US3309882A - Employing bottoms of a second fractionator as a refrigerant medium for first fractionator overhead - Google Patents

Description

March 21, 1967 E. J. CABANAW gfiwggggg EMPLOYING BOTTOMS OF A SECOND FRACTIONATOR AS A REFRIGERANT MEDIUM FOR FIRST FRACTIONATQR OVERHEAD Filed Dec. 17, 1964 33 F 55 REFRIGERATION X i J 45 FUELS JCONDENSER FEED] :l Q E; 23

(ALKYLATION EFF LU E NT) DEPROPAN IZER PROPANE (LPG) 4O COOLER INVENTOR E J. CA BANAW Qua :7

ATTORNEYS United States Patent EMPLOYENG BOTTOIWS OF A SECOND FRAC- TIONATOR AS A REFRIGERANT MEDIUM FOR FIRST FRACTIONATOR OVERHEAD Eldred J. Cahanaw, Bartlesville, Okla, assignor to Phillips Petroleum Company, a corporation of Delaware Filed Dec. 17, 1964, Ser. No. 418,981 3 Claims. (Cl. 6228) This invention relates to an improved fractionation operation. In one of its aspects this invention relates to an improved method for removal of non-condensibles from a fractionation operation. In another of its aspects, the invention relates to the removal of non-condensibles from a fractionation operation by way of a two-stage condenser from which non-condensibles are periodically removed. In another of its aspects the invention relates to a method and apparatus wherein a portion of'the coolant required to cool overhead from a fractionator is provided from a product of the process.

In the operation of a fractionation system such as disclosed by Van Pool et al. in US. Patent 2,988,894, it is known to apply refrigeration to the accumulator condenser to avoid the passage of entrained materials overhead. While such a system is operative, it has been found that by utilizing an external refrigeration unit as the only source of refrigeration there is required a significant investment in cooling equipment such as compressors, heat exchangers and the like. In addition, when propane is desired to be incorporated into the fuel gas being taken overhead from the accumulator, additional handling is required to reduce the temperature and pressure of the liquid propane from the stripping unit to a state wherein it can be readily employed as an additive to the fuel gas, or as a fuel gas.

In the venting of non-condensibles from a system, to avoid undesired accumulation of non-condensibles therein and undesired pressure rises due to said non-condensibles, there is unavoidably vented together with the noncondensibles a portion of usually desirable product or distillate. Thus, in the fractionation of an alkylation effluent, it is known that non-condensibles such as nitrogen, methane and ethane accumulate in the system and must be vented therefrom. In venting non-condensibles from the system, there is unavoidably entrained therewith an important amount or proportion of HF which is a desired chemical costing about $100 per barrel. Thus, when the non-condensibles go to flare or to use as fuel gas components, HF is lost.

In addition, the presence of HF in the gas system results not only in the waste of desired HF but also in the creation of a corrosive stream which is extremely harmful to that portion of the system further downstream.

I have now discovered that the presence of HF in an off-gas stream can be substantially avoided while at the same time achieving a substantial reduction in the amount of expensive refrigeration required in contrast with that previously required by utilizing as a primary refrigerant, or higher temperature level refrigerant, a product of the process.

It is thus an object of this invention to provide an improved fractionation system. Another object of this invention is to provide a method for the economical operation of a fractionation process. A further object of the invention is to substantially reduce the amount of expensive refrigeration heretofore required in the operation of a fractionation process.

Other aspects, objects and the several advantages of the invention will be readily apparent to those skilled in the art fiom the following description, the drawing, and the appended claims.

In accordance with the present invention I have discovered that by utilizing propane recovered from a stripping unit as a primary coolant or relatively high temperature refrigerant in the condenser employed to vent non-condensibles from the accumulator for alkylation products, there is achieved a substantial reduction in the amount of expensive, low temperature refrigeration which is otherwise required to prevent the passage of HF overhead with the noncondensibles being vented from the system. In addition, I have discovered that the liquid propane effluent from the stripping unit is conveniently converted to a gaseous form at desired pressure and temperature suitable for use as a fuel gas, either as an additive or alone.

The invention will be described in conjunction with the accompanying figure, which diagrammatically illustrates the invention as applied to the fractionation of an alkylation efiluent.

As shown in the drawing, an alkylate efiluent is fed by way of pipe 1 to fractionator 2 from which isobutane and heavier are recovered as bottoms by way of pipe 3. A reboiler 4 is provided in the bottom of the fractionator. Overhead is passed in part by way of pipe 5, pipe 6 to accumulator 10 and, in part, by way of coolercondenser 7 and pipe 8 into accumulator 10. From accumulator 10 there is recovered an alkylation catalyst phase, in this instance hydrofluoric acid, which is returned to the reactors from keg 13 by way of liquid level control 14 actuating control of the valve in pipe 15. Hydrocarbon phase is removed from accumulator 10 by way of pipe 16 and fed by way of pump 17 and pipe 18 to fractionator 2 as reflux therefor. A flow recorder controller 19 adapted to control reflux flow through valve 20 is provided. Production quantity of hydrocarbon phase from accumulator 10 is passed by way of pipe 21 to HF stripper 22. Pipe 21 is provided with a flow recorder controller 23 which operates upon valve 24. Flow recorder controller 23 is adapted to be reset by liquid recorder controller 25 on accumulator 1i 'Accumulator hydrocarbon phase from accumulator 10 is stripped in stripper 22 to provide propane which is removed by way of pipe 26. Overhead from stripper 22 passes by way of pipe 27 and thence by way of cooler-condenser 7 and pipe 8 to accumulator 10.

Accumulator It has provided thereon a condenser 30 provided with a temperature recorder controller 31 which is effective in controlling a secondary refrigeration unit 32 from which low temperature refrigerant medium passes by way of pipe 33 into a heat exchange coil 34 in the top of condenser 30. The thusly heated refrigerant medium is returned to refrigeration unit 32 by way of pipe 35. Non-condensibles are removed from accumulator 10 by way of condenser 36 through pipe 36 and valve 37 on pipe 36.

The liquid propane from HF stripper 22 is removed through line 26 to cooler 40 wherein a portion of same is removed through line 42. The remainder of the propane is removed through line 41. The propane in line 42 having a temperature of about F. is introduced as primary refrigerant by means of line 43 and expansion valve 46 to primary heat exchange coil 44. Valve 46 can be controlled by temperature control means 48. The vaporized used primary heat exchange medium is then removed through line 45 for use as fuel, either alone or in combination with the overhead from condenser 30 removed through line 36.

The following table shows operating data for one days operation, using the primary cooling system according to the invention.

TABLE I Stream and Number Component 1 3 5 21 26 27 36 43 D0 DC; DC; Stripper Stripper Stripper Accumulator Accumulator Propane to Feed Bottoms Overhead 1 Feed Bottoms Overhead ent HF Yield Refrigeration Nitrogen, lbs/day.

Total, b./d

1 Does not include reflux.

The following table shows operating conditions for the various units of the operation as described above.

TABLE II Depropanizer (2):

Top pressure, p.s.i.g 285 Bottom pressure, p.s.i.g.

Top temperature, F 136 Bottom temperature, F. 233 Overhead accumulator (10):

Pressure, p.s.i.g 280 Temperature, F. 100 Condenser unit (30):

Temperatures, F

Bottom 100 Above lower coil (44) 60 Above refrigerator coil (34) 40 HF stripper (22) Pressure, p.s.i.g 285 Top temperature, F 136 Bottom temperature, F. 141 Refrigeration propane (43, 44, 45)

barrels/day 75 (43) Temperature before expansion valve (46),

F. 100 (44) After expansion:

Temperature, F. 50 Pressure, p.s.i.g 75 (45) Expanded efiluent (to fuel):

Temperature, F. 55 Pressure, p.s.i.g 70

Without the coil 44 of the invention, the secondary refrigeration unit 32, which uses an expensive, low temperature (e.g., Freon) type refrigerant, requires 5.5-ton size whereas with the coil 44 of the invention, only 1.4- ton size of refrigeration unit 32 is required.

It will be understood by one skilled in the art in possession of this disclosure that certain pieces of apparatus and details have been omitted for the sake of simplicity. Such apparatus and details will be supplied routinely by one skilled in the art in possession of this disclosure seeking to place the same into actual operation.

Reasonable variations and modifications of this invention can be made, or followed, in view of the foregoing, without departing from the spirit and scope thereof.

I claim:

1. In a method for operating a fractionation system which comprises charging to a fractionation zone a feed to be fractionated therein, fractionating said feed to produce a bottoms product which is withdrawn therefrom and an overhead containing gas or vapor, passing said overhead to a cooling -zone, therein cooling said overhead, passing the thus cooled overhead to an accumulation zone, in said accumulation zone separating a liquid phase and a gas or vapor phase containing said desired gas or vapor, removing the liquid phase from said accumulation zone, passing at least a portion of the removed liquid phase to said fractionation zone as reflux therefor and passing the remainder thereof to a second fractionation zone wherein a bottoms product and an overhead product are obtained, passing gas or vapor phase in said accumulation zone to a condensing zone wherein desired gas or vapor is condensed and recovered and the remaining gas or vapor removed overhead from said condensing zone, the improvement which comprises utilizing at least a portion of the bottoms product from said second fractionation zone as a primary coolant in said condensing zone.

2. In a method for fractionating an alkylation effluent containing propane and gases lighter than propane which comprises fractionating in a fractionation zone said effiuent to remove propane, hydrogen fluoride, and lighter gases overhead therefrom, passing said overhead to a cooling zone, therein cooling said overhead, passing the cooled overhead to an accumulation zone, in said last zone separating a liquid phase and a gas or vapor phase containing propane and said lighter gases, removing liquid phase from said last zone, passing at least a portion of the removed liquid phase to said fractionation zone and the remaining portion to a second fractionation zone, passing vapor phase in said accumulation zone into a condensing zone wherein desired hydrogen fluoride is condensed and removed and the remaining uncondensed gas or vapor is passed overhead therefrom, the improvement which comprises utilizing the bottoms from said second fractionation zone as a primary coolant in said condensing zone.

3. The process of claim 2 wherein said condensing zone has a primary and secondary cooling section disposed therein.

References Cited by the Examiner UNITED STATES PATENTS 2,933,901 4/1960 Davison 624O X 2,988,894 6/1961 Van Pool et al 62-28 3,229,471 12/1961 Palen et al. 6231 X NORMAN YUDKOFF, Primary Examiner.

V. W. PRETKA, Assistant Examiner.

Claims (1)

1. IN A METHOD FOR OPERATING A FRACTIONATION SYSTEM WHICH COMPRISES CHARGING TO A FRACTIONATION ZONE A FEED TO BE FRACTIONATED THEREIN, FRACTIONATING SAID FEED TO PRODUCE A BOTTOMS PRODUCT WHICH IS WITHDRAWN THEREFROM AND AN OVERHEAD CONTAINING GAS OR VAPOR, PASSING SAID OVERHEAD TO A COOLING ZONE, THEREIN COOLING SAID OVERHEAD, PASSING THE THUS COOLED OVERHEAD TO AN ACCUMULATION ZONE, IN SAID ACCUMULATION ZONE SEPARATING A LIQUID PHASE AND A GAS OR VAPOR PHASE CONTAINING SAID DESIRED GAS OR VAPOR, REMOVING THE LIQUID PHASE FROM SAID ACCUMULATION ZONE, PASSING AT LEAST A PORTION OF THE REMOVED LIQUID PHASE TO SAID FRACTIONATION ZONE AS REFLUX THEREFOR AND PASSING THE REMAINDER THEREOF TO A SECOND FRACTIONATION ZONE WHEREIN A BOTTOMS PRODUCT AND AN OVERHEAD PRODUCT ARE OBTAINED, PASSING GAS OR VAPOR PHASE IN SAID ACCUMULATION ZONE TO A CONDENSING ZONE WHEREIN DESIRED GAS OR VAPOR IS CONDENSED AND RECOVERED AND THE REMAINING GAS OR VAPOR REMOVED OVERHEAD FROM SAID CONDENSING ZONE, THE IMPROVEMENT WHICH COMPRISES UTILIZING AT LEAST A PORTION OF THE BOTTOMS PRODUCT FROM SAID SECOND FRACTIONATION ZONE AS A PRIMARY COOLANT IN SAID CONDENSING ZONE.

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