Cooling of recycle hydrocarbon and/or alkylate product in isoparaffin-olefin alkylation
BACKGROUND OF THE INVENTION 
     This invention relates to an improved process of alkylation of at least one isoparaffin with at least one olefin in the presence of an acid catalyst. In particular, it relates to an alkylation process in which at least a portion of the hydrocarbon liquid withdrawn from a settler is flashed to provide indirect cooling for the alkylation process. 
     Since the temperature at which alkylation of isoparaffin with at least one olefin in the presence of an acid catalyst (such as HF) is inversely proportional to the octane value of the alkylate produced, it is desirable to carry out the alkylation at a minimum attainable temperature. The cooling of the reaction zone can be provided by any suitable means, but it is economically advantageous to utilize the cooling from within the system. One method proposed for cooling of the alkylation zone, disclosed, for example, in U.S. Pat. No. 3,925,501, is by flashing hydrocarbon liquid withdrawn from the settling zone. The flashing provides cooling which reduces the temperature in the reaction zone. The flashed vapor which contains HF therein is transported into a compressor where its pressure is increased to a level sufficient to pass it to a depropanizer. One problem with this process is the cost associated with the use of a compressor. The usually expensive piece of process equipment is made economically unaffordable by the requirement that those parts which come in contact with hydrofluoric acid must be made of materials which are not corroded by vaporized HF. Since materials that are not corroded by acids such as HF are extremely difficult to obtain and their costs are enormous, the compressors made from these are not readily available and must be custom-made. If the material is even slightly corrosive, frequent replacements of parts increase maintenance costs. 
     The present invention obviates some of the problems encountered in the prior art. Thus, one object of the present invention is to provide an improved alkylation process. 
     Another object of the invention is to provide an alkylation process which requires a minimum amount of energy. 
     Still another object of the invention is to provide a process which will reduce the equipment expenses and maintenance expenses. 
     A still further object of the invention is to provide a process which utilizes commercially available, mass-produced, inexpensive components. 
     Other objects of the invention will become apparent to those skilled in the art upon studying this disclosure. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the invention, in a process for alkylation of at least one isoparaffin with at least one olefin in the presence of an acid catalyst, at least a portion of the acid-containing hydrocarbon effluent liquid from the settling zone is flashed and used as indirect coolant in the alkylation process. The liquid taken off from the settling zone is pressurized to a sufficiently high level to allow the vapor obtained by flashing the liquid to enter a fractionating zone, after it cools a portion of the alkylation system, without the use of a compressor. 
     In accordance with another aspect of the invention, in a process for alkylation of at least one isoparaffin with at least one olefin in the presence of HF catalyst, a portion of the HF-containing hydrocarbon effluent liquid from the settler is partially vaporized or flashed and used as indirect coolant in the alkylation process. The portion of the liquid which is to be flashed is first pressurized by pump means to a level sufficient to allow the vapor formed upon flashing to enter a fractionating zone after it cools a portion of the alkylation system without the use of a compressor. The amount of liquid that is flashed is the amount that contains the amount of propane equal to the sum of the amount of propane in the feed stream and the amount of propane produced in the alkylation process. 
     Other aspects of the invention will become apparent upon studying this specification and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 depicts the schematic of an HF alkylation process in which the present invention is utilized. 
     FIG. 2 depicts the schematic of a portion of another HF alkylation process in which the present invention is utilized. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the process of alkylation of an isoparaffin and olefin in the presence of an acid catalyst (such as HF) the reaction temperature is inversely proportional to the octane number of the produced alkylate. It is therefore essential to conduct the alkylation reaction at a minimum permissible temperature. In order to save energy supplied to the system, at least a portion of the cooling can be provided by flashing effluent liquid withdrawn from the settling zone. It has been discovered that if a sufficient pressure is imparted to the liquid, as by pumping, before it is vaporized or flashed, the resulting vapors can be charged to the depropanizer without employing any additional means to promote the transport. The pressure that must be imparted to the liquid prior to vaporizing or flashing varies, depending on the particular system. Usually the minimum pressure is in the range from about 250 psig to about 350 psig (1800-2500 kPa). The upper limit of the pressure is defined only by the structural strength of the equipment. Depending upon liquid composition, when the pressure is suddenly reduced to about 250-300 psig (1800-2150 kPa), rapid evaporation of a portion of the liquid ensues. The transition from liquid to vaporous state utilizes energy present in the system and thereby provides a cooling effect. The cooling can be utilized to lower the temperature in the alkylation system. The heat can be withdrawn from the desired portion of the alkylation system by indirect heat exchange with the reaction mass, one of the streams flowing to the reaction zone, the alkylate stream prior to its storage, or any of these. Under usual operating conditions, it is preferable to utilize the cooling produced by flashing a portion of the pressurized hydrocarbon liquid effluent for cooling the recycle isoparaffin stream or streams. The reason for the preference is that the recycle isoparaffin stream from fractionation is normally at a higher temperature, the temperature difference being about 50° F (28° C) to about 250° F (139° C), than that of the other streams entering the reaction zone or the reaction mass, thus making heat exchange more efficient. Recycle isobutane also is the largest volume hydrocarbon stream. 
     Alternatively, at least some of the cooling produced by flashing of the compressed liquid can be used for cooling the alkylate stream taken off as product from the fractionator. Usually that stream is at a temperature from about 300° F (149° C) to about 400° F (204° C). Since in normal operation the alkylate is stored at ambient temperature and pressure equal or only slightly above atmospheric, to prevent rapid evaporation of the hot alkylate stream the stream is cooled prior to storing to a temperature of about 90° F (32° C). The cooling of the alkylate can at least partially be accomplished by heat exchange with the pressurized hydrocarbon effluent. The pressurized hydrocarbon liquid used in indirect cooling of isoparaffin and of alkylate is at the same time heated to the proper temperature (while already at its proper pressure) for being charged to fractionation from a final vapor liquid separation zone. 
     In the process, the isoparaffin to olefin mole ratio can vary considerably, but it generally is in the range from about 2:1 up to about 25:1, usually from about 4:1 to about 15:1. The total hydrocarbon to HF catalyst volume ratio can vary considerably depending on the specific system, but the usual range is from about 1/10 to about 10/1. The HF alkylation reaction can be carried out in a wide temperature range, usually in the range from about 0° F (-18° C) to about 150° F (66° C), and at a pressure sufficient to maintain liquid phase conditions. The lower the reaction temperature, the higher the octane number of the alkylate. Since it is less expensive to use cooling water, the normal reaction zone temperature is about 90° C (32° C). 
     Any isoparaffin, alone or in admixture with another isoparaffin, is suitable for use with the present invention including isobutane, isopentane, and isohexanes. Among olefins that can be used are propylene, butylenes, amylenes, and many others, alone or in admixture with other olefins. 
     The invention is particularly suitable for the process of alkylation of isobutane with propylene and butylenes. Any portion of the hydrocarbon liquid effluent from the settler can be pumped to high pressure, used as heat exchange (coolant) and partially vaporized; however, in some embodiments, it is preferred to vaporize only that portion of the hydrocarbon liquid effluent which contains the amount of propane equal to the sum of the amount fed into the reaction zone and the amount produced by the reaction so this propane can be removed from the system. 
     The invention can be further explained by referring to the specific embodiments depicted in FIGS. 1 and 2. 
     Referring now to FIG. 1, the isoparaffin feed passed through 10 combines with isoparaffin recycle streams 15 and 20. The combined isoparaffin streams 10, 15, and 20 are introduced into the riser reactor 25 together with olefin feed passed by 30 and hydrofluoric acid passed by 35. The conditions in the riser reactor, including temperature and pressure, are maintained at such levels as to achieve liquid phase alkylation of the isoparaffin with the olefin in the presence of HF. From the riser reactor 25 the reaction mass is passed to a phase separator 40 which is also maintained at conditions which retain the reaction mass in the liquid state. In separator 40 the alkylation reactor effluent is allowed to form an upper phase containing primarily hydrocarbons and a lower phase containing mainly hydrofluoric acid. The lower phase is passed via 42 from the phase separator 40 to a cooler 45 and therefrom through 50 and 35 to the riser reactor 25. Make-up hydrofluoric acid can be supplied to the system through 55. 
     The upper phase from the phase separator 40 is passed by 60 to a transport pump 65 which causes one portion of the hydrocarbon stream to flow to deisobutanizer 70 via 75 and another portion of that stream via 85 to a pump 80 where the pressure on the liquid is increased, leaving pump 80 via 90. The pump 80 causes the pressure of the liquid to rise to a sufficiently high level to allow vapors obtained by heating and flashing a portion of hydrocarbon stream 90 to enter the depropanizer 130 without any HF-containing vapor compression. The pressured liquid flashed in line 90 is utilized in indirect cooling of recycle isobutane streams 15 and 20 using heat exchangers 105 and 110, respectively, and cooling of alkylate 190 in indirect exchanger 192. The heated and partially vaporized stream 90 is then passed to a liquid-vapor separator 115. The liquid phase separated in the separator 115 which comprises alkylate and isobutane is passed to the top section of deisobutanizer 70 via 125. The vapor phase comprising propane and HF is introduced into depropanizer 130 via 135. The conditions in the depropanizer are such as to separate the materials introduced therein into a depropanizer overhead and depropanizer bottoms. The depropanizer overhead is taken off as a depropanizer overhead stream 140 which is condensed in an exchanger 145 and passed therefrom to an accumulator 150. In the accumulator cooled overhead stream 140 is allowed to separate into a liquid HF phase, which is withdrawn by 155 and combined with 35, and a liquid hydrocarbon phase, carried by 158, one portion of which is pumped by a depropanizer pump 160 via 162 into the top of, as reflux for, depropanizer 130 and another portion of which is passed by the same pump via 164 to the top of HF stripper 165. Therein conditions are such as to separate propane from HF. Propane liquid is removed from the bottom of HF stripper 165 via 170; the stripper overhead vapor comprising HF and propane is passed via 175 to condenser 145. 
     The depropanizer bottoms liquid, composed essentially of isobutane, is withdrawn from depropanizer 130 via 176 and passed by recycle pump 177 into heat exchanger 110. Therein the recycle stream is cooled by indirect heat exchange with the fluid in 90. The cooled isobutane stream is then directed to recycle stream cooler 178 in which the stream is further cooled and then passed via 20 to the reaction zone. 
     The materials introduced into deisobutanizer 70 are separated as the result of conditions maintained therein into a deisobutanizer overhead vapor comprising isobutane and propane, an intermediate cut comprising mainly normal butane, and deisobutanizer bottoms comprising mainly alkylate product. These components are withdrawn from deisobutanizer 70 by 180, 185, and 190, respectively. The recycle isoparaffin stream 180 is cooled in heat exchanger 105 and further cooled by a recycle cooler 191. The other two streams, 185 and 190, are withdrawn from the deisobutanizer as products. Alkylate stream 190, as described above, is cooled in alkylate heat exchanger 192 and therefrom passed by 195 to additional coolers and storage (not shown). 
     FIG. 2 depicts the schematic of a portion of the alkylation process utilizing the same reaction as shown in FIG. 1. Starting at the point of the alkylation process where liquid hydrocarbon effluent exits from the phase separator (not shown), the entire hydrocarbon effluent phase is passed to a hydrocarbon effluent pump 200 via 205. The pressure of the liquid is increased to a level sufficient to permit passing of vapor obtained by subsequent heating of the hydrocarbon effluent stream into a fractionator without any means of HF-containing vapor compression. In this particular system, the pressure of the liquid is increased to about 230 psig (1680 kPa). 
     The pressured liquid in line 210 is used for indirectly cooling a recycle isobutane stream 215 by means of a heat exchanger 220 and the alkylate stream 225 using heat exchanger 230. From line 210 after heat exchanger 230, the partially vaporized hydrocarbon stream 210 is introduced into a liquid-vapor separator 235. Vapor stream 240 and liquid stream 245 are removed from separator 235. Vapor stream 240 is introduced near the top to a fractionator 250; liquid stream 245 is fed to the same fractionator near its midsection. The temperature and pressure in the fractionator are such as to separate the materials introduced thereto into an overhead comprising mainly propane and some HF, intermediate cuts, one comprising isobutane and the other comprising mainly normal butane, and bottoms comprising essentially alkylate. These fractions having temperatures of 150° F (66° C), 190° F (88° C), 200° F (93° C), and 420° F (216° C) are withdrawn from fractionator 250, which is maintained at about 200 psig (1480 kPa), via 255, 215, 260, and 225, respectively. The propane-HF stream 255 can be further passed to an HF stripper (not shown) to recover HF and propane contained therein. HF can then be recycled to the reaction zone (not shown) and propane can be recovered as product. 
     The following example merely illustrates the practice of the invention and is not intended to limit in any manner the scope of the invention. 
     EXAMPLE 
     Using the process of the invention shown in FIG. 1, the following flow rates and the following operating conditions in specific components of the systems were calculated: 
     
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Typical Flow Rates                                                        
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Fresh Isobutane (10), B/D                                                 
95% iC.sub.4 by volume 4,800                                              
Olefin Feed (30), B/D                                                     
54% C.sub.3 =, C.sub.4 = olefins by volume                                
                       9,200                                              
Recycle Total Isobutane                                                   
(15 and 20), B/D                                                          
86% iC.sub.4 by volume 78,000                                             
To Deisobutanizer (70)                                                    
via (75), B/D          50,000                                             
Pressure, psig         150                                                
 kPa                   1,130                                              
To Exchanger (105)                                                        
via (85), B/D          41,400                                             
Vapor (135) to DeC.sub.3 (130), B/D                                       
                       20,700                                             
nC.sub.4 Yield (185), B/D                                                 
                       800                                                
Propane Yield (170), B/D                                                  
                       1,500                                              
OPERATING CONDITIONS IN SPECIFIC COMPONENTS                               
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Reactor Riser (25):                                                       
Temperature, ° F (inlet)                                           
                       85                                                 
Pressure, psig         105                                                
Pressure, kPa          825                                                
HF/Total Hydrocarbon Vol. Ratio                                           
                       4                                                  
iC.sub.4 /Olefin Mol Ratio, About                                         
                       14                                                 
Separator (40):                                                           
Temperature, ° F                                                   
                       100                                                
Pressure, psig         90                                                 
Pressure, kPa          720                                                
Deisobutanizer (70):                                                      
Top Zone:                                                                 
Temperature, ° F                                                   
                       150                                                
Pressure, psig         120                                                
Pressure, kPa          930                                                
Bottom Zone:                                                              
Temperature, ° F                                                   
                       350                                                
Pressure, psig         125                                                
Pressure, kPa          960                                                
Liquid-Vapor Separator (115):                                             
Temperature, ° F                                                   
                       220                                                
Pressure, psig         285                                                
Pressure, kPa          2070                                               
Depropanizer (130):                                                       
Top Zone:                                                                 
Temperature, ° F                                                   
                       125                                                
Pressure, psig         260                                                
Pressure, kPa          1895                                               
Bottom Zone:                                                              
Temperature, ° F                                                   
                       245                                                
Pressure, psig         265                                                
Pressure, kPa          1930                                               
HF Stripper (165):                                                        
Top Zone:                                                                 
Temperature, ° F                                                   
                       127                                                
Pressure, psig         295                                                
Pressure, kPa          2135                                               
Bottom Zone:                                                              
Temperature, ° F                                                   
                       145                                                
Pressure, psig         300                                                
Pressure, kPa          2170                                               
Material in Conduit (85):                                                 
Outlet at Pump (80):                                                      
Temperature, ° F                                                   
                       100                                                
Pressure, psig         325                                                
Pressure, kPa          2340                                               
Inlet to Separator (115):                                                 
Temperature, ° F                                                   
                       220                                                
Pressure, psig         285                                                
Pressure, kPa          2070                                               
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     Using the process shown in FIG. 1 and flow rates and operating conditions specified in this example, estimated heat savings over the conventional process are in the range from about 20-25 million Btu per hour. The savings result from: 
     1. Utilization of heating, vaporizing, or flashing liquid hydrocarbon effluent for cooling the reaction zone system. 
     2. Smaller size of depropanizer 130 as the result of priorly separating about 45 percent of the feed 135 (normally introduced to the depropanizer) in the separator 115. 
     3. Smaller size of the deisobutanizer 70 as the result of recycling stream 176, containing only about one percent of alkylate, directly to the reaction zone instead of introducing it to the deisobutanizer. 
     All kPa pressures are reported as absolute pressures.