump 30, described in greater detail belvw, through a tapPred outle~ por~ion 22A. The level of the liquid polymer in the vent sta~k is prefera~ly m~intained such that polyethylene-ethylene interface is above the pump : ~ inlet~ant the diameter of ~he vent stack is such that the pressure drop caused by ths fl~win~ polymer pool is equal .
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, Il 1~66~6 to, preferably less than the pressure head formed by the liquid level of the polymer. Thus, gear pump 30 is flood-fed by the liquid polymer P in the vent stack.
Gear pump 30 is driven by a variable speed drive 32 and torque coupling 34 to withdraw polymer from the lower portion of the vent stack and pu~p it under pressure through conduit 36 into a pelletizer 38 for final process-ing and delivery of the polymer to product hopper 40.
Control of the system is provided by a central process control 42 which, in addition to monitoring the function and operation of the reactor, as is known in the industry, monitors and controls the above-mentioned equip-ment. More specifically, a liquid level controller 44, wh~ch may be either of the displacement or rate types, as discussed ~ore fully below, senses the level (or rate of change of level) of polymer in the vent stack. This variable is utilized by the process control 42 to set ~he variable Ppeed drive 32 to insure flood feed operation and full capacity capabili~y of the pump. Also, the level control may be utilized to provide an indication of production rates, as discussed below.
Additionally, the process control unit monitors power input ~o the pump, and inlet and outlet pressures and temperatures at the pump through lines 46. The exact instrument for monitoring these variables are well known in the $nstrumentation art and include torque meters, pressure transducers, either mechanical or electrical, e.g., piezoelectrictransducers and thermocouples or thermistors, respectively. From these measured variables on-line 10 .
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viscosity monitoring is achieved. Viscosity is inversely related to ~le product mel~ index which is a primary control variable for reactor control in the production of polymers. Thus, these measured variables are fed back in the process control unit 42 to provide control of polymer production in the reactor.
The Gear Pump Figs. 2-4 illustrate gear pump 30 specifically designed to provide a minimum net positive suction h~ad and improved polymer pumping ca~ability. Ptm~p 30 is modular in construction and i~cludes an inlet structure 50, gear housing 52, a~d steam jacket 54. Inlet structure 50 includes two plate-like elements 50A and 50B of generally L-shaped configuration and having interlocking end portions 50C and 50D as best illustrated in Fig. 4. Inlet s~ruc-ture 50 ~s coupled to a peripheral flange 22B on the out-let of ~ent stack 22A and to gear housing 52 by a plurality of fas teners 56, Fig. 2.
Gear housing 52 is generally rectangular in outer dimension and includes an upper flange por~ion 52A, -~ which abuts against ~he lower surface of inlet structure - 50, and sidewalls which define a gear chamber 58 at their inner surface and ~aper downwardly at two opposite outer surfaces 60 to define the inner surface sf a steam chest.
The steam jackets 54 are removably moun~et to the gear housing 52 by means, not shown, and complement the tapered sides 60 to form a steam chest about the gear pump. The steam jackets include a steam inlet 54A and condensate outlet 54B ~or admitting s~eam from a source (not ~hown) 11 .
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to maintain the pump at a temperature above thP melting temperature of the polymer. Alternatively, pass~geways ~uch as drilling, described below, may be utilized in a solid pump body configuration.
Referring particularly to Figs. 2 and 3, inlet structure S0 defines an inle~ 66 whose width (~arallel to the gear axis) is approximately equal to the gear width, Fig. 3, and whose minimum length (normal to the gear axis) taper down~ardly to complement the gear chamber 58.
A pair of rotating, inter~eshing gears 70 are positioned in gear housing 52. ~ny one of a wide variety of gear pairs m~y be employed, intermeshing herringbone gears being preferred.
As 8hown in Fig. 3, a pair of intermeshing herringbone gears 70 are rota~a~ly mounted in gear hous- -ing 52 by means of journal b arings 71 and end pla~es 72.
One of the gears (the drive gear) includes a drive shaft 70A which extends outwardly from the gear pump to be coupled to the torque coupling 34 ant thereby provide power input to ~he pump. The o~her gear is driven by the intermeshing relationship of the ~erringbone gears.
Journal bearings 71~ are lubricated by the polymer ~elt, and as discussed b~low provide a means for monitoring the viscosity of ~he polymer melt. The length and diameter of the bearings and clearances are such that high pressure leakage is mi~imized by a throttling effect.
The chamber 58 in which the inte D eshing gears 70 are loca~ed is d~signed to achieve an inlet of minimum restriction of poly~er flow ~o improve volumetric efficiency.
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, Substantially all of the faces of the gears are exposed to the media free space (the volume between the gear faces and walls of chamber 58). This configuration permits the development of a pressure gradient within the media free space to facilitate the filling of the gear tooth cavities with polymer.
To achieve the necessa~y large media free space and still provide the necessary seal between the outlet 103 and the low pressure inlet of the pump, the pump discharge opening 103 and seal zone 74 must be minimized.
The portion of outlet 103 in direct communication with the gear chamber defines a generally rectangular shaped open-ing having a width 103A, Fig. 3, equal to the gear face width and a length 103B, Fig. 2, equal to about the gear tooth height.
The outlet is defined, in part, by high wear resistant seal zone inserts 75, which are removably secured to a seat 58A in the gear housing as by bolts, not shown.
Each seal zone extends from the ~ip 75A of the inserts 75 for a circumferential distance at least equal to th~
distance hetween adJ cent teeth across the full axial length of ~he gears to provide an effeetive seal between the gear teeth and gear housing. The star~ of the seal zone is generally indicated by numeral 75B, and the seal extends to tip 75A. It should be noted that a seal leng~h larger than that ~ust specified does not significantly increas~ ~eal effectiveness.
The inner surface of chamber 58 increases in dimension from the start of the ~eal 75B in a continuous 13.
smooth curve to join ehe inlet structure chamb r 66 just above the top of gears 70.- In one pump embodiment the gear chamber or media free space boundary extended from a location 75B 53 bel~w the horizontal passing through the : center of gear 8XiS in a circular arc to a location 15 b~low the horizontal to provide a radial distance between : . the tooth tip and cha~ber surface of 1/2 inch at that point, and extended linearly from the 15 point to the top of the pump. In general, the ratio of free media space expansion to the circumferen~ial distance tas measured from the end of seal zone 75B) is optimized in accordance with lubri-cation theory to achieve an increasing hydraulic radius in the media free space to fill the gear too~h cavi~ies with polymer. The media free ~pace increases from the seal zone 75B to produce a maximum pressure gradient at a point preferably below the horizontal elevation of the ~ear axes. The optimal media free space configuration is a function of the particular polymer being pumped. A ratio of the clearance (gear tooth to chamber wall~ to the circumferen~ial distance from the seal zone in the range o~ about 0.2 to 0.9 is a typical value for polymers having viscosities in the ran~e of 0.2 to 1.4 lb. sec/in2, ~`; ; respec~ively.
: : With particular reference to Fi~s. 2 and 3, the outlet 103 changes rom the slot-like opening adjacent the gear~ to a circular discharge outlet 103C at its lower : end, which is in fluid communication with condui~ 36.
: The herringbone gears 70 may have helix angles ran8ing up to 30; however> smaller helix angles, 15 or 14.
, less, are preferred to avoid leakage at gear intermesh.
Most preferably, herringb~ne gears with a helix angle o 7-10 are used to avoid trapping polymer in the tooth cavities. The use of herringbone pa~tern gears is preferred to constant helical or spur gears due to reduced stress loading on the gears and housing realized with the herringbone gears.
The use of a gear pump and more particularly a gear pump with herringbone gears provides economic savings in capital cost, operating cost and improved performance in comparison ~o screw extruders used for the polymer ~` recovery system. More specifically, for a recovery system capable of handling 30-50,000 lbs/hr, system investment cost may be up to 50% less for a gear pump equipped system. In gear pump recov ry systems for the indicated capacities, h~rsepower requirements ranged from 7 to 17 horsepower as opposed to 35 to 60 horsepower for a comparable extruder system. A comparable reduction in shaft horsepower equates to an annual cost savings for a 50,000 lb/hr system in the order of ~150,000/year assuming 3.6 cents/kw. hr. eleotric costs.
Finally, the viscous shear dissipation work is greatly redueed by the use of ~he gear pump as compared ~o a screw extruder. The reduction in this work is about one order of magnitude. This reduc~ion, of course, results in a reduction in the temperature rise of the poly~er as it passes through the gear pump. Steady state temperature rises of 1 to 4 C have been experieneed with the gear pump of this invention, as compared to 10 15.
to 35 C for a comparable screw extruder. The reduced temperature rise provides better produc~ property control and less thermal degradation of the polymer product.
The gear tooth helix angle and media free space of the gear pump provide improved handling and volumetric efficiency. Table I indicates a comparison of operating parameters for se~eral configurations which illustrate the effects of gear tooth angle and media free space on gear pump performance. The body type~ include the one described above and designated as "FULL" and one designated : "MIN" which had a media free space inlet dimension located 25 above the horizontal gear axis and extended therefrom in a circular arc to a location 20 below the horizontal gear axis (the start of the seal zone). Each unit was equipped with herringbone gears with the indi-cated helix angles and identical outlet shapes.
From Table I, it will be noted that ~he FULL
media space configuration, i.e., the least restricted inlet configuration, provided volume~ric capacities which were essentially constant for the range of produc~ tested.
The gear pump of the present in~ention is essentially insensitive to vi~cosity and pressure conditions with respect to i~s pumping capacity, and its volumetric efficiencies are essentially 100% over a range of gear ~pitch line velocities up to 150 ft/min.
The designations MIN and FULL as well as the characteristics of the pump structure given in connection with Tabl~ I are illustrative embodiments and should not be interpreted as se.tting forth the limits of applicants' :~ :
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inventi~n. The values listed in the first six columns from left to right represent short tests for peak per-formance of the gear pump, while ~he last two columns represent long term steady-state operating conditions for a reactor system having the indicated production rates.
An additional advantage of the gear pump is increased recovery system operation. Unlike a screw extruder system which is highly sensitive to polymer viscosity, the gear pump being a posi~ive displacement pump will continue to pump even if large changes in viscosity are experienced. An extruder on the other hand, would have to be shut down and cleaned, when high or low viscosity are introduced into it. Thus, reactor down time is decreased and reliability is greatly increased with the gear pump.
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On-Line Viscosity Monitoring and Reactor Control As previously described, the gear pump recovery system includes ins~rumentation that measures pump input power and inlet and outlet pressures and temperatures.
From this information, the signals are conditioned to provide on-line viscosity monitoring system which is related to the product melt index as is known in the art.
This signal is then used to monitor the product melt index ~ia ~iscosi~y monit~rin~ and tied bac~ ~nto the reaction control system for closed-loop melt index con~rol. Since the component instrumentation for carrying out the indi-vidual measurement of the~e variables is well known in the instrumentation art, a detailed dcscription of them will not be presented. However, the manipulation of these da~a to achieve viscosity monitoring and production rate ron-trol will be described in detail.
The gear pump horsepower is ~he su~ of the volumetric pumping work and viscous di~sipation work. The viscous dissipation work is primarily due to viscous dissipation work in the pump bearings. Assuming all the viscous dissipation takes place ~n the pump bearings, the total horsep~wer can be represented by:
HP - ~ ~VA t Q ~p [1 where: ~ ~ ma~erial visc05ity in bearings;
shear rate in bearings;
V ~ surface velocity of shaft in bearings;
A ~- ~otal surface area of shaft in bearing - ~LD
Q ~ volume of ~terial being pumped; and ~-~p ~ pressure dlfferential across the pump.
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The total horsepower can also be repre3ented by:
HP - 2~TN ~2]
where: T = torquei N ~ rotational speed.
Furthermore, the shear rate (~) can be repre-sented by:
~ ~ ~DN ,' C ~3]
where: D - bearing diameteri C ~ bearin~ clearance; and Q can be expressed by:
Q ~ qN [4]
where: q ~ tisplacement/rev.
Combining these equations and using constants for the fixed parameters results in the following relationship:
klT k2 P [5]
where:
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kl _?C _ snd k2 ~ qC
~2D3L ~D]3L
The last term (k2 P) can be elimina~ed for a simplified .~ model o provide:
~ klT [63 -: ~ : In plant experiments, the ~orque and speed measurements produced ViBcosity determinations which agreed closely with the laboratory melt index ~nalysis of the materials being processed.
:; The mod~l can:be modified to include produc~
temperatu~e e~fects on the viscosity monitoring systems by ~ measuring the product temperature at the i~let or outlet '.':
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, L,,r~ ~ ~,¢~6 of the pump. This modification has only a slight effect on the viscosity determinations since the temperature of the material in the bearings will reach a temperature which is dependent primarily upon the pump speed and mater-ial viscosity rather than the pump inlet or outlet temper-atures. In effect, the temperature variations of the product being processed will have a small effec~ upon the on-line viscosity determinations.
Due to non-linear relationship between ~elt index and viscosity and simplif.ying ~ssumptions in the model, the viscosity may not always be a true viscosity but it will provide a trend indication, which is one of the main criteria needed for closed-loop control systems.
Production Rate Indication .
The level L in the vent stack 22 can be main-tained at a set level by varyi.ng the pump speed as neces-sary to maintain a set level using conventional set point control equipment. When using this type of ~ent stack level c~ntrol, the instantaneous production rate is directly proportional to the pump ~peed and can be measured by a tachometer to provide production rate data to con~rol 42.
The gear pump speed can also be controlled by using the continuous vent stack level signal to directly ontrol the gear pump speed. In ~his mode of operation, a higher production rate would tend to fi~l the vent stack to a higher level and then the direct use of ~he continu-ous lev~l slgnal to the gear pump speed control would increase the pump speed proportionately to the change in 21.
level. In normal operation, this leads to a higher vent stack level and higher pu~lp speed for higher production rates which are associated with the higher melt index materials. Thereore, the vent stack level can be used as an indication of the instantaneous production rate.
Modifi_d Gear Pump Figs. 5-7 illustrate a modified gear pump 130 of this invention for use in polymer recovery systems in which the polymer is discharged in the form of a tacky rope R, shown in phanton line in Fig. 5.
The use of gear pump 30, described above, in a polymer rope discharge system would tend to force ~he rope against one side of the pump inlet due to the rotation of gears 70. This would lead to reduced pumping capacity since the rope would feed only one gear or possibly bridge the pump inlet resulting in loss of pumping.
To prevent these situations from occuring and to pro~ide a positive inlet pressure to the gears, pump 30 is modified to achieve pump 130. Common elements of pump 30 and 130 are correspondingly numbered, and a detailed description of these common elements will~not be repeated.
Due to the modular ~onstruction of the gear pump~ 30, only inlet strueture 50 need be removed and re-; placed with modified inlet structure 150 to achieve the -alteration of thP pump.
So handle rope R, a set of rollers 152 is posi- - -t~oned i~ the pump inlet to pull the rope into the pump and provide a positive inlet pressure for filling the pump , ~
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tooth cavities and also densifying th~ polymer, if it is in a foamed condition.
With particular reference to Fig. 6, each roller 152 includes a cylindrical roll portion 152A, a pair of helical end gears 152B which mesh with the herringbone gears of the associated gear 70 below each roller, and axially extending stub shafts.l52C. Sha~ts 152C are rota~ably mounted in inlet structure 150 by bearings 154.
In this manner, each roller 152 is driven by its associ-ated herringbone gear in the directions indicated in Fig. 5.
For foam materials o~ very low density it is preferahle to drive rollers 150 independently of gears 70 to obtain desired feed rates to the pump. This may be accomplished by positioning the juxtaposed gear 152B on ~-each roller such that they intermesh with each other but -not with the h~rringbone gears 70, and providing an external drive.
As best illustrated in Fig. 5, inlet structure ~:~ 20 15a defines an inlet 166 which is com~lementary to the cylindrical surface of roller 152A, and is provided with cutouts 167, Fig. 7,for receiving end gear 152B. The , ~ ~
~ : : minimu~ width of the inlet`(parall21 to gear axis) is .~ ~
~ : equal to the width o gears 70.
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:~ The bottom surface 151 of inlet stru ture 150 :.
1: overhangs the gear chamber 58 at each side, Fig. 5, ~nd a pair of wipers 168 having a generally wedge-shaped ~ross section abut against these overhanging portions and : are secured w~thin chamber 58 by bolts, not shown. Each ' i :' 23. .
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wiper 168 includes a wiper surface 168A positioned at zero clearance with ~he surface of roller 152A. Wipers 168 thus prevent polymer from being recirculated about the periphery of the rollers as they rotate in the direc-tions indicated.
In operation, rope R is fed between the nip of rollers 152 which press the polymer into a charging zone 170 to fill the tooth cavities of gears 70. The polymer is carried by the gears 70 into the gear chamber or free media space 58 and then through the seal zone 74 (shown enlar~ed for clarity) for positive displacement tpumping) înto outlet 103, as the herringbone t~eth of the gears mesh.
On-Line Direct Additive Injection ; Both the gear pump 30, Fig. 2, and modified gear pump with rollers 130, Fig. 5, include provision or the direc~ injection of additives into the pump for mixture with the polymer. The additive in liquid fvrm may be pumped through plural conduits 130 (only one of which i5 ~; 20 illustrated) in the inle~ ~ection of the pump ei~her para-llel or perpendicular to the gear axis, Tig. 5, or into contact with roller 152A, which doctors the additive into : the charging zone for mixture with the polymer. Solid additives, which cannot be melted for introduction into the pump via condui~s 100, may be fed into the pump through a screw ex~ruder 102, built into the 8ear housing and extending laerally outwardly from the pump.
Extruder 102 includes a screw 102A, which is driven by motor 102B and extends coaxîally within a ~4.
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tubular housing 102C from an input hopper 102D atone end to the media free space adjacent the gears. Preliminary mixture of the additive and melt is achieved in the terminal end of the extruder, by means of a high pressure melt feedback conduit 104 which extends from the outlet of the gear pump to the intermediate portion of the extruder. In this manner the melt and additives are mixed in the last few flights of the extruder screw and intro-duced into the media free space for final mixing with the 10 polym~r in the gear pump. Conduit 104 may be either sized to provide proper feedback ra~e or may be equipped with an orifice plate or valve to provide variable feedback.
In addition to additive injection at the gear pump, additives may also be introduced through ~he top of vent stac~ 22. In the case of ~olid additives, they may be sprinkled over the surface of the polymer mel~ where the heat of the polymer melts ~he additives. The thus melted additives are mixed with the polymer as the ; material passes through the shear field in the gear pump.
The mixlng of addi~ives and polymer, whether introduced through ven~ stack or gear pump, is accomplished in the pump by the shear actions induced in the flow patterns wi~hin the pum~, particularly as the material enters and is 6ubsequently displaced rom the gear tooth cavities.
For materials requiring a higher degree of mixing than performed by the gear pump, a s~atic mixer, e.g. , one of the type known co~mercially as a Kenics mixer, may be connected to the discharge of ~he ~ear pump 25.
,~,,r~ 5"~j to provide further mixing.
Compounding Systems The gear pumps described above may also be utilized in compounding systems to increase the capacity and lower the operating costs. Compounding systems are similar to a recovery system in that more than one opera-tion is performed by the compounding system. Compounding involves fluxing and mixing and in some cases pumping.
Compounding systems using extruders, whether single-screw or multiple-screw, are modified so that only the fluxing and mixing operations are performed by the extruder. The material is then fed to a gear pump for the pumping operation.
Figs 8A and 8B illustrate schematically two compounder-gear pump systems, 200 and 250, respectively.
System 200 includes a compounding mixer 205 which may be of the batch or continuous discharge type. Compounder 205 discharges material through outlet 206 into a gear pump 30 -of the type described above. By relegating the pumping ; operation to gear pump 30, which 1s insensitive to viscosity fluctuation, increased compo~nding capacity and production rate are achieved with a reduced product temperature rise. Final temperature reductions of up to 80~ C may be achieved by the utilization of gear pump 30 for the pumping operation.
, The system 250 includes a compounder 252 of --either the batch or continuous process type which dis-charges the polymer in the form of rope R from outlet 252A.
~ ~ The polymer rope is enclosed within a conduit 254 which `~ 26.
. , !l extends from outlet 252A to ~he inlet of a roller equipped gear pump 130, described above, for pumping. An inert atmosphere such as nitrogen gas is supplied to the interior of conduit 254 from source 256 ~hrough line 258. Any excess gas is withdrawn from conduit 254 through line 254A.
lt will be appreciated that the inert atmosphere reduces the oxidative degradation of the polymer as it passes from the compounder to the gear pump.
Fabricating Systems The gear pump of ~he present inven~ion may also be used in the ul~imate fabrication of material from polymers. Fig..9 illustrates schematically a gear pump 30 closely coupled to a plasticating extruder 260 by con-duit 262. Die or mold 265 is used generically to cover ; both a restrictor for the fabrication of continuous products 266 having a cross section determined by the restrictor, e.g. film, pipes, structural beams, and a mold for the ~abrication of molded articles. In the latter case, a control valve 264 is utilized in line 263 connecting pump 30 ~o die 265 to pulse polymer into the die mold ~65.
The gear pump 30 is the inal element for promot-ing polymer melt flow into the die 265. The use of the gear pump to provide either con~inuous pressurized polymer i for continuous extrusion or discrete charges of pressurized polymer ~or molding articles provides lo~er energy con sumption for the fabrication system than comparable screw extruder or pressure cylinder rethod, respectively.
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Alternative Modular Embodiments The basic modular gear pumps 30 and 130, described above, may be further formed by modular compon-ents to increase the utility and flexibility. Various other modular configurations will now be described.
Modularity may be achie~ed in a number of ways including: (a) gear housing casting with modular body insert, Figs. 10 and 11; (b) gear housing casting which is split to i~crease the axial bore width with modular inserts, Figs. 12 and 13; (c) pump body fabricated of independent modular sections, Figs. 17 and 18; and (d) modular and roller configurations, Figs. 14-16A.
Figs. 10 and 11 illustrate a gear pump 30 similar to the pump 130, described above, which is equipped with feed rollers 152. Corresponding parts are designated by the same part designations described with pump 130.
As previously dascribed, the contour of the ~ -; media free space is a function of the viscosity of ~he particular polymer ~eing pumped. It will therefore be ~20 appreciated that the ability to easily vary the media free space of a pump is highly desirable. Additionally, the .
~ ability t~ change the gear diam~ters (and thus the media :; ~ : :
ree space expansion) is also desirable to accomodate the production rate of the reactor. To this end, pump 330 includes an inlet/cylinder insert 380 of generally :
trapezoidal shape which is~nestably received in a comple-men~ary cutou~ 381 within gear housing 52 which extends through ~he entire housing. Insert 380 is retained within ~ cutout 381 by the overhanging portion lSl of inlet struc~ure : ~ :
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150 ant fasteners (not shown) and defines the media free space chanber 358 at its inner surface.
Insert 380 also defines wiper portions 368 immediately adjacent each roller 152A and seal zones 375 adjacent the discharge outlet 103. Thus, the insert 380 ser~es the functions of several of the previously de-scribed structural elements. But, in addition, insert 380 also mounts the bearing 71 within cylindrical bores 371A
and defines the discharge outlet 103.
Thus, insert 380 together with herringbone gears 70, bearing 71 and end plates 72 form a remov~ble package which can be custom designed for a particular polymer viscosity and production rate. Insert 380 may also be easily heat treated or plated for desired characteristics.
P~mp 330 has certain other modifications over pump 130 including the deletion of feedback conduit 104 which, if desired, may be included. Additionally, additive extruder 102 has been relocated to the position of additive port 100 and an additive port 390, which may also include a screw ex~ruder, has been located at the i ~ mixing zone parallel to the axis of gear rotation.
Fig.s. 12 and 13 iIlustrate a pump 430 in which the gear housing 452 is cast in ~wo half sections which are symmetrical axially about a plane perpendicular to the gear axis to per~it axial variation of gear width.
The two halves are secured together by axially extending bolts or the like (not ~hown). Housing 452 includes a thickened ~ide wall 460 to accommodate a plurality of heat transfer passages 461 which extend laterally there-, ~ :
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through and are interconnec~ed by vertical passages 462, Fig. 13. Steam or other heat transfer (heating or cool-ing) is circulated through passages 461 and 462 to control the temperature of the pump.
Similar to pump 330, pump 43~ includes an insert 480 which is retained in a complementary shaped cutout 481 within housing 452 by the overhanging portion 151 of inlet struc~ure 150 and fasteners. Insert 480 defines the media free space chamber, seal zone 475 at its inner surface 458, and outlet 103. Additionally, insert 480 mounts the herringbone gear shafts and bearing 71 in bores 471 to provide a removable assembly. In the - event that an axially larger assembly is to be inserted within housing cutou~ 481, the two halves of housing 452 are detached and a modular housing insert is positioned ~-; therebetween. These modular housing inserts a~e described below in connection with Fig. 14.
Since an axial enlargement of the housing 452 also enlarges the outlet 103, a modular outlet insert 403 may also be utilized. Insert 403 is retained in a rectangular cutout 403A at the base o~ housing 452 and held therein by fasteners (not shown). Insert 403 inrlud~ heat transfer conduits 461 and 462 which mate with those in hous~ing 452. The use of the insert 403 provides a eontinuous mating surface for the 1ange connection of the outlet pipe 36 or other equipment to which the polymer is discharged, The split housing 452 may also be formed to eliminate end plates 72. This is accomplished by machining , ~
30.
bores 471 from the inside of insert 480 to provide a shoulder at the exterior end of the bore.
Figs. 14-16 illustrate a further modular pump design in which not only the housing but also the gearing and feed rollers are modules. Pump 530 includes a split inlet structure 550, and split housing casing 552.
Inlet structure 550, Fig 14, includes a pair of plate-like structures 550A and 550B having L-shaped interlock end portions 550C and 550D between which is located a pair of complementary-shaped inserts 550E which extend the axial dimension of the inlet structure. Plates 550A and 550B and inserts 550E are secured to the under-lying gear housing 552 by fasteners 556.
Housing 552 is a modified version of housing 52 and includes an întegral steam chest 554. Housing 552 is a pair of symmetrical "hal~es" 552A and 552B as de-scribed above in connectîon with Fig. 13 and an inter-mediate insert 552C. The housing "halves" and insert underlie the inlet structure and ar~ interconnected by fasteners 552D.
The interior surface of housing 552 defines a trapezoidal receiver 581 in whieh an insert 580 is ~- located. In~ert 580 i9 retained in receiver 581 by the overlying end portions 551 of inlet structure 553 and fasteners. The interior surace of insert 580 defines wiper portion 5&8, media free apace chamber 558 and seal zone~ 575, as previously described. Insert 580 ex~ends between the end portions 552A and 552B, Fig. 16, across insert S52C. A modular outlet insert 503 is retained in 31.
cutout 503A in the gear housing portions by fasteners not shown. Insert 503 defines the outlet discharge 503B at its inner surface and abuts against a bottom flange portion 552E of the gear housing. Insert 580 in turn overlies the top surface of the flange portion 552E.
With reference to Fig. 15, additive injection is provided by extruder 102 and conduit 390 at the inlet structure and may also be provided through conduit 590 (shown plugged3.
The roller assemblies 652 and gear assemblies 670 are also modular in construction and are mounted on splined shafts 652A and 670A, respectively.
The modular arrangement of roller assemblies 652 and gear 670 enables fhe performance characteristics of the pump to be varied and also permits ready replace-ment of parts. More particularly, the use of helical - gears to form a herrin8bone gear pattern has several advantages: -(a3 The choice of a~y helix angle can be accommodated due to the reduction of fabrication difficulties.
(b) The expanded choice of helix angles permits optimization of the gears with respect to varying physical properties of the reactor product mix.
(c) Gear segmen~s can be varied to match reaotor conversion rate.s.
(d) Xn order to facilitate additive dispersion, a smaller circular pitch gear can be employed.
With particular reference to Figs. 14 and 16, 32.
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each roller assembly 652 is formed by a pair of end rollers 652B having integral end gears 652C and a central cylindrical roller 652D. To increase the axial length of roller assembly 652 additional rollers 652D or different length rollers 652D are added to the assembly. Shafts 652A include stub shafts rotatably mounted in inlet structure 550 by bearing 653 having end thrust collars.
As described above,gears 652C mesh with the underlying gear to provide conjoint rotation therewith.
Gear assemblies 670 are also modular and may be formed completely by helical gears 670B as illustrated by the right side gear assembly (as viewed in Fig. 14).
In this case, a central pair of helical gears are posi-tioned symmetrically about the center of shaft 670A to form a herringbone pattern and additional helical gears are stacked axial ~herefrom. Thus an even number of helical gears form the herringbone pattern.
Alternatively, a central herringbone gear 670C
as illustrated on the left gear assembly in Fig. 14 may be used. In this case, hetical gears 670B are positioned axially on both æides of ~he central herringbon~ gear 670C to form the herringbone pattern.
Thus, an unlimited ~ariatio~ of axial lengths for the roller and gPar assemblies may be provided by the addition of roller and gear modules, respec~ively.
Gear shafts 670A are rotatably mounted in hous~ng 552 by bearings which may be a collar bearing 653 ; ~ or a journal bearing 671.
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Fig. 16A illustrates an alternative pump structure similar to Fig. 16. Roller assembly 75~ incudes a segment shaft 752A formed by a plurality of threadably connec~ed or butted segments 760 to allow axial enlarg~-ment by the addi~ion of segments thereto. Each roller shaft 752A is driven independently of the gears 770 by drive 761. The roller surface is provided by multiple - cylinders 752B which are keyed ~o the shaft segments for rotation therewith.
Gear assembly 770 includes a segment shaft 770A
having hollow shaft segments 770B interconnected to end shaft 770D by a longitudinally extending bolt 770E. The exterior surface of shaft 770A is splined to retain helieal gears 670B therein.
Figs. 17 and 18 illustrate a further modular pump 830 which is close coupled to vent stack 22 by means ; of the inlet structure 850, thereby eliminating the necessity for section 22A and the attendant pressure loss. Xnlet structure 850 includes an additi~e injection tube 829 which extends across and above the mesh line of the gears 870.
The internal configuration of ~he gear assembly may be of~any of the types previously described, i,e~., one piece or modular. Therefore, a detailed description of the assembly will not be repeated.
The basic attribute of modular pump 830 is the provision of five basic modules, a pair of gear hous-ings 852 which are symme~rical about ~he mesh line of gears 870; a pair of plate-like end structures 860 which :
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abut against each end of the housing 852 and extend downward therefrom; and an outlet structure 803 which abuts against the bottom of the housing 852 and is positioned between the end structures 860.
With reference to Fig. 17, the pair of housings 852 have thickened side walls which include a plurality of axially extending heat transfer fluid passageways 853.
The interior surface 858 of housing 850 defines the media free spAce chamber and seal zone 875, and outlet aperture, as previously described.
The end structures 860 carry the gear shafts 870A within bearings 871 and are fastened to the housings 850 and outlet structure 803 by longitudinally extending bolts 876. As illustrated, outlet structure 803 is close coupled to a mix~r module 900, which is also fastened ~o end structures 860 by bolts 876.
In addition, end s~ructures 860 include a manifold network 901 having an inlet 901A and outlet 901B
for interconnecting the passages 853 in housings850 and ~0 supplying the heat transfer medium thereto and to passage 903 in outlet structure 803. The manifold 901 may be directly abutted agalnst the passages, or a nipple coupling 905 may be provided. An additional heat transfer ne~work 904, having an inlet 904~ and outle~ 904B is provided for the depending por~ions of the end structures.
Pump 830 provides several advantages including:
(a~ Interchangeable media free space configur-ations for particular product mixes.
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!l (b) The individual replacemPnt of worn o.
corroded sections.
(c) Facilitation of finishing and treatment of internal surfaces.
(d) Provision of mixing/dispersion systems in the outlet module as required for additive applications.
These and other modifications may be made to the present invention by those skilled in the art without departing from the scope and spirit of the present inven- -tion as poineed out in the appended claims.
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As employed herein, the term "polymer(s)" is ~derstood to mean organic homopolymers, copolymers, and polymeric mixtures either alone or containing additives of the type normally encountered in the plastics field, e.g., stabilizers, fillers, processing aids, colorants and other additives (such as anti-block, anti-oxidation, anti-static and the like).
Background In the typical production of low density poly-ethylene, a reactor discharges a ~tream which i~ a mixture of polymer and unreac~ed ma~erials to a product reeeiver.
The product receiver operates at a pressure substantially below the reactor pressur~ and flow of the reactor dis-~ 20 charge is controlled by the product valve. In the product I receiver, the m~jor portio~ of the unreacted materials are removed due to flashing which results from the trop in pressure experienced by ~che mixture. The flashed material, commonly referred o as the return gas, is sub-sequen~ly returned to the reac~or. The remaining polymer-lzed material se~tles in the product receiver and still contains some unreacted m~teria~ which are removed in the remainder of ~he polymer recovery system.
~2 '.1 ~ ~ ~ 6 ~
The polymer discharged from the product receiver is fed to an extruder through a polymer flow control system. The extruder performs two functions in this system: final devolatilization to remove the remaining unreacted material; and pumping of the polymer through a screen pack, if one is being used, and a pelletizer die plate.
The material enters the side of the extruder and the unreacted materials flash and form a foam having a very low density. Therefore, an extruder having a very large volumetric conveying capacity in the feed section is necessary ~o handle the material as final devolatili-zation ls occurri~g. Normally, an ex~ruder having a two~diameter screw or an oversize single-diameter screw ; is required to obtain the con~eyi~g capacity to handle - the material entering the extruder. In some installation~, a portion of the flashed material is removed from the extruder through a vent stack.
As the production rate o~ single low density polyethylene (LDPE) reactors are increased, larger and larger extruders, which become prohibitively expensive, are needed. In an effort to eliminate the u~e of two-diameter extruders or oversized extruders, some existing units have been modified to include a secondary e~hylene separation (flashing) operation up~tream of the extruder inlet subseq~ent to the primary product receiver ethylene separation ~flashing) oper~tion.
is ~ystem differs from the side-fed extruder, 3.
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top mounted vent stack type in that the m~erial is fed into the top of the vent stack and essentially all of the remaining unreacted ma~erials are released before the polymer stream enters the extruder. This provides a material to the extruder which has a much greater density and eliminates the need for two-diameter extruders or large single-diameter extruders. The devolatilization an~
pumping functions of the original, two-diameter extruder system have now been separated, i.e. the final devolatil-lzation is performed in the vent stack and only the polymer pumping is performed by the extruder. However, extruders pump polymer by developing viscous drag, and are very ine~ficient pumps.
ummary of the Invention The present inven~ion has utility in a plasticsrecovery system in which polymer produced in a reactor is partially devolatilized in a product receiver, trans-ferred to the top of a vent stack for further devolatili-zation and flood feed to a low ne~ positive suction head gear pump.
: The use of the specially dPsigned gear pump of the preseDt invention ha~ing a free filling space surround-ing substantially the entire periphery of the gears allows an interaction of the material being pumped caused by the rotating gears and the pump body above the sealing zones to generate pressure above ~che pump inlet pressure, similAr to the pressure generation tha~ occurs in a hydromatic housing, ~o facili~ate filling of the gear 4.
~ 6 ~ ~ ~
tooth cavities. Additionally, the shear areas and energy dissipation in shear is an order of magnitude lower than comparable positive displacement pumps.
Specifically, the gear pump includes a pair of intermeshing, rotatable gears surrounded by a filling chamber increasing in volume ~oward the pump inlet.
Horizontal gear axes orientation is preferred, but not essential. The gears seal with the housing at a minimum seal area sized to seal substantially the distance between adjacent teeth across ~he full axial length near the outlet of the pump, said outlet having a cross-sectional area from 0.2 to 2.0 times the product of the gear tooth height and ~he ~ear face width.
The outlet, which is sized to interface with the gear pump, is preferably rectangular in shape with the major dimension parallel to the axes of the gears. The outlet may, however, be circular, oval or of any other cross-sectional shape desired from the operating standpoint.
In some polymer processing systems wh~re the polymer is discharged in the form of a rope, a pair of rollers is provided directly above the gears. The rollers pull the polymer rope in~o a charging area between -the rollers and gears to fil} the gear cavities and also ;~
densify the foamed polymer.
Suitable instrumentation is disclosed for ;~ monitoring pump parameters ~o provide on line ~iscosity and production rate control.
Thu8, the features to be described in greater detail ~elow include n comprehensive low density polymer 5.
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recovery system which includes a novel gear pump.
Another feature is a novel gear pump structure for handling a broad range of production rates.
A fur~her feature is a gear pump having roller charging capabilities for pumping polymer material produced in the form of strand or rope.
Yet another fea~ure is the provision of on line additive injec~ion at the pump of a recovery system and modular construc~ion of the pump body, feed gears and rollers.
These and still other features will be readily apparent from ~he drawings and the following disclosure.
Brief Description of the Drawings Fig. 1 is a schematic view of a low energy ~
recovery system in accordance with ~he present invention;
Fig. 2 is an enlarged cross-sectional view of a gear pump for use in the low energy recovery system of Pig, l;
Fig. 3 is a eross-sectional view with certain portions broken away for clarity taken generally alsng line 3-3 in Fig. 2;
Fig. 4 is a top plane view taken generally long line 4-4 of the pump in Fig. 2;
~` Fig. 5 is a cross-sectional view of a gear pump - with feed~rollers for handling feed in rope form;
Fig. 6 is a sectional view taken along line 6-6 i~ Fig~ 5;
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Fig. 7 is a top plane Yiew of the pump of Fig. 5;
Figs. 8A and 8~ are schematic views illustrating compounding sys~ems equipped with the pump of Figs.2 and 5, respectively;
Fi~. 9 is a schematic view illustrating the use of a gear pu~p of the present invention in combination with an extrusion die ~r mold in a fabrication processi Fig. 10 is a cross-sectional view of a modular gear pum~ with feed rollers wherein the entire inlet/gear cylinder section is replaceable;
Flg. ll ls a sectional view taken alung line ll-ll in Fig. 10;
Fig. 12 is a cross-sectional view of a modular gear pump wherein the body is formed by two halves for increasing t~e gear width by modular additions;
Fig. 13 is a sectional view taken along line 13-13 in Fig. 12;
Fig. 14 is a top plane view of a modular gear pump with feed rollers wherein the gears and rollers are : 20 modular for increasing the capacity of th2 pump;
Fig. lS is a cross-sectional view taken generally along line 15-15 in Fig. 14;
Fig. 16 ~s a cross-sectional view taken generally along line 16 16 ~n Fig. 15;
Fig. 16A is a cross-sectional view similar to Fig. 16, illustra~ing an external drive for the feed rollers;
Fig. 17 is a side elevation view with certain portlcns broken away in cr~ee-sectl~n illustrsting a 7.
completely modular 8ear pump embodiment; and Fig. 18 is a ront Plevation view of the pump of Fig, 17 wi~h certain portions broken away in cross-section.
Detailed Description of a Preferred Embodiment While this invention is susceptible of embodiment in many differene forms, there is shown ~n the drawings ; and will hereina~ter be described in detail a preferred embodimert of the invention, and modifications thereto, with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is n~t intended to limit the ihvention to the embodiments illustrated.
The ~ecovery 5vstem Fig. 1 illustrates an integrated system for the production of low density polyethylene in accordance with ~-the prese~t invention. A composition of liquid polyethylene :
and entrained ethylene is produced in a reactor (not ~hown), as is well known in the art. The composition is ~; 20 discharged from he reartor through line 10 into a product receiver 12. The mixture flow to produet receiver 12 is controlled by a product valve 14 and a pressure relief valve 16 in line~10.~ The bulk of ~he entrained e~hylene gas i removed from the mixture by flashing in product recei~er 12 and ~he thus removed ethylene gas is condensed , ;~ ~ and recycled to the reactor through line 12A for fur~her use. The partially devolatilized polye~hylene collects at the lower portion of produe~ reeeiver 12 and is transferred through line 20 o the i~let of ven~ s~ack 22.
8.
: , ~ ~ ~ 6 ~ ~ 6 Transfer of the liquid polyethylene to the vent stack is accomplished by a pressure gradient which exists between product receiver 12 ant vent stack 22. The product receiver operates at pressures in the order of 1000 p.s.i.
or higher and the vent stack is operated at pressures at or slightly above or below a~mospheric pressure. The fl~w rate of polyethylene to vent stack 22 is controlled by a feed control valve ~24 in line 20, and a Rufficient le~el of liquid polyethylene is main~ained in the pr~duct receiver to prevent blow through of ethylene directly to the vent stack.
As the polyethylene enters vent stack 22, sub-stantially all of the remaining entrained ethylene flashes from the mixture due to ~he pressure-equilibrium relation-~hip at the pressure of the vent stack. The low pressure in the vent stack and removal of the ethylene is accomp-lished through a return line 26 which includes a pumping means 28, such as a venturi nozzl~ or vacuum line. The ~: return fluid in line 26 is collected for further process-ing in the reactor.
Polyethylene melt P ~e tles in the vent stack and 18 sll~wet ~o achie~e a l~quid level L. The l~wer portion of ~ent stack 22 is i~ fluid communication with a gear pump 30, described in greater detail belvw, through a tapPred outle~ por~ion 22A. The level of the liquid polymer in the vent sta~k is prefera~ly m~intained such that polyethylene-ethylene interface is above the pump : ~ inlet~ant the diameter of ~he vent stack is such that the pressure drop caused by ths fl~win~ polymer pool is equal .
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, Il 1~66~6 to, preferably less than the pressure head formed by the liquid level of the polymer. Thus, gear pump 30 is flood-fed by the liquid polymer P in the vent stack.
Gear pump 30 is driven by a variable speed drive 32 and torque coupling 34 to withdraw polymer from the lower portion of the vent stack and pu~p it under pressure through conduit 36 into a pelletizer 38 for final process-ing and delivery of the polymer to product hopper 40.
Control of the system is provided by a central process control 42 which, in addition to monitoring the function and operation of the reactor, as is known in the industry, monitors and controls the above-mentioned equip-ment. More specifically, a liquid level controller 44, wh~ch may be either of the displacement or rate types, as discussed ~ore fully below, senses the level (or rate of change of level) of polymer in the vent stack. This variable is utilized by the process control 42 to set ~he variable Ppeed drive 32 to insure flood feed operation and full capacity capabili~y of the pump. Also, the level control may be utilized to provide an indication of production rates, as discussed below.
Additionally, the process control unit monitors power input ~o the pump, and inlet and outlet pressures and temperatures at the pump through lines 46. The exact instrument for monitoring these variables are well known in the $nstrumentation art and include torque meters, pressure transducers, either mechanical or electrical, e.g., piezoelectrictransducers and thermocouples or thermistors, respectively. From these measured variables on-line 10 .
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viscosity monitoring is achieved. Viscosity is inversely related to ~le product mel~ index which is a primary control variable for reactor control in the production of polymers. Thus, these measured variables are fed back in the process control unit 42 to provide control of polymer production in the reactor.
The Gear Pump Figs. 2-4 illustrate gear pump 30 specifically designed to provide a minimum net positive suction h~ad and improved polymer pumping ca~ability. Ptm~p 30 is modular in construction and i~cludes an inlet structure 50, gear housing 52, a~d steam jacket 54. Inlet structure 50 includes two plate-like elements 50A and 50B of generally L-shaped configuration and having interlocking end portions 50C and 50D as best illustrated in Fig. 4. Inlet s~ruc-ture 50 ~s coupled to a peripheral flange 22B on the out-let of ~ent stack 22A and to gear housing 52 by a plurality of fas teners 56, Fig. 2.
Gear housing 52 is generally rectangular in outer dimension and includes an upper flange por~ion 52A, -~ which abuts against ~he lower surface of inlet structure - 50, and sidewalls which define a gear chamber 58 at their inner surface and ~aper downwardly at two opposite outer surfaces 60 to define the inner surface sf a steam chest.
The steam jackets 54 are removably moun~et to the gear housing 52 by means, not shown, and complement the tapered sides 60 to form a steam chest about the gear pump. The steam jackets include a steam inlet 54A and condensate outlet 54B ~or admitting s~eam from a source (not ~hown) 11 .
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to maintain the pump at a temperature above thP melting temperature of the polymer. Alternatively, pass~geways ~uch as drilling, described below, may be utilized in a solid pump body configuration.
Referring particularly to Figs. 2 and 3, inlet structure S0 defines an inle~ 66 whose width (~arallel to the gear axis) is approximately equal to the gear width, Fig. 3, and whose minimum length (normal to the gear axis) taper down~ardly to complement the gear chamber 58.
A pair of rotating, inter~eshing gears 70 are positioned in gear housing 52. ~ny one of a wide variety of gear pairs m~y be employed, intermeshing herringbone gears being preferred.
As 8hown in Fig. 3, a pair of intermeshing herringbone gears 70 are rota~a~ly mounted in gear hous- -ing 52 by means of journal b arings 71 and end pla~es 72.
One of the gears (the drive gear) includes a drive shaft 70A which extends outwardly from the gear pump to be coupled to the torque coupling 34 ant thereby provide power input to ~he pump. The o~her gear is driven by the intermeshing relationship of the ~erringbone gears.
Journal bearings 71~ are lubricated by the polymer ~elt, and as discussed b~low provide a means for monitoring the viscosity of ~he polymer melt. The length and diameter of the bearings and clearances are such that high pressure leakage is mi~imized by a throttling effect.
The chamber 58 in which the inte D eshing gears 70 are loca~ed is d~signed to achieve an inlet of minimum restriction of poly~er flow ~o improve volumetric efficiency.
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, Substantially all of the faces of the gears are exposed to the media free space (the volume between the gear faces and walls of chamber 58). This configuration permits the development of a pressure gradient within the media free space to facilitate the filling of the gear tooth cavities with polymer.
To achieve the necessa~y large media free space and still provide the necessary seal between the outlet 103 and the low pressure inlet of the pump, the pump discharge opening 103 and seal zone 74 must be minimized.
The portion of outlet 103 in direct communication with the gear chamber defines a generally rectangular shaped open-ing having a width 103A, Fig. 3, equal to the gear face width and a length 103B, Fig. 2, equal to about the gear tooth height.
The outlet is defined, in part, by high wear resistant seal zone inserts 75, which are removably secured to a seat 58A in the gear housing as by bolts, not shown.
Each seal zone extends from the ~ip 75A of the inserts 75 for a circumferential distance at least equal to th~
distance hetween adJ cent teeth across the full axial length of ~he gears to provide an effeetive seal between the gear teeth and gear housing. The star~ of the seal zone is generally indicated by numeral 75B, and the seal extends to tip 75A. It should be noted that a seal leng~h larger than that ~ust specified does not significantly increas~ ~eal effectiveness.
The inner surface of chamber 58 increases in dimension from the start of the ~eal 75B in a continuous 13.
smooth curve to join ehe inlet structure chamb r 66 just above the top of gears 70.- In one pump embodiment the gear chamber or media free space boundary extended from a location 75B 53 bel~w the horizontal passing through the : center of gear 8XiS in a circular arc to a location 15 b~low the horizontal to provide a radial distance between : . the tooth tip and cha~ber surface of 1/2 inch at that point, and extended linearly from the 15 point to the top of the pump. In general, the ratio of free media space expansion to the circumferen~ial distance tas measured from the end of seal zone 75B) is optimized in accordance with lubri-cation theory to achieve an increasing hydraulic radius in the media free space to fill the gear too~h cavi~ies with polymer. The media free ~pace increases from the seal zone 75B to produce a maximum pressure gradient at a point preferably below the horizontal elevation of the ~ear axes. The optimal media free space configuration is a function of the particular polymer being pumped. A ratio of the clearance (gear tooth to chamber wall~ to the circumferen~ial distance from the seal zone in the range o~ about 0.2 to 0.9 is a typical value for polymers having viscosities in the ran~e of 0.2 to 1.4 lb. sec/in2, ~`; ; respec~ively.
: : With particular reference to Fi~s. 2 and 3, the outlet 103 changes rom the slot-like opening adjacent the gear~ to a circular discharge outlet 103C at its lower : end, which is in fluid communication with condui~ 36.
: The herringbone gears 70 may have helix angles ran8ing up to 30; however> smaller helix angles, 15 or 14.
, less, are preferred to avoid leakage at gear intermesh.
Most preferably, herringb~ne gears with a helix angle o 7-10 are used to avoid trapping polymer in the tooth cavities. The use of herringbone pa~tern gears is preferred to constant helical or spur gears due to reduced stress loading on the gears and housing realized with the herringbone gears.
The use of a gear pump and more particularly a gear pump with herringbone gears provides economic savings in capital cost, operating cost and improved performance in comparison ~o screw extruders used for the polymer ~` recovery system. More specifically, for a recovery system capable of handling 30-50,000 lbs/hr, system investment cost may be up to 50% less for a gear pump equipped system. In gear pump recov ry systems for the indicated capacities, h~rsepower requirements ranged from 7 to 17 horsepower as opposed to 35 to 60 horsepower for a comparable extruder system. A comparable reduction in shaft horsepower equates to an annual cost savings for a 50,000 lb/hr system in the order of ~150,000/year assuming 3.6 cents/kw. hr. eleotric costs.
Finally, the viscous shear dissipation work is greatly redueed by the use of ~he gear pump as compared ~o a screw extruder. The reduction in this work is about one order of magnitude. This reduc~ion, of course, results in a reduction in the temperature rise of the poly~er as it passes through the gear pump. Steady state temperature rises of 1 to 4 C have been experieneed with the gear pump of this invention, as compared to 10 15.
to 35 C for a comparable screw extruder. The reduced temperature rise provides better produc~ property control and less thermal degradation of the polymer product.
The gear tooth helix angle and media free space of the gear pump provide improved handling and volumetric efficiency. Table I indicates a comparison of operating parameters for se~eral configurations which illustrate the effects of gear tooth angle and media free space on gear pump performance. The body type~ include the one described above and designated as "FULL" and one designated : "MIN" which had a media free space inlet dimension located 25 above the horizontal gear axis and extended therefrom in a circular arc to a location 20 below the horizontal gear axis (the start of the seal zone). Each unit was equipped with herringbone gears with the indi-cated helix angles and identical outlet shapes.
From Table I, it will be noted that ~he FULL
media space configuration, i.e., the least restricted inlet configuration, provided volume~ric capacities which were essentially constant for the range of produc~ tested.
The gear pump of the present in~ention is essentially insensitive to vi~cosity and pressure conditions with respect to i~s pumping capacity, and its volumetric efficiencies are essentially 100% over a range of gear ~pitch line velocities up to 150 ft/min.
The designations MIN and FULL as well as the characteristics of the pump structure given in connection with Tabl~ I are illustrative embodiments and should not be interpreted as se.tting forth the limits of applicants' :~ :
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inventi~n. The values listed in the first six columns from left to right represent short tests for peak per-formance of the gear pump, while ~he last two columns represent long term steady-state operating conditions for a reactor system having the indicated production rates.
An additional advantage of the gear pump is increased recovery system operation. Unlike a screw extruder system which is highly sensitive to polymer viscosity, the gear pump being a posi~ive displacement pump will continue to pump even if large changes in viscosity are experienced. An extruder on the other hand, would have to be shut down and cleaned, when high or low viscosity are introduced into it. Thus, reactor down time is decreased and reliability is greatly increased with the gear pump.
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On-Line Viscosity Monitoring and Reactor Control As previously described, the gear pump recovery system includes ins~rumentation that measures pump input power and inlet and outlet pressures and temperatures.
From this information, the signals are conditioned to provide on-line viscosity monitoring system which is related to the product melt index as is known in the art.
This signal is then used to monitor the product melt index ~ia ~iscosi~y monit~rin~ and tied bac~ ~nto the reaction control system for closed-loop melt index con~rol. Since the component instrumentation for carrying out the indi-vidual measurement of the~e variables is well known in the instrumentation art, a detailed dcscription of them will not be presented. However, the manipulation of these da~a to achieve viscosity monitoring and production rate ron-trol will be described in detail.
The gear pump horsepower is ~he su~ of the volumetric pumping work and viscous di~sipation work. The viscous dissipation work is primarily due to viscous dissipation work in the pump bearings. Assuming all the viscous dissipation takes place ~n the pump bearings, the total horsep~wer can be represented by:
HP - ~ ~VA t Q ~p [1 where: ~ ~ ma~erial visc05ity in bearings;
shear rate in bearings;
V ~ surface velocity of shaft in bearings;
A ~- ~otal surface area of shaft in bearing - ~LD
Q ~ volume of ~terial being pumped; and ~-~p ~ pressure dlfferential across the pump.
19 .
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The total horsepower can also be repre3ented by:
HP - 2~TN ~2]
where: T = torquei N ~ rotational speed.
Furthermore, the shear rate (~) can be repre-sented by:
~ ~ ~DN ,' C ~3]
where: D - bearing diameteri C ~ bearin~ clearance; and Q can be expressed by:
Q ~ qN [4]
where: q ~ tisplacement/rev.
Combining these equations and using constants for the fixed parameters results in the following relationship:
klT k2 P [5]
where:
.
kl _?C _ snd k2 ~ qC
~2D3L ~D]3L
The last term (k2 P) can be elimina~ed for a simplified .~ model o provide:
~ klT [63 -: ~ : In plant experiments, the ~orque and speed measurements produced ViBcosity determinations which agreed closely with the laboratory melt index ~nalysis of the materials being processed.
:; The mod~l can:be modified to include produc~
temperatu~e e~fects on the viscosity monitoring systems by ~ measuring the product temperature at the i~let or outlet '.':
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, L,,r~ ~ ~,¢~6 of the pump. This modification has only a slight effect on the viscosity determinations since the temperature of the material in the bearings will reach a temperature which is dependent primarily upon the pump speed and mater-ial viscosity rather than the pump inlet or outlet temper-atures. In effect, the temperature variations of the product being processed will have a small effec~ upon the on-line viscosity determinations.
Due to non-linear relationship between ~elt index and viscosity and simplif.ying ~ssumptions in the model, the viscosity may not always be a true viscosity but it will provide a trend indication, which is one of the main criteria needed for closed-loop control systems.
Production Rate Indication .
The level L in the vent stack 22 can be main-tained at a set level by varyi.ng the pump speed as neces-sary to maintain a set level using conventional set point control equipment. When using this type of ~ent stack level c~ntrol, the instantaneous production rate is directly proportional to the pump ~peed and can be measured by a tachometer to provide production rate data to con~rol 42.
The gear pump speed can also be controlled by using the continuous vent stack level signal to directly ontrol the gear pump speed. In ~his mode of operation, a higher production rate would tend to fi~l the vent stack to a higher level and then the direct use of ~he continu-ous lev~l slgnal to the gear pump speed control would increase the pump speed proportionately to the change in 21.
level. In normal operation, this leads to a higher vent stack level and higher pu~lp speed for higher production rates which are associated with the higher melt index materials. Thereore, the vent stack level can be used as an indication of the instantaneous production rate.
Modifi_d Gear Pump Figs. 5-7 illustrate a modified gear pump 130 of this invention for use in polymer recovery systems in which the polymer is discharged in the form of a tacky rope R, shown in phanton line in Fig. 5.
The use of gear pump 30, described above, in a polymer rope discharge system would tend to force ~he rope against one side of the pump inlet due to the rotation of gears 70. This would lead to reduced pumping capacity since the rope would feed only one gear or possibly bridge the pump inlet resulting in loss of pumping.
To prevent these situations from occuring and to pro~ide a positive inlet pressure to the gears, pump 30 is modified to achieve pump 130. Common elements of pump 30 and 130 are correspondingly numbered, and a detailed description of these common elements will~not be repeated.
Due to the modular ~onstruction of the gear pump~ 30, only inlet strueture 50 need be removed and re-; placed with modified inlet structure 150 to achieve the -alteration of thP pump.
So handle rope R, a set of rollers 152 is posi- - -t~oned i~ the pump inlet to pull the rope into the pump and provide a positive inlet pressure for filling the pump , ~
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tooth cavities and also densifying th~ polymer, if it is in a foamed condition.
With particular reference to Fig. 6, each roller 152 includes a cylindrical roll portion 152A, a pair of helical end gears 152B which mesh with the herringbone gears of the associated gear 70 below each roller, and axially extending stub shafts.l52C. Sha~ts 152C are rota~ably mounted in inlet structure 150 by bearings 154.
In this manner, each roller 152 is driven by its associ-ated herringbone gear in the directions indicated in Fig. 5.
For foam materials o~ very low density it is preferahle to drive rollers 150 independently of gears 70 to obtain desired feed rates to the pump. This may be accomplished by positioning the juxtaposed gear 152B on ~-each roller such that they intermesh with each other but -not with the h~rringbone gears 70, and providing an external drive.
As best illustrated in Fig. 5, inlet structure ~:~ 20 15a defines an inlet 166 which is com~lementary to the cylindrical surface of roller 152A, and is provided with cutouts 167, Fig. 7,for receiving end gear 152B. The , ~ ~
~ : : minimu~ width of the inlet`(parall21 to gear axis) is .~ ~
~ : equal to the width o gears 70.
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:~ The bottom surface 151 of inlet stru ture 150 :.
1: overhangs the gear chamber 58 at each side, Fig. 5, ~nd a pair of wipers 168 having a generally wedge-shaped ~ross section abut against these overhanging portions and : are secured w~thin chamber 58 by bolts, not shown. Each ' i :' 23. .
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wiper 168 includes a wiper surface 168A positioned at zero clearance with ~he surface of roller 152A. Wipers 168 thus prevent polymer from being recirculated about the periphery of the rollers as they rotate in the direc-tions indicated.
In operation, rope R is fed between the nip of rollers 152 which press the polymer into a charging zone 170 to fill the tooth cavities of gears 70. The polymer is carried by the gears 70 into the gear chamber or free media space 58 and then through the seal zone 74 (shown enlar~ed for clarity) for positive displacement tpumping) înto outlet 103, as the herringbone t~eth of the gears mesh.
On-Line Direct Additive Injection ; Both the gear pump 30, Fig. 2, and modified gear pump with rollers 130, Fig. 5, include provision or the direc~ injection of additives into the pump for mixture with the polymer. The additive in liquid fvrm may be pumped through plural conduits 130 (only one of which i5 ~; 20 illustrated) in the inle~ ~ection of the pump ei~her para-llel or perpendicular to the gear axis, Tig. 5, or into contact with roller 152A, which doctors the additive into : the charging zone for mixture with the polymer. Solid additives, which cannot be melted for introduction into the pump via condui~s 100, may be fed into the pump through a screw ex~ruder 102, built into the 8ear housing and extending laerally outwardly from the pump.
Extruder 102 includes a screw 102A, which is driven by motor 102B and extends coaxîally within a ~4.
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tubular housing 102C from an input hopper 102D atone end to the media free space adjacent the gears. Preliminary mixture of the additive and melt is achieved in the terminal end of the extruder, by means of a high pressure melt feedback conduit 104 which extends from the outlet of the gear pump to the intermediate portion of the extruder. In this manner the melt and additives are mixed in the last few flights of the extruder screw and intro-duced into the media free space for final mixing with the 10 polym~r in the gear pump. Conduit 104 may be either sized to provide proper feedback ra~e or may be equipped with an orifice plate or valve to provide variable feedback.
In addition to additive injection at the gear pump, additives may also be introduced through ~he top of vent stac~ 22. In the case of ~olid additives, they may be sprinkled over the surface of the polymer mel~ where the heat of the polymer melts ~he additives. The thus melted additives are mixed with the polymer as the ; material passes through the shear field in the gear pump.
The mixlng of addi~ives and polymer, whether introduced through ven~ stack or gear pump, is accomplished in the pump by the shear actions induced in the flow patterns wi~hin the pum~, particularly as the material enters and is 6ubsequently displaced rom the gear tooth cavities.
For materials requiring a higher degree of mixing than performed by the gear pump, a s~atic mixer, e.g. , one of the type known co~mercially as a Kenics mixer, may be connected to the discharge of ~he ~ear pump 25.
,~,,r~ 5"~j to provide further mixing.
Compounding Systems The gear pumps described above may also be utilized in compounding systems to increase the capacity and lower the operating costs. Compounding systems are similar to a recovery system in that more than one opera-tion is performed by the compounding system. Compounding involves fluxing and mixing and in some cases pumping.
Compounding systems using extruders, whether single-screw or multiple-screw, are modified so that only the fluxing and mixing operations are performed by the extruder. The material is then fed to a gear pump for the pumping operation.
Figs 8A and 8B illustrate schematically two compounder-gear pump systems, 200 and 250, respectively.
System 200 includes a compounding mixer 205 which may be of the batch or continuous discharge type. Compounder 205 discharges material through outlet 206 into a gear pump 30 -of the type described above. By relegating the pumping ; operation to gear pump 30, which 1s insensitive to viscosity fluctuation, increased compo~nding capacity and production rate are achieved with a reduced product temperature rise. Final temperature reductions of up to 80~ C may be achieved by the utilization of gear pump 30 for the pumping operation.
, The system 250 includes a compounder 252 of --either the batch or continuous process type which dis-charges the polymer in the form of rope R from outlet 252A.
~ ~ The polymer rope is enclosed within a conduit 254 which `~ 26.
. , !l extends from outlet 252A to ~he inlet of a roller equipped gear pump 130, described above, for pumping. An inert atmosphere such as nitrogen gas is supplied to the interior of conduit 254 from source 256 ~hrough line 258. Any excess gas is withdrawn from conduit 254 through line 254A.
lt will be appreciated that the inert atmosphere reduces the oxidative degradation of the polymer as it passes from the compounder to the gear pump.
Fabricating Systems The gear pump of ~he present inven~ion may also be used in the ul~imate fabrication of material from polymers. Fig..9 illustrates schematically a gear pump 30 closely coupled to a plasticating extruder 260 by con-duit 262. Die or mold 265 is used generically to cover ; both a restrictor for the fabrication of continuous products 266 having a cross section determined by the restrictor, e.g. film, pipes, structural beams, and a mold for the ~abrication of molded articles. In the latter case, a control valve 264 is utilized in line 263 connecting pump 30 ~o die 265 to pulse polymer into the die mold ~65.
The gear pump 30 is the inal element for promot-ing polymer melt flow into the die 265. The use of the gear pump to provide either con~inuous pressurized polymer i for continuous extrusion or discrete charges of pressurized polymer ~or molding articles provides lo~er energy con sumption for the fabrication system than comparable screw extruder or pressure cylinder rethod, respectively.
27, .
Alternative Modular Embodiments The basic modular gear pumps 30 and 130, described above, may be further formed by modular compon-ents to increase the utility and flexibility. Various other modular configurations will now be described.
Modularity may be achie~ed in a number of ways including: (a) gear housing casting with modular body insert, Figs. 10 and 11; (b) gear housing casting which is split to i~crease the axial bore width with modular inserts, Figs. 12 and 13; (c) pump body fabricated of independent modular sections, Figs. 17 and 18; and (d) modular and roller configurations, Figs. 14-16A.
Figs. 10 and 11 illustrate a gear pump 30 similar to the pump 130, described above, which is equipped with feed rollers 152. Corresponding parts are designated by the same part designations described with pump 130.
As previously dascribed, the contour of the ~ -; media free space is a function of the viscosity of ~he particular polymer ~eing pumped. It will therefore be ~20 appreciated that the ability to easily vary the media free space of a pump is highly desirable. Additionally, the .
~ ability t~ change the gear diam~ters (and thus the media :; ~ : :
ree space expansion) is also desirable to accomodate the production rate of the reactor. To this end, pump 330 includes an inlet/cylinder insert 380 of generally :
trapezoidal shape which is~nestably received in a comple-men~ary cutou~ 381 within gear housing 52 which extends through ~he entire housing. Insert 380 is retained within ~ cutout 381 by the overhanging portion lSl of inlet struc~ure : ~ :
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28.
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150 ant fasteners (not shown) and defines the media free space chanber 358 at its inner surface.
Insert 380 also defines wiper portions 368 immediately adjacent each roller 152A and seal zones 375 adjacent the discharge outlet 103. Thus, the insert 380 ser~es the functions of several of the previously de-scribed structural elements. But, in addition, insert 380 also mounts the bearing 71 within cylindrical bores 371A
and defines the discharge outlet 103.
Thus, insert 380 together with herringbone gears 70, bearing 71 and end plates 72 form a remov~ble package which can be custom designed for a particular polymer viscosity and production rate. Insert 380 may also be easily heat treated or plated for desired characteristics.
P~mp 330 has certain other modifications over pump 130 including the deletion of feedback conduit 104 which, if desired, may be included. Additionally, additive extruder 102 has been relocated to the position of additive port 100 and an additive port 390, which may also include a screw ex~ruder, has been located at the i ~ mixing zone parallel to the axis of gear rotation.
Fig.s. 12 and 13 iIlustrate a pump 430 in which the gear housing 452 is cast in ~wo half sections which are symmetrical axially about a plane perpendicular to the gear axis to per~it axial variation of gear width.
The two halves are secured together by axially extending bolts or the like (not ~hown). Housing 452 includes a thickened ~ide wall 460 to accommodate a plurality of heat transfer passages 461 which extend laterally there-, ~ :
' 2g.
through and are interconnec~ed by vertical passages 462, Fig. 13. Steam or other heat transfer (heating or cool-ing) is circulated through passages 461 and 462 to control the temperature of the pump.
Similar to pump 330, pump 43~ includes an insert 480 which is retained in a complementary shaped cutout 481 within housing 452 by the overhanging portion 151 of inlet struc~ure 150 and fasteners. Insert 480 defines the media free space chamber, seal zone 475 at its inner surface 458, and outlet 103. Additionally, insert 480 mounts the herringbone gear shafts and bearing 71 in bores 471 to provide a removable assembly. In the - event that an axially larger assembly is to be inserted within housing cutou~ 481, the two halves of housing 452 are detached and a modular housing insert is positioned ~-; therebetween. These modular housing inserts a~e described below in connection with Fig. 14.
Since an axial enlargement of the housing 452 also enlarges the outlet 103, a modular outlet insert 403 may also be utilized. Insert 403 is retained in a rectangular cutout 403A at the base o~ housing 452 and held therein by fasteners (not shown). Insert 403 inrlud~ heat transfer conduits 461 and 462 which mate with those in hous~ing 452. The use of the insert 403 provides a eontinuous mating surface for the 1ange connection of the outlet pipe 36 or other equipment to which the polymer is discharged, The split housing 452 may also be formed to eliminate end plates 72. This is accomplished by machining , ~
30.
bores 471 from the inside of insert 480 to provide a shoulder at the exterior end of the bore.
Figs. 14-16 illustrate a further modular pump design in which not only the housing but also the gearing and feed rollers are modules. Pump 530 includes a split inlet structure 550, and split housing casing 552.
Inlet structure 550, Fig 14, includes a pair of plate-like structures 550A and 550B having L-shaped interlock end portions 550C and 550D between which is located a pair of complementary-shaped inserts 550E which extend the axial dimension of the inlet structure. Plates 550A and 550B and inserts 550E are secured to the under-lying gear housing 552 by fasteners 556.
Housing 552 is a modified version of housing 52 and includes an întegral steam chest 554. Housing 552 is a pair of symmetrical "hal~es" 552A and 552B as de-scribed above in connectîon with Fig. 13 and an inter-mediate insert 552C. The housing "halves" and insert underlie the inlet structure and ar~ interconnected by fasteners 552D.
The interior surface of housing 552 defines a trapezoidal receiver 581 in whieh an insert 580 is ~- located. In~ert 580 i9 retained in receiver 581 by the overlying end portions 551 of inlet structure 553 and fasteners. The interior surace of insert 580 defines wiper portion 5&8, media free apace chamber 558 and seal zone~ 575, as previously described. Insert 580 ex~ends between the end portions 552A and 552B, Fig. 16, across insert S52C. A modular outlet insert 503 is retained in 31.
cutout 503A in the gear housing portions by fasteners not shown. Insert 503 defines the outlet discharge 503B at its inner surface and abuts against a bottom flange portion 552E of the gear housing. Insert 580 in turn overlies the top surface of the flange portion 552E.
With reference to Fig. 15, additive injection is provided by extruder 102 and conduit 390 at the inlet structure and may also be provided through conduit 590 (shown plugged3.
The roller assemblies 652 and gear assemblies 670 are also modular in construction and are mounted on splined shafts 652A and 670A, respectively.
The modular arrangement of roller assemblies 652 and gear 670 enables fhe performance characteristics of the pump to be varied and also permits ready replace-ment of parts. More particularly, the use of helical - gears to form a herrin8bone gear pattern has several advantages: -(a3 The choice of a~y helix angle can be accommodated due to the reduction of fabrication difficulties.
(b) The expanded choice of helix angles permits optimization of the gears with respect to varying physical properties of the reactor product mix.
(c) Gear segmen~s can be varied to match reaotor conversion rate.s.
(d) Xn order to facilitate additive dispersion, a smaller circular pitch gear can be employed.
With particular reference to Figs. 14 and 16, 32.
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each roller assembly 652 is formed by a pair of end rollers 652B having integral end gears 652C and a central cylindrical roller 652D. To increase the axial length of roller assembly 652 additional rollers 652D or different length rollers 652D are added to the assembly. Shafts 652A include stub shafts rotatably mounted in inlet structure 550 by bearing 653 having end thrust collars.
As described above,gears 652C mesh with the underlying gear to provide conjoint rotation therewith.
Gear assemblies 670 are also modular and may be formed completely by helical gears 670B as illustrated by the right side gear assembly (as viewed in Fig. 14).
In this case, a central pair of helical gears are posi-tioned symmetrically about the center of shaft 670A to form a herringbone pattern and additional helical gears are stacked axial ~herefrom. Thus an even number of helical gears form the herringbone pattern.
Alternatively, a central herringbone gear 670C
as illustrated on the left gear assembly in Fig. 14 may be used. In this case, hetical gears 670B are positioned axially on both æides of ~he central herringbon~ gear 670C to form the herringbone pattern.
Thus, an unlimited ~ariatio~ of axial lengths for the roller and gPar assemblies may be provided by the addition of roller and gear modules, respec~ively.
Gear shafts 670A are rotatably mounted in hous~ng 552 by bearings which may be a collar bearing 653 ; ~ or a journal bearing 671.
' 33.
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Fig. 16A illustrates an alternative pump structure similar to Fig. 16. Roller assembly 75~ incudes a segment shaft 752A formed by a plurality of threadably connec~ed or butted segments 760 to allow axial enlarg~-ment by the addi~ion of segments thereto. Each roller shaft 752A is driven independently of the gears 770 by drive 761. The roller surface is provided by multiple - cylinders 752B which are keyed ~o the shaft segments for rotation therewith.
Gear assembly 770 includes a segment shaft 770A
having hollow shaft segments 770B interconnected to end shaft 770D by a longitudinally extending bolt 770E. The exterior surface of shaft 770A is splined to retain helieal gears 670B therein.
Figs. 17 and 18 illustrate a further modular pump 830 which is close coupled to vent stack 22 by means ; of the inlet structure 850, thereby eliminating the necessity for section 22A and the attendant pressure loss. Xnlet structure 850 includes an additi~e injection tube 829 which extends across and above the mesh line of the gears 870.
The internal configuration of ~he gear assembly may be of~any of the types previously described, i,e~., one piece or modular. Therefore, a detailed description of the assembly will not be repeated.
The basic attribute of modular pump 830 is the provision of five basic modules, a pair of gear hous-ings 852 which are symme~rical about ~he mesh line of gears 870; a pair of plate-like end structures 860 which :
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abut against each end of the housing 852 and extend downward therefrom; and an outlet structure 803 which abuts against the bottom of the housing 852 and is positioned between the end structures 860.
With reference to Fig. 17, the pair of housings 852 have thickened side walls which include a plurality of axially extending heat transfer fluid passageways 853.
The interior surface 858 of housing 850 defines the media free spAce chamber and seal zone 875, and outlet aperture, as previously described.
The end structures 860 carry the gear shafts 870A within bearings 871 and are fastened to the housings 850 and outlet structure 803 by longitudinally extending bolts 876. As illustrated, outlet structure 803 is close coupled to a mix~r module 900, which is also fastened ~o end structures 860 by bolts 876.
In addition, end s~ructures 860 include a manifold network 901 having an inlet 901A and outlet 901B
for interconnecting the passages 853 in housings850 and ~0 supplying the heat transfer medium thereto and to passage 903 in outlet structure 803. The manifold 901 may be directly abutted agalnst the passages, or a nipple coupling 905 may be provided. An additional heat transfer ne~work 904, having an inlet 904~ and outle~ 904B is provided for the depending por~ions of the end structures.
Pump 830 provides several advantages including:
(a~ Interchangeable media free space configur-ations for particular product mixes.
35, ,. .
!l (b) The individual replacemPnt of worn o.
corroded sections.
(c) Facilitation of finishing and treatment of internal surfaces.
(d) Provision of mixing/dispersion systems in the outlet module as required for additive applications.
These and other modifications may be made to the present invention by those skilled in the art without departing from the scope and spirit of the present inven- -tion as poineed out in the appended claims.
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Claims (26)
1. A gear pump for pumping highly viscous media comprising: housing means defining an inlet passage at one end and an outlet passage at an opposite end, and a chamber therebetween; a pair of pump gears generally horizontally positioned within said chamber having inter-meshing teeth and being rotatably mounted in said housing, said chamber and said pump gears defining therebetween a media free space, said chamber and each of said gears defining gear sealing zones adjacent said outlet passage, each sealing zone having a minimum length equal to the circumferential distance between two adjacent gear teeth across the full axial length of the gears at the outside diameter of said gears; said media free space expanding from the end of said seal zones remote from said outlet passage such that an increasing hydraulic radius is pro-duced at least to the plane of the tops of said gears, said increasing hydraulic radius being produced by an increasing horizontal dimension of the chamber from the end of said seal zones remote from said outlet passage to the plane of the tops of said gears, whereby a pressure gradient is created to fill the gear cavities with said viscous media; said outlet passage adjacent said sealing zones having a cross-sectional area of from 0.2 to 2.0 times the product of the gear width and the gear tooth height; and said chamber increasing in cross-sectional area above the tops of said gears to the top of said chamber, thereby enhancing the pumping capacity and volumetric efficiency of said gear pump.
37.
37.
2. The pump in accordance with claim 1, wherein said outlet is generally rectangular in shape having a width equal to the gear face width and a length equal to the gear tooth height with the major dimension being parallel to the gear axis.
3. The pump of claim 1, wherein the ratio of media free space clearance to the circumferential distance from said seal zone is in the range of about 0.2 to 0.9
4. The pump of claim 1, wherein said gears have a herringbone pattern.
5. The pump of claim 4, wherein the helix angle of said gears is between 5° and 30°.
6. The pump of claim 5, wherein said helix angle is between 7° and 10°.
7. The pump of claim 4, wherein each of said herringbone pattern gears includes a central herringbone gear and a pair of helical gears, each of said helical gears having a helix angle equal to the herringbone gear helix angle and being positioned in coaxial end-to-end relationship with said herringbone gear to form said herringbone pattern.
8. The pump of claim 4, wherein each of said herringbone pattern gears comprises a plurality of helical gears; said gears being arranged symmetrically to form a herringbone pattern.
9. The pump of claim 4, wherein said gears are mount d on a segmented shaft.
38.
38.
10. The pump of claim 1, further including means in said housing means for introducing an additive to said viscous media.
11. The pump of claim 10, wherein said intro-ducing means includes an extruder having its outlet positioned adjacent said free space and its inlet exterior of said housing.
12. The pump of claim 11, wherein said housing means defines a feedback chamber extending from said pump outlet to a point upstream of said extruder outlet, whereby polymer may be initially mixed with the additive in said extruder,
13. The pump of claim 10, wherein introducing means is positioned to introduce the additive at a location upstream of said chamber.
140 The pump of claim 1, further including a pair of feed rollers rotatably mounted upstream of said pair of gears, the axis of each roller being substantially parallel to the axis of the associated gear, and said rollers being spaced apart to define a media nip substan-tially coplanar with the mesh line of said gears; said housing means including wiper means adjacent each roller;
and means for rotating said rollers to draw said media into and feed said gears.
and means for rotating said rollers to draw said media into and feed said gears.
15. The pump of claim 14, wherein said means for rotating said rollers includes a gear train operatively coupling each roller to its associated gear, whereby the rollers are driven conjointly with said gears.
39.
39.
16. The pump of claim 14, wherein said means for rotating said rollers includes a drive means operatively coupled to said rollers, whereby said rollers are driven independently of said gears.
17. The pump of claim 14, wherein said rollers are modular in construction, whereby the axial length thereof may be varied.
18. The pump of claim 1, wherein said media free space is defined by the interior surface of said housing such that the free space increases in cross section from said seal zone in a smooth continuous curve, to a location upstream of said gears.
19. The pump of claim 1, wherein said housing is modular and includes two interconnected portions.
20. The pump of claim 19, wherein said housing modular portions are symmetrical about the plane extending transversely to said gear axes.
21. The pump of claim 20, further including modular housing insert means interposed between and complementary to said modular portions.
22. The pump of claim 19, wherein said housing modular portion are symmetrical about the mesh line of the gears.
23. The pump of claim 19, wherein said housing includes removable central insert means, said insert means defining said media free space at its inner surface.
40.
40.
24. The pump of claim 23, wherein said insert means extends the full axial dimension of said gear face width.
25. The pump of claim 1, wherein said housing comprises a pair of modular housing portions defining said media free space at their interior surfaces and symmetrical about the mesh line of said gears; a pair of end structure modules abutting the axial ends of said housing portions and rotatably mounting said gears, said end structures extending beyond said housing portions;
and a modular outlet structure nested with the outlet of said housing portions between said end structures, and means inter-connecting said modules.
and a modular outlet structure nested with the outlet of said housing portions between said end structures, and means inter-connecting said modules.
26. The pump of claim 1, wherein said outlet communicates with the inlet of a mixer.
41.
41.
Priority Applications (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA275,715A CA1106696A (en) | 1977-04-06 | 1977-04-06 | Gear pump |
| CA350,116A CA1131849A (en) | 1977-04-06 | 1980-04-17 | Low energy recovery compounding and fabricating systems for plastic materials |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA275,715A CA1106696A (en) | 1977-04-06 | 1977-04-06 | Gear pump |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1106696A true CA1106696A (en) | 1981-08-11 |
Family
ID=4108342
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA275,715A Expired CA1106696A (en) | 1977-04-06 | 1977-04-06 | Gear pump |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1106696A (en) |
-
1977
- 1977-04-06 CA CA275,715A patent/CA1106696A/en not_active Expired
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Legal Events
| Date | Code | Title | Description |
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| MKEX | Expiry |