FI61408B - FOER FARTA OCH ANORDNING FOER SEPARERING AV KVARBLIVANDE FLYKTIGT LOESNINGSMEDEL FRAON FASTA POLYMERKULOR - Google Patents

Description

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Patent and registration authorities AimBIcu uttagd td utl. «Krtfun pubikurad (32) (33) (31) Pyydetty« tuolkuui —a «jird priority (71) E.I. du Pont de Nemours and Company, Wilmington, Delaware 19898, USA (72) Robert Garnett Humkey, Orange, Texas, Donald James Ryan, Port Neches, Texas, USA (7M Oy Kolster Ab (54) Method and device for separating residual volatile solvent from solid polymer granules - Förfarande och anordning för separering av kvarblivande flyktig lösriingsmedel fran fasta polymerkulor

The invention relates to a continuous process for separating the remaining volatile solvent from solid polymer granules by passing a hot, vapor-containing gas through a layer formed by the polymer granules.

The method according to the invention is characterized in that a) the polymer granules are continuously fed as a falling ring to the upper end of a cylindrical column of granules moving downwards due to gravity, maintained mainly in a plug flow state, said granules containing said , and its diameter is about half the diameter of a cylindrical statue? 2 f · -1 Δ O 8 b) a gas containing steam is continuously introduced into the lower part of said downwardly moving column and through the moving column against the direction of movement of the granules; c) continuously removing the granules sy1 from the lower end of the temporary moving column to the conically downwardly tapering portion of the column leading to the axis-centered cartridge discharge point, while the granules are guided from the center of the cylindrical column base to the d) the granules are fed to the upper end of the sy1 interim column at the same rate at which the granules are removed from the axle-centered batch outlet: e) the flow rate of the to a certain level; and f) passing the gas containing the volatiles removed from the granules from the vapor space above the moving column to a condenser and a separator, and recovering the volatiles separated therefrom.

The invention also relates to a device for use in the method,

The doka consists of a cylindrical main part with a length to diameter ratio of more than 3: 1, a conical bottom part, a shaft-centric feeder located at the top, an axle-centric discharge device located at the bottom, a first shaft-centric conical granule feeder at the upper part a distributor located at the bottom and having a conical upper surface and a degassing tube at the top, characterized in that a) the diameter of the first axial granular distributor is about half the diameter of the cylindrical part; 3 61408 (b) the second axle-centered granule distributor is hollow and located completely inside the conical bottom part and is formed by two cones connected at their bases, the apex angles of which are approximately equal to the apex angle of the conical part at the bottom of the column; the upper tip is at the level of the fold line at which the cylindrical top and the conical bottom are connected, leaving an annular gap between the distributor and the wall of the conical bottom, said second granule distributor comprising circumferentially divided devices at its top for discharging gas therefrom; and c) the gas supply pipes are circumferentially and evenly arranged near and around the top of the conical base.

The steam stream is diluted with an inert gas (e.g. N 2) when a temperature below 100 ° C is desired. The steam containing the solvent separated from the granules is passed to a conventional condensing and separation system to recover the solvent.

The device is a cylindrical tower with an inverted cone-shaped base; a guide plate is placed at the upper end of the column to guide the granules entering the column so as to form a ring-shaped surface protrusion; a second flow guide in the form of two cones connected at the bottom is placed in the conical bottom of the column; an outlet at the tip of the bottom cone on the central axis to reduce or eliminate backmixing of the granules as they flow through the bottom into the outlet.

Polymerizations carried out in the presence of a solvent in the preparation of a solid polymer leave, even after extrusion and / or granulation, residual solvent, monomer and / or other residual volatiles which must be removed.

The removal of solvent or other residual volatiles from the polymer granules has been performed in batches by passing hot gas and / or steam through a solid mattress of dry granules with or without mixing with drum, circulating or otherwise mixing. However, in the past, it has not been possible to remove residual volatiles in a continuous process from solid particles without contamination or mixing, due to the inability to adjust the residence time of the granule during removal. The relatively large volume of separation medium required by previous methods and the low and variable concentration of residual solvent in the granules and vapor have hitherto made separation and recovery of volatiles inefficient and expensive.

Examples of prior methods that have generally been substantially batchwise using either granules with dry surfaces or suspensions of granules in liquids can be found in CA Patent No. 836,977, U.S. Patent No. 3,227,703, and U.S. Patent No. 3,539,539.

The method of the present invention overcomes these problems. It also avoids the economic loss and air pollution caused by the release of residual volatile substances into the atmosphere, and requires substantially less energy to operate, thus providing substantial energy savings that result in more economical operations.

Figure 1 is a schematic drawing of a separation apparatus of the present invention that may be used in the wet separation process of the invention. It is described in more detail below.

Figure 2 is a graph showing a comparison of the equilibria between the wet separation process (curve A) and dry separation (curve B) of the present invention, previously used, applied at 95 ° C to HD polyethylene granules containing residual cyclohexane solvent. The equilibrium weight% (at 95 ° C) of the cyclohexane remaining in the HDPE granules is placed on the vertical ordinate, while the molarity of the cyclohexane in the separation vapor is placed on the horizontal abscissa.

Fig. 3 is a graph showing the relationship between the backmixing of downwardly moving granules in the conical bottom of an open column (denoted as 454 kg of HDPE back-mixed granules, 366 cm in diameter in a device against the vertical of the vertical cone Θ), and the column cone of the column Θ) between (marked on the horizontal abscissa). Curve C is for the case where the rolling angle of the granules, oc. at the top of the column, is 35 ° while curve D is for the case where & is 45 °. An open column with a conical bottom and granules filling the column in a conical pile at the top of the column are shown in a schematic drawing, 23, in a graphical view; this figure shows a, the angle of extraction of the granules at the upper end and Θ, the apex-r angle of the conical bottom of the column.

Fig. 4 is a graph showing the backmixing of the dependence of the granules running on the conical bottom of the device of the present invention (Fig. 1) using polyethylene granules (applied to the vertical ordinate in units of 454 kg HDPE backmixed granules in a 366 cm diameter cone) and θ: between the apex angles of the base, (marked on the horizontal abscissa). Curve E is for the case where the rolling angle of the granules, Oi, at the top of the column is 35 °, while F shows the case where OC is 45. The new separation column used for this case, which includes granule dividers, 3 and 4, at the top and bottom of the column, the top surface of the grain mattress being annular, is shown in the graphical diagram by schematic drawing 24; this diagram shows OC, the rolling angle of the granules at the upper end and Θ, the apex angle of the conical bottom of the separation column.

The rolling angle, a, is determined primarily by the geometric shape of the granules (waffle, oval ball, dog bone, etc.);

Oi is also dependent on the rate at which the granules are fed and dropped onto the surface of the granular column and the state of the surface - wet or dry.

1. Method

A new continuous process has been invented for removing volatiles (e.g., solvent remaining from the polymerization process or solvent support used for additives) from polymer granules or pelletized catalyst mats and the like, which method has the advantage of reduced air backmixing over other previously known processes. avoidance, 6 61408 significant savings in energy consumption, resulting in more economical operations and reduced investment in equipment. This new method achieves maximum volatility removal rates from solid granules even when the concentration of volatiles in the separation gas leaving the process is high and the gas flow rates are relatively low. The method further allows for controlled and equal residence times for essentially all solid granules and prevents the required contact time to obtain a very low content of volatile substances in the granules from being shortened. This results in a very high separation efficiency, while greatly facilitating the recovery of volatiles and avoiding contamination or mixing of different batches or degrees of granules. The method is readily applicable to deliver any desired amount of production while providing granules that are substantially completely free of volatiles.

According to the new method of the present invention, granules whose surfaces are wetted by a water film (which may come from steam condensed on their surfaces at the top of the column or from pre-irrigation or both) are fed continuously to the top of a vertical, downwardly flowing mattress, the granules moving in a plug flow and removing through the tip of the cone while feeding steam to the column near the bottom of the mattress and flowing upstream against the moist grain stream, entraining the remaining solvent vapor by separating the volatile solvent from the water film surrounding each grain; the steam containing the separated volatiles in a concentration of up to 25-50 mol / L then exits the top of the column to a conventional condensing and separation system where the solvent separated from the granules is recovered. The granules, from which the remaining volatiles have been separated, are continuously removed from the tip of the conical bottom of the granular bed flowing downwards at the same rate as new granules are added to the upper end. Preferably, the granules are added at the top end in an aqueous slurry, while the excess water carrying them is removed from the top end of the granule column. Preferably, the backmixing of the granules at the bottom of the column is reduced by feeding the granules to the top of the column in a free-flowing, vertical, cylindrical stream forming a circular pile ring-shaped at the top of the grain column from which the granules are removed, the downward-flowing grain bed is deflected from the direction of the central axis of the column towards the outer circumference of the conical bottom at such a rate that back-mixing is reduced and approaches zero.

0 7 61408 2. Device The new device of the present invention is particularly suitable for carrying out the preferred method of the invention. It is shown in Figure 1.

Figure 1 is a schematic diagram of a cylindrical separation column of the present invention including a downwardly moving grain bed. According to Figure 1, the separation vessel has a cylindrical column head 1 of sufficient volume to provide the required residence time for complete separation of volatiles at the desired production volume and with a large cross-sectional area to allow separation of the separation gas and wetted granules backflow and flooding ; the bottom of the column is formed by an inverted conical portion 2. Typically, the length / diameter ratio of the cylindrical portion is greater than about 2: 1 (e.g., usually between about 5-7: 1, depending on the residence time required to completely separate the solvent at the desired production rate; factor is the cost of very large columns) and has an inverted cone-shaped base portion 2, the cone peak angle preferably between 20-60 °, to ensure plug flow in the cylindrical top 1. Near the top of the column is a conical grain feed distributor 5t located directly below the granule inlet 10. The upper end of the conical feed distributor includes a device (5a) for passing liquid (water); preferably it is made of a heavy sieve net which drains the water coming through the inlet 20, bringing the granules coming through the inlet 21 through the S-liquid lock 19 to enter and out of the deflector 5 through the opening 12. The level of the water surface 22 in the S-liquid lock prevents the escape of steam. Centrally located in the conical bottom part of the separation column is a second grain dividing device 4, comprising two cones connected at their bases, above the grain discharge opening 11; the upper tip is also on the central axis and the fold line between the cylindrical column 1 and the conical bottom part 2. This divider forms an annular opening 9 between its surfaces and the conical inner surface of the base part 2. Around the upper cone of the distributor 4 are arranged devices 5 for introducing separating gas (steam) through a line 8 to pass into a granular mattress 15 descending from inside the distributor 4. Preferably these devices comprise a strong, supported pulp screen with a smooth outer surface running flush with the upper cone of the distributor 4. The sieve 4a at the tip of the bottom 4tn lowers any condensed water in 4 to flow out. The device 6 is also intended to discharge the separating gas which enters the outer circumference of the granular mattress 15 descending through the opening 7. The device 6 may comprise, for example, a hollow belt or multiple inlet gas manifolds sufficiently close along the outer circumference of the conical base part 2 to prevent flooding, and multiple circumferentially spaced windows made of a strong pulping screen drawn through and flush with the conical base part 2. along and near the top circumference, the top of the separation column 1 also includes a steam outlet 18 through which a separation gas containing solvent separated from the granules) is passed to a conventional series of condensation and separation (not shown) to recover the organic solvent separated from the granules. The upper part of the granular mattress 13 inside the separator 1 is annular 14, the low points being a ring along the circumference and the axis of the low point separation column and the high points being a ring below the outer circumference of the feed distributor 3 »with a bottom diameter of about half the column diameter.

With the method of the present invention operating in the apparatus of Figure 1, the granules flow downward as a plug flow (without mixing) so that the individual granules retain their original annular appearance 14 until they reach position 15 at the bottom of the separation column 1 cylinder. As the granules flow downwards from position 15 to the annular slot 9, their shape gradually evens out, as shown by 16 and 17, due to the flow pattern resulting from the structure of the base 2 and the shape and position of the bottom distributor 4, reducing and replacing the backmixing normally associated with the converging part.

In carrying out the method of the present invention using the apparatus of Figure 1, the granules, the surfaces of which are wetted by water carrying them through the S-liquid lock 19 and condensing upward steam, run downward by gravity countercurrently upstream of a vapor gas, optionally diluted with N , kdn temperatures below 100 ° C are desired. The separating gas enters the device and comes into contact with the downstream flowing pellets via the gas distribution devices 5a and 6a, which are fed through the inlets 7 and 8. The granules from which the remaining solvent has been separated are continuously removed through the outlet 11 at the same rate as the granules are fed to the top of the column, while the separating gas containing organic solvent vapors is conveyed through the outlet 18 to a conventional condensing and separating device. After leaving the separation 1 through the discharge opening 11, the granules are conveyed to a conventional drying device. The temperature of the granules in the separation column is kept lower than the tack temperature of the solid particles used, for example by diluting the vapor with an inert gas such as NgSO 4 or working under reduced pressure where the tack temperature of the granules is below 100 ° C.

9 61408

Thus, the process of the present invention provides a novel continuous process for the removal and recovery of residual volatiles from granules (polymers, catalysts, natural products) that facilitates the recovery of residual volatiles and avoids atmospheric pollution while achieving substantial energy and energy savings. The flow of the separation medium, the size of the apparatus and the back-mixing of solids are reduced by the countercurrent flow of the separation medium, e.g. plug flow and having a tapered bottom portion that is geometrically shaped (peak angle 20-60 °) and structurally designed to ensure solid plug flow at the top and reduce back-mixing that would otherwise occur in the tapered portion. The separating medium (steam and, optionally * gas) enters the separating column through inlet openings at its lower end adapted and arranged to avoid disturbing the plug flow of the downwardly flowing granules and exits through the steam outlet at its upper end, leaving the separation column at its lower end.

In the method, cold (<100 ° C) and / or moistened granules are continuously added to this separation column to keep the surface of the mattress formed by the moistened granules above the separation medium inlets, the moistened a rectilinear vertical portion and substantially continues through the tapered bottom portion, while the flow of separation medium moves upwardly through the mattress formed by the moistened particles until it reaches the vapor space above the mattress surface. The separation medium containing volatiles removed from the granules in the column is removed through a steam outlet to the apparatus to separate and recover the volatiles remaining from this separation medium. The verses, the concentration of volatiles in the weight - # has decreased to a sufficiently low value to allow further processing, are assembled on the basis of a tapered section, the temperature of the granules is kept lower than the sticking temperature of the granules to be treated by adjusting the vapor partial pressure. A device for continuously removing granules from this solids outlet is arranged at the bottom of the converging part.

10 614 0 8 In the process of this invention, volatiles are effectively separated from polymer granules in any suitable form, such as produced by melt extrusion and product cutting, in the apparatus of this invention, in a motion bed separator operating with minimal back-mixing, and the volatiles are recovered to avoid air contamination. . In addition, the method of the present invention makes it possible to separate the use of substantially smaller separation devices and smaller volatile matter recovery devices due to the improved efficiency.

Detailed description of the invention

It is well known that the removal of volatiles from solid particles, particles or polymer granules involves the diffusion of volatiles through the solid and then through the vapor film which tends to surround each particle. The maximum diffusion rates in the solid and the maximum separation rates are generally reached at the maximum temperature at which the solid can be treated without melting or decomposing. In previous processes in the art, maximum separation rates also require that the barrier layer be reduced at high separation media rates. In general, as the concentration of volatiles in the film increases, the driving force for the diffusion of volatiles in the granule decreases. In a "dry" system, as has been commonly used in the past, the granule is surrounded by a gas film containing some volatiles * High flow rates of separation medium are required to keep the volatiles concentration at a low level approaching zero.

Surprisingly, it has been found that the equilibrium conditions preferably change when the granules are wetted, e.g. by condensing steam and / or immersing in water before being introduced into the separator, forming a water film around each granule. In the method of the present invention, such an aqueous film is formed around each granule. The concentration of volatiles in this aqueous film is very low, which depends not only on the low solubility of the volatiles in water, but also on the relatively high volatility of the resulting mixture of volatiles and water. Therefore, the effect of all vapor barrier films is reduced and maximum separation rates are still reached using low separator (steam) flow.

To ensure the equilibrium change that occurred by switching from a dry (anhydrous) to a wet system, equilibrium values were collected. For the "dry" case, nitrogen gas was passed through bubble-adjusted cyclohexane to saturate the nitrogen gas, and this gas was then passed through a matrix of dry granules at a temperature of 95 ° £ 6βη 11 6-: 403. The saturated role of cyclohexane in the nitrogen gas was varied by adjusting the temperature of the cyclohexane. The granules were given sufficient time to equilibrate (3 hours). In the "wet" test, saturated nitrogen gas was mixed with steam and sprayed onto the bottom of a column of wetted granules adjusted to 95 ° C. Comparisons of equilibrium values are shown in Figure 2. The wet system component showed a strong change in equilibrium conditions compared to the dry system. In addition, the equilibrium concentration of the granules in the wet system was very low and was independent of the concentration of cyclohexane in the separating gas.

The separation medium of the present invention comprises steam alone and steam together with an inert gas. The preferred inert gas is nitrogen. The most preferred medium is steam, usually slightly above 100 ° C, depending on the nature of the granules to be treated and the tack temperature.

Steam is also an excellent heat transfer medium. The polymer granules heat very rapidly to the desired temperature using the bound heat of condensation. Thus, the size of the device is reduced because the maximum separation rates at the desired temperature are reached very quickly in contact with the steam.

When inert gas is used alone, heat transfer is generally poor and large gas flows are required. The inert gas can be used with steam to lower the partial pressure and temperature; however, the higher the concentration of inert gas, the more expensive the equipment for processing the process.

The present invention is also applicable to a simplified volatile matter recovery system. Water-insoluble volatiles are easily separated by conventional condensing and decanting equipment. The size of the equipment required for separation is reduced by the effect of low vapor flows and relatively high concentrations of essentially unchanged volatile substances.

Once the polyolefin granules have been separated, the remaining volatiles may include hexane, cyclohexane, henzene, toluene, vinyl acetate, and generally all volatiles normally present in their use in the process of synthesizing the polymer and having a solubility in the liquid particle. or the volatility of the volatile matter / water-azeotropic mixture is high enough so that the concentration of volatile substances remains low in the liquid film. This invention is particularly directed to residual volatiles of cyclohexane and / or hexane due to their presence in high density polyethylene.

The solid particles treated by this invention are polymer granules of all sizes and shapes commonly used in the molding and extrusion industries, as well as non-polymeric solid particles of the same size. The process is not suitable for finely divided powders, such as those initially formed in the polymerization of olefins, because such powders tend to float under the action of countercurrent gas or steam and do not flow uniformly like a plug flow. Representative examples of polymer granules include polyethylene plastics such as high density polyethylene plastics and low density polyethylene plastics, ionomeric resins and polypropylene. Representative examples of non-polymeric particles are catalyst mats of coarse particles. Preferred solid particles to which this invention can be applied are polymer granules. Most preferred are crystalline polyolefins, especially high density polyethylenes. The preparation of polyethylene granules as compression or extrusion granules is well known in the art and has been described in several patents.

In the process of the present invention, volatiles are removed from the solid granules as the granules settle as a plug flow in the vertical portion of the separation column. As the granules reach the converging portion of the bottom of the separation column, the plug flow tends to deviate due to the oblique sides. The magnitude of the deviation from the plug flow depends on the angle of the oblique sides of the vertical axis Buhteen and the flow properties of the granules. This deviation, defined as the back-mixing coefficient, β, as described below, can be reduced or eliminated completely using the preferred method and the apparatus of Figure 1.

The term "plug flow" as used herein is defined as the flow of solid pelletized materials in which all the granules move at the same speed in each cross-section of the mattress, leaving all the granules out of the mattress after a substantially equal length of time in the mattress.

The plug flow at the top of the separation column is essential for this process. Deviation from the plug flow would require a larger equipment size, as sufficient residence time must be set aside to process the fastest moving granules; even small deviations from the plug flow can decisively increase backmixing, leading to excessive contamination of the quality product with substandard and varying degrees of products. For example, if the ratio of the residence times of the fastest and slowest grains were 10/1 and the distribution of the residence time were straight from the center to the wall of a typical cylindrical device, the volume of the device would have to be increased by about 50 falls. compared to a plug-flow device, to provide sufficient separation and back-mixing would increase to a level many times higher than the plug-flow system. Existing processes for separating the remaining 13 51408 volatiles from solid particles do not provide continuous operation by plug flow; therefore, the aforementioned residence time ratios of 10/1 are not uncommon in prior art practice.

Backmixing in a moving bed of solid particles through the converging bottom of the separation column of the present invention can be reduced by installing in the separation column: (l) an upper internal feed divider located near the a column diameter, the purpose of this baffle being to distribute solid particles to the mattress to form an annular surface to balance any differences in plug flow in the converging bottom of the column, and (2) an internal bottom flow divider usually comprising two cones connected by two conical ends, each is approximately the same as the converging bottom angle of the column and is positioned in the center of the column so that its tip is vertically cylindrical; at the level of the fold line between the part and the converging bottom part. Thus, for the flow of granules, an annular gap is formed between the surfaces of the intra-bottom flow divider and the converging bottom portion and is approximately concentric with the annular annular surface at the upper end of the grain mattress by the inner end distributor.

The apparatus of the present invention can be used to carry out the method of the present invention to remove residual volatiles from polymer granules or other solid particles of the same size and shape.

The following expressions have been obtained experimentally with different device parameters and flow parameters and 3.2 mm low pressure polyethylene granules.

They can be used to estimate the amount of mixing (the amount of good product contaminated with non-quality product) or to determine the parameters of the device for a given flow and tolerable mixing situation.

X = Q (t'-t) (1)

For plate devices DqY - tg 0 Y2 (2) (rectangular cross section) t = - D V o o f - t / β (3)

For cylindrical devices:

DqY-2 D tg 0 Y2 + 4 (tg 0) 2γ3 t = ° _! _ (4) D2 V o o o f = t / p * (5) 14 614 0 8

Where: X = kg mixed or contaminated Q = granule flow rate, kg / min.

t = time in minutes required for the grain to travel vertically at the point in the converging part of the distance Y at the point where the vertical velocity is greatest (eg in a cylindrical column without internal dividers, this is in the vessel centerline) t '= time in minutes required for the granules to travel in the vessel, or on the surfaces of the distributor a vertical distance Y.

Vg f = ratio = -, backmixing factor

CL

Vg * vertical component of the grain velocity on the wall surface VCL = vertical velocity of the granules at the center line of the discharge opening Dq = diameter or width of the part above the fold line (rectangular or cylindrical), except when using a bottom distributor. For this case, Dq »l / 2 times the diameter or width of the vessel © - the apex angle of the converging part at the bottom of the column 0 = half the apex angle © Y = the total height of the mixing part (below the fold line)

Vq - grain velocity on the fold line

The backmixing that occurs in the conical funnel of the cylindrical devices can be calculated using Equations 4 and 5 above.

In all the calculations so far, it has been assumed that a uniform surface profile for the mattress is achieved when the conical hopper is fed. In reality, this type of profile is not easily achieved. Reducing backmixing can be accomplished by feeding the granules centrally and allowing the bump to form depending on the rolling angle of the granules. The reduction in backmixing occurs because the granules in the center of the mattress, which move through the fastest converging portion, have a longer transition path in the vertical cylindrical portion. As can be seen in Figure 3, the optimum peak angle is highly dependent on the rolling angle of the granules. Because the wet bed rolling angle of typical HD polyethylene granules varies from 30 to 50 ° depending on the geometric shape and flow characteristics of the granules, it was difficult to select the peak angle Θ that would consistently give the lowest possible backmixing. In addition, the slope at the height of the mattress at the top of the vessel tends to cause a privileged flow of separation steam to the outside of the device, which results in uneven separation in the granules and inefficient use of the volume of the device. Thus, the exact formula for calculating © cannot be given.

However, the preferred method of the present invention uses C i 3 3. The dispensers comprise (l) a feed distributor at the upper end, typically formed by a conical (for devices having a circular cross-sectional surface) guide plate axially centered near the top of the cylindrical column and having a diameter of approximately half the diameter an annular surface buoyancy is formed, and thus smoothing for a non-plug-like flow pattern in the tapered bottom portion of the device, resulting from (2) an in-bottom flow divider comprising two cones (for a conical tapered portion) connected at their bases and the apex angle apex angle and is positioned in the center of the device so that its upper tip is at the level of the fold line. For grain flow, an annular gap is held between the surfaces of the distributor and the converging bottom portion.

Figure 3 shows the calculation of backmixing in the case where the centralized feed of granules forms a protrusion in which the surface profile of the mattress has different rolling angles. Mixing in such cases was calculated using the equation 1: kg

Mixing = * Q -. (t * - t) (l) based on devices with flat mattress profiles. For flat profiles t * = t / p. When the rolling angle α of the solid particles dominates the mattress profile, t1 - (t / β) -t ", where t" is the additional time required for the granules to have the maximum vertical velocity in the converging section to reach the fold line compared to granules entering the apparatus simultaneously but flowing through the funnel near the wall. For a given rolling angle a:

D

o tg a t - -

2 V

o

For some a-values, At in Equation 1 is negative. Using Equations 1, 4, 5, and 6, the mixing that occurs in the round funnel can be calculated for different «values.

ie 6140 8 The reduced back-mixing obtained using the preferred method of the present invention using internal granule distributors at the top and bottom of the column is illustrated in Figure 4 · As can be seen not for peak angles, © = 20-60 °, rolling angle α is less critical and low back-mixing rates are obtained regardless of the apex angle © of the cone and the rolling angle α.

It is well known that the steeper the peak angle © of the conical funnel, the more certain it can be assured that a uniform flow of solids can be achieved without the formation of dead spots or slow-moving zones in the device. However, the height of the tapered portion becomes continuously larger as the apex angle decreases. Therefore, the choice of the peak angle depends on the economic balance between backmixing and equipment cost.

The following examples are illustrative only and are not intended to limit the invention described above. Several variations that do not alter the nature of the disclosed process or apparatus will be apparent to those skilled in the art.

The polyethylene granules used in the following exemplary embodiments of the present invention are granules having cross-sectional dimensions roughly between 1/6 and 10 mm in all commercially conventional ways "cut" - spherical, diamond, cubic, cylindrical, etc. The term "polyethylene" includes "polyethylene". both homopolymers of ethylene and copolymers of ethylene with mixed monomers such as vinyl acetate or higher olefins (e.g. butene, octene, decene) and have a density between about 0.91 and 0.97 · Preferred polyethylene granules are 3.2 mm melt-cut "spheres" of high density polyethylene (HDPE). The production of polyethylene granules by melt extrusion and cutting is well known in the art and has been described in several patents.

Example 1

To demonstrate maximum diffusion rates, recovery of residual volatiles, and the ability to provide plug flow, a transparent polypropylene model separation column with a diameter of 58.4 cm and a total height of 226 cm and a tapered bottom apex angle of 33 °, about 181 kg mat, about 181 kg, was packed. , 2 mm. Approximately 16 kg per hour of saturated steam at about 100-150 torr pressures were injected into four 3.8 mm nozzles located 74 mm above the cone discharge port and 90 ° apart. The nozzles were level with the surface of the device and were covered by a sieve to prevent the ingress of granules. The operation took place in batches, so that the granules were continuously returned at a rate of 272 kg / hour by directing the granules from the bottom back to the top. The plug flow was reached each time the granules passed through the cylindrical part of the column, which could be determined by visual observations with a black layer of granules. The top end steam was condensed and collected. The mattress was sampled from a height of one foot above the fold line periodically over two hours and analyzed for cyclohexane content by weight. The results obtained are shown in the following Table 1, using: 181 kg of HDPE granules, density <96 granule temperature => 100 ° C pressure = 1 atm (absolute).

Table I

Solvent content of the granules Sample Collected cyclo- Weight- # cyclohex- Weight- # cyclohexane uptake time hexane (ml) slag in granules granular calculated (min) lab analyzes 0 0 1.11 1.35 10 600 0.92 Oi, 09 20 1200 0, 89 0.83 40 1750 0.55 0.60 70 2270 0.39 0.38 80 2360 0.34 0.34 90 2490 0.25 0.28 100 2550 0.29 0.26 130 2880 0.13 0 , 15

Example 2

To ensure maximum diffusion rates and plug flow in a full scale device, a 411 cm diameter vertical cylindrical device with a truncated cone base with an upper diameter of 411 cm and a lower diameter d2 of 152 cm and a peak angle of 40 ° was added to the truncated cone base β: a conical portion with a peak angle of 90 °, about 56,700 kg of low pressure polyethylene, j0fca containing 1.5-1> 75 wt.% of cyclohexane. Hoi 11,300 kg of black tracer was placed on the surface of this mattress. Steam was introduced into the apparatus through three nozzles located in a conical bottom. The continuous unloading of the mattress from the device and the feeding of fresh inseparable granules to the upper end of the mattress was started at a typical speed of 18 61 408 at 13,600 kg / h. Exhaust gas with steam and cyclohexane was vented from the top of the apparatus. The grain sampling site was located on the discharge line. Samples were taken periodically and analyzed to determine the amount of separator and the mixing of the granules. The granules left by the apparatus were removed with 0.05-0.07 wt% cyclohexane after 5 hours in the apparatus. The plug flow was proved by the fact that the predicted mixing using expressions (1), (4) and (5) above, which assumes plug flow above the fold line, was quite close to the actual mixing values obtained in the tests. The separation values and plug flow values are shown in Tables II and III,

The solvent half-life used here is the time difference (t-to) at which the concentration (c) of the remaining volatile constituents at time "t" in the granules is half the initial concentration (CQ) of these components at some earlier time t ..

o

Table II

Separation values with steam Separation temperature 100 ° C

Resin HDPE granule spherical Cyclohexane half-life, type part effective radius (cm) Actual Calculated (min) HDPE 0.202 (measured) 62-641 62.52

Obtained from actual difference values collected during the test in a 15.5-foot-diameter device.

Calculated half-life using laboratory values.

Table III Plug flow values

Source of contamination Contaminated amount (kg) A. Color charge 11340 B. Mixing in a cone, calculated 14150 "Total calculated1 25490

Actual pollution 25400

The calculated total requires perfect conditions. Mixing1 2 due to downcomers in the device, rough points at the cone joint, and mixing due to steam spraying are not included in the predicted value.

19 C Ί 4 O 8

The actual contamination may be somewhat higher because the sampling method does not allow for a true assessment of the last mixed nails.

Example III

An example of a full-scale implementation of the process of the present invention in the novel apparatus of the present invention is shown herein, indicating maximum diffusion rates, solvent recovery rates, and plug flow.

The separator consists of a cylindrical top with a diameter of 366 cm and a conical bottom with a peak angle of about 33 °. · Internal bottom and top splitters (Fig. 1, 3 and 4) are used to reduce backmixing in the conical bottom. The base divider consists of two cones connected at their bases, the bases of which have a diameter of approximately half the diameter of the cylindrical top and each cone has an apex angle of about 33 °. The tip of the bottom cone of the distributor is in the center of the device and is located just above the grain removal nozzle. The distributor is dimensioned to provide an annular gap for grain flow between the bottom distributor and the conical bottom portion in the device. When the granules reach the fold line, those in the center of the annular slot (at a distance of about 25 n, calculated from the diameter of the cylindrical top, from the wall of the device) begin to move faster relative to the other granules. An upper end distributor comprising a cone having an apex angle of 100-120 ° and a diameter of about $ 50 from the diameter of the cylindrical top and a cylindrical ring located around the outer edge of the conical manifold leaving an annular opening between the cone and the ring at the upper extrudate feed . The fed granules fall onto a centrally located top end distributor, slide and / or pop down along an oblique surface of the distributor and fall to the top of the granule mattress in the device and form an annular protrusion coaxially with the device, the highest point being directly in the center of the This somewhat extends the path through the mattress to those particles that move most rapidly in the conical bottom. Possible back-mixing in the conical bottom is reduced.

The separator is filled with about 74840 kg of low pressure polyethylene (HDPE) granules containing about 2.5 weight percent of residual cyclohexane and 200-250 torr of saturated steam is passed through the mattress countercurrently against the grain flow / at a rate of about 2270 kg / H. The steam injection points located in the conical 20 61 4 0 8 bottom part are circumferentially located just below the fold line to reduce disturbances in the grain flow.

The device is continuously filled and discharged with granules from the lower end of the conical part, while wet granules are fed to the upper end of the device at the same speed to maintain a constant mattress surface level. The feed rate is 16,300 kg / h and the residence time is 4 »6 hours (approx. 275 minutes). The maximum half-life of cyclohexane in IIDPE granules with an effective spherical particle radius of about 0.17 cm in a system with fully controlled diffusion in the solid state is 50 minutes. The process of the present invention makes it possible to reduce the residual amount of cyclohexane to about 0.055% by weight (550 pp) when leaving the device. The remaining volatile substances must be less than 1000 ppm (parts per million) for safe packaging. Under the conditions of this example, this reduction is achieved with a residence time of the granules which is 5-6 times the maximum half-life of cyclohexane in HDPE granules. Should a disturbance in the reaction system occur which would cause the substandard plastic to be fed into the separator, this can be diverted to the side after the separator outlet nozzle, whereby the loss of quality plastic is small. The deterioration is further reduced by the presence of the above-mentioned distribution devices. The loss of quality plastic due to mixing resulting from such disturbance is calculated to be 0 to 2270 kg.

If cyclohexane is replaced by n-hexane, similar results are obtained with the same residence time in relation to the half-life of n-hexane in HDPE granules. If the granules entering the separation system contain more or less residual solvent (or if the transport requirements require more or less residual solvent in the granules), this ratio should be set accordingly to give the desired final concentration of volatiles. Thus, a lower value, such as less than about 400 ppm, is usually required for n-hexane.

The novel pulp transfer and separation system (method and apparatus) of the present invention, as described above, offers a number of unexpected advantages that make it very useful and advantageous in plastic manufacturing processes. This new system provides a way to separate and recover residual volatiles from plastic granules without contaminating the environment, which has been a negative feature of previous methods. In addition, this new system provides a way to remove these volatiles from the granules with lower energy consumption than prior art processes, such as the dry-vapor separation of Canadian Patent No. 836,977. The dry separation method of said patent requires that all steam be passed through as steam in air. Nor can such a system implement the new mass transfer provided by the method and apparatus of the present invention. Thus, it has previously been thought that it would be impossible to achieve a plug flow of granules in large scale devices, while the novel device of the present invention makes it possible to achieve this goal in commercial scale devices.

The energy savings and reduction in air pollution achieved by the novel method of the present invention can be calculated and compared with the best previously obtained results in the art. The results are shown in Table IV: 22 61 * 08 I <d c 4-> o + j 4-1 <u m o 4J id -d * C C O in Φ Φ in 3 I I T-.

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The energy efficiency values shown in Table IV were calculated using the sum of the heat demand of the heating, the heat loss, and the energy required to separate the volatiles in each case. The energy required for the batch air process was calculated as the electrical energy required to run the fans that compress and heat the air and push the air through the grain column + the heat losses associated with this type of system. The energy required for the dry steam process was calculated as the electrical energy required to run the blowers + heat units (BTEUs) in the exhaust steam. The energy required by the mobile bed wet separation process of this invention was calculated in simple thermal units (BTUs) required to generate the steam used.

The volume efficiencies shown in Table IV for the batch air separation process and the batch 1 dry steam separation process have been calculated by dividing the number of granules Gg, by the time required for one complete separation cycle (filling, separation and discharging), all divided by the volume of the device. In the continuous wet moving mattress steam separation method of the present invention, the volume efficiency was calculated by dividing the amount of grain fed in one hour by the volume of the device, which gives an average residence time of 5 hours.

From the values in Table IV, it is apparent that the new process of this invention provides significant savings in both energy consumption and solvent recovery, that it eliminates hydrocarbon air pollution, and reduces thermal pollution compared to previous methods in the art. Such benefits, or even the possibility of achieving them, have not been suggested in any previous teachings in the field.

Claims (7)

24 £ ί 4 Ω 8 Claims: '1 ^ A continuous process for separating residual volatile solvent from solid polymer granules by passing a hot, steam-containing fiber through a layer of polymer granules, characterized in that (a) the polymer granules are continuously fed as a falling ring in a plug flow state, wherein said granules contained in the statue are surrounded by a vapor-containing gas and moistened with a water film, and said falling ring forms a toroid and has a diameter of about half the diameter of the cylindrical statue; (b) continuously introducing a vapor-containing gas into the lower part of said downwardly moving column and through the moving column against the direction of movement of the granules; the center of the base of the statue is directed towards the outer walls of the conical part and the granules from the periphery of the base of the cylindrical statue are directed towards the inner walls of the conically tapered statue; provide that the residence time of the granules in the cylindrical column in relation to the half-life of the volatile substances in the granules is sufficient to reduce the concentration of volatile substances in the granules; to a pre-determined level; and (f) passing the gas containing the volatiles removed from the granules from the vapor space above the moving column to the condenser and separator, and recovering the volatiles separated therefrom. Process according to Claim 1, characterized in that the polymer granules are ethylene polymers and in that the volatile solvent is a hydrocarbon mixture retained in the polymerization which forms an azeotropic mixture with water having a boiling point of less than 100 ° C at atmospheric pressure. Method according to Claim 1 or 2, characterized in that the gas consists of water vapor and nitrogen gas. * 1. Method according to Claim 1 or 2, characterized in that the gas is water vapor. Device for use in the method according to claims 1-1 *, comprising a cylindrical main part with a length to diameter ratio of more than 3: 1, a conical bottom part, an axle-centric feeder located at the top, an axle-centric discharge device located at the bottom, 25 61 4 0 8 from the first axis-centered conical granule divider ', oo. located immediately below the feeder at the top, from a second shaft-centric granule distributor located at the bottom, and the surface is cariolar, and from the degassing tube at the top. characterized in that (a) the first axis of the first grain distribution device (3) has a haiku ii j a. or: about half the diameter of the cylindrical part (1); (ib) the second axial granule distributor (h) is hollow and: I left tse ·; completely inside the conical base part (2), and is formed by two; a cone connected to the front, the apex angles of which are approximately equal to the apex angle of the conical part (2) at the bottom of the column, and this second dividing device is arranged axially so that its upper tip is in the plane of the fold line at which the cylindrical upper part (1) and the conical base part (2) is connected to each other, leaving an annular gap (9) between the distributor (k) and the wall of the conical base part (2), this second granule distributor (M comprising circumferentially divided devices (5) at its top for discharging gas from inside it, and ( c) the gas supply pipes are circumferentially and evenly arranged close to and around the upper part of the conical base part (2). Device according to Claim 5, characterized in that the apex angle of the angular base part (2) is 20 ° to 60 °. Device according to claims 5 and 6, characterized in that the shaft-centered feed device (10) at the top is connected to a C-liquid lock with a T-mixing body at one end, and that the first shaft-centered conical granule distributor (3) is hollow, and in its conical at the top there is a device (3a) for passing a flow of liquid into the interior of the dispenser, which has a liquid discharge device (12) which directs the liquid out of the interior of the dispenser (3). 26 61 4 0 8