US2915462A

US2915462A - Distillation column control

Info

Publication number: US2915462A
Application number: US595336A
Authority: US
Inventors: Salmon Royes
Original assignee: Union Oil Company of California
Current assignee: Union Oil Company of California
Priority date: 1956-07-02
Filing date: 1956-07-02
Publication date: 1959-12-01
Anticipated expiration: 1976-12-01

Description

Dec' l, 1959 R. sALMoN 2,915,462

DISTILLATION COLUMN CONTROL Filed July 2, 195e 2,915,462 DismLArroN coLUMN coNrRor.

Royes Salmon, Fullerton, Calif., assigner to Union Oil Company of California, Los Angeles, Calif., a corporation of California Application July 2, 1956, Serial No. 595,336 4 Claims. (Cl. 208-353) liowing stream of reliux liquid. The reflux is obtained.

both by cooling and at least partially condensing the overhead vapor. Heat is in this Way introduced at the bottom and removed from the top of the column to maintain the countercurrent ow of vapor and liquid. In the column a vapor-liquid exchange occurs so that the lower boiling materials accumulate at the top of the column, the higher boiling materials accumulate at the bottom, and the composition varies more or less uniformly between the two points. The feed stream is introduced into the column at a point where the composition of material in the column is approximately the same as that of the feed.

Where the overhead product is only partially condensed and a gas phase remains for production from the top of the column, pressure control is readily accomplished by any form of back pressure controller operating on the gas stream tlowing from the receiver into which the cooled distillate is introduced. However, where the overhead stream is totally condensed the problem becomes more diliicult. The pressure can be maintained by bleeding fuel gas or other non-condensable material into the overhead receiver, but this requires that gas be bled from the receiver every time the liquid level rises. This can result in many thousands of dollars loss annually in normally condensable materials being carried out with the bleed gas.

Additional problems arise when the liquid material is to be maintained in a heated condition so that it may be used eiiiciently in a subsequent heating process. In this instance cooling below the condensate bubble point is useless. The condensate temperature can be controlled by varying the llow of coolant such as cooling water through the overhead condensers. However, at low flow rates, the cooling water rate becomes so low that its control becomes diicult, the effluent water temperature from the condenser rises `to very high values, and the corrosion attack on the heat exchange surfaces is greatly accelerated. Also in this situation when a temperature controller is employed, a change of the control point or control temperature is required every time the feed composition changes to one having a different boiling range.

The present invention is therefore directed to an improved distillation process having a control system which successfully overcomes either one or both of the foregoing problems.

Elt is a primary object of this invention to provide an States Patent O ICC improved pressure control for distillation columns and the like.

A specic object is to provide a process for pressure control in distillation columns when the overhead vapors therefrom are totally condensed. v

lIt is also an object of this invention to provide for the control of overhead reflux and product liquid temperatures at or only slightly below their bubble point in continuous distillation processes.

It is also an object of this invention to provide an irnproved apparatus for accomplishing the foregoing objects.

Other objects and advantages of the present invention will become apparent to those skilled in the art as the description and illustration thereof proceed.

Briey the present invention comprises an improved distillation process and an improved method for the control of distillation processes as shown on the accompanying drawings and which are best described in conjunction therewith.

Figure 1 shows a schematic vprocess flow diagram of the present invention incorporating the improved control steps referred to briefly above, and

Fig. 2 shows in detail the structure of one element of the apparatus shown schematically in Figure l.

Referring now more particularly to Figure l, the process employs distillation column 10 provided with a plurality of contacting trays 12. If desired, the column may be provided instead with any of the Well-known forms of contacting column packing, and of course any form of trays may be used. The feed material is introduced into the column through line 14 controlled by valve 16 and may be either in the vapor or liquid phase, or a mixture of the two.

The high boiling material collects as a body of liquid 18 in the bottom of column 10 and is removed therefrom through line 20. A portion of this liquid flows by means of a thermal syphon elect, or by means of a pump not shown, through line 22 through reboiler 24. Heat is introduced by means of coil 26, through which is passed any suitable heat exchange medium, and the bottoms product is reboiled. The vapor is returned to the bottom of column 10 through line 28 and passes upwardly countercurrent to the descending stream of liquid. The remainder of the high boiling material either from the reboiler or directly from the bottom of the column, is removed from the system as a bottoms product through line 30 at a rate controlled by valve 32 which in turn is actuated by liquid level controller 34. i The overhead vapor from the column flows through line 36 into a bank of coolers and condensers indicated generally as 3d and 40. A suitable cooling medium,l such as cooling water, is passed through these heat exchange units in indirect heat exchange relationship with the overhead vapor. The overhead vapor is cooled and totally condensed, and the liquid flows through rundown line 42 into reflux drum or condensate receiver 44. The overhead condensate is removed through line 46,. and a portion is pumped by means of retlux pump 48' through line 5t) at a rate controlled by valve 52 intol the top of column 10 as reflux. If desired, recording or indicating temperature controller 54 may be employed to control the overhead temperature by varying the redux ow rate. The remaining portion of the overhead' condensate is pumped as an overhead product of thel process by means of pump 55 through line 58 at a rate controlled by valve 60 and liquid level controlled 62 which in turn is actuated by the liquid level in reflux drum 44.

The pressure control according to the present invention is accomplished by automatically vvarying the area available for condensation of the overhead vapor in coolers 38 and 40. Reflux drum 44 is maintained at a pressure:

which is less than the desired column pressure. The rundown line 42 opens into reflux receiver 44 and extends as indicated generally at 64 to a point below the liquid level therein and adjacent the bottom of the receiver. In this Way the condensate can be forced to stand up in rundown line 42 and submerge at least part of the heat transfer surfaces in coolers 38 and 40. The degree to which the heat transfer surfaces are so covered is controlled by valve 66 which is actuated by pressure controller 68 in accordance with the column pressure. When thecolumn pres-- sure tends to drop below the desired value, pressure controller 68 closes valve 66 causing a liquid level 70 to rise in condenser 40, reducing the exposed tube surface, forcing the complete condensation to take place at a higher temperature, and causes the column pressure to rise. At. pressures above the desired value, pressure controller 68 opens valve 66, level 70 in condenser 40 drops exposing more tube surface permitting complete condensation of the overhead vapor at a lower temperature, and the column pressure is lowered.

The liquid present in condensate receiver 44, and ofcourse the liquid streams removed as product and as reflux therefrom, are all maintained at or near the bubble point of the condensate by differential pressure controller '72. This instrument detects the difference in pressure between, column and overhead receiver 44 and actuates overhead vapor bypass valve 74 connected in vapor bypass line 76. This line opens directly into rundown line 42 and serves to mix the condensate flowing from condenser 40 into receiver 44 with substantially uncooled vapors bypassing the coolers through line 76. The temperature of the liquid introduced into receiver 44 is raised or lowered by directly condensing or quenching more or less of the overhead vapors in the condensate from the bank of coolers 38 and 40. With higher temperatures of the liquid in receiver 44, a higher vapor pressure is exerted, raising the receiver pressure relative to that of the column, and thus changes in temperature may be detected by a differential pressure instrument.

In operation, differential pressure controller 72 is set to hold a specific pressure differential between column 10 and receiver 44. With the column pressure controlled as. previously described, the receiver 44 pressure is thus maintained at a fixed value less than the column pressure and which is established by differential pressure controller 72. If the receiver pressure becomes too low caused by excessive cooling of the condensate below its bubble point at this pressure, differential pressure controller 72 opens valve 74 allowing hot vapors to mix with condensate rundown to receiver 44, raising the temperature of the material in the drum until a vapor pressure is exerted which is sufficient to maintain the fixed pressure difference between the column and the receiver. When the pressure rises, controller 72 closes valve 74 allowing the material in receiver 44 to cool thus lowering its vapor pressure. This control system thus maintains the condensate in receiver 44 at its bubble point temperature for the particular pressure at which it is maintained.

Controller 72 is preferably a differential pressure controller in this invention thus making operation completely independent of changes in the column operating pressure and always produces reflux and overhead liquid product as hot as possible, i.e. at a high pressure bubble point. An ordinary pressure controller actuated by the receiver pressure can be substituted, but it requires a change in the set pressure each time a change in the column pressure is effected.

Referring now more particularly to Figure 2, an elevation view in partial cross section of an apparatus ernployed in this invention for combining bypass overhead vapors in line 76 with rundown condensate in line 42 is shown. The end of bypass line 76 opens directly into the lower end of rundown line 42 as indicated. Bypass line. 76 is provided at its end with a plurality of openings 78 through which the hot overhead vapors ow in all directions thus thoroughly mixing with and being condensed by the rundown liquid inline 42. The mixture thus produced is a liquid having a higher temperature than the rundown liquid depending upon the amount of vapors thus bypassed and quenched in the condensate.

The present invention is applied in the continuous distillation of a naphtha having an end point of 450 F. to distill overhead a lighter gasoline having a 380 F. end point as feed to a catalytic reforming process. In this process the reformer feed is necessarily heated to temperatures of the order of 900 F. and thus the overhead product of the prefractionation is desirably cooled only sufficiently to produce a liquid condensate phase. The distillation column is operated at 20 p.s.i.g., the control point of pressure controller 68. The differential pressure controller 72 is set to hold a differential of l0 p.s.i. between the distillation column and the overhead receiver. The overhead product is totally condensed. Pressure controller 68 maintains the 2O p.s.i.g. column pressure by varying the heat exchange area in the overhead coolers and condensers through variation of liquid level 70. Differential pressure controller 72, in maintaining the fixed differential of l0 p.s.i. between the column and the receiver, keeps the liquid present in the receiver hot, that is, at its bubble point of about 220 F. for a pressure of l0 p.s.i.g. This gasoline is pumped from receiver 44 as indicated in Figure 1 and is pumped directly into the preheating and vaporizing furnaces in the catalytic reforming system.

The system performs very satisfactorily, controls the distillation column pressure within very narrow limits, and maintains the overhead liquid product at its maximum temperature to produce a material in the liquid phase. As indicated previously, the control system is cornpletely independent of changes in boiling range of the distillation column feed and the composition of the overhead liquid. When the feed naphtha composition is substantially constant over long periods, a pressure controller set for l0 p.s.i.g. can be substituted for differential controller 72.

If desired, in certain particular instances, the pressure control and overhead liquid temperature control methods here described may be used independently, although in the present case in which the overhead vapors are totally condensed it is preferable to use them simultaneously. The means for maintaining the overhead liquid at its bubble point may be used in conjunction with a distillation in which only part of the overhead vapors is condensed. In this latter situation, the usual back pressure control on the stream of uncondensed gases will maintain the distillation column pressure in the usual way. Similarly in those situations in which a distillate vapor is to be totally condensed and where there are no requirements that it be produced at its bubble point or other elevated temperature, then the column pressure control step of the present invention may be applied to advantage without the temperature control method also disclosed.

A particular embodiment of the present invention has been hereinabove described in considerable detail by way of illustration. It should be understood that various other modifications and adaptations thereof may be made by those skilled in this particular art without departing from the spirit and scope of this invention as set forth in the appended claims.

I claim:

l. A method of fractional distillation of a mixture which comprises passing at least part of the distillate vapor through a cooling zone to condense substantially all of said distillate, continuously detecting the pressure of distillation, varying the cross-sectional area open to heat exchange in said cooling zone in accordance with the detected pressure of distillation to maintain said pressure at a predetermined value, said cross-sectional area open to heat exchange being varied by the step of controlling the rate of removal of condensate therefrom to maintain a body of condensate therein of variable level to submerge at least part of said area, collecting condensed distillate in a receiving zone, removing condensate therefrom as retiux in the distillation, continuously detecting the pressure in said receiving zone, bypassing the remaining part of said distillate vapor around said cooling zone in accordance with the detected pressure of said receiving zone, and mixing this bypassed vapor thoroughly with condensate flowing between said cooling zone and said receiving zone to maintain in said receiving zone a condensate substantially at its bubble point and at a pressure below that of the distillation.

2. A method according to claim 1 in combination with the step of removing condensate from said receiving zone as a distillate product at a rate controlled to maintain therein a substantially constant liquid level.

3. A method according to claim 1 wherein the detected pressure of said receiving zone is measured relative to said pressure of distillation by a differential pressure controller, in combination with the step of maintaining the pressure of said receiving zone at a lixed difference below said pressure of distillation whereby the condensate produced from said receiving zone as product is always at its bubble point irrespective of the boiling range of the material fed to the distillation.

4. In a continuous fractional distillation in which a feed mixture is introduced into a rectification zone, one portion of the liquid is removed at the bottom thereof and heated and the vapor portion reintroduced into said zone and the remaining liquid portion is produced as a bottoms product, the vapor is removed at the top of said rectification zone and is condensed in a cooling zone and one liquid portion returned to said rectification zone as reflux and the rest is produced as an overhead product, the improved method for controlling the pressure in said rectification zone which comprises continuously detecting the pressure in said rectification zone, and controlling the rate of flow of the condensate at a point between said cooling zone and a condensate receiving zone so as to maintain in said cooling zone a variable liquid level of condensate submerging at least part of the heat exchange area to raise said level in response to detected pressures below the desired value and to lower said level in response to detected pressures above the desired value and whereby the condensate receiving zone is not flooded so as to maintain the desired pressure in said rectification zone in spite of the total condensation of the overhead vapor.

References Cited in the file of this patent UNITED STATES PATENTS 1,730,892 Leslie Oct. 8, 1929 2,104,310 Roelfsema Jan. 4, 1938 2,357,113 Houghland et al. Aug. 29, 1944 2,369,058 Legatski Feb. 6, 1945 2,709,678 Berger May 31, 1955 2,711,992 Kerner June 28, 1955 OTHER REFERENCES American Petroleum Institute Proceedings 1946, vol. 26, section III, pages 1535-198.

Claims

1. A METHOD OF FRACTIONAL DISTILLATION OF A MIXTURE WHICH COMPRISES PASSING AT LEAST PERT OF THE DISTILLATE VAPOR THROUGH A COOLING ZONE TO CONDENSE SUBSTANTIALLY ALL OF SAID DISTILLATE, CONTINOUSLY DETECTING THE PRESSURE OF DISTILLATION, VARYING THE CROSS-SECTIONAL AREA OPEN TO HEAT EXCHANGE IN SAID COOLING ZONE IN ACCORDANCE WITH THE DETECTED PRESSURE OF DISTILLATION TO MAINTAIN SAID PRESSURE OF A PERDETERMINED VALUE,SAID CROSS-SECTIONAL AREA OPEN TO HEAT EXCHANGE BEING VARIED BY THE STEP OF CONTROLLING THE RATE OF REMOVAL OF CONSENSATE THEREFORM TO MAINTAIN A BODY OF CONDENSATE THEREIN OF VARIABLE LEVEL TO SUBMERGE AT LEAST PART OF SAID AREA, COLLECTING CONDENSED DISTILLATE IN A RECEIVING ZONE, REMOVING CONDENSATE THEREFROM AS REFLUX IN THE DISTILLATION, CONTINUOUSLY DETECTING THE PRESSURE IN SAID RECEIVING ZONE, BYPASSING THE REMAINING PART OF SAID DISTILLATE VAPOR AROUND SAID COOLING ZONE IN OCCORDANCE WITH THE DETECTED PRESSURE OF SAID RECEIVING ZONE, AND MIXING THIS BYPASSED VAPOR