US3225551A

US3225551A - Fractional distillation system

Info

Publication number: US3225551A
Application number: US262736A
Authority: US
Inventors: Carl S Kelley
Original assignee: Phillips Petroleum Co
Current assignee: Phillips Petroleum Co
Priority date: 1963-03-04
Filing date: 1963-03-04
Publication date: 1965-12-28
Anticipated expiration: 1982-12-28

Description

Dec. 28, 1965 c. s. KELLEY 3,2

FRACTIONAL DISTILLATION SYSTEM Filed March 4, 1963 2 Sheets-Sheet 2 TV T INVENTOR. C. S. K ELLEY A 7' TORNE Y5 United States Patent 3,225,551 FRACTIONAL DIFaTILLATIGN SYSTEM Carl S. Kelley, Bartiesviiie, Okla, assignor to Philiips Petroleum Company, a corporation of Delaware Filed Mar. 4, 1963, Ser. No. 262,736 6 Claims. (Cl. 62-21) This invention relates to fractional distillation. In one aspect the invention relates to the control of a fractional distillation operation wherein a component to be distilled as overhead product is present in the feed stock in such a low concentration as to provide too small a stream from the reflux accumulator to serve for control of fractionator pressure. In another aspect the invention relates to a novel heat exchanger-vent gas absorber.

Prior art fractional distillation has usually treated charge stocks containing a sufiicient proportion of the lowest boiling component or composition so that ample gas flow was obtained for use in regulation of fractionator pressure. In one instance, however, when accumulator off-gas flow was too small, an extraneous gas was added to the overhead gas stream to increase the off-gas flow. In some instances accumulator otf-gases have specific utilities and it is undesirable to add an extraneous gas thereto, or to dilute one component of a mixture with added amounts of another component,

According to this invention, I compress the overhead vaporous stream from a fractionator and regulate the reflux accumlator off-gas withdrawal in response to the pressure dilferential across the compressor. The cornpressor is operated for a minimum of compression to produce just sufficient condensate with use of the avail able cooling to reflux the fractionator. In one specific embodiment the available cooling is the sum of the cooling available from the fractionators reboiler and a small amount from a small stream of refrigerant.

The rate of compression is regulated in response to the overhead fractionator pressure. Thus, upon an increase in overhead pressure, the compressor is speeded up, thereby to return the pressure to its normal operating pressure. This increase in the rate of compression is accompanied by an increase in pressure differential across the compressor. This increase in pressure dilferential then resets the set point of a fiow controller on the accumulator off-gas line. The converse of the above is also effected. That is, if the overhead pressure decreases, the compressor speed is decreased which results in returning the overhead pressure to normal at a lower compressor speed; this results in a decrease in differential pressure across the compressor, and ultimately a decrease in flow of light gases from the system.

Accordingly, it is an object of the invention to provide novel method and means for controlling a fractional distillation system. Another object of the invention is to provide method and means for regulating the overhead pressure in a fractionation column. A still further object of the invention is to provide method and means for regulating the overhead pressure in a fractionation system wherein the overhead product is a very small percentage of the feed stream. Yet another object of the invention is to provide a novel self-refluxing gas absorber.

Other aspects, objects, and advantages of the invention will be apparent from a study of the disclosure, the drawing and the appended claims to the drawing.

In the drawings, FIGURE 1 is a schematic representation of a fractional distillation system embodying the control features of the invention;

FIGURE 2 is an elevation view, partly in cross section, of a self-refluxing gas absorber in accordance with the invention; and

FIGURE 3 is a view of the apparatus of FIGURE 2 taken along line 33.

Referring now to the drawing and to FIGURE 1 in particular there is shown a system for the upgrading of the content of a component of a feed stock boiling intermediate the boiling points of the lowest boiling and highest boiling components of the feed stock wherein the lowest boiling component is present in the feed stock in too low a concentration, for example on the order of three percent by weight or less of the total feed stock, to serve as a means for controlling the pressure in the fractional distillation column. While the invention is applicable to the fractional distillation of any mixture wherein the overhead product is present in the feed stock in too low a concentration to serve as a means for controlling the pressure in the fractional distillation column, the invention will be described in terms of a separation of a feed stock comprising ethane, propylene, and propane as the lowest boiling component, intermediate boiling component, and highest boiling component, respectively. A feed stream of ethane, propylene, and propane is passed through conduit 11 into a fractional distillation column 13 and fractionally distilled therein. An overhead vapor stream containing a portion of the ethane is withdrawn from fractional distillation column 13 by way of conduit 15, while a product stream enriched in propylene can be withdrawn by way of side draw conduit 16. The rate of withdrawal of product by way of conduit 16 can be controlled by any suitable means. For example, the rate of flow through conduit 16 can be controlled by a valve 17 which is manipulated by a flow recorder controller 18 responsive to a comparison of a set point value and the differential pressure across an orifice 19 located in conduit 16. The rate of withdrawal of product through conduit 16 can be controlled by valve 17 responsive to the output of an analyzer controller 22. If desired the set point value of flow recorder controller 18 can be manipulated by analyzer recorder 22 responsive to the composition of the product in conduit 16.

A liquid stream is withdrawn from the bottom of fractional distillation column 13 by way of conduit 23 and introduced into reboiler 24 wherein the withdrawn liquid is subjected to indirect heat exchange to eifect the vaporization of the liquid. The vapors thus produced are withdrawn from reboiler 24 and passed by way of conduit 25 into the lower portion of column 13, preferably below the lowest tray. A vapor stream containing substantially all of the propane and only a small portion of the propylene is Withdrawn from the bottom section of column 13 by way of conduit 27 and is passed to a point of utilization, recovery, or further treatment. The rate of flow through conduit 27 can be controlled by valve 28 which can be manipulated by liquid level controller 29 responsive to the level of liquid kettle material in reboiler 24 to maintain such level substantially constant. In one particular embodiment the vapor stream withdrawn by way of conduit 27 is utilized as a fuel gas.

The overhead vapor stream in conduit 15 is introduced into a surge tank 30 from which a vapor stream is withdrawn by way of conduit 31 and subjected to compression in a compressor 33, which is driven by a suitable means such as motor 35. The speed of motor 35, and thus the rate of compression, is controlled by a speed control means 37 responsive to a comparison of the pressure in conduit 31, as indicated by a pressure sensing means 39, and a value applied to set point input 41. The resulting hot vaporous compressed stream is passed by way of conduit 43 into reboiler 24 in indirect heat eXchange with the kettle contents contained therein. The cooled fluid stream is withdrawn by way of conduit 44 and passed by way of conduit 45 through heat exchanger 46 into accumulator 47. If desired, a portion of the cooled fluid stream can be withdrawn from conduit 44 and bypassed around heat exchanger 46 by way of conduit 48. A suitable coolant is passed by way of conduit 49 and valve 51 into heat exchanger 46 with the warmed coolant being withdrawn by way of conduit 52. Valve 51 can be controlled by pressure recorder controller 53 responsive to the pressure in accumulator 47 to maintain such pressure substantially constant by varying the rate of flow of coolant through heat exchanger 46. If desired, the ratio of the rates of flow in

conduits

45 and 48 can be regulated responsive to the pressure in accumulator 47 to maintain the pressure substantially constant instead of or in addition to the regulation of the rate of flow of coolant through conduit 49. The hot vaporous stream from conduit 43 is partially condensed by its passage through reboiler 24 and heat exchanger 46 to produce a condensate and an uncondensed gas enriched in ethane. The condensate is withdrawn from accumulator 47 and is passed by way of pump 55 and conduit 56 into an upper portion of column 13 as reflux therefor. The rate of flow of reflux through conduit 56 is controlled by valve 57 which is manipulated by flow rate recorder controller 58 responsive to a comparison of the differential pressure across an orifice 59 in conduit 56 and the set point value on controller 58. The set point value to controller 58 is in turn manipulated by liquid level controller 61, which is mounted on accumulator 47, to maintain the liquid level in accumulator 47 substantially constant.

A self-refluxing gas absorber 63 is connected to the vapor section of acumulator 47 and comprises a contactor section 64 and a refrigeration section 65. A vapor stream is withdrawn from accumulator 47 and passed into contactor section 64 wherein the ascending vapor is contacted with descending liquid resulting from a condensation of a portion of the ascending vapor in refrigeration section 65. A suitable refrigerant is passed by way of conduit 66 into indirect heat exchange relationship with refrigeration section 65 and is withdrawn therefrom by way of conduit 67. The remaining unc-ondensed gas is withdrawn from absorber 63 and passed by way of conduit 68 and valve 69 to vent, recovery, or further treatment. Valve 69 is manipulated by a flow recorder controller 71 responsive to a comparison of the differential pressure across an orifice 72 in conduit 68 and a set point value. In accordance with the invention the set point value to How recorder controller 71 is manipulated by a differential pressure recorder controller 73 responsive to the difference in pressure of the overhead vapor stream in conduit 31 and the hot vaporous compressed stream in conduit 43. Thus for an incrase in the overhead pressure in column 13 speed control means 37 causes motor 35 to speed up and increase the rate of compression in compressor 33 in order to return the pressure in conduit 31 to its normal operating value. This increase in the rate of compression is accompanied by an increase in the diflerential pressure across compressor 33, that is, an increase in the difference in pressure between the overhead vapor streamin conduit 31 and the hot vaporous compressed stream in conduit 43. This increase in pressure differential is utilized to reset flow recorder controller 71 to permit an increase in the rate of withdrawal of off-gas by way of conduit 68. Similarly upon a decrease in the overhead pressure in column 13 the speed of the motor 35 is reduced by control means 37, causing a reduction in the rate of compression in compressor 33. This reduction in the rate of compression is accompanied by a decrease in differential pressure across compressor 33, and this decrease in pressure differential is utilized to reset flow recorder controller 71 to reduce the rate of withdrawal of olT-gas by way of conduit 68. Accordingly it is readily seen that the control system of the invention provides for maintaining the overhead pressure of the fractional distillation column substantially constant at the desired value while regulating the rate of flow of ofl-gas without incurring any excessive loss of propylene through conduit 68. This control system reduces the consumption of power by the compressor to a minimum, decreases possibility of ofi-specification product, and relieves the operator from having to continuously watch and reset controls.

A first signal representative of the rate of flow of fluid through conduit 27 is transmitted by way of line 75 to a first input of differential flow rate recorder controller 76. A second signal representative of the rate of flow of fluid through conduit 25 is transmitted by way of line 77 to a second input of controller 76. Controller 76 produces an output signal responsive to the difierence between the first and second signals. The output signal is transmitted by way of line 78 to the reset input of pressure recorder controller 79. A valve 81 in conduit 44 is manipulated by pressure recorder controller 79 responsive to a comparison of the actual pressure in conduit 44 and the output signal of controller 76. Valve 81 thus controls the amount of heat which is transferred from the hot vaporous compressed stream of conduit 43 into the kettle material in reboiler 24, and thereby the heat input to column 13. For example, if too much liquid is produced in the bottom of column 13, the liquid level of kettle material in reboiler 24 will tend to rise. Liquid level controller 29 will actuate valve 28 to increase the rate of withdrawal of vapor through conduit 27. This results in a decrease in the difference between the flow rates in conduits 25 and 27, and differential flow rate controller 76 accordingly resets the set point value on pressure controller 79 to actuate valve 81 to raise the pressure upstream of valve 81. This increase in pressure of the compressed vapors passing from conduit 43 into reboiler 24 raises the condensation pressure and hence the condensation temperature of the compressed vapors. This permits additional heat to be transferred from the compressed vapors to the kettle material in reboiler 24 due to the higher differential temperature. The increase in heat transferred results in an increase in flow rate through conduit 25 until the system returns to desired operating conditions. A reverse action, that is, a drop in the liquid level in reboiler 24, will cause the reverse effect. Consequently the control system regulates the heat input to the column and thereby stabilizes operation of the column.

Referring now to FIGURE 2 self-refluxing gas absorber 63 comprises contacting section 64, refrigeration section 65 and cap 90. Contacting section 64 comprises a vertically positioned cylindrical vessel 91 having its lower end in vapor communication with accumulator 47, and one or more liquid-vapor contacting means 92, such as bubble cap tray, sieve tray or plate tray, positioned within vessel 91. Refrigeration section 65 comprises a vertically positioned cylindrical vessel 93, a perforate plate 94 positioned across the entire diameter of vessel 93 and mounted between

vessels

91 and 93, a perforate plate positioned across the entire diameter of vessel 93 and mounted between vessel 93 and cap 90, and a plurality of separate tubes extending in fluid-tight relationship from each perforation in lower plate 94 into corresponding perforations in upper plate 95. The upper ends of the tubes 96 are substantially flush with the upper surface of plate 95 while the upper ends of tubes 97 extend above plate 95 a suitable distance, for example on the order of one inch.

Tubes

96 and 97 are each spaced from one another to provide free space between them into which the coolant is passed by way of conduit 66 with the warmed coolant being withdrawn by way of conduit 67. While a plurality of tubes 96 have been illustrated in the drawing, it is within the contemplation of the invention to utilize one or more tubes 96. The vapors ascending through section 91 are contacted with the downflowing liquid which is obtained by condensation of a portion of the vapors passing through

tubes

96 and 97. Substantially all of the entrained liquid in the vapor

streams leaving tubes

96 and 97 is separated out in cap 90 and falls on plate 95. The liquid collecting on plate 95 drains through tubes 96, thereby preventing any flooding of tubes 97.

The following example is presented in further illustration of the invention and should not be construed unduly in limitation thereof. In the operation of a particular fractional distillation system in accordance with the invention for the separation of a feedstock comprising ethane, propane, and propylene, under the conditions set forth in Table I, the process streams have the compositions set forth in Table II.

Table 1 Pressure, Temperature,

p.s.i.g. F.

Feed Stock 110 45 Column 13:

Kettle 91 55 Overhead- 83 38 Surge Tank 30- 8O 38 Compressor 33:

Suction 80 38 Discharge 135 95 Conduit 44 132 68 Accumulator 47 130 64 Conduit 68 129 15 Conduit l6..- 85 45 91 55 91. 5 57 91 55 Table II 11 16 27 68 Component/Stream Lbs/hr. Lbs/hr. Lbs/hr Lbs/hr Ethane 150 5 145 Propylene 17, 000 16, 155 700 145 Propane 9, 850 S0 9, 770

Total 27, 000 16, 240 10, 470 290 The effects of a sli ht increase in C content of feed stream is illustrated in Table III.

The slight increase in C content illustrated in Table III requires that almost twice as much off-gas be vented. The control system of the invention maintains the loss of propylene through conduit 68 at a minimum while maintaining the power consumption of compressor 33 at a minimum.

Reasonable variations and modifications are possible within the scope of the foregoing disclosure, the drawings, and the appended claims to the invention.

I claim:

1. A process for the regulation of pressure in a fractional distillation zone operating for the purification of a feed component having a boiling point intermediate the boiling points of the lowest boiling and highest boiling components of the feed to be distilled and wherein the lowest boiling component is present in said feed in too low a concentration to serve as a means for regulation of pressure in said fractional distillation zone, comprising fractionally distilling said feed in said fractional distillation zone thereby producing an overhead vapor stream containing the major portion of said lowest boiling component, compressing this overhead vapor stream thereby producing a hot vaporous compressed stream, partially condensing this hot vaporous compressed stream by indirect heat exchange with kettle contents of said zone and by further indirect heat exchange with a coolant thereby leaving uncondensed gas enriched in said lowest boiling component, separating the resulting condensate from said uncondensed gas, returning the condensate to the distilling step as reflux, sensing the differential pressure between said overhead vapor stream and said hot vaporous compressed stream, and regulating the rate of withdrawal of said uncondensed gas in response to the sensed differential pressure.

2. The process of claim 1 wherein the lowest, intermediate and highest boiling components are ethane, propylene, and propane, respectively.

3. A process for the regulation of pressure in a fractional distillation zone operating for the upgrading of the content of a component of a feed stock boiling intermediate the boiling points of the lowest boiling and highest boiling components of the feed to be distilled, said feed being rich in the intermediate boiling component, and the lowest boiling component being present in the feed in too low a concentration to serve as a means for control of pressure in said fractional distillation zone, comprising fractionally distilling said feed in said fractional distillation zone thereby producing an overhead vapor stream containing the major portion of said lowest boiling component, compressing this overhead vapor stream thereby producing a hot vaporous compressed stream, partially condensing this hot stream by indirect heat exchange with kettle contents of said fractional distillation zone and by further indirect heat exchange with a coolant thereby leaving an uncondensed gas enriched in said lowest boiling component, separating the result ing condensate from said uncondensed gas, returning said condensate to the fractional distillation zone as reflux, sensing the pressure differential between said overhead vapor stream and said hot vaporous compressed stream, withdrawing the uncondensed gas from the system in response to the sensed pressure differential, sensing the pressure of said overhead vaporous stream, regulating the rate of compressing in response to the sensed pressure, and withdrawing a sidestream from said fractional distillation zone, said sidestream being richer in said intermediate boiling component than the feed stock.

4. The process of claim 3 wherein the lowest, intermediate, and highest boiling components are ethane, proplene, and propane, respectively.

5. A process for the regulation of pressure in a fractional distillation zone operating for the upgrading of the content of a component of a feed stock boiling intermediate the boiling points of the lowest and highest boiling components of the feed to be distilled, said feed containing from an infinitesimal amount up to about 3 percent by weight of the lowest boiling components, said amount of lowest boiling components being too small to provide sufficient accumulator gas flow for pressure control of the fractional distillation zone, comprising fractionally distilling said feed stock in said fractional distillation zone thereby producing an overhead vaporous stream containing the major portion of said lowest boiling components, compressing this overhead vapor stream thereby producing a hot vaporous compressed stream, partially condensing this hot vaporous compressed stream by indirect heat exchange with kettle contents of said zone and by further indirect heat exchange with a coolant thereby leaving an uncondensed gas enriched in said lowest boiling components, separating the resulting condensate from said uncondensed gas, returning said condensate to the fractional distillation zone as reflux therefor, sensing the pressure differential between said overhead .vaporous stream and said hot vaporous compressed stream, withdrawing the uncondensed gas in response to the sensed pressure differential, sensing the pressure of said overhead vaporous stream, regulating the rate of compressing in response to the sensed pressure, and Withdrawing a sidestream from said distillation zone, said sidestream being richer in said intermediate boiling component than said feed stock.

6. The process of claim 5 wherein said lowest boiling constituents comprise ethane and ethylene, and said intermediate and highest boiling constituents comprise a major proportion of propylene and a minor proportion of propane.

References Cited by the Examiner UNITED STATES PATENTS Schlitt 6226 XR Haynes 6226 XR Haldernan 261153 Hackmuth 6231 XR Moore 261-153 Sattler 6221 Greca 62-31 XR Cabbage 6221 NORMAN YUDKOFF, Primary Examiner.

Claims

1. A PROCESS FOR THE REGULATION OF PRESSURE IN A FRACTIONAL DISTILLATION ZONE OPERATING FOR THE PURIFICATION OF A FEED COMPONENT HAVING A BOILING POINT INTERMEDIATE THE BOILING POINTS OF THE LOWEST BOILING AND HIGHEST BOILING COMPONENTS OF THE FEED TO BE DISTILLED AND WHEREIN THE LOWEST BOILING COMPONENT IS PRESENT IN SAID FEED IN TOO LOW A CONCENTRATION TO SERVE AS A MEANS FOR REGULATIONS OF PRESSURE IN SAID FRACTIONAL DISTILLATION ZONE, COMPRISING FRACTIONALLY DISTILLING SAID FEED IN SAID FRACTIONAL DISTILLATION ZONE THEREBY PRODUCING AN OVERHEAD VAPOR STREAM CONTAINING THE MAJOR PORTION OF SAID LOWEST BOILING COMPONENT, COMPRESSING THIS OVERHEAD VAPOR STREAM THEREBY PRODUCING A HOT VAPOROUS COMPRESSED STREAM, PARTIALLY CONDENSING THIS HOT VAPOROUS COMPRESSED STREAM BY INDIRECT HEAT EXCHANGE WITH KETTLE CONTENTS OF SAID ZONE AND BY FURTHER INDIRECT HEAT EXCHANGE WITH A COOLANT THEREBY LEAVING UNCONDENSED GAS ENRICHED IN SAID LOWEST BOILING COMPONENT, SEPARATING THE RESULTING CONDENSATE FROM SAID UNCONDENSED GAS, RETURNING THE CONDENSATE TO THE DISTILLING STEP AS REFLUX, SENSING THE DIFFERENTIAL PRESSURE BETWEEN SAID OVERHEAD VAPOR STREAM AND SAID HOT VAPOROUS COMPRESSED STREAM, AND REGULATING THE RATE OF WITHDRAWAL OF SAID UNCONDENSED GAS IN RESPONSE TO THE SENSED DIFFERENTIAL PRESSURE.