US3475288A - Fractionator overhead and reflux control system with optional accumulator by-pass - Google Patents

Description

INVENTOR. R C. EZZELL A TTORNE V5 United States Patent O 3,475,288 FRACTIONATOR OVERHEAD AND REFLUX CON- TROL SYSTEM WITH OPTIONAL ACCUMULA- TOR BY-PASS Robert C. Ezzell, Borger, Tex., assignor to Phillips Petroleum Company, a corporation of. Delaware Filed Mar. 18, 1968, Ser. No. 713,681

Int. Cl. B01d 3/20 U.S. Cl. 203-1 8 Claims ABSTRACT OF THE DISCLOSURE The condensate from a fractionator overhead ilooded condenser is passed directly to the inlet of the overhead condensate r or reux pump at a rate responsive to column overhead pressure; while only a vapor stream is passed to the overhead accumulator, at a rate responsive to accumulator pressure. A conduit between the accumulator and the pump inlet permits liquid tlow in either direction. A first portion of the condensate is passed to the column as reflux while the remainder is withdrawn as product at a rate responsive to the liquid level in the accumulator.

This invention relates to improved method and apparatus for controlling fractional distillation columns. In many of the present systems difliculties have been encountered in that frequently the automatic controls perform at cross purposes with one another. The controlling of column overhead pressure and the controlling of reliux accumulator pressure are examples of conventional control loops whose criteria occasionally conllict. This conict can become acute in a system employing a high reux to feed ratio wherein it is desirable to operate the column so as to condense in the column the maximum amount of high boiling material, which would otherwise contaminate the overhead product, while maintaining the reux accumulator pressure as close as possible to the column overhead pressure to be able to liquefy all of the column overhead vapor stream at minimum cooling and to maintain as high as possible the inlet pressure to the reflux pump to minimize cavitation problems, while at the same time it is desirable for the reflux to be as cold as possible to aid in the condensation of the higher boiling material in the column. A common method of holding a desired pressure on a fractionating column accumulator is to bypass a portion of the hot vapors from the overhead Vapor line directly to the accumulator without cooling, with the remainder being passed through the overhead condenser to produce subcooled condensate which is passed to the accumulator. This method is employed in operations where minute quantities of the light components in the normally used (from an extraneous source) pressure maintenance gases would contaminate the overhead product. However, the hot bypassed vapors tend to condense in the accumulator, because of the presence of the subcooled liquid, heating up the liquid thereby causing a loss of part of the energy required for condensation and subcooling in the overhead condenser and limiting the cooling capacity of the external reux stream. In other words, the minimum external reflux temperature obtainable is the equilibrium temperature corresponding to the accumulator pressure.

It has beenv found that the objectives of such a control system can be accomplished, while avoiding the disadvantages, by operating the overhead condenser under flooded conditions and passing the condensate from the condenser directly to the overhead pump inlet instead of through the accumulator. The tiow rate of condensate 3,475,288 Patented Oct. 28, 1969 from the condenser can be controlled responsive to column overhead pressure, and the flow rate of vapor to the accumulator can still be manipulated to maintain the desired accumulator pressure. A conduit provides communication between the pump inlet and the bottom of the accumulator for flow in either direction as required by the operating condition. The accumulator liquid level is utilized to regulate overhead liquid product withdrawal rate.

Accordingly, it is an object of the invention to provide an improved control system for a fractionato-r. Another object of the invention is to increase the cooling power of the external reux. Another object of the invention is to increase the purity of the column overhead for a given reilux flow rate. A further object of the invention is to increase the eciency of the utilization of the fractionator overhead condenser. Other objects, aspects and advantages of the invention will be apparent from a study of the specification, the drawing andthe appended claims to the invention.

The drawing is a diagrammatic representation of a fractionator control system in accordance with one embodiments of the invention. A stream to be fractionated is passed through conduit 11 into fractional distillation column 12. Heat is supplied to the bottom .section of column 12 by passing a heating fluid through conduit 13 into and through internal indirect reboiler 14, with the Withdrawal of the heating iluid being by way of conduit 15. While an internal indirect reboiler has been illustrated, any suitable means of supplying heat to the column bottom section can be employed. A kettle product is withdrawn by way of conduit 16.

A vaporous overhead stream is withdrawn from the top section of column 12 and a first portion thereof is passed by way of conduits 17 and 18 to the inlet of overhead condenser 19. A second portion of the vaporous overhead stream passes by way of conduits 17 and 21 directly to accumulator 22. Conduits 23 and 24 connect the outlet of condenser 19 to the inlet of overhead pump 25. Valve 26 is positioned in conduit 23 to maintain at least a partially flooded condition in condenser 19. In other words, all of the overhead vapor stream entering condenser 19 is liquefied and only condensate passes through conduit 23. This permits the operation of condenser 19 to subcool the condensate to as low a temperature as is desired or to the limit of the cooling capacity of condenser 19. Valve 26 is manipulated by pressure recorder controller 27 responsive to a comparison of the actual column overhead pressure as indicated by pressure sensor 28 and the desired column overhead pressure represented by setpoint 29. A decrease in column overhead causes pressure controller 27 to decrease the opening of valve 26 while an increase in column overhead pressure results in controller 27 increasing the opening of valve 26.

Valve 31 is positioned in vapor bypass conduit 21 and is manipulated by pressure recorder controller 32 responsive to a comparison of the actual accumulator pressure as indicated by pressure sensor 33 and the desired accumulator pressure represented by setpoint 34. An increase in accumulator pressure results in controller 32 decreasing the opening of valve 31 while a decrease in accumulator pressure causes controller 32 to open valve 31 further. Conduit 35 is connected between the bottom of accumulator 22 and the junction of conduits 23 and 24 to provide liquid ow from conduit 23 through conduit 35 into accumulator 22 or liquid flow from accumulator 22 through conduits 35 and 24 to the inlet of pump 25. Conduit 36 is connected between the outlet of pump 25 and an upper portion of column 12 to pass condensate into column 12 as reux therefor. Valve 37, located in conduit 36, is manipulated by flow recorder controller 38 responsive to a comparison of the actual flow rate of external reflux as indicated by flow sensor 39 with the desired flow rate represented by setpoint 41. Internal reflux computer 42 can be utilized to adjust flow setpoint 41 responsive to a change in the actual temperature of the external reflux, as indicated by temperature sensor 43, or the actual temperature of the column overhead, as indicated by temperature sensor 44, or both.

Conduit 45 is connected to the system for the withdrawal of the overhead liquid product stream. While conduit 45 can be connected to the outlet of pump 25 or to conduit 24, it is presently preferred that conduit 45 be connected to conduit 35 or directly to accumulator 22 as illustrated. Valve 46, located in conduit 45, is manipulated by liquid level controller 47 responsive to a comparison of the actual liquid level in accumulator 22 with the desired liquid level represented by setpoint 48. Analyzer 51 can be connected to conduit 17 to vary the measurement input of analyzer recorder controller 53 responsive to a variation in the actual concentration of one or more components of the overhead vapor stream. Controller 53 compares the measurement with the desired concentration for such components represented by setpoint 54 and manipulates the desired internal reflux setpoint 55 on computer 42. For an increase in temperature of the external reflux, a greater flow rate of external reflux through conduit 36 is required to obtain the same purity of the overhead product stream. Similarly a smaller flow rate of external reflux can be used with a lower external reflux temperature to obtain the same product purity. If the product purity changes from the desired value, analyzer controller 53 adjusts the setpoint 55 of internal reflux computer 42, resulting in a manipulation of the setpoint 41 of external reflux flow controller 38. An increase in impurity would cause analyzer controller 53 to vary setpoint 41 to cause an increase in the reflux flow rate in conduit 36, assuming no change in reflux temperature.

The system is generally operated under conditions Where a small flow rate of vapor through conduit 21 is maintained. Where this vapor flow rate equals the flow rate of overhead liquid product through line 45, there is no liquid flow through conduit 35 in either direction. If conduit 45 were to be connected to conduit 35 or the outlet of pump 25, there would then be a flow rate through conduit 35 equal to the overhead liquid product flow rate.

A change in feedstock composition or other column upset which results in a decrease in the column overhead pressure results in a decrease in the flow rate of condensate through conduit 23. This causes pump 24 to draw part of its supply from accumulator 22. However, the accompanying drop in accumulator liquid level causes controller 47 to adjust valve 46 to provide a corresponding decrease in the overhead liquid product stream flow rate, thereby permitting a portion of the condensate from accumulator 22 to flow through conduit 35 to pump 25 to make up the loss in condensate from conduit 23. The combination of condensate from accumulator 22 with condensate from conduit 23 increases the temperature of the external reflux resulting in a faster vaporization of the reflux in the column, thereby tending to increase column overhead pressure. The use of internal reflux computer 42 would result in an increase in the external reflux flow rate, calling for more hot condensate from accumulator 22. As the desired column overhead pressure is regained, valve 26 is opened further, decreasing the temperature of the reflux and decreasing the flow rate of hot condensate through conduit 35 until the original conditions are achieved. If the column overhead pressure increases above the desired value, valve 26 is opened further, causing an excess over that required for the external reflux. This excess flows upwardly through conduit 35 into accumulator 22. The resulting increase in accumulator liquid level causes valve 46 to be opened further to withdraw the excess as part of the overhead liquid product.

The following example is presented in further illustration of the invention and should not be construed in undue limitation of the invention.

EXAMPLE A fractional distillation system of the type illustrated in the drawing, except that the eflluent conduit of the overhead condenser is connected to the accumulator instead of the pump inlet and computer 42 and controller 53 are omitted, is utilized to fractionate a feed stream to obtain an overhead product and bottoms product having the compositions set forth in Table I. The fractional distillation system is then modified by connecting conduit 23 directly to the pump inlet as illustrated in the drawing and is operated to fractionate the same feed stream to obtain overhead and bottom product stream having the compositions set forth in Table I. The necessary operating conditions of the first fractional distillation system of the prior art and the second fractional distillation system in accordance with the invention are set forth in Table II.

Prior Method Invention Tower pressure, p.s.i.g 70 70 Tower top temperature, F.. 120 120 Redux temperature, F i12 100 Reflux volume, gallons/hour.. 85, 000 80, 000 Accumulator pressure, p.s.i.g. 60 60 Accumulator temperature, F 112 112 Feed, gallons/hour 5, 500 5, 500 Overhead product, gallons/hour. 5, 5,190 Mol percent 1,3-butadiene 99. 7 99. 7 Bottom temperature, F 150 150 The operation in accordance with the invention permits obtaining the same product stream compositions at a lower reflux rate and thus a lower cost. The use of the same reflux flow rate for the system of the invention would result in a higher purity butadiene overhead product stream.

Reasonable variation and modication are possible within the scope of the foregoing disclosure, the drawing and the appended claims to the invention.

I claim:

1. Apparatus comprising a fractional distillation column, first conduit means for introducing into an intermediate section of said column a feed stream to be fractionated, second conduit means connected in communication with a lower portion of said column for withdrawal of a kettle product stream, a condenser having an inlet and an outlet, third conduit means connected between an upper portion of said column and said inlet of said condenser, an accumulator having a vapor connection and a liquid connection, fourth conduit means connected between said third conduit means and said vapor connection of said accumulator, a pump having an inlet and an outlet, fifth conduit means connected between said liquid connection of said accumulator and said inlet of said pump, sixth conduit means connected directly between said outlet of said condenser and said fifth conduit means to bypass said accumulator, seventh conduit means connected between said outlet of said pump and an upper portion of said column to pass condensate from said pump to said column as reflux therefor, eighth conduit means for withdrawal of an overhead liquid product stream, first valve means operatively positioned in said sixth conduit means, means for manipulating said first valve means responsive to the overhead pressure of said column, second valve means operatively positioned in said fourth conduit means, means for manipulating said second valve means responsive to the pressure in said accumulator, third valve means operatively positioned in said eighth conduit means, means for manipulating said third VAIvalve means responsive to the liquid level of condensate in said accumulator, and means associated with said seventh conduit means for regulating the rate of ow of reliux therethrough.

2. Apparatus in accordance with claim 1 whereint said eighth conduit means is connected to one of said pump outlet and said accumulator.

3. Apparatus in accordance with claim 2 wherein said means for regulating the rate of ow of reux comprises fourth valve means operatively located in said seventh conduit means, a flow controller for manipulating said fourth valve means responsive to the rate of lowv of reux through said seventh conduit means, and an internal reflux computer controller for manipulating the setpoint of said flow controller responsive to the temperature of the liquid in said seventh conduit means.

4. Apparatus in accordance with claim 3 further comprising means for manipulating the setpoint of said internal redux computer controller responsive to the composition of the stream flowing through said third conduit means.

5. A method comprising passing a feed stream tobe fractionated into an intermediate portion of a fractional distillation zone, withdrawing a kettle product stream from a lower portion of said fractional distillation zone, withdrawing an overhead vaporous stream from an upper portion of said fractional distillation zone, passing a first portion of said overhead vaporous stream into an accumulation zone at a rate responsive to the pressure in said accumulation zone, passing a second portion of said overhead vaporous stream into an at least partially ooded condensing zone and therein condensing said second portion, withdrawing condensate from said condensing zone at a rate responsive to the pressure in the overhead of said fractional distillation zone, passing at least a portion of the thus withdrawn condensate directly from said condensing zone into an upper portion of said fractional distillation zone as external reux therefor and thereby bypassing said accumulation zone, passing a portion of said condensate withdrawn from said condensing zone into said accumulation zone only when the rate of Withdrawal of condensate from said condensing zone exceeds the requirements for condensate from said condensing zone and` withdrawing a portion of at least one of the condensate from said condensing zone and the condensate from said accumulation zone as an overhead liquid product stream at a rate responsive to the liquid level of condensate in said accumulation zone.

6. A method in accordance with claim 5 further comprising passing condensate from said accumulation zone into said upper portion of said fractional distillation zone when the rate of withdrawal of condensate from said condensing zone is less than the desired reux flow rate.

7. A method in accordance with claim 6 further comprising varying the rate of introduction of reflux into said upper portion of said fractional distillation zone responsive to the actual temperature of the reflux.

8. A method in accordance with claim 7 further comprising varying said rate of introduction of reflux responsive to the composition of said overhead vaporous stream.

References Cited UNITED STATES PATENTS 2,813,594 11/1957 Gantt 203-2 2,915,462 12/ 1959 Salmon 203-2 3,039,941 6/ 1962 Sweeney et al 203-2 3,049,886 8/ 1962 Cabbage 62-37 3,296,241 1/ 1967 Berger 202-260 3,301,778 1/ 1967 Cabbage 203-2 3,309,288 3/ 1967 iButterbaugh 203-2 3,340,160 9/ 1967 Waldby 203-1 3,361,646 1/ 1968 MacMullan et al 20'3-2 WILBUR L. BASCOMB, JR., Primary Examiner U.S. C1. X.R.

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