Abstract:
Embodiments of the present invention generally disclose membrane-based systems and methods for the separation of propylene and propane that overcome certain issues associated with prior art devices and take advantage of a temperature drop across the associated separation membrane.

Description:
RELATED APPLICATIONS 
       [0001]    The present application is a divisional of U.S. patent application Ser. No. 11/968,507, filed Jan. 2, 2008, which claims priority to U.S. provisional application 60/884,013, filed Jan. 8, 2007, titled “Improved Systems and Methods for the Separation of Propylene and Propane.” The entire contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Olefins, particularly ethylene and propylene, are important chemical feedstocks. Typically they are found in nature or are produced as primary products or byproducts in mixtures that contain saturated hydrocarbons and other components. Before the raw olefins can be used, they usually must be purified from these mixtures. Numerous difficulties have been experienced in this type of separation. Due to their similar relative volatilities, energy-intensive, capital-intensive, multi-trayed distillation columns typically have been used for the purification of light olefins. 
         [0003]    An example of a prior art distillation column for the separation of propylene and propane is illustrated in  FIG. 1 . In an exemplary method of operation, a raw feedstock of Refinery Grade Propylene (RGP) comprising 70% propylene and 30% propane is introduced to distillation column  205  along feed pipe or pipes  200 . Distillation column  205  generally comprises multiple trays, or levels. In an embodiment, distillation column  205  comprises 135 trays. Operation of a distillation column is primarily determined by a combination of the number of trays and the reflux ratio. In general, the more trays in a distillation column, the greater the separation at a constant reflux ratio, but also the greater the capital cost. Conversely, fewer trays can be used if reflux is increased, but operating cost is increased. In the distillation column, the lighter components tend to rise and the heavier components tend to sink. Accordingly, a propylene product stream is capable of being extracted along piping  235 . In an embodiment, the propylene is extracted as Commercial Grade Propylene (CGP nominally 93% propylene). 
         [0004]    A second stream comprising propylene and other lighter components can be extracted from column  205 , re-condensed and injected back into column  205  along piping  210 . A propane product typically greater than 95% propane, can be extracted about piping  215 . The extracted products can be passed through heat exchangers  245  and/or  225  to aid in the control or change of various streams within the process. 
         [0005]    Column  205  is merely one example of a propylene and propane distillation column. One of ordinary skill in the art would readily understand that many variations are possible. 
         [0006]    Typical results from a single distillation column, with a 70% propylene feed, produces a propylene product of 93% purity and a propane product of 95%. Accordingly, an improved system would produce results at least as good as a typical prior art distillation column. 
         [0007]    The art field has desired to use a membrane-based system. U.S. Pat. No. 3,758,603 and U.S. Pat. No. 3,864,418 in the names of Robert D. Hughes and Edward F. Steigelmann describe membranes used in conjunction with metal complexing techniques to facilitate the separation of ethylene from ethane and methane. Similar metal complex and membrane hybrid processes, called facilitated transport membranes, have been described in U.S. Pat. No. 4,060,566 in the name of Robert L. Yahnke and in U.S. Pat. No. 4,614,524 in the name of Menahem A. Kraus. 
         [0008]    Further membranes have been considered for the separation of olefins from paraffins as an alternative to distillation. However, the separation is difficult largely because of the similar molecular sizes and condensability of the components desired to be separated. The membrane must operate in a hydrocarbon environment under conditions of high pressure and temperature, often resulting in plasticization and can cause loss of selectivity and/or permeation rate. Such harsh conditions tend to adversely affect the durability and stability of separation performance of many membrane materials. A membrane system with sufficiently high olefin/paraffin selectivity, high productivity, and sufficient durability in long-term contact with hydrocarbon streams under high pressure and temperature is highly desired. 
         [0009]    The art is replete with processes said to fabricate membranes possessing both high selectivity and high fluxes. Without sufficiently high fluxes the required membrane areas required would be so large as to make the technique uneconomical. It is now well known that numerous polymers are much more permeable to polar gases (examples include H 2 O, CO 2 , H 2 S, and SO 2 ) than to nonpolar gases (N 2 , O 2 , and CH 4 ), and that gases of small molecular size (He, H 2 ) permeate more readily through polymers than large molecules (CH 4 , C 2 H 6 ). 
         [0010]    However, even considering these difficulties, utilization of membrane separation has taken an important place in chemical technology for use in a broad range application. Gas separation has become a major industrial application of membrane technology in the last 15 years. Membrane based technology for the production of nitrogen from air, removal of carbon dioxide from natural gas, and purification of hydrogen now occupy significant shares of the markets for these processes. 
         [0011]    Membrane materials and systems for separating olefinic hydrocarbons from a mixture of olefinic and saturated hydrocarbons have been reported, but none can be easily or economically fabricated into membranes that offer the unique combination of high selectivity and durability under industrial process conditions to provide economic viability. 
         [0012]    For example, several inorganic and polymer/inorganic membrane materials with good propylene/propane selectivity have been studied. However, the art filed has found it difficult to fabricate these membranes into practical industrial membranes. Likewise, liquid facilitated-transport membranes have been demonstrated to have attractive separation performance in the lab, but have been difficult to scale up, and have exhibited declining performance in environments typical of an industrial propylene/propane stream. 
         [0013]    Solid polymer-electrolyte facilitated-transport membranes have shown to be capable of fabrication into more stable thin film membranes for ethylene/ethane separation. See Ingo Pinnau and L. G. Toy, Solid polymer electrolyte composite membranes for olefin/paraffin separation, J. Membrane Science, 184 (2001) 39-48. However, these membranes are severely limited by their chemical stability in the olefin/paraffin industrial environment. 
         [0014]    Carbon hollow-fiber membranes have shown promise in laboratory tests (“Propylene/Propane Separation”, Product Information from Carbon Membranes, Ltd., Israel), but are vulnerable to degradation caused by condensable organics or water present in industrial streams. Moreover, carbon membranes are brittle and difficult to form into membrane modules of commercial relevance. 
         [0015]    Membranes based on rubbery polymers typically have olefin/paraffin selectivity too low for an economically useful separation. For example, Tanaka et al. report that the single-gas propylene/propane selectivity is only 1.7 for a polybutadiene membrane at 50° C. (K. Tanaka, A. Taguchi, Jianquiang Hao, H. Kita, K. Okamoto, J. Membrane Science 121 (1996) 197-207) and Ito reports a propylene/propane selectivity only slightly over 1.0 in silicone rubber at 40° C. (Akira Ito and Sun-Tak Hwang, J. Applied Polymer Science, 38 (1989) 483-490). 
         [0016]    Membranes based on glassy polymers have the potential for providing usefully high olefin/paraffin selectivity because of the preferential diffusivity of the olefin, which has smaller molecular size than the paraffin. 
         [0017]    Membrane films of poly(2,6-dimethyl-1,4-phenylene oxide) exhibited pure gas propylene/propane selectivity of 9.1 (Ito and Hwang, Ibid.) Higher selectivity has been reported by Ilinitch et al. (J. Membrane Science 98 (1995) 287-290, J. Membrane Science 82 (1993) 149-155, and J. Membrane Science 66 (1992) 1-8). However, the membrane exhibited plasticization, most likely due to the presence of hydrocarbons. 
         [0018]    Polyimide membranes have been studied extensively for the separation of gases. Lee et al. (Kwang-Rae Lee and Sun-Tak Hwang, Separation of propylene and propane by polyimide hollow-fiber membrane module, J. Membrane Science 73 (1992) 37-45). The article discloses a hollow fiber membrane of a polyimide that exhibits a mixed-gas propylene/propane selectivity in the range of 5-8 with low feed pressure (2-4 bar). 
         [0019]    Krol et al. (J. J. Krol, M. Boerrigter, G. H. Koops, Polyimide hollow fiber gas separation membranes: preparation and the suppression of plasticization in propane/propylene environments, J. Membrane Science. 184 (2001) 275-286) report a hollow fiber membrane of a polyimide composed of biphenyltetracarboxylic dianhydride and diaminophenylindane which exhibited a pure-gas propylene/propane selectivity of 12. However, this membrane was plasticized at even low pressures. 
         [0020]    Many of these prior art membrane-based techniques of propane/propylene separation utilize a pervaporation mode (liquid feed, gaseous permeate) or gas separation mode (gaseous feed, gaseous permeate). In the pervaporation mode of operation, the liquid feed is “evaporated” to the permeate stream. The latent heat of vaporization is large and a large temperature decrease accompanies the separation. Pervaporation based separation has similarities to propane refrigeration. The high pressure feed permeates the membrane to low pressure. This process is similar to the expansion of propane across an expansion valve in s propylene refrigeration system. 
         [0021]    A convenient mathematical method of describing pervaporation is to divide the separation into two steps. The first is evaporation of the feed liquid to form a hypothetical saturated vapor phase on the feed side of the membrane. The second is permeation of this vapor through the membrane to the low pressure permeate side of the membrane. Although no evaporation actually takes place on the feed side of the membrane during pervaporation, this approach is mathematically simple and is thermodynamically equivalent to the physical process. 
         [0022]    In pervaporation, transmembrane permeation is typically induced by maintaining the pressure on the permeate side lower than the vapor pressure of the feed liquid. The permeate side pressure can be reduced, for example, by drawing a vacuum on the permeate side of the membrane, by sweeping the permeate side to continuously remove permeating vapor, or by cooling the permeate vapor stream to induce condensation. The feed may also be heated to raise the vapor pressure on the feed side or to at least partially compensate for the temperature drop on permeation. 
         [0023]    Certain issues accompany the use of pervaporation, namely, and typically regarded as most important, the temperature decrease causes membrane productivity to decrease and further increase membrane selectivity. This is an inherent problem for pervaporation. It is very difficult to supply sufficient heat at the correct location (face of the membrane) to maintain constant temperature. As a result, productivity decreases exponentially and module count becomes unacceptably large. Further, the large temperature drop of the feed to the membrane due to the pressure drop often condenses the feed. 
         [0024]    At least one prior art patent has identified that temperature has an effect on membrane performance. U.S. Pat. No. 5,679,133 discloses a glassy polymer membrane that is operated at temperatures of less than about 5° C. for separation of gas components. The patent claims, in an embodiment, a permeation method comprising contacting a first side of a gas separation membrane comprising a discriminating layer or region of a glassy polymer, wherein for the polymer cellulose triacetate is excluded, with the gas mixture while maintaining a difference in chemical potential from the first side of the membrane to a second side of the membrane, such that at least one first component gas of the gas mixture selectively permeates relative to at least one second component gas in the gas mixture from the first side of the membrane through the membrane to the second side of the membrane, the contacting occurring at a temperature of −5° C. or lower, wherein the membrane having a glassy region is selected so that, when using a mixture of 80 mole percent nitrogen and 20 mole percent oxygen as a feed at 30° C. with a pressure of 30 psia on the first side of the membrane and a vacuum of less than 1 mm Hg on the second side of the membrane, the permeability of oxygen in barriers is less than 2000. This patent illustrated that temperatures below ambient could be utilized in permeation procedures. 
         [0025]    US Patent Application 2004/0000513A1 discloses a plurality of membrane modules disposed in a first product group, a second product group, and optionally one or more intermediate groups used for simultaneous recovery of a very pure permeate product and a desired non-permeate product from a mixture containing organic compounds. Examples of propylene/propane separation are given as simulated by a computer model. The preferred embodiment is a system of three membranes with propylene selectivity over propane. The embodiments disclosed pump feed stock (about 70% propylene/30% propane) in liquid form to a vaporizer and then to a first membrane. The permeate flows through, is compressed, cooled to 200° C. and passed through a second membrane. The permeate is collected as a stream comprising greater than 95% propylene. The non-permeate from the first membrane is passed to a third membrane. Permeate from the third membrane is compressed and passed back through the second membrane. Non-permeate from the third membrane is collected as a propane product. 
         [0026]    U.S. Pat. No. 6,986,802 discloses a membrane device comprising multiple perm-selective membranes that are capable of effecting separation of a mixture of two or more compounds in a feed stock which when subjected to appropriately altered conditions of temperature and/or pressure exhibit a bubble point. The enthalpy of the feed stock is adjusted by a heat exchanger. Membrane Efficiency Index of the non-permeate fluid, when withdrawn, is within a range from about 0.5 to about 1.5. The Membrane Efficiency Index is defined as a ratio of the difference between the specific enthalpy of the feed stream entering the membrane device and specific enthalpy of the non-permeate fluid effluent to the difference between the specific enthalpy of the feed stream and the bubble point specific enthalpy of the non-permeate fluid at the non-permeate product pressure and composition. At an MEI of one, the non-permeate is disclosed as being a liquid at its bubblepoint. 
         [0027]    U.S. Pat. No. 7,070,694 discloses an apparatus comprising a fractional distillation column and one or more membrane devices utilizing solid perm-selective membranes. The processes are stated as capable of use in simultaneous recovery of a very pure permeate product, a desired non-permeate stream, and one or more distillate products from a fluid mixture containing at least two compounds of different boiling point temperatures. The patent discloses the cooling effect produced by a membrane when a low pressure permeate is produced from a high pressure feed stock This is due to the Joule-Thompson effect. The patent further expresses the need for the incorporation heat integrated membrane apparatuses with pressure driven membrane separations. 
         [0028]    However, these various prior art approaches require excessive capital costs and a multitude of membrane modules. Accordingly, there is a need in the art field for an improved membrane and/or membrane system for the separation of propylene and propane that uses fewer membrane modules and requires less capital cost. 
       SUMMARY OF THE INVENTION 
       [0029]    The present invention provides a method of separating nonideal organic streams. For the purposes of the patent, nonideality is defined by the inability of the ideal gas law to describe the PVT (pressure volume temperature) behavior of the gas. Non ideality for organic gases increases with molecular weight:methane is more ideal than ethane, ethane is more ideal than propane, propane is more ideal than butane, and/or the like. Membrane based separation of nonideal organic gases leads to a temperature decrease on separation. The temperature change increases with increasing non-ideality of the stream. The temperature change arises due to reduction in pressure of the permeated gas from feed pressure to permeate pressure. This temperature change can be approximated by a Joule-Thompson expansion (dT/dP)H. The present invention provides a means to minimize the temperature drop on separation for gases. This method is particularly useful in the separation of organics, wherein the permeation process causes a significant temperature drop. This invention is particularly applicable to separation of organic streams, more particularly propylene and propane, wherein many negative aspects of membrane-based separation of propylene and propane are eliminated. 
         [0030]    The current systems and methods allow membrane-based separation of propylene and propane, without a large change in temperature within or at a surface the membrane separator, thereby allowing a permeate stream and residue stream to be withdrawn at or about the temperature of the feed stream. The near isothermal operation greatly simplifies design, and operation of the system. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0031]    In order that the manner in which the above recited and other advantages and objects of the invention are obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof, which are illustrated, in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope, the invention will be described with additional specificity and detail through the use of the accompanying drawings in which: 
           [0032]      FIG. 1  is an illustration of a flow diagram showing a prior art distillation column for the separation of propylene and propane. 
           [0033]      FIG. 2  is an illustration of an embodiment of a flow diagram showing an embodiment of the current system and/or method using a generally athermal membrane separation approach applied to a propylene/propane separation. 
           [0034]      FIG. 3  is an illustration of an alternate embodiment of a flow diagram showing an embodiment of the current system and/or method. 
           [0035]      FIG. 4  is an illustration of an alternate embodiment of a flow diagram showing an embodiment of the current system and/or method. 
           [0036]      FIG. 5  is an illustration of an alternate embodiment of a flow diagram showing an embodiment of the current system and/or method. 
           [0037]      FIG. 6  is a Table illustrating Condensation and Temperature Change at various operating pressures in Propylene Separation. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0038]    The following definitions and explanations are meant and intended to be controlling in any future construction unless clearly and unambiguously modified in the following Description or when application of the meaning renders any construction meaningless or essentially meaningless. In cases where the construction of the term would render it meaningless or essentially meaningless, the definition should be taken from Webster&#39;s Dictionary, 3 rd  Edition. Definitions and/or interpretations should not be incorporated from other patent applications, patents, or publications, related or not, unless specifically stated in this specification or if the incorporation is necessary for maintaining validity. 
         [0039]    Exemplary, non-limiting embodiments and/or disclosures of propane/propylene separation membranes that can be used with various embodiments of the present invention are disclosed in U.S. Pat. No. 4,374,657; U.S. Pat. No. 4,444,571; U.S. Pat. No. 4,857,078; U.S. Pat. No. 4,952,751; U.S. Pat. Nos. 4,978,430; 5,057,641; U.S. Pat. No. 5,273,572; U.S. Pat. No. 5,326,385; U.S. Pat. No. 5,679,133; U.S. Pat. No. 6,187,196; U.S. Pat. No. 6,187,987; U.S. Pat. No. 6,517,611; U.S. Pat. No. 6,986,802; U.S. Pat. No. 6,693,018; U.S. Pat. No. 7,025,804; and, U.S. Pat. No. 7,070,694, the contents of which are hereby incorporated by reference, as if they were presented herein in their entirety. In general, any membrane apparatus for use in the separation of propylene and propane is capable of use with the various embodiments of the present invention with varying degrees of performance, as would be readily apparent to one of ordinary skill in the art. 
         [0040]    As used herein, a “fluid” is a continuous, amorphous substance whose molecules move freely past one another and that has the tendency to assume the shape of its container, for example, a liquid or a gas. 
         [0041]    As used herein, “membrane apparatus” means and refers to flat sheet membranes, spiral wound flat sheet membranes, tubesheets, tubular tube membranes, hollow fiber membranes, and/or other membranes commonly used in industry. 
         [0042]    As used herein, “RGP” means and refers to refinery grade propylene. Refinery grade propylene ins nominally 70% propylene with propane as the other major component. RGP propylene composition varies widely. For the purposes of this invention RGP can range from 50% to 90% 
         [0043]    As used herein, “CGP” means and refers to Chemical Grade Propylene. Chemical grade propylene nominally 93% propylene with propane as the other major component. The propylene content of CGP can be higher than 93% but is typically less than 99% propylene. 
         [0044]    Various embodiments of the present invention have wide applicability across the art field for systems and methods for the separation of propylene and propane. This disclosure will discuss application of the improvements of the present invention as it relates to primarily to a membrane-based system for the separation of propylene and propane wherein the feed fluid is capable of being a liquid and the produced product streams are capable of being liquid. 
         [0045]    Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of components used herein are to be understood as modified in all instances by the term “about”. 
         [0046]    In general, various embodiments of a system of the present invention comprise at least a first membrane apparatus, a pump, and an evaporator, to produce a CGP product and a propane product from a feed stream comprising propylene and propane, such as, but not limited to a RGP feed stock. More specifically, in an embodiment of the present invention, a liquid feed comprising propylene and propane is introduced into an embodiment of a system of the present invention and a purified propylene product and a propane enriched product is extracted. 
         [0047]    In the present invention, many of the negative aspects of membrane-based separation of a nonideal fluid stream are eliminated. In various embodiments of this process the feed is initially pumped or compressed to a relatively high pressure. A factor for selection of a pressure for the feed is such that the Joule Thompson coefficient is minimized. 
         [0048]    The pressurized feed stream is then vaporized. Vaporization of the stream after pressurization reduces the heat required for vaporization relative to the vaporization prior to pressurization. This is due to the fact that the heat of vaporization decreases with increasing pressure. Pressurization of the feed has additional benefits. Higher pressure reduces total membrane count (i.e., the number of membranes required). Higher pressure increases the pressure-ratio (i.e., feed pressure to the membrane divided by permeate pressure from the membrane). Higher pressure ratio improves separation performance, in various embodiments. 
         [0049]    In various embodiments of this process, the non-permeate portion of the feed stream may condense or at least partially condense on separation. As the non-permeate portion condenses, it evolves heat. This is commonly referred to as the heat of condensation. The evolution of heat at least partially offsets the temperature reduction created by the permeation process to keep the temperature of the membrane separator essentially constant. Through optimization of the feed pressure, the separation can be performed essentially athermally, i.e., remain at an essentially constant temperature. However, various embodiments of the present invention do allow for some temperature change, such that the separation is not essentially athermal. 
         [0050]    Now referring to  FIG. 2 , an illustration of an embodiment of a system of the present invention for the separation of propylene and propane is disclosed. System  2  comprises various elements, such as, but not limited to, pump  5 , heat exchanger  15 , and first membrane  25 . These elements are interconnected by any means for connection common in the art, such as, but not limited to line(s), piping, valves, and/or the like. For example, in  FIG. 3 , a line introduces feed fluid  1  to pump  5 , a line conveys the pressurized feed fluid  10  to heat exchanger  15 , a line conveys the at least partially gaseous feed  20  to first membrane  25 . From first membrane  25 , a line conveys propylene enriched stream  30  and/or a line conveys propane enriched stream  40 . 
         [0051]    In an embodiment of operation of system  2 , a feed stock or feed stream comprising at least propylene and propane is introduced or injected into system  2 . In an embodiment, the feed stream is a refinery grade propylene (RGP) comprising about 70% propylene. However, RGP comprising other concentrations of propylene is possible and acceptable. In general, any feed stock comprising any concentration propane and propylene can be used in various embodiments of the present invention. 
         [0052]    As disclosed, feed stream  1  is pumped to pressure in pump  5 . In various embodiments, the pressure of feed stream  1  is pumped to about 300 psia to about 700 psia prior to introduction to membrane  25 . In an embodiment, the pressure of feed stream  1  is pumped to about 300 psia to about 600 psia prior to introduction to membrane  25 . In an alternate embodiment, the pressure of feed stream  1  is pumped to about 400 psia to about 500 psia prior to introduction to membrane  25 . Further embodiments of a feed stream  1  are gas or at least partially gas and the pressure is raised by compression. 
         [0053]    Membrane  25  is selective for propylene over propane. In general, any membrane capable of effecting a propylene/propane separation can be used. In various embodiments, membranes capable of operating in a condensing hydrocarbon environment and effecting a propylene/propane separation are preferred. An example of a membrane capable of operating in a hydrocarbon condensing environment is a polyimide membrane, and particularly a polyimide membrane made of polymers sold under the tradenames P84 or P84HT from HP Polymers GmbH. Preferred membranes of P84 or P84HT are disclosed in U.S. Pat. No. 7,018,445, titled POLYIMIDE BLENDS FOR GAS SEPARATION MEMBRANES, and U.S. Patent Application titled IMPROVED SEPARATION MEMBRANE BY CONTROLLED ANNEALING OF POLYIMIDE POLYMERS, filed on Mar. 2, 2005. The entire disclosures of these applications are incorporated herein by this reference. 
         [0054]    In general, the selectivity of the propylene/propane membrane for propylene should be at least about 3.0 to about 20.0. In an embodiment, the selectivity for propylene is about 6.0 to about 15.0. In an alternate embodiment, the selectivity for propylene is about 8.0 to about 13.0. In yet an alternate embodiment, the selectivity for propylene is about 10.0 to about 12.0. 
         [0055]    In an alternate embodiment, the selectivity of the propylene/propane membrane should be greater than at least about 2.5 for propylene. In an alternate embodiment, the selectivity for propylene is greater than about 6.0. In an alternate embodiment, the selectivity for propylene is greater than about 8.0. In yet an alternate embodiment, the selectivity for propylene is greater than about 10.0. 
         [0056]    Non-permeate stream  30  will exit membrane  25  at about the temperature at which stream  20  entered membrane  25 . However, due to the cooling effect of membrane  25 , stream  40  can be expected, in various embodiments, to be cooler than stream  20 . Either or both of stream  30  and/or stream  40  may be further processed as is desired. Further embodiments will disclose various further processing. 
         [0057]    In an embodiment, the permeate stream, propylene enriched, will pass from membrane  25  and be collected as a product. In an embodiment, purity of the stream  30  is about at least 93% propylene. However, in various embodiments, the purity can vary as designed. In an embodiment, the propylene product extracted is used as a CGP product. In various alternate embodiments, a gas product is capable of being collected. 
         [0058]    In various embodiments, the percentage recovery of propylene is from about 50% to about 99%. In an alternate embodiment, the percentage recovery of propylene is from about 75% to about 99%. In an alternate embodiment, the percentage recovery of propylene is from about 85% to about 99%. 
         [0059]    In various embodiments, the percentage recovery of propane is from about 75% to about 99. In an alternate embodiment, the percentage recovery of propane is from about 80% to about 92%. 
         [0060]    Now referring to  FIG. 3 , an illustration of an alternate embodiment of a system of the present invention for the separation of propylene and propane is disclosed. System  300  comprises various elements, such as, but not limited to, pump  315 , heat exchanger  325 , first membrane  335 , second membrane  345 , first compressor  395 , second compressor  365 , first evaporator  325 , and first condenser  375 . 
         [0061]    These elements are interconnected by any means for connection common in the art, such as, but not limited to line(s), piping, valves, and/or the like. For example, in  FIG. 4 , a line introduces feed fluid  301  to pump  315 , a line conveys the pressurized feed fluid  320  to first heat exchanger  325 , a line conveys the at least partially gaseous feed  330  to first membrane  335 . From first membrane  335 , a line conveys non-permeate stream  340  and/or a line conveys propylene enriched stream  390 . 
         [0062]    Stream  390  is conveyed to first compressor  395  and conveyed as a high pressure propylene stream  397 . Stream  397  can be collected as a product, sent for further processing, used elsewhere in the process, and/or the like. 
         [0063]    Stream  340  is typically depleted in propylene as compared to stream  330 . However, a large component of stream is capable of being propylene. 
         [0064]    In an embodiment, stream  340  is conveyed to a second membrane  345 . Second membrane  345  is selective for propylene over propane, such that a propane-enriched stream  350  is extracted and a propylene-enriched stream  360  is extracted. 
         [0065]    In general, the selectivity of the propylene/propane membrane for propylene should be at least about 3.0 to about 20.0. In an embodiment, the selectivity of propylene over propane is about 6.0 to about 15.0. In an alternate embodiment, the selectivity of propylene over propane is about 8.0 to about 13.0. In yet an alternate embodiment, the selectivity of propylene over propane is about 10.0 to about 12.0. 
         [0066]    In an alternate embodiment, the selectivity of the propylene/propane membrane should be greater than at least about 3.0. In an alternate embodiment, the selectivity of propylene over propane is greater than about 6.0. In an alternate embodiment, the selectivity of propylene over propane is greater than about 8.0. In yet an alternate embodiment, the selectivity of propylene over propane is greater than about 10.0. 
         [0067]    Non-permeate stream  340  will exit membrane  335  at about the temperature at which stream  330  entered membrane  335 . However, due to the cooling effect of membrane  335 , stream  350  can be expected, in various embodiments, to be cooler than stream  330 . 
         [0068]    The permeate stream  360 , propylene enriched, will pass from membrane  345  and into second compressor  365 . High pressure stream  370  is then conveyed to condenser  375 , wherein a liquid propylene-enriched stream is capable of being injected into stream  1 , forming a combined stream  310 . 
         [0069]    Now referring to  FIG. 4 , an illustration of an alternate embodiment of a system of the present invention for the separation of propylene and propane is disclosed. System  400  comprises various elements, such as, but not limited to, pump  415 , first heat exchanger  425 , first membrane  435 , second membrane  445 , first compressor  465 , and first condenser  475 . 
         [0070]    First membrane  435  and second membrane  445  are capable of being constructed/formed to be selective for a variety of components. In an embodiment, membrane  435  and membrane  445  are selective for propylene over propane. 
         [0071]    Now referring to  FIG. 5 , an illustration of an alternate embodiment of a system of the present invention for the separation of propylene and propane is disclosed. System  501  discloses a feed stream  505 , pump  520 , first heat exchanger  515 , second heat exchanger  525 , third heat exchanger  530 , evaporator  535 , first membrane  550 , heat exchanger  560 , second membrane  570 , compressor  590 , first condenser  615 , second condenser  620 , and third condenser  625 . 
         [0072]    Stream  505  is typically introduced to system  501  as a liquid and pumped to pressure in pump  520 , producing some heat. Stream  510  is passed through various heat exchangers for cooling and/or heating. In an embodiment, stream  510  is cooled against streams  580 ,  550 , and/or  595 . However, in various embodiments, one or more of streams  580 ,  550 , and/or  595  act to heat stream  510 . 
         [0073]    Evaporator  535  evaporates stream  510  into at least partially gaseous stream  545 . Stream  545  is conveyed to first membrane  550 . The non-permeate stream  520  is passed across a heat exchanger  560  with permeate stream  550 . Heat exchanger acts to heat stream  550  and cool stream  520 . However, little cooling of stream  520  will be realized considering the volume of stream  520  being greater than that of stream  550 . Stream  520  can then be collected as a liquid propylene product stream  605  after passing through condenser  615 . 
         [0074]    In an embodiment, there is no cooling of stream  520  in heat exchanger  560 . In an alternate embodiment, there is minimal cooling of stream  520  in exchanger  550 . In an alternate embodiment, stream  520  is cooled by more than a minimal amount. 
         [0075]    Stream  520  is then conveyed to second membrane  570  wherein a further propylene-depleted stream  580  is removed and a further propylene-enriched stream  575  is removed. Stream  580  will be at least partially gaseous and therefore act to heat stream  510  in exchanger  515 . Stream  580  is capable also of being condensed in condenser  620  and collected as a propane/propylene liquid product. 
         [0076]    Stream  575  will be pressure depleted and propylene-enriched. In an embodiment, for process enhancement, at least a portion of stream  575  is compressed as stream  595 , condensed in condenser  625  and reintroduced into system  501  into stream  501 . 
         [0077]    Further embodiments further comprise additional membranes as desired. For example, a further membrane may be added to further separate propylene from stream  575 , such as to produce a Polymer Grade Propylene Product (PGP) product. Further, a different arrangement of heat exchangers, evaporators, compressors, and condensers can be used within the general inventive guidelines of the present invention. The addition of another membrane allows improving propylene purity without the necessity of improving or modifying membrane structure, i.e., membrane selectivity and/or the like. In an embodiment, adding at least one additional membrane increases the purity of propylene to at 99%. 
         [0078]    Further embodiments of the preset invention contemplate a process for the separation of propylene and propane. In an embodiment, the feed stream is a propylene/propane stream at a temperature T 1 . The pressure P 1  of the liquid propylene/propane stream is increased to a pressure P 1  of about 400 to 600 psia. The liquid stream is then vaporized and heated to temperature sufficient to establish process conditions of P 2  and T 2  at the membrane separator inlet or feed port that is just above or at the dewpoint. The propylene/propane stream enters the membrane separator and partially or fully condenses in the membrane separator as the propylene permeates through the membrane and the remaining stream (primarily propane) is removed as a non-permeate liquid or two-phase stream gas/liquid stream. The propylene permeate is compressed and condensed as required to form a propylene liquid product. The non-permeate stream is then passed through another membrane separator wherein the non-permeate is collected as a propane product and the permeate propylene-rich stream is recycled back to the feed stream. 
         [0079]    Accordingly, in an embodiment, a process of the present invention generally comprises the steps of feeding a feed stream comprising propylene and propane to a first membrane at a temperature at or slightly above the dewpoint of propylene, said membrane having a selectivity for propylene of at least 5.0, whereby at least a portion of a first non-permeate stream condenses on or about the first membrane; extracting a permeate propylene enriched stream, wherein the permeate stream is extracted at or about the same temperature as the feed stream; condensing the permeate stream; and, recovering the permeate stream as a propylene product stream. 
       EXAMPLES 
       [0080]    It has been discovered that higher pressure reduces the module count and the higher pressure-ratio (feed pressure to the membrane divided by permeate pressure from the membrane) improves separation performance. Further, the evolution of heat by the condensation of the process feed fluid helps to keep the temperature constant and reduce capital cost and operational cost of the system. 
         [0081]    It has been found that traditional gas separation systems require 95% more membranes, 12% more horsepower and 9.5% more energy for processing RGP than embodiments of the current inventive process. The current inventive process uses ⅓ more pump horsepower than the traditional gas separation approach. 
         [0082]    Reference to  FIG. 6  illustrates the effect of varying pressure of a feed gas in the embodiment of a propylene separation membrane, Refinery Grade Propylene (RGP), at a temperature of 90° C. on the temperature of the permeate gas and the associated vapor/liquid (V/L) proportion of the permeate gas. The results indicate that feed pressures between about 400 psia and about 500 psia (2.76e+3 kPa to 3.45e+3 kPa) produce a substantial liquid permeate without a significant drop in temperature. 
         [0083]    Application of these concepts is illustrated in the following examples. For each of these examples, a membrane with a selectivity of propylene to propane of 11 is assumed. 
       Example 1 
       [0084]    A simulation was performed based upon the system illustrated in  FIG. 2 . The intent of this system is the recovery of higher purity propylene. The system incorporates a membrane ( 25 ) with a propylene to propane selectivity of 11.3 MMscfd. Now referring to  FIG. 2 , an example of a system of the present invention for the separation of propylene and propane is disclosed. A feed of 3 MMscfd at 200 psia and  30 C is introduced to System  2 . Pump  5  pressurizes the stream 500 psia. Evaporator ( 15 ) vaporizes the feed reaching an ultimate temperature of 90° C. Membrane ( 25 ) with an inherent propylene to propane selectivity of 11 is used to separate the feed into a propylene enriched product ( 40 ) and a propane enriched product ( 30 ). The propylene product is recovered at 93% propylene purity at 40 psia. Propylene recovery is 70%. The propane purity is 57% and propane recovery is 88%. The pump for this system requires 44 kw and the evaporator (heat exchanger) requires 662 kW. Table 1 lists the various properties of the streams for Example 1. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Stream Properties for Example 1 
               
             
          
           
               
                   
                 Stream # 
                 1 
                 10 
                 20 
                 30 
                 40 
               
               
                   
                   
               
             
          
           
               
                   
                 T (° C.) 
                 30 
                 33 
                 95 
                 77 
                 77 
               
               
                   
                 P (psia) 
                 200 
                 500 
                 500 
                 40 
                 499 
               
               
                   
                 V/L 
                 0 
                 0 
                 1 
                 1 
                 0.8 
               
               
                   
                 Flow (MMscfd) 
                 3 
                 3 
                 3 
                 1.6 
                 1.4 
               
               
                   
                 % propylene 
                 70 
                 70 
                 70 
                 93.1 
                 43 
               
               
                   
                 % propane 
                 30 
                 30 
                 30 
                 6.9 
                 57 
               
               
                   
                   
               
             
          
         
       
     
       Example 2 
       [0085]    A simulation was also performed based upon the system illustrated in  FIG. 3 . The system is capable of high recovery of increased purity propylene and high recovery of increased purity propane. A feed of 3 MMscfd at 200 psia and 30C is introduced to the system outline in  FIG. 2 . Pump ( 315 ) pressurizes the stream 500 psia. Evaporator (heat exchanger) ( 325 ) vaporizes the feed reaching an ultimate temperature of 90° C. Membrane ( 335 ) separates the feed into a propylene enriched product at 40 psia ( 390 ). The propylene product is compressed to system feed pressure by Compressor ( 395 ). The non-permeate stream ( 340 ) is reheated to 9° C. and further processed by membrane ( 335 ) into a 95% propane product ( 50 ) and a low pressure propylene-enriched stream. Stream  360  is compressed by compressor ( 364 ) and subsequently condensed by condenser ( 375 ). The resulting stream is comingled with the feed ( 301 ). The combined stream is fed to the membrane system. 
         [0000]    A system based on  FIG. 3  is capable of producing 93% propylene at 98% recovery and a 95% propane product at 83% recovery. Energy requirements for the system are as follows. 
         [0086]    Pump ( 315 ): 13.8 kW 
         [0087]    Evaporator (heat exchanger) ( 325 ): 921 kW 
         [0088]    Condenser ( 375 ): 344 kW 
         [0089]    Compressor ( 365 ): 96 kW 
         [0090]    Compressor ( 395 ): 175 kW 
         [0000]    Table 2 lists the various properties of the streams for Example 2.
 
Compared with Example 1 the current system increases propylene recovery and produces a high purity propane stream.
 
         [0000]    
       
         
               
             
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Stream Properties for Example 2 
               
             
          
           
               
                   
                 Stream # 
               
             
          
           
               
                   
                 301 
                 320 
                 330 
                 390 
                 340 
                 341 
                 350 
                 360 
               
               
                   
                   
               
             
          
           
               
                 T (° C.) 
                 30 
                 33 
                 90 
                 80 
                 80 
                 90 
                 35 
                 35 
               
               
                 P (psia) 
                 200 
                 500 
                 500 
                 40 
                 499 
                 499 
                 499 
                 30 
               
               
                 V/L 
                 0 
                 0 
                 1 
                 1 
                 0.8 
                 1 
                 0.5 
                 1 
               
               
                 Flow (MMscfd) 
                 3 
                 4.17 
                 4.17 
                 2.22 
                 1.95 
                 1.95 
                 0.79 
                 1.16 
               
               
                 % propylene 
                 70 
                 69.8 
                 69.8 
                 93.1 
                 43 
                 43 
                 4.9 
                 69.7 
               
               
                 % propane 
                 30 
                 30.2 
                 30.2 
                 6.9 
                 57 
                 57 
                 95.1 
                 30.3 
               
               
                   
               
             
          
         
       
     
       Example 3 
       [0091]    A simulation was also performed based upon the system illustrated in  FIG. 4 . The system is capable of high recovery of increased purity propylene and high recovery of increased purity propane and elimination of the propylene product compressor. A feed of 3 MMscfd at 200 psia and  30 C is introduced to the system outline in  FIG. 3 . Pump ( 415 ) pressurizes the stream 500 psia. Evaporator ( 325 ) vaporizes the feed reaching an ultimate temperature of 90° C. Membrane ( 435 ) separates the feed into a propylene enriched product at 200 psia ( 490 ). No compression is required to achieve system feed pressure. The non-permeate stream ( 340 ) is reheated to 90C. and further processed by membrane ( 435 ) into a 95% propane product ( 50 ) and a low pressure propylene-enriched stream. Stream  460  is compressed by compressor ( 465 ) and subsequently condensed by condenser ( 475 ). The resulting stream is comingled with the feed ( 301 ). The combined stream is fed to the membrane system. 
         [0000]    A system based on  FIG. 3  is capable of producing 93% propylene at 98% recovery and a 95% propane product at 83% recovery. Energy requirements for the system are as follows. 
         [0092]    Pump ( 315 ): 18 kW 
         [0093]    Evaporator (heat exchanger) ( 325 ): 1210 kW 
         [0094]    Condenser ( 375 ): 737 kW 
         [0095]    Compressor ( 365 ): 203 kW 
         [0000]    Example 3 demonstrates identical performance to Example 2 without the necessity of propylene product compression. 
         [0000]    
       
         
               
             
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Stream Properties for Example 3 
               
             
          
           
               
                   
                 Stream # 
               
             
          
           
               
                   
                 401 
                 420 
                 430 
                 490 
                 440 
                 441 
                 450 
                 460 
               
               
                   
                   
               
             
          
           
               
                 T (° C.) 
                 30 
                 32 
                 90 
                 82 
                 82 
                 90 
                 35 
                 35 
               
               
                 P (psia) 
                 200 
                 500 
                 500 
                 40 
                 499 
                 499 
                 499 
                 30 
               
               
                 V/L 
                 0 
                 0 
                 1 
                 1 
                 0.8 
                 1 
                 0.5 
                 1 
               
               
                 Flow (MMscfd) 
                 3 
                 5.49 
                 5.49 
                 2.2 
                 3.27 
                 3.27 
                 0.78 
                 2.49 
               
               
                 % propylene 
                 70 
                 76.29 
                 76.29 
                 93.1 
                 63.7 
                 63.7 
                 4.9 
                 82.1 
               
               
                 % propane 
                 30 
                 23.71 
                 23.71 
                 6.9 
                 36.3 
                 36.3 
                 95.1 
                 17.8 
               
               
                   
               
             
          
         
       
     
         [0096]    The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes to the claims that come within the meaning and range of equivalency of the claims are to be embraced within their scope. Further, all published documents, patents, and applications mentioned herein are hereby incorporated by reference, as if presented in their entirety.