Abstract:
A process for producing alkylated hydrocarbons includes the steps of: (a) combining a first feed stream comprising an olefin and an isoparaffin with an alkylation catalyst stream in a first alkylation reactor, (b) removing heat of reaction from the first alkylation reactor, (c) passing an effluent of the first alkylation reactor to a first reaction zone of a second alkylation reactor operating adiabatically to thereby form a first reaction zone effluent, (d) passing the first reaction zone effluent to a second reaction zone of the second alkylation reactor for mixture with a second feed stream comprising an olefin and an isoparaffin, and (e) passing an effluent of the second alkylation reactor to a settler for separation into a hydrocarbon stream and an alkylation catalyst effluent stream. An alkylation unit for carrying out the process is also disclosed.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     Not Applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The disclosure relates to an alkylation unit and process. 
     2. Description of the Related Art 
     Generally, alkylation is a process that can be used to produce a high quality, high octane gasoline from lower boiling feeds. Usually, commercial refinery plants alkylate a feed including an isoparaffin stream, typically including isobutane, and an olefin stream, typically including one or more C 3 -C 5  olefins, to form branched chain paraffin products boiling in the gasoline range, which can include hydrocarbons boiling up to about 200° C. 
     Increasing capacity of an alkylation unit may be implemented by various methods. The problem with increasing the capacity of an alkylation unit is that simply increasing the flow of olefin lowers the isobutane/olefin ratio, which depresses the octane number of the alkylate product, which is an undesirable result. 
     Therefore, what is needed is an improved alkylation unit and process that maintain the desired isobutane/olefin ratio thereby minimizing octane loss. 
     SUMMARY OF THE INVENTION 
     The foregoing needs are met by an alkylation unit including an olefin source, an isoparaffin source, a first alkylation reactor, a second alkylation reactor, and a settler. The olefin source is in fluid communication with a first isoparaffin-olefin admixture conduit and a second isoparaffin-olefin admixture conduit. The isoparaffin source is in fluid communication with the first isoparaffin-olefin admixture conduit and the second isoparaffin-olefin admixture conduit. The first alkylation reactor is in fluid communication with the first isoparaffin-olefin admixture conduit and an alkylation catalyst feed conduit supplying an alkylation catalyst. The first alkylation reactor includes a heat transfer device for removing heat of reaction from the first alkylation reactor. The second alkylation reactor has a plurality of reaction zones. A first reaction zone of the plurality of reaction zones is in fluid communication with the first alkylation reactor for receiving an effluent of the first alkylation reactor. A second reaction zone of the plurality of reaction zones is in fluid communication with the first reaction zone for receiving a first reaction zone effluent. The second reaction zone of the plurality of reaction zones is also in fluid communication with the second isoparaffin-olefin admixture conduit. The settler is in fluid communication with the second alkylation reactor, and the settler receives an effluent of the second alkylation reactor. The settler separates the effluent of the second alkylation reactor into a hydrocarbon stream and an alkylation catalyst effluent stream. Preferably, the isoparaffin source comprises a C 4 -C 5  alkane and the olefin source comprises a C 3 -C 5  alkene. Preferably, the second alkylation reactor is operated adiabatically. 
     The alkylation unit can further include a fractionation unit in fluid communication with the settler. The fractionation unit receives the hydrocarbon stream from the settler and separates the hydrocarbon stream into an alkylate stream and an isoparaffin recycle stream in fluid communication with the first and second isoparaffin-olefin admixture conduit. 
     In one version of the alkylation unit, a ratio of isoparaffin to olefin in terms of total flow rate in the first isoparaffin-olefin admixture conduit and the second isoparaffin-olefin admixture conduit is about 3:1 and about 1:3. A molar ratio of isoparaffin to olefin in the first isoparaffin-olefin admixture conduit and the second isoparaffin-olefin admixture conduit can be between about 5:1 and about 20:1. A volume ratio of the alkylation catalyst to a first feed stream in the first isoparaffin-olefin admixture conduit can be between about 1:1 and about 4:1. 
     In one version of the alkylation unit, the first isoparaffin-olefin admixture conduit is in fluid communication with the first alkylation reactor via a plurality of inlet loci comprising four to eight inlet points longitudinally spaced intermediate ends of the first alkylation reactor. In one version of the alkylation unit, the settler is in fluid communication with the first alkylation reactor, and the first alkylation reactor receives the alkylation catalyst effluent stream. In one version of the alkylation unit, the second isoparaffin-olefin admixture conduit is in fluid communication with a third isoparaffin-olefin admixture conduit, and a third reaction zone of the plurality of reaction zones of the second alkylation reactor is in fluid communication with the second reaction zone for receiving a second reaction zone effluent wherein the third reaction zone is also in fluid communication with the third isoparaffin-olefin admixture conduit. 
     The foregoing needs are also met by a process for producing alkylated hydrocarbons. The process comprising includes the steps of (a) combining a first feed stream comprising an olefin and an isoparaffin with an alkylation catalyst stream in a first alkylation reactor; (b) removing heat of reaction from the first alkylation reactor; (c) passing an effluent of the first alkylation reactor to a first reaction zone of a second alkylation reactor to thereby form a first reaction zone effluent; (d) passing the first reaction zone effluent to a second reaction zone of the second alkylation reactor for mixture with a second feed stream comprising an olefin and an isoparaffin; and (e) passing an effluent of the second alkylation reactor to a settler for separation into a hydrocarbon stream and an alkylation catalyst effluent stream. Preferably, the isoparaffin comprises a C 4 -C 5  alkane, and the olefin comprises a C 3 -C 5  alkene. The second alkylation reactor can be operated adiabatically. 
     The process can further include the steps of (f) passing the hydrocarbon stream to a fractionation unit, (g) separating the hydrocarbon stream into an alkylate stream and an isoparaffin recycle stream, and (h) combining the isoparaffin recycle stream with the first feed stream and the second feed stream. The process can further include the step of controlling a ratio of a first volumetric flow rate of the first feed stream to a second volumetric flow rate of the second feed stream to be between about 3:1 and about 1:3. The process can further include the step of directing the first feed stream into the first alkylation reactor through a plurality of inlet loci comprising four to eight inlet points longitudinally spaced intermediate ends of the first alkylation reactor. The process can further include the step of passing the alkylation catalyst effluent stream from the settler to the first alkylation reactor. The process can further include the step of passing a second reaction zone effluent to a third reaction zone of the second alkylation reactor for mixture with a third feed stream comprising an olefin and an isoparaffin. 
     The process can further include the steps of controlling a first molar ratio of isoparaffin to olefin in the first feed stream to be between about 5:1 and about 20:1, and controlling a second molar ratio of isoparaffin to olefin in the second feed stream to be between about 5:1 and about 20:1. The process can further include the step of controlling a volume ratio of the alkylation catalyst to the first feed stream to be between about 1:1 and about 4:1. 
     It is therefore an advantage of the invention to provide an improved alkylation unit and process that achieve an increase in unit capacity while at the same time minimizing octane loss. 
     These and other features, aspects, and advantages of the present invention will become better understood upon consideration of the following detailed description, drawings and appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic process flow diagram of an alkylation unit according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As used herein, the hydrocarbon molecules may be abbreviated C 1 , C 2 , C 3  . . . C n  where “n” represents the number of carbon atoms in the one or more hydrocarbon molecules. Additionally, characterizing a stream as, e.g., an “olefin stream” can mean a stream including or rich in at least one olefin. 
     As used herein, the term “unit” or “system” can refer to an area including one or more equipment items and/or one or more sub-zones. Equipment items can include one or more reactors or reactor vessels, heaters, exchangers, pipes, pumps, compressors, and controllers. Additionally, an equipment item, such as a reactor, dryer, or vessel, can further include one or more zones or sub-zones. 
     As used herein, the term “rich” can mean an amount of at least generally about 30%, preferably about 50%, and optimally about 70%, by mole, of a compound or class of compounds in a feed, an effluent, or a stream. 
     As used herein, the term “substantially” can mean an amount of at least generally about 80%, preferably about 90%, and optimally about 99%, by mole, of a compound or class of compounds in a feed, an effluent, or a stream. 
     As used herein, the term “vapor” can mean a gas or a dispersion that may include or consist of one or more hydrocarbons. 
     As used herein, hydrofluoric acid can disassociate and may form ions of H 3 O + , H + , FHF − , and F − . 
     The term “communication” means that material flow is operatively permitted between enumerated components. 
     As depicted, process flow lines in the figures can be referred to as lines, conduits, feeds, effluents, or streams. Particularly, a line or a conduit can contain one or more feeds, effluents, or streams, and one or more feeds, effluents, and streams can be contained by a line or a conduit. 
     Referring to  FIG. 1 , an acid alkylation unit or system  10  can include a first alkylation reactor  25 , preferably a cooler-reactor, and a second alkylation reactor  30 , preferably a multi-zone reactor arranged in series for the purposes of carrying out an alkylation reaction. 
     Usually, the alkylation reaction can include the reaction of an isoparaffin, such as isobutane, with an olefin such as propylene, isobutylene, 1-butene, 2-butene, and pentenes. Generally, the reaction of an isoparaffin with a C 3  or a C 4  olefin, such as isobutylene, 1-butene, and/or 2-butene, is an example of a preferred reaction involving these specified materials and mixture. One preferred mixture is an isoparaffin and typically a mixed “butene” produced from a catalytic cracking operation, which can include about 25%, by volume, of 1-butene, about 30%, by volume, of isobutylene and about 45%, by volume, of 2-butene. Usually, the stream rich in isobutane can at least be partially provided by recycling isobutane from a downstream fractionation zone and include make-up isobutane from refinery or chemical manufacturing units. The recycle isobutane provides a molar excess of isoparaffin as described below. 
     Typically, the alkylation catalyst can include a hydrofluoric acid, a sulfuric acid, a phosphoric acid, a metal halide, or other suitable alkylation catalyst. Preferably, the catalyst is a hydrofluoric acid. Furthermore, the catalyst may be substantially anhydrous hydrogen fluoride containing various additives or promoters such as boron trifluoride. Ordinarily, commercially available anhydrous hydrogen fluoride will be charged to the alkylation system as fresh catalyst. It is however, possible to use hydrogen fluoride containing as much as about 2.5% water by weight. 
     Generally, the alkylation reaction is carried out with substantial molar excess of isoparaffin:olefin, typically in excess of about 1:1, usually about 4:1 to about 70:1, preferably about 5:1 to about 20:1, more preferably about 7:1 to about 15:1. 
     Usually, the system or unit  10  can maintain an acid:hydrocarbon volume ratio of 1:1 to 4:1, with one preferred range of 2:1 to 3:1. Another preferred range for the acid:hydrocarbon volume ratio is not lower than about 2.5:1. The acid:hydrocarbon volume ratio will be higher in the first reactor  25 , and the acid:hydrocarbon volume ratio depends on how the feed and recycle isobutane are split between the two reactors. The acid:hydrocarbon volume ratio in second reactor  30  will be the overall ratio, so in the preferred range. 
     The pressure in the reactors has to be at least high enough to keep the hydrocarbon in the liquid phase. The minimum pressure is about 600 kPa, though this depends on the temperature in the reactor and settler. The temperature range can be 15° C. to 50° C. with a preferred range of about 25° C. to 40° C. Preferred pressure in the settler is about 600 kPa to 850 kPa. 
     Olefin hydrocarbon via line  11  is premixed with an isoparaffin make-up stream  12  to form a first isoparaffin-olefin admixture stream  13 . Typically, at least a portion  14  of the stream  13  may be bypassed around the cooler-reactor  25 . Typically, at least about 25% to about 75%, by volume, optimally about 50%, by volume, of the stream  13  can be bypassed as the portion  14 . The remainder can pass as a stream  15  to the cooler-reactor  25 . A stream  23 , including one or more isoparaffins, typically isobutane, can be combined with the stream  15 . Usually, the stream  23 , including one or more isoparaffins, can include isoparaffins obtained from a recycled stream  21  from downstream fractionation unit  36 . The combined streams  15  and  23  can form a hydrocarbon feed  16 . This hydrocarbon feed  16  can be split into several streams, namely stream  17 , stream  18 , stream  19 , and stream  20  before entering the cooler-reactor  25  at inlet loci (a), (b), (c) and (d). Inlet loci (a), (b), (c) and (d) may be, for example, spray nozzles and the like. Thus, the vertically spaced streams  17 ,  18 ,  19 , and  20  may ensure good dispersion of the hydrocarbons through an acid phase in the cooler-reactor  25 . 
     As described above with reference to the preferred embodiment, the purpose of a plurality of inlet loci is to inject the feed mixture into a cooling section of the reactor cooler through the alkylation catalyst phase already present in the cooler-reactor  25  in such a manner that the evolved heat of reaction may be removed almost immediately by a heat exchanger using coolant, such as relatively cold water, flowing through the heat exchange lines  39  and  40 . Typically, the heat exchanger is a conventional heat exchanger using tubes for the passage of the coolant with the reaction taking place on the shell side of the heat exchanger. By operation in this manner, the alkylation reaction takes place in the immediate vicinity of the heat removal so that regulated isothermal conditions are maintained in reaction cooler  25 . 
     The hydrocarbon and alkylation catalyst may react within the cooler-reactor  25  to provide a cooler-reactor effluent  26 . The effluent  26  can enter the multi-zone alkylation reactor  30  with a first reaction zone  30   a , a second reaction zone  30   b  and, optionally, a third reaction zone  30   c . Such first reaction zone  30   a , second reaction zone  30   b  and third reaction zone  30   c  are operably related to each other such that a first reaction zone effluent passes to a second reaction zone  30   b  from the first reaction zone  30   a  and a second reaction zone effluent passes to a third reaction zone  30   c  from the second reaction zone  30   b . The cooler reactor effluent  26  enters the multi-zone alkylation reactor  30  at the first reaction zone  30   a . Cooler-reactor bypass stream  14  can be combined with the remaining portion  22  of isoparaffin recycle stream  21  resulting in stream  27 . A portion of the second isoparaffin-olefin admixture stream  27  may enter the multi-zone reactor  30  via line  29  at reaction zone  30   b . Optionally, the remainder of stream  27  may pass as stream  28  to the multi-zone reactor at reaction zone  30   c.    
     The ratio of isobutane to olefin is calculated for various streams. This ratio is calculated by taking the flow rate of isobutane in a particular stream and dividing by the flow rate of the total olefins in that same stream. The ratio can be calculated on either a volumetric or a molar basis. Preferably, the ratio (on a volumetric basis) of isobutane to olefin in stream  16  is in the range of 7 to 12, the ratio for stream  27  is in the range of 5 to 10, and the ratio for combined streams  13  and  21  is in the range of 7 to 15. 
     In one embodiment of the invention, the multi-zone reactor is operated adiabatically. Therefore, the multi-zone reactor is dissimilar to the cooler-reactor in that that the evolved heat of reaction is not removed by heat exchange. 
     The hydrocarbons and alkylation catalyst may react within the multi-zone reactor  30  to provide a multi-zone reactor effluent  31 . The multi-zone reactor  30  overcomes the reducing of octane caused by a lower isoparaffin-olefin ratio by increasing the localized isoparaffin-olefin ratio using unreacted isobutane from the cooler-reactor  25  contained in stream  26  in subsequent multi-zone reactor  30 . 
     The multi-zone reactor effluent  31  can enter acid settler  32 . Generally, the acid settler  32  can allow the second reactor effluent  31  to split into three phases, namely, a vapor phase, a hydrocarbon phase, and an alkylation catalyst or acid phase. The hydrocarbon phase can be drawn through a stream  33 , passed through the fluid transfer device  34 , and provided to downstream fractionation unit  36  via stream  35  for recovering the alkylate product  38 , isoparaffins  21  and other hydrocarbons  37  such as propane. The acid phase can be provided via a stream  24  to the cooler-reactor  25 . The acid phase in stream  24  can be combined with additional make-up acid, and the combination can flow to the cooler-reactor  25 . The acid phase in stream  24  can be supplied to a regeneration zone for regenerating the alkylation catalyst. The regenerated acid catalyst can be returned via a stream to the cooler reactor  25 , although the regenerated catalyst can be provided to other locations within the system  10 . 
     Generally, the embodiments provided herein can provide a method of expanding capacity of an alkylation unit, such as a hydrogen fluoride alkylation unit, by adding a second reactor in series with a first reactor wherein the first reactor is preferably a cooler-reactor  25  and the second reactor is preferably a multi-zone reactor  30  operating adiabatically. 
     Although the invention has been described in considerable detail with reference to certain embodiments, one skilled in the art will appreciate that the present invention can be practiced by other than the described embodiments, which have been presented for purposes of illustration and not of limitation. Therefore, the scope of the appended claims should not be limited to the description of the embodiments contained herein.