Abstract:
Methods for recovering energy from a mother liquor stream in paraxylene crystallization processes are disclosed herein. The low temperature energy from the mother liquor is optimally utilized to reduce the refrigeration burden on the crystallization process.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This Application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Patent Application Ser. No. 61/454,337 filed Mar. 18, 2011, which is incorporated herein by reference in its entirety as if fully set forth herein. 
     
    
     TECHNICAL FIELD OF THE INVENTION 
       [0002]    The claimed invention is directed to methods for recovering energy from a mother liquor stream in paraxylene crystallization processes. In the claimed invention, the energy from the mother liquor is optimally utilized to reduce the refrigeration burden on the crystallization process. 
       BACKGROUND OF THE INVENTION 
       [0003]    Xylene isomers, ortho-xylene (OX), meta-xylene (MX), and para-xylene (PX), and ethylbenzene (EB) are C8 aromatics from a reforming process or other petrochemical processes. Generally, the product distribution for an equilibrium xylene mixture is about 40% MX, 20% PX, 20% OX and 20% EB. These amounts can vary by ±10%. The purified individual xylene products are used on a large scale as industrial solvents and intermediates for many products. The most important isomer, PX, is used for the production of terephthalic acid (TPA) and dimethyl terephthalate (DMT), which are used for the production of fibers, films and polyethylene terephthalate (PET) bottles. In these applications, high purity (&gt;99.7%) PX is required. Demand for high purity PX has increased greatly over the past years to meet rapidly growing markets. 
         [0004]    Many physical properties of the individual xylene isomers are similar, such as boiling points, which makes the separation of high purity xylene isomers extremely difficult by conventional distillation. Two methods are currently used commercially to separate and produce high purity PX: adsorption and crystallization. A third method, a hybrid adsorption/crystallization process, was successfully field-demonstrated in 1990s. 
         [0005]    Before the commercialization of PX adsorption process, low temperature fractional crystallization was the first and for many years the only commercial technology for separating PX from C8 aromatics. The xylene system is an extremely favorable system for melt crystallization. The melting points of PX, MX, OX, and EB are 13.3° C., −47.9° C., −25.2° C., and −95.0° C. respectively, and the system does not form solid solutions above the eutectic temperature. Thus, the crystals are essentially pure PX. Several commercial crystallization processes have been developed to separate PX from its isomer mixture. PX crystals are typically produced in two or more crystallization stages, with PX recovery of about 60-65% per pass. In commercial practice, PX crystallization is carried out at a temperature just above the eutectic point, which is about −50° C. to about −70° C. for an equilibrium xylene mixture feed. The equilibrium of PX in C8 aromatics liquid (mother liquor) limits the efficiency of the crystallization process. The solid PX crystals are typically separated from the mother liquor by filtration or centrifugation. 
         [0006]    For PX production with equilibrium xylene feed, the mother liquor is separated from PX solid at low temperature. Thus, the mother liquor from the process contains significant amount of refrigeration duty due to its low temperature and high flow rate. This invention is related to the efficient energy recovery from the mother liquid in this low temperature crystallization process. 
         [0007]    In view of the foregoing, methods to recover energy from mother liquor in a low temperature crystallization process for PX production would be of considerable benefit. Such methods would allow more efficient operation of crystallization processes. 
       SUMMARY OF THE INVENTION 
       [0008]    In various embodiments, methods for recovering energy from mother liquor in paraxylene crystallization process are disclosed. The methods comprise: 1) providing a crystallizer or heat exchanger to recover energy from low temperature mother liquor; 2) providing a second heat exchanger to recovery energy from intermediate temperature mother liquor; 3) providing a third heat exchanger to recovery energy from high temperature mother liquor. The feed stream is the media on the other side of the heat exchangers/crystallizers that carries the energy, and the feed stream is cooled down by the mother liquor. One option is to have a fourth heat exchanger for the feed stream between the first crystallizer/heat exchanger and second heat exchanger to further optimize the energy recovery. 
         [0009]    The foregoing has outlined rather broadly the features of the present disclosure in order that the detailed description that follows may be better understood. Additional features and advantages of the disclosure will be described hereinafter, which form the subject of the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    For a more complete understanding of the present disclosure, and the advantages thereof, reference is now made to the following description to be taken in conjunction with the accompanying drawings describing specific embodiments of the disclosure, wherein: 
           [0011]      FIG. 1  shows an illustrative energy recovery system from mother liquor; and 
           [0012]      FIG. 2  shows an illustrative energy recovery system from mother liquor with an optional fourth heat exchanger between first crystallizer and second heat exchanger. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0013]    In the following description, certain details are set forth such as specific quantities, and temperature, so as to provide a thorough understanding of the present embodiments disclosed herein. However, it will be obvious to those skilled in the art that the present disclosure may be practiced without such specific details. In many cases, details concerning such considerations and the like have been omitted inasmuch as such details are not necessary to obtain a complete understanding of the present disclosure and are within the skills of persons of ordinary skill in the relevant art. 
         [0014]    In the PX crystallization process, the main energy consumption is from the refrigeration station compressors, which are used to provide the low temperature refrigerant duty to cool the feed streams to desired temperature. It is desirable to minimize the refrigeration duty by recovering energy from different streams within the crystallization unit before discharge. 
         [0015]    For a PX crystallization process with equilibrium xylene feed, the lowest operating temperature is limited by the eutectic point, which is between −50° C. and −70° C. The mother liquor is at this temperature before discharge. Because the equilibrium xylene feed contains only about 20% PX, the quantity of the mother liquor is significant. Therefore, there is significant amount of low temperature refrigeration duty available in the mother liquor. The optimum recovery of the energy from the mother liquor improves the energy efficiency of the process. 
         [0016]    An embodiment of the invention is directed to a method for recovering energy from a mother liquor in a PX crystallization process, the method comprising providing a feed stream to a PX crystallization unit; providing a first crystallizer or heat exchanger to recover low temperature energy from low temperature mother liquor; providing a second heat exchanger to recovery energy from intermediate temperature mother liquor; providing a third heat exchanger to recovery energy from high temperature mother liquor; wherein the feed stream to the PX crystallization unit is cooled down by the energy extracted from the mother liquor. 
         [0017]    Crystallizers or crystallization units are based on the use of vertical vessel, scraped-surface crystallizers, and wash columns. The crystallizers create a slurry of high-purity para-xylene crystals in a mother liquor. This slurry is fed to wash columns where the crystals are separated from the mother liquor, and melted for the final product. 
         [0018]    In certain embodiments of the invention, the low temperature mother liquor temperature is from −50° C. to −70° C. In other embodiments of the invention, the crystallizer is a screw type crystallizer, scrape surface crystallizer, or part of the crystallizer in the main PX crystallization section. In a further embodiment of the invention, the crystallizer can be a single crystallizer, or multiple crystallizers operated in serial or in parallel. In other embodiments of the invention, the heat exchanger can be a shell/tube type heat exchanger, or more advantageously a double pipe heat exchanger. 
         [0019]    A further embodiment of the invention is directed to a method for recovering energy from a mother liquor in PX crystallization process by providing a first crystallizer or heat exchanger to recover energy from low temperature mother liquor; providing a second heat exchanger to recovery energy from intermediate temperature mother liquor; providing a third heat exchanger to recovery energy from high temperature mother liquor; and providing a fourth heat exchanger to further reduce the temperature of feed stream, wherein the feed stream to the PX crystallization unit is cooled down by the energy extracted from the mother liquor. In certain embodiments of the invention, a heat exchanger may be used to cool a feed stream. 
         [0020]    In the method illustrated in  FIG. 1 , the energy of mother liquor is first recovered in a first crystallizer or heat exchanger  101 . The crystallizer can be a screw type crystallizer, or scrape surface crystallizer, or part or a portion of the crystallizers in the crystallization section shown in  FIG. 1 . It can also be multiple crystallizers operated in serial or in parallel. The reason to use a crystallizer is that when the temperature drops below the PX freezing point and PX crystals are formed, it is necessary to remove the crystals continuously to prevent the accumulation of solid that may cause plugging of the equipment. In the example illustrated in  FIG. 1 , mother liquor is warmed up from −63° C. to −54° C. in  101 , and the feed stream is cooled down from −35° C. to −40° C. Mother liquor from  101  is further warmed up in a second heat exchanger  102  to recover additional energy for cooling the feed stream.  102  can be a regular shell/tube type heat exchanger, or more advantageously a double pipe heat exchanger to minimize the equipment plugging problems. Mother liquor from  102  is further warmed up in a third heat exchanger  103  to about 35° C. as illustrated in the example before exit from the PX crystallization process. This warm stream is ready to be processed in the down stream units, such as a xylene isomerization unit. Feed stream is cooled down from 40° C. to about −17° C. in  103  as illustrated in the example. The energy from mother liquor is thus fully recovered. 
         [0021]    In the method illustrated in  FIG. 2 , which is similar to the method illustrated in  FIG. 1  except a fourth heat exchanger  104  is introduced between the first crystallizer  101  and a second heat exchanger  102 . The addition of the fourth heat exchanger is to utilize a high temperature energy source so that the energy from the mother liquor can be better utilized. The shift of a high temperature energy source from a low temperature energy source means the overall refrigeration station power is decreased. This is illustrated with a high temperature energy source to the fourth heat exchanger  104  to cool the feed stream temperature just above the feed freezing point; thus, the first crystallizer  101  has been maximized in utilizing the low temperature energy from the mother liquor. The cooling media for  104  can be a refrigerant from the refrigeration station, or other suitable media. 
         [0022]    From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this disclosure, and without departing from the spirit and scope thereof, can make various changes and modifications to adapt the disclosure to various usages and conditions. The embodiments described hereinabove are meant to be illustrative only and should not be taken as limiting of the scope of the disclosure, which is defined in the following claims.