Abstract:
One exemplary embodiment can be a process for cooling a vent stream from a receiver. Generally, the process may include providing a refrigerant including at least one compound contained in the receiver so the refrigerant leaking into the receiver can be compatible with the process.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention generally relates to an alkylation system, and a process for cooling a vent stream from a receiver of a depropanizer column in the system. 
       DESCRIPTION OF THE RELATED ART 
       [0002]    Often a fractionation zone is positioned downstream of an acid alkylation unit to separate the hydrocarbons into various streams and any remaining acid. In the fractionation zone, often one or more columns are utilized for providing these separate streams. Usually, the first column in the series receiving the alkylation zone effluent provides an overhead stream that can include light hydrocarbons and the acid. Often, the acid can be recycled back to the alkylation unit. 
         [0003]    In the receiver of the first column, a chiller can be provided to cool vent gas exiting the receiver. Often a light hydrocarbon can be used as a refrigerant to cool the vent gas. 
         [0004]    Typically, the vent gas is at a higher pressure than the refrigerant within the coils. As a consequence, the acid alkylation catalyst can leak into the refrigerant system. As a result, the acid can corrode equipment and lines, such as the refrigerant compressor. Replacing the compressor and other equipment can incur costs and increase hazards. Thus, it would be desirable to eliminate this risk and lower operating costs. 
       SUMMARY OF THE INVENTION 
       [0005]    One exemplary embodiment can be a process for cooling a vent stream from a receiver. Generally, the process may include providing a refrigerant including at least one compound contained in the receiver so the refrigerant leaking into the receiver can be compatible with the process. 
         [0006]    Another exemplary embodiment may be an alkylation system. Generally, the alkylation system may include an acid alkylation zone and a fractionation zone. Usually, the fractionation zone has a depropanizer column, which can include a receiver. The receiver can include a vent condenser containing one or more cooling coils, which may contain a refrigerant at a pressure greater than the pressure in the receiver. 
         [0007]    Yet another embodiment can include a process for cooling a vent stream from a receiver of a depropanizer column in an acid alkylation unit. Generally, the process includes providing a refrigerant including propane to one or more cooling coils contained in a vent condenser of the receiver. Typically, the propane in the one or more cooling coils is at a pressure greater than the receiver. 
         [0008]    The embodiments provided herein can provide a refrigerant at a higher pressure than the gases in the receiver. Typically, leaks can happen due to cracking of the one or more cooling coils. As a result, the refrigerant can leak into the process rather than the vent gases leaking into the one or more cooling coils. Typically, the refrigerant can be selected from a light hydrocarbon that is compatible with the process, such as propane, so any leak from the refrigerant into the vent system can simply be processed along with the other fluids. Particularly, the refrigerant can be compatible with the overall fractionation zone fluids so as to be merely separated along with the other products. 
       DEFINITIONS 
       [0009]    As used herein, the term “stream” can be a stream including various hydrocarbon molecules, such as straight-chain, branched, or cyclic alkanes, alkenes, alkadienes, and alkynes, and optionally other substances, such as gases, e.g., hydrogen, or impurities, such as heavy metals, and sulfur and nitrogen compounds. The stream can also include aromatic and non-aromatic hydrocarbons. Moreover, the hydrocarbon molecules may be abbreviated C1, C2, C3 . . . Cn where “n” represents the number of carbon atoms in the one or more hydrocarbon molecules. Additionally, characterizing a stream as, e.g., a “vent stream”, can mean a stream rich in one or more substances representative of the characterizing adjective, such as a vent stream being rich in at least one fluid typically present in a receiver. 
         [0010]    As used herein, the term “zone” 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. 
         [0011]    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 stream or an effluent. 
         [0012]    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 stream or an effluent. 
         [0013]    As used herein, the term “vapor” can mean at least one of a gas or a dispersion that may include or consist of one or more hydrocarbons. 
         [0014]    As used herein, the term “hydrogen fluoride” can include at least one of a hydrogen fluoride or a hydrofluoric acid. Generally, a hydrofluoric acid is a solution of a hydrogen fluoride in water, where the hydrogen fluoride can disassociate and may form ions of H 3 O + , H + , FHF − , and F − . In some preferred embodiments, a substantially anhydrous hydrogen fluoride can be utilized. 
         [0015]    As depicted, process flow lines in the figures can be referred to as lines, effluents, or streams. Particularly, a line can contain one or more effluents or streams, and one or more effluents and streams can be contained by a line. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0016]      FIG. 1  is a schematic depiction of an exemplary alkylation system or unit. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Referring to  FIG. 1 , an acid alkylation system or unit  100  can include an acid alkylation zone  140  and a fractionation zone  200 . Typically, the acid alkylation zone  140  can be any suitable alkylation zone, typically utilizing an acid alkylation catalyst. Usually, the acid alkylation catalyst can include a hydrogen fluoride. Exemplary alkylation zones are disclosed in, e.g., U.S. Pat. No. 5,098,668. 
         [0018]    The fractionation zone  200  can include one or more distillation columns, such as a depropanizer column  240 . Exemplary distillation columns of the fractionation zone  200  are disclosed in, e.g., U.S. Pat. No. 4,348,544. The depropanizer column  240  can provide an overhead stream  260 , a side-stream  270 , and a bottom stream  280 . Usually, the side-stream  270  can include or be rich in one or more hydrocarbons, such as C3-C5 hydrocarbons, more typically, C4 hydrocarbons. The bottom stream  280  can include or be rich in C4 +  hydrocarbons, typically an alkylate product. The overhead stream  260  can include or be rich in one or more C4 −  hydrocarbons and an alkylation catalyst, typically an acid such as hydrogen fluoride. The overhead stream  260  can be received within a receiver  300  forming a boot  304 . The boot  304  can collect an alkylation catalyst and provide an alkylation catalyst stream  308  that can be recycled to the acid alkylation zone  140 . 
         [0019]    The receiver  300  can also provide a hydrocarbon product in a stream  262  that can be split into a reflux stream  264  sent back to the depropanizer column  240  and an overhead product stream  266 , typically including propane. In addition, the receiver  300  can form a vent condenser or a stack  310  that can provide a vent stream  314  including C2 −  hydrocarbons with a reduced level of an acid alkylation catalyst. The vent stream  314  can be provided to a scrubber  380  to provide a scrubber effluent stream  384  that can be sent to any suitable destination, such as fuel gas or the flare. Typically, the receiver  300  can be at a pressure of no more than about 1,720 kPa. 
         [0020]    In the vent condenser  310 , one or more cooling coils  320  can be provided to cool the vent stream  314  prior to exiting the vent condenser  310  to partially recover propane and the acid alkylation catalyst. Typically, the one or more cooling coils  320  can contain at least a portion of the refrigerant, and the refrigerant can be at a pressure greater than the pressure in the receiver  300 . The vent stream  314  can be at a temperature of no more than about −20° C. and a pressure of no more than about 1,720 kPa. The refrigerant can enter the one or more cooling coils  320  and then exit in a line  322 . 
         [0021]    Typically, the one or more cooling coils  320  can include any suitable refrigerant, such as an olefin or a paraffin, in a liquid phase. Usually, a suitable olefin can include a C2-C4 olefin, and a suitable paraffin can include propane. Preferably, the refrigerant can be a dry propane obtained from, e.g., a propane stripper or a product dryer, chilled, and pumped to about 60-about 140 kPa above the pressure in the receiver  300 . In the event of a coil leak, the high-pressure cold propane as the working fluid in the vent condenser  310  can leak into the depropanizer column  240 , rather than the alkylation catalyst leaking into the one or more cooling coils  320 . Hence, the alkylation catalyst, typically hydrogen fluoride, cannot contaminate the wet gas or fuel systems. Moreover, a leak can be detected quickly and fixed by monitoring the refrigerant level in the one or more cooling coils  320  or a surge drum  330 . 
         [0022]    The line  322  can communicate with the surge drum  330 . The refrigerant can pass through a line  324  to a pump  340 , which can pump the refrigerant up to a pressure of about 1,900 kPa. 
         [0023]    Afterwards, the refrigerant can pass through a line  326  to the exchanger  360 . Chilling can be provided by vaporizing the same or different refrigerant. Typically, a liquefied petroleum gas stream  364  at a pressure of about 300-about 500 kPa and a temperature of about −5-about 5° C. can be provided to cool the vaporized refrigerant. After exiting the exchanger  360 , the refrigerant can be at a temperature of no more than about 20° C., preferably below about 0° C. The refrigerant may return to the vent condenser  310  via a line  328 . 
         [0024]    Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever. 
         [0025]    In the foregoing, all temperatures are set forth in degrees Celsius and, all parts and percentages are by weight, unless otherwise indicated. 
         [0026]    From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.