Abstract:
Recovering high purity benzene from hydrocarbon feedstock containing aromatics and non-aromatics is implemented by simple and low-cost modifications to conventional extractive distillation columns (EDCs). Methyl cyclohexane (MCH) that is generated through non-selective hydrogenation of toluene in hydrodesulfurization (HDS) units is a major contaminant in benzene production. To meet MCH specifications, often times the extractive distillation (ED) process for recovering purified benzene is operated with excessive benzene loss to the overhead raffinate stream, producing a lower quality non-aromatic product. Novel techniques (1) remove operational constrictions of the HDS unit on MCH production, thus lengthening the catalyst life and (2) allow the EDC to drive essentially any amount of MCH away from the bottom benzene product without concerns with benzene loss to the overhead raffinate stream and (3) recover benzene from the overhead raffinate stream to upgrade the quality of non-aromatic product and increase the benzene product recovery.

Description:
FIELD OF THE INVENTION 
     The present invention is directed to improved processes for recovering high purity benzene from hydrocarbon feedstock that contains aromatics and non-aromatics where the processes can be implemented with minimal modifications to conventional extractive distillation columns in current benzene recovery extractive distillation processes. 
     BACKGROUND OF THE INVENTION 
     Commercial extraction processes for aromatic hydrocarbon recovery typically involves hydrotreating feedstock in a hydrodesulfurization unit to remove sulfur and nitrogen before undergoing extractive distillation (ED) with a non-aqueous selective solvent to produce a high purity benzene product. For example, U.S. Pat. No. 5,215,629 to Skatulla et al. discloses an ED process that uses non-aqueous N-substituted morpholine solvent to separate aromatics from mixtures containing non-aromatics that entails distilling a raffinate (non-aromatics) stream from the overhead of an extractive distillation column (EDC) in a separate distillation column. U.S. Pat. No. 5,252,200 to Skatulla et al. simplifies the configuration and operation of the &#39;629 patent process by including the feature of distilling the raffinate (non-aromatics) stream from an overhead of the EDC in an internal distillation column that is installed on top of the EDC to recover a selective solvent residue from the raffinate. The distillation column is installed as an integral upper portion of the EDC for recovery of the selective solvent residue from the overhead raffinate stream. 
     Hydrotreatment of feedstock generates methyl cyclohexane (MCH) through non-selective hydrogenation of toluene. Since MCH is one of the major contaminants in the final benzene product, the hydrodesulfurization (HDS) unit is operated under stringent conditions in order to maintain the required balance between removing sulfur and nitrogen contaminants and minimizing the formation of MCH. Consequently, in order to meet MCH specifications for benzene products, the ED operates under process conditions that result in excessive benzene loss to the overhead raffinate stream and yields lower quality non-aromatic products. 
     SUMMARY OF THE INVENTION 
     The present invention is based in part on the recognition that current extractive distillation processes, as exemplified by U.S. Pat. No. 5,252,200, for recovering benzene from feedstock such as the C 6  fraction of hydrotreated coal tar, are deficient in many respects. The deficiencies can be attributed to the accumulation of MCH in the raffinate distillation column. The MCH is pushed downward toward the lower portion of the EDC and contaminates the benzene that is present in the rich solvent at the bottom. To prevent MCH from descending into the column, the EDC reboiler duty can be increased but this causes additional benzene loss to the overhead raffinate stream and higher energy consumption in the EDC. A related problem is the small loading that is in the upper EDC and the integral raffinate distillation column, especially above the EDC solvent feed tray, which is caused by the low non-aromatic content in the feed. Consequently, a high external reflux is required to prevent channeling which would otherwise lead to low column efficiency in the upper portion of the integrated EDC. This is the case even when employing a top distillation column with a reduced diameter. 
     The present invention provides improved methods for processing the raffinate stream that is produced at the top of the EDC in order to minimize benzene loss to the raffinate product and to prevent (or minimize) MCH from contaminating the benzene product at the bottom of the EDC. The inventive techniques can be realized by simple and low-cost modifications to conventional extractive distillation columns. One feature of the invention eliminates the requirement of limiting MCH generation in the HDS unit; this accomplished by improved operations in the subsequent purification processes that improve the separation between MCH and benzene. The HDS unit operation is more flexible and, as a result, the HDS catalyst life can be significantly increased as the HDS unit is configured to focus on the removal of thiophene (another benchmark contaminant) and other sulfur and nitrogen compounds. Other features of the invention are the reduction of MCH contaminants in the benzene product, reduction of benzene loss to the non-aromatic product to maintain its quality as a gasoline blend stock, increased recovery of the benzene product, and reduced energy consumption of the purification process. Extractive distillation (ED) is the preferred process for separating benzene from the non-aromatics, including MCH and uses solvent such as N-substituted morpholines with substituents containing not more than 7 carbon atoms, sulfolane, glycols, N-methyl pyrrolidone and their mixtures, all with or without water. Preferred solvents are N-formyl morpholine (NFM) and sulfolane/water mixture containing 0 to 1.0% water; a particularly preferred solvent is non-aqueous NFM. 
     Accordingly, in one aspect, the invention is directed to a process for recovering high purity benzene from a feedstock containing aromatic hydrocarbons and non-aromatic hydrocarbons that includes the steps of: 
     (a) removing sulfur and nitrogen contaminants from the feedstock through a hydrodesulfurization (HDS) unit to yield a hydrotreated feedstock; 
     (b) introducing the hydrotreated feedstock into in a distillation column to remove a heavy portion of the hydrotreated feedstock and to recover a C 6  fraction and lighter portion of the hydrotreated feedstock that contains benzene which is introduced to a middle portion of an extractive distillation column (EDC); 
     (c) introducing a lean solvent stream that is recycled from the bottom of a solvent recovery column (SRC) into an upper portion of the EDC as a selective solvent feed; 
     (d) recovering a first non-aromatic hydrocarbon-rich stream from the top of the EDC and withdrawing a first solvent-rich stream containing the solvent and substantially all the benzene that was present in the C 6  fraction and lighter portion of the hydrotreated feedstock from the bottom of the EDC; 
     (e) introducing the first solvent-rich stream into a middle portion of the SRC and recovering an aromatic hydrocarbon stream consisting essentially of benzene and that is substantially free of the solvent and non-aromatic hydrocarbons from the top of the SRC and removing the lean solvent stream from the bottom of the SRC; 
     (f) introducing the first non-aromatic hydrocarbon-rich stream from step (d) into a lower portion of a liquid-liquid extraction (LLE) column and introducing a slip stream from the lean solvent stream from step (c) into an upper portion of the LLE column as the solvent feed stream; 
     (g) recovering a second non-aromatic hydrocarbon-rich stream from the top of the LLE column as a raffinate product and removing a second solvent-rich stream from the bottom of the LLE column, which is recycled into the middle portion of the EDC; and, optionally, 
     (h) introducing a stripping agent into a lower portion of the EDC to prevent channeling in an upper portion of the EDC that is caused by low column loading to thereby enhance its separation efficiency. 
     In another aspect, the invention is directed to a process for recovering high purity benzene from a feedstock containing aromatic hydrocarbons and non-aromatic hydrocarbons that includes the steps of: 
     (a) removing sulfur and nitrogen contaminants from the feedstock through a hydrodesulfurization (HDS) unit to yield a hydrotreated feedstock; 
     (b) introducing the hydrotreated feedstock into a distillation column to remove a heavy portion of the hydrotreated feedstock and to remove a C 6  fraction and lighter portion of the feedstock which is introduced into a middle portion of an EDC which contains a raffinate distillation column as an integral upper portion of the EDC for recovering benzene from ascending raffinate vapor stream to provide an internal reflux for the EDC; 
     (c) introducing a lean solvent stream recycled from the bottom of a solvent recovery column (SRC) into an upper portion of the EDC below the raffinate distillation column as a primary selective solvent feed; 
     (d) introducing a slip stream from the primary selective solvent stream to the upper portion of the raffinate distillation column in step (b) as a secondary selective solvent feed to convert the raffinate distillation column into an auxiliary EDC to recover additional benzene from the first non-aromatic hydrocarbon-rich stream; 
     (e) recovering a first non-aromatic hydrocarbon-rich stream that is low in benzene content from the top of the EDC and withdrawing a first solvent-rich stream containing the solvent and substantially pure benzene from the bottom of the EDC; 
     (f) introducing the first solvent-rich stream into a middle portion of the SRC, recovering an aromatic hydrocarbon stream containing essentially pure benzene, that is substantially free of the solvent and non-aromatic hydrocarbons, from the top of the SRC, and removing the lean solvent stream from the bottom of the SRC; 
     (g) introducing the first non-aromatic hydrocarbon-rich stream into a middle portion of a distillation column to recover a second non-aromatic hydrocarbon-rich stream from the top of the distillation column as a raffinate product, and to remove a bottom stream from said distillation column containing benzene, non-aromatic hydrocarbons and entrained solvent, which is recycled to the EDC as a mixture with the EDC primary solvent feed stream; and optionally, 
     (h) introducing a stripping agent to the lower portion of the EDC to prevent channeling in an upper portion of the EDC caused by low column loading, to increase its separation efficiency. 
     The invention can be readily implemented by revamping existing ED processes for benzene recovery from coal tar or other similar feedstocks. The only additions to the existing equipment are a small LLE column and a gas blower (optional), or a small distillation and a gas blower (optional), along with some process piping modifications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an extractive distillation (ED) process using NFM solvent for benzene recovery; 
         FIG. 2  is a schematic diagram of an improved ED process using a liquid-liquid extraction column for recovering additional benzene from the ED raffinate; and 
         FIG. 3  is a schematic diagram of an improved ED process using a distillation column for recovering additional benzene from the ED raffinate. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  illustrates a commercial process, for producing high purity benzene and non-aromatic hydrocarbons for gasoline blending or other applications, which employs a HDS unit  100 , a splitter column  101  that is equipped with reboiler  114 , an integrated EDC  102  that is equipped with reboiler  120 , and a solvent recovery column (SRC)  104  that is equipped with reboiler  126 . In operation, a suitable hydrocarbon feed such a light fraction of coal tar containing mainly benzene, toluene and C 6 -C 7  non-aromatic hydrocarbons is fed via line  1  into a HDS unit  100 . The hydrosulfurization process removes sulfur and nitrogen contaminants in the fraction but also generates MCH so, in a conventional benzene recovery process, HDS unit  100  is operated under conditions that remove the sulfur and nitrogen contaminants and, at the same time, minimize the formation MCH. MCH is the major contaminant in the benzene product, which is typically kept below 1,000 ppm and preferably below 200 ppm in the final product. 
     In a conventional process, HDS unit  100  typically operates with reactor inlet and outlet temperatures of approximately, 230-235° C. and 250-255° C., respectively and under a reactor pressure of 28-29 Kg/cm 2  (gauge). Under these conditions, the concentration of MCH in the reactor effluent is controlled within a range of 1,500 to 2,500 ppm. If the end-of-run (EOR) reactor temperature is allowed to exceed 255° C. which leads to more MCH in the reactor effluent, the HDS catalyst regeneration cycle can be lengthened beyond 2 to 3 years, and subsequently, catalyst life can be prolonged beyond 3 to 5 years. With the present invention, it is possible to eliminate the restriction of generating excess MCH in HDS  100  unit by improving the operation of subsequent purification processes for a better separation between MCH and benzene as depicted in  FIGS. 2 and 3 . 
     As further shown in  FIG. 1 , a hydrotreated light fraction  2  from HDS unit is introduced to a C 6 /C 7  splitter  101  where toluene and C 7   +  non-aromatics are removed from the bottom of the column via line  6 , while the C 6  concentrate (which contains 90 to 98 wt %, preferably 96 to 98 wt % benzene and 2 to 10 wt %, preferably 2 to 4 wt % non-aromatics, including approximately 1,000 to 5,000 ppm MCH and preferably 2,500 to 5,000 ppm MCH) is withdrawn via line  3  from the top of the column  101 , condensed through cooler  110 , and transferred into accumulator  112 . A portion of the condensate from  112  is recycled to the top of column  101  as the reflux through line  4  and the other portion is introduced via line  5  into the middle portion of the EDC  102 . 
     A portion of the lean solvent from the bottom of the SRC  104  is fed to the upper portion of the EDC  102  via line  15  as the primary solvent feed. Raffinate vapor stream from the top of EDC  102  ascends into the raffinate distillation column  103  to separate the solvent and aromatic hydrocarbons (mainly benzene) from the non-aromatics. Conditions in the distillation column are adjusted so that a sump product recovering all the solvent in the raffinate, descends into the EDC  102  to provide an internal reflux to the top of EDC  102 , while the overhead vapor from column  103  is transferred via line  7  and condensed in condenser  116  before entering accumulator  118 . A portion of the solvent-free raffinate stream is recycled to the top of column  103  as reflux via  8  and the other portion is withdrawn as raffinate product through line  9 . Rich solvent exiting the bottom of EDC  102  is transferred to the middle portion of SRC  104  via line  10 . 
     Purified aromatic vapor is stripped from the solvent and withdrawn from the top of SRC  104  via line  12  and condensed in condenser  122  before entering accumulator  124 . A portion of the aromatic hydrocarbon is recycled to the top of SRC  104  as reflux via line  13  and the other portion is withdrawn as the aromatic product (substantially all benzene) with trace of MCH through line  14 . Lean solvent from the bottom of SRC  104  is recycled to upper part of EDC  102  via line  15 . 
     The major consideration in operating EDC  102  and column  103  is to keep MCH away from the bottom of EDC  102  so that its concentration in the bottom benzene product can be maintained within specifications, without losing a substantial amount of benzene to the overhead raffinate product. Raffinate with high benzene concentration is less desired as a gasoline blending stock and cannot be used as a more valuable solvent; the presence of benzene in the raffinate also lowers the yield of benzene. 
     The present invention affords a simple and low cost process scheme that keeps MCH away from the bottom of EDC  102  and maintains the MCH content in the benzene product  14  in  FIG. 1  to below 200 ppm, and reduces benzene concentration in the raffinate product taken from line  9  of  FIG. 1 , substantially from 40-50 wt % to 0-5 wt % and preferably to 0-2 wt %. 
       FIG. 2  depicts an improved ED process which employs a liquid-liquid extraction column to recover additional benzene from the ED raffinate. The hydrotreated C 6  concentrate from the overhead of splitter  101  ( FIG. 1 ) is fed to the middle portion of EDC  202 , that is equipped with reboiler  220 , via lines  5 A and  16 , while lean solvent is fed to upper portion of EDC  202  via lines  15  and  18 . Raffinate vapor stream from the top of EDC ascends into the integral raffinate distillation column  203  to separate the solvent and aromatic hydrocarbon (mainly benzene) from the non-aromatic hydrocarbons. By “integral” is meant that column  203  is constructed contiguous with or built within EDC  202  and no external piping is needed to connect the fluid flow between the two units. Conditions in column  203  are adjusted so that a sump product recovering all the solvent in the raffinate, which descends into the EDC to provide an internal reflux to the top of the EDC, while the overhead vapor from column  203  is transferred via line  19  and condensed in condenser  212  before entering accumulator  214 . 
     The solvent-free raffinate stream withdrawn from  214  is cooled in cooler  216  and fed to the lower portion of liquid-liquid extraction (LLE) column  205  via line  20 . Lean solvent in line  17 , a slip stream from primary lean solvent line  15 , is cooled in cooler  210  and fed to the upper portion of LLE column  205  to counter-currently extract benzene from the raffinate to reduce its benzene concentration substantially from 40-50 wt % to 0-5 wt %, preferably 0-2 wt %. The benzene reduced raffinate product is withdrawn from line  21  as a solvent-rich stream  22  is recovered from the bottom of LLE column  205  and recycled to EDC  202  via line  16 . The ratio of lean solvent feed between the EDC and the LLE column preferably ranges from 50:1 to 150:1. 
     Because LLE column  205  can remove any amount of benzene from the raffinate stream to an extremely low level, the operation of the EDC  202  can focus primarily on driving essentially all the MCH from the bottom benzene product without concern with the loss of benzene to the raffinate. The cost of the LLE column operation is quite insignificant because weight ratio of the lean solvent between lines  15  and  17  is approximately 200:1, preferably 100:1, so the amount of solvent required by LLE column  205  is quite small. Also, the weight ratio of the hydrocarbons between line  16  (to EDC  202 ) and line  20  (to LLE column  205 ) is roughly 60:1 and preferably 40:1, so the diameter of LLE column  205  is quite small, only around 6 to 12 inches (15.2 to 30.4 cm) Furthermore, no additional energy is required for the LLE operation. 
     Optionally, the loading of upper portion of the EDC  202  can be increased by introducing an operational stripping agent to the lower portion of the EDC to prevent channeling in the upper portion of the EDC, including the integral part of column  203 , thus, increasing their separation efficiencies. Overhead condenser  212  can be operated to condense only the raffinate to allow the stripping agent uncondensed and recycled to the lower portion of the EDC through line  24  and blower  218 . The stripping agent is selected from the list comprising nitrogen, hydrogen, natural gas (methane), ethane, propane, C 4  and C 5  paraffins and combination thereof. A preferred stripping agent is natural gas and a particularly preferred stripping agent is nitrogen. Rich solvent exiting the bottom of EDC  202  is transferred to the middle portion of SRC  204  via line  23  to strip off the purified benzene product and recycle the lean solvent to EDC  202  through primary solvent line  15  via line  18 . 
     The process shown in  FIG. 2 , exhibit, among other novel features: (1) preventing MCH accumulation in the raffinate distillation column (an integral upper portion of the EDC) by eliminating the external reflux of the EDC; (2) minimizing benzene loss to the overhead raffinate by contacting the raffinate with the lean solvent from a slip stream of the solvent loop in an extraction zone counter-currently to extract benzene from the raffinate stream; (3) recycling the extract phase from the extraction zone to the EDC at one or more entry points between the solvent feed and hydrocarbon feed entry points, and withdrawing the hydrocarbon phase as the raffinate product from the extraction zone without solvent removal; (4) optionally, increasing loading of upper portion of the EDC by introducing a stripping agent to the lower portion of the EDC to prevent channeling in the upper portion of the EDC, thus, increasing its separation efficiency; and (5) optionally, adjusting the overhead condenser for condensing only the raffinate to allow the stripping agent uncondensed, and recycling the gaseous stripping agent to the lower portion of the EDC through a blower. 
       FIG. 3  depicts the improved ED process which employs a distillation column to recover additional benzene from the ED raffinate. The hydrotreated C 6  concentrate from the overhead of splitter  101  ( FIG. 1 ) is fed to the middle portion of EDC  302  via line  5 A, and lean solvent that is supplied from the bottom of SRC  104  ( FIG. 1 ) via lines  15 A (from  FIG. 1) and 38  is fed to the upper portion of EDC  302  which is equipped with reboiler  318 . Raffinate vapor stream from the top of EDC ascends into integral raffinate distillation column  303  to separate the solvent and aromatic hydrocarbon (mainly benzene) from the non-aromatics. Conditions in column  303  are adjusted so that a sump product recovering most of all the solvent in the raffinate, which descends into EDC  302  to provide an internal reflux to the top of EDC  302 . A slip stream of the lean solvent is fed to the top of column  303  via line  26  as a secondary solvent feed to assist the recovery of additional benzene from the raffinate vapor at the top of column  303 . The benzene-free (or benzene reduced) overhead vapor from column  303  is transferred via line  27  and condensed in condenser  310  before entering accumulator  312  via line  28 . 
     The raffinate equivalent to 2 to 4 wt % of the feedstock to EDC  302  is withdrawn from accumulator  312  and introduced to the middle portion of distillation column  306  via line  29  to remove the trace amount of entrained solvent from the bottoms of said column, which is circulated in a closed loop around the column bottom through lines  33  and  34 . A slip stream is withdrawn from the loop via line  35 , which is then combined with the primary lean solvent stream and fed to the top of EDC  320  via line  38 . A solvent-free raffinate vapor stream is recovered from the top of column  306 , condensed in cooler  319  and introduced to accumulator  320  via line  30 . A portion of the raffinate from accumulator  320  is recycled to the top of column  306  as the reflux through line  31 , while the other portion is withdrawn as the raffinate product via line  32 . The size of distillation column  306 , which is equipped reboiler  322 , is substantially smaller than that of the major columns  302  and  303 , so its operational costs are comparatively lower. 
     Again, because distillation column  303  functions like an EDC with a secondary solvent to recover any amount of benzene from the raffinate stream to an extremely low level, and distillation column  306 , which equipped with roboiler  322 , eliminates any entrained solvent from the raffinate product, the operation of EDC  302  can focus on driving essentially all the MCH from the bottom benzene product without concern with loss of benzene to the raffinate. 
     As an optionally, loading of tipper portion of EDC  302  can be increased by introducing a stripping agent to the lower portion of the EDC to prevent channeling in the upper portion of EDC  302  as well as the integral raffinate distillation column  303 , thus, increasing their separation efficiencies. Overhead condenser  310  can be operated to condense only the raffinate to allow the stripping agent uncondensed and recycled to the lower portion of the EDC through line  37  and blower  314 . The operational stripping agent is selected from the list comprising nitrogen, hydrogen, natural gas (methane), ethane, propane, C 4  and C 5  paraffins, and the combination thereof. A referred stripping agent is natural gas and particularly preferred stripping agent is nitrogen. Rich solvent exiting the bottom of EDC  302  is transferred to the middle portion of SRC  104  ( FIG. 1 ) via line  36  to strip off the purified benzene product and recycle the lean solvent to EDC  302  through line  15 A. 
     The process shown in  FIG. 3  exhibit, among other novel features: (2) preventing MCH accumulation in the raffinate distillation column by eliminating the external reflux to the EDC; (2) minimizing benzene loss to the overhead raffinate by feeding a secondary solvent feed to the top of the integral raffinate distillation column (at the original external reflux entry point) and by reducing the reboiler duty; (3) optionally, increasing loading of upper portion of the EDC by introducing a stripping agent to lower portion of the EDC to prevent channeling in upper portion of the EDC, thus, increasing its separation efficiency; (4) optionally, adjusting the overhead condenser for condensing only the raffinate to allow the stripping agent uncondensed, and recycling the gaseous stripping agent to lower portion of the EDC through a blower; (5) feeding liquid raffinate (from the overhead accumulator) to a small separate distillation column and withdrawing the solvent-free raffinate product from the overhead of said distillation column and circulating the bottom stream of said distillation column in a closed loop between reboiler and the bottom of the column; and (6) withdrawing a slip stream from the bottom loop of said distillation column and recycling said stream to the EDC with the solvent feed. 
     EXAMPLES 
     The following examples further illustrate different aspects and embodiments of the invention and are not to be considered as limiting the scope of the invention. 
     Example 1 
     To demonstrate effectiveness the of N-formyl morpholine (NFM) as a selective solvent for extracting benzene from overhead stream of the raffinate distillation column  203  (which an integrated upper part of EDC  202 ) shown in  FIG. 2 , extraction experiment was conducted in laboratory using a glass separatory funnel. A sample of the overhead raffinate stream was analyzed in a gas chromatography/mass spectrometer (GC/MS) analyzer and the composition of the stream is summarized in Table 1. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Composition of Overhead Raffinate from Column 203/EDC 202 
               
             
          
           
               
                 Component 
                 Wt % 
                 Component 
                 Wt % 
               
               
                   
               
             
          
           
               
                 i-Pentane 
                 0.49 
                 n-Pentane 
                 1.75 
               
               
                 Cyclopentane 
                 31.79 
                 2-Methyl Pentane 
                 0.38 
               
               
                 3-Methyl Pentane 
                 0.23 
                 n-Hexane 
                 0.94 
               
               
                 Methylcyclopentane 
                 4.33 
                 Benzene 
                 45.11 
               
               
                 Cyclohexane 
                 6.06 
                 2-Methyl Hexane 
                 0.18 
               
               
                 Dimethylcyclopentanes 
                 1.43 
                 3-Methyl Hexane 
                 0.26 
               
               
                 n-Heptane 
                 1.72 
                 Methylcyclohexane 
                 4.07 
               
               
                 Ethylcyclopentane 
                 0.90 
                 Trimethyl Pentanes 
                 0.30 
               
               
                   
               
             
          
         
       
     
     The analysis demonstrated that the overhead raffinate stream, besides containing benzene, is extremely rich in cyclic paraffins, including cyclopentane, cyclohexane, and MCH. The NFM solvent for this test was obtained from the lean solvent of a commercial ED plant wherein NFM was used as the selective solvent for recovering high purity benzene from hydrotreated C 6  fraction of coal tar. This NFM solvent contained more than 97 wt % NFM and less than 3 wt % benzene with essentially no other impurities. 
     Approximately 360.0 grams of the NFM solvent and 180.0 grams of the raffinate with composition shown in Table 1, under a solvent-to-feed weight ratio (S/F) of 2:1, were added to a separatory funnel and well mixed to create two liquid phases after settling at room temperature. The liquid phases were then separated and samples of both the raffinate (hydrocarbon) phase and the extract (solvent) phase were analyzed to yield the compositions in Tables 2 and 3, respectively: 
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Composition of Raffinate Phase from Extraction 
               
               
                 with 2.0 of Solvent-to-Feed Ratio 
               
             
          
           
               
                 Component 
                 Wt % 
                 Wt %* 
                 Component 
                 Wt % 
                 Wt %* 
               
               
                   
               
             
          
           
               
                 i-Pentane 
                 0.72 
                 0.73 
                 n-Pentane 
                 2.62 
                 2.67 
               
               
                 Cyclopentane 
                 40.67 
                 41.44 
                 2-Methyl Pentane 
                 0.63 
                 0.64 
               
               
                 3-Methyl Pentane 
                 0.37 
                 0.38 
                 n-Hexane 
                 1.59 
                 1.62 
               
               
                 Methylcyclopentane 
                 6.37 
                 6.49 
                 Benzene 
                 21.77 
                 22.18 
               
               
                 Cyclohexane 
                 8.77 
                 8.94 
                 2-Methyl Hexane 
                 0.33 
                 0.34 
               
               
                 Dimethylcyclo- 
                 1.55 
                 1.58 
                 3-Methyl Hexane 
                 0.47 
                 0.48 
               
               
                 pentanes 
                   
                   
                   
                   
                   
               
               
                 3-Ethyl Pentane 
                 0.32 
                 0.33 
                 n-Heptane 
                 3.16 
                 3.22 
               
               
                 Methylcyclohexane 
                 6.55 
                 6.67 
                 Dimehtylhexanes 
                 0.25 
                 0.25 
               
               
                 Ethylcyclopentane 
                 1.46 
                 1.49 
                 Trimethyl 
                 0.21 
                 0.21 
               
               
                   
                   
                   
                 Pentanes 
                   
                   
               
               
                 NFM (Solvent) 
                 1.90 
               
               
                   
               
               
                 *Composition on a solvent-free basis 
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Composition of Extract Phase from Extraction 
               
               
                 with 2.0 Solvent-to-Feed ratio 
               
             
          
           
               
                 Component 
                 Wt % 
                 Wt %* 
                 Component 
                 Wt % 
                 Wt %* 
               
               
                   
               
             
          
           
               
                 i-Pentane 
                 0.14 
                 0.28 
                 n-Pentane 
                 0.51 
                 1.04 
               
               
                 Cyclopentane 
                 12.03 
                 24.42 
                 2-Methyl Pentane 
                 0.10 
                 0.20 
               
               
                 3-Methyl Pentane 
                 0.06 
                 0.12 
                 n-Hexane 
                 0.25 
                 0.51 
               
               
                 Methylcyclopentane 
                 1.45 
                 2.94 
                 Benzene 
                 29.90 
                 60.70 
               
               
                 Cyclohexane 
                 2.08 
                 4.22 
                 Dimethylcyclo- 
                 0.27 
                 0.55 
               
               
                   
                   
                   
                 pentanes 
                   
                   
               
               
                 3-Methyl Hexane 
                 0.19 
                 0.39 
                 n-Heptane 
                 0.40 
                 0.81 
               
               
                 Methylcyclohexane 
                 1.23 
                 2.50 
                 Ethylcyclo- 
                 0.27 
                 0.55 
               
               
                   
                   
                   
                 pentane 
                   
                   
               
               
                 NFM (Solvent) 
                 50.78 
               
               
                   
               
               
                 *Composition on a solvent-free basis 
               
             
          
         
       
     
     Based on the experimental data from a one-theoretical stage extraction under a S/F of 2.0 as presented in Tables 2 and 3, the separation factor (SF) between MCH and benzene (BZ) on solvent-free phase compositions can be as high as 7.3 according to the following calculation: 
                   SF   =       ⁢             [       (     MCH   ⁢           ⁢   in   ⁢           ⁢   Raffinate     )     /     (     MCH   ⁢           ⁢   in   ⁢           ⁢   Extract     )       ]     /               [       (     BZ   ⁢           ⁢   in   ⁢           ⁢   Raffinate     )     /     (     BZ   ⁢           ⁢   in   ⁢           ⁢   Extract     )       ]                       =       ⁢       [       (   6.67   )     /     (   2.50   )       ]     /     [       (   22.18   )     /     (   60.70   )       ]                   =       ⁢   7.3               
This result suggests that NFM solvent is very effective in extracting benzene from the EDC overhead raffinate stream, in which the non-aromatic hydrocarbons are mainly cycloparaffins, including MCH, which have higher polarities than other types of non-aromatic hydrocarbons.
 
     Example 2 
     To test the effect of the solvent-to-feed ratio on the separation factor for separating benzene and cycloparaffins, especially MCH, the lab experimental procedures of Example 1 were repeated under S/F of 1.0, 4.0, 5.0, and 6.0. The test results are summarized in Tables 4 to 9. 
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Composition of Raffinate Phase from Extraction 
               
               
                 with 1.0 of Solvent-to-Feed Ratio 
               
             
          
           
               
                 Component 
                 Wt % 
                 Wt %* 
                 Component 
                 Wt % 
                 Wt %* 
               
               
                   
               
             
          
           
               
                 i-Pentane 
                 0.60 
                 0.62 
                 n-Pentane 
                 2.19 
                 2.27 
               
               
                 Cyclopentane 
                 36.79 
                 38.08 
                 2-Methyl Pentane 
                 0.51 
                 0.53 
               
               
                 3-Methyl Pentane 
                 0.31 
                 0.32 
                 n-Hexane 
                 1.29 
                 1.34 
               
               
                 Methylcyclopentane 
                 5.46 
                 5.65 
                 Benzene 
                 29.92 
                 30.97 
               
               
                 Cyclohexane 
                 7.58 
                 7.85 
                 2-Methyl Hexane 
                 0.26 
                 0.27 
               
               
                 Dimethylcyclo- 
                 1.26 
                 1.30 
                 3-Methyl Hexane 
                 0.46 
                 0.48 
               
               
                 pentanes 
                   
                   
                   
                   
                   
               
               
                 3-Ethyl Pentane 
                 0.25 
                 0.26 
                 n-Heptane 
                 2.47 
                 2.56 
               
               
                 Methylcyclohexane 
                 5.43 
                 5.62 
                 Dimehtylhexanes 
                 0.19 
                 0.20 
               
               
                 Ethylcyclopentane 
                 1.20 
                 1.24 
                 Trimethyl 
                 0.23 
                 0.24 
               
               
                   
                   
                   
                 Pentanes 
                   
                   
               
               
                 NFM (Solvent) 
                 3.37 
               
               
                   
               
               
                 *Composition on a solvent-free basis 
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Composition of Extract Phase from Extraction 
               
               
                 with 1.0 of Solvent-to-Feed Ratio 
               
             
          
           
               
                 Component 
                 Wt % 
                 Wt %* 
                 Component 
                 Wt % 
                 Wt % 
               
               
                   
               
             
          
           
               
                 i-Pentane 
                 0.17 
                 0.28 
                 n-Pentane 
                 0.62 
                 1.01 
               
               
                 Cyclopentane 
                 14.43 
                 23.59 
                 2-Methyl Pentane 
                 0.13 
                 0.22 
               
               
                 3-Methyl Pentane 
                 0.08 
                 0.13 
                 n-Hexane 
                 0.32 
                 0.52 
               
               
                 Methylcyclopentane 
                 1.77 
                 2.89 
                 Benzene 
                 37.86 
                 61.90 
               
               
                 Cyclohexane 
                 2.54 
                 4.15 
                 Dimethylcyclo- 
                 0.34 
                 0.56 
               
               
                   
                   
                   
                 pentanes 
                   
                   
               
               
                 3-Methyl Hexane 
                 0.22 
                 0.36 
                 n-Heptane 
                 0.52 
                 0.85 
               
               
                 Methylcyclohexane 
                 1.53 
                 2.50 
                 Ethylcyclopentane 
                 0.33 
                 0.54 
               
               
                 NFM (Solvent) 
                 38.85 
               
               
                   
               
               
                 *Composition on a solvent-free basis 
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 6 
               
             
             
               
                   
               
               
                 Composition of Raffinate Phase from Extraction 
               
               
                 with 4.0 of Solvent-to-Feed Ratio 
               
             
          
           
               
                   
                   
                   
                   
                   
                 Wt 
               
               
                 Component 
                 Wt % 
                 Wt %* 
                 Component 
                 Wt % 
                 %* 
               
               
                   
               
             
          
           
               
                 i-Pentane 
                 0.84 
                 0.85 
                 n-Pentane 
                 3.08 
                 3.12 
               
               
                 Cyclopentane 
                 41.97 
                 42.52 
                 2-Methyl Pentane 
                 0.79 
                 0.80 
               
               
                 3-Methyl Pentane 
                 0.46 
                 0.47 
                 n-Hexane 
                 2.02 
                 2.05 
               
               
                 Methylcyclopentane 
                 7.18 
                 7.27 
                 Benzene 
                 14.59 
                 14.78 
               
               
                 Cyclohexane 
                 9.757 
                 9.88 
                 2-Methyl Hexane 
                 0.44 
                 0.45 
               
               
                 Dimethylcyclo- 
                 1.89 
                 1.91 
                 3-Methyl Hexane 
                 0.44 
                 0.45 
               
               
                 pentanes 
                   
                   
                   
                   
                   
               
               
                 3-Ethyl Pentane 
                 0.42 
                 0.43 
                 n-Heptane 
                 4.27 
                 4.33 
               
               
                 Methylcyclohexane 
                 7.86 
                 7.96 
                 Dimehtylhexanes 
                 0.32 
                 0.32 
               
               
                 Ethylcyclopentane 
                 1.76 
                 1.78 
                 Trimethyl 
                 0.29 
                 0.29 
               
               
                   
                   
                   
                 Pentanes 
                   
                   
               
               
                 NFM (Solvent) 
                 1.26 
               
               
                   
               
               
                 *Composition on a solvent-free basis 
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 7 
               
             
             
               
                   
               
               
                 Composition of Extract Phase from Extraction 
               
               
                 with 4.0 of Solvent-to-Feed Ratio 
               
             
          
           
               
                 Component 
                 Wt % 
                 Wt %* 
                 Component 
                 Wt % 
                 Wt % 
               
               
                   
               
             
          
           
               
                 i-Pentane 
                 0.13 
                 0.33 
                 n-Pentane 
                 0.48 
                 1.22 
               
               
                 Cyclopentane 
                 10.71 
                 27.29 
                 2-Methyl Pentane 
                 0.10 
                 0.25 
               
               
                 3-Methyl Pentane 
                 0.06 
                 0.15 
                 n-Hexane 
                 0.25 
                 0.64 
               
               
                 Methylcyclopentane 
                 1.36 
                 3.47 
                 Benzene 
                 21.68 
                 55.24 
               
               
                 Cyclohexane 
                 1.93 
                 6.85 
                 Dimethylcyclo- 
                 0.17 
                 0.43 
               
               
                   
                   
                   
                 pentanes 
                   
                   
               
               
                 3-Methyl Hexane 
                 0.16 
                 0.41 
                 n-Heptane 
                 0.41 
                 1.04 
               
               
                 Methylcyclohexane 
                 1.19 
                 3.03 
                 Ethylcyclopentane 
                 0.26 
                 0.66 
               
               
                 NFM (Solvent) 
                 60.75 
               
               
                   
               
               
                 *Composition on a solvent-free basis 
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 8 
               
             
             
               
                   
               
               
                 Composition of Raffinate Phase from Extraction 
               
               
                 with 5.0 of Solvent-to-Feed Ratio 
               
             
          
           
               
                   
                   
                   
                   
                   
                 Wt 
               
               
                 Component 
                 Wt % 
                 Wt %* 
                 Component 
                 Wt % 
                 %* 
               
               
                   
               
             
          
           
               
                 i-Pentane 
                 0.82 
                 0.83 
                 n-Pentane 
                 3.07 
                 3.10 
               
               
                 Cyclopentane 
                 41.89 
                 42.31 
                 2-Methyl Pentane 
                 0.85 
                 0.86 
               
               
                 3-Methyl Pentane 
                 0.50 
                 0.51 
                 n-Hexane 
                 2.21 
                 2.23 
               
               
                 Methylcyclopentane 
                 7.47 
                 7.54 
                 Benzene 
                 12.11 
                 12.23 
               
               
                 Cyclohexane 
                 10.17 
                 10.27 
                 2-Methyl Hexane 
                 0.50 
                 0.51 
               
               
                 Dimethylcyclo- 
                 2.09 
                 2.11 
                 3-Methyl Hexane 
                 0.43 
                 0.43 
               
               
                 pentanes 
                   
                   
                   
                   
                   
               
               
                 3-Ethyl Pentane 
                 0.48 
                 0.48 
                 n-Heptane 
                 4.93 
                 4.33 
               
               
                 Methylcyclohexane 
                 8.58 
                 8.67 
                 Dimethylhexanes 
                 0.35 
                 0.35 
               
               
                 Ethylcyclopentane 
                 1.92 
                 1.94 
                 Trimethyl Pentanes 
                 0.22 
                 0.22 
               
               
                 NFM (Solvent) 
                 1.01 
               
               
                   
               
               
                 *Composition on a solvent-free basis 
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 9 
               
             
             
               
                   
               
               
                 Composition of Extract Phase from Extraction with 
               
               
                 5.0 of Solvent-to-Feed Ratio 
               
             
          
           
               
                 Component 
                 Wt % 
                 Wt %* 
                 Component 
                 Wt % 
                 Wt % 
               
               
                   
               
             
          
           
               
                 i-Pentane 
                 0.12 
                 0.35 
                 n-Pentane 
                 0.42 
                 1.21 
               
               
                 Cyclopentane 
                 9.51 
                 27.39 
                 2-Methyl Pentane 
                 0.10 
                 0.29 
               
               
                 3-Methyl Pentane 
                 0.06 
                 0.17 
                 n-Hexane 
                 0.23 
                 0.66 
               
               
                 Methylcyclopentane 
                 1.25 
                 3.60 
                 Benzene 
                 18.63 
                 53.65 
               
               
                 Cyclohexane 
                 1.79 
                 5.16 
                 Dimethylcyclo- 
                 0.16 
                 0.46 
               
               
                   
                   
                   
                 pentanes 
                   
                   
               
               
                 3-Ethylpentane 
                 0.18 
                 0.52 
                 n-Heptane 
                 0.39 
                 1.12 
               
               
                 Methylcyclohexane 
                 1.13 
                 3.25 
                 Ethylcyclopentane 
                 0.25 
                 0.72 
               
               
                 NFM (Solvent) 
                 65.28 
               
               
                   
               
               
                 *Composition on a solvent-free basis 
               
             
          
         
       
     
     From the experimental results for the raffinate and extract phase compositions that were derived under different solvent-to-feed ratios, the separation factors between MCH and benzene were next calculated in accordance with the previously defined SF formula. The results are summarized in Table 10. 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 10 
               
             
             
               
                   
               
               
                 Separation Factor Between MCH and Benzene versus S/F of Extraction 
               
             
          
           
               
                   
                 S/F (solvent-to-feed weight ratio) 
                 SF (separation factor) 
               
               
                   
               
             
          
           
               
                   
                 1.0 
                 4.49 
               
               
                   
                 2.0 
                 7.30 
               
               
                   
                 4.0 
                 9.82 
               
               
                   
                 5.0 
                 11.70 
               
               
                   
               
             
          
         
       
     
     As expected, a higher solvent-to-feed ratio yields a higher the separation factor but the incremental increase diminishes at higher S/F ratios. Another important observation from this investigation is that a clean phase separation can be achieved between the raffinate and the extract phases even under a high solvent-to-feed weight ratio (S/F=5.0), although the non-aromatic components of the hydrocarbon feed, which are primarily the more polar cycloparaffins, tend to dissolve into the solvent phase and cause difficulty in phase separation. Nevertheless, under an S/F of 6.0 or higher, it was found that no phase separation could be formed since the hydrocarbon feed mixture was totally dissolved in the solvent phase. 
     Example 3 
     The test results from Examples 1 and 2 were incorporated into a simulation model of the process illustrated in  FIG. 2  and employed to design an improved process for recovering high purity benzene with acceptable an MCH content from a hydrotreated C 6  fraction of coal tar or other similar feedstock that features various aspects of the invention. Specifically, referring to  FIG. 2 , a NFM solvent feed that is at temperature in the range of 90 to 110° C. is fed to the upper portion of EDC  202  via lines  15  and  18  at a rate of approximately 21,600 kg/hr, while roughly 3,700 kg/hr of a hydrotreated C 6  fraction from the top of splitter column  201  is introduced to the middle portion of EDC  202  through lines  5 A and  16  at temperature in the range of 70 to 90° C. Reboiler  220  of EDC  202  is operated at a temperature range of 130 to 150° C. to drive substantially all of the MCH from the bottom rich solvent, which consists of essentially pure benzene and the solvent, and exits the bottom of EDC  202  via line  23  at a rate of approximately 25,500 kg/hr. Raffinate vapor from the top of column  203  which contains excess amounts of aromatics that consists mainly benzene (about 48 wt %) is condensed through condenser  212  and transferred to accumulator  214  at a rate 110 kg/hr at a temperature in the range of 70 to 80° C. via line  19 . Column  203  is an integral part of the upper distillation portion of EDC  202 . 
     A raffinate stream which withdrawn from the bottom of accumulator  214  is further cooled to approximately 40° C. in cooler  216  and fed to the lower portion of LLE column  210 . A slip stream of 225 kg/hr from the lean solvent feed via line  17  is cooled through cooler  210  to 40° C. and fed to the upper portion of LLE column  205  to counter-currently extract aromatics (mainly benzene) from the raffinate stream. A raffinate product via line  21  is withdrawn at a rate in the range of 50 to 60 kg/hr from the top of column  205  and it contains 1.0 to 2.0 wt % benzene with no more than 0.5 wt % solvent. An extract stream from column  205  containing roughly 80 wt % solvent, 20 wt % aromatics (mainly benzene) and less than 1 wt % non-aromatics is taken from the column bottom through line  22  at a flow rate in the range of 270 to 290 kg/hr. This stream is then combined with the EDC hydrocarbon feed from line  5 A ( FIG. 1 ) and fed to the middle portion of EDC  202  via line  16 . The composition of the feed stream in line  16  contains 93 to 95 wt % aromatics (benzene), 1 to 2 wt % non-aromatics, and about 5 wt % solvent. 
     This example demonstrates that with the improved process 1 wt % of the lean solvent (225 kg/hr) is sufficient to extract benzene in the raffinate stream and to reduce its benzene concentration from as high as 50 wt % to less than 2 wt % even when using in a very small extraction column that is 8 inches (20.3 cm) in diameter with no extra energy requirements. Benzene that is recovered in the extract phase is recycled back to EDC  202 . 
     The configuration of this low-cost raffinate extraction column affords a number of process benefits. First, the HDS unit  100  ( FIG. 1 ) can operate under conditions to maximize desulfurization and denitrogenation of the hydrocarbon feed without impeded by the undesirable generation of MCH from the non-selective hydrogenation of toluene; consequently, the HDS requires lower catalyst activity and fewer catalyst replacements. Second, EDC  202  ( FIG. 2 ) can operate with fewer restrictions in order drive essentially all the MCH into the overhead raffinate stream and away from the bottom product, benzene. Moreover, excess amounts of benzene in the raffinate can be readily recovered and recycled by low-cost LLE column  205 . 
     Example 4 
     Sulfolane is another important extraction solvent for recovering aromatic hydrocarbons from reformate and pyrolysis gasoline, although it is not commonly used in the ED process for benzene recovery from the hydrotreated C 6  fraction of coal tar. To demonstrate the effectiveness of sulfolane for extracting benzene from the overhead stream of the raffinate distillation column  203  (which is an integral upper part of EDC  202 ) shown in  FIG. 2 , an extraction experiment was conducted in a laboratory separatory funnel which is equivalent to one-theoretical extraction stage. 
     The sulfolane solvent for this test was obtained from the lean solvent of a commercial extractor for recovering benzene, toluene, and xylenes from pyrolysis gasoline. This solvent contained 98.5 wt % sulfolane (analyzed on water-free basis) and less than 0.8 wt % water. Approximately 300.0 grams of the sulfolane solvent and 75.0 grams of the raffinate with composition shown in Table 1, under a solvent-to-feed weight ratio (S/F) of 4:1, was added to the separatory funnel and well mixed to create two liquid phases after settling at room temperature. The liquid phases were then separated and samples of both the raffinate (hydrocarbon) phase and the extract (solvent) phase were analyzed to yield the compositions in Tables 11 and 12, respectively: 
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 11 
               
             
             
               
                   
               
               
                 Composition of Raffinate Phase Extracted with 
               
               
                 the Sulfolane Solvent Under S/F of 4.0 
               
             
          
           
               
                 Component 
                 Wt % 
                 Wt %* 
                 Component 
                 Wt % 
                 Wt %* 
               
               
                   
               
             
          
           
               
                 i-Pentane 
                 0.71 
                 0.71 
                 n-Pentane 
                 2.60 
                 2.61 
               
               
                 Cyclopentane 
                 42.39 
                 42.60 
                 2-Methyl Pentane 
                 0.62 
                 0.62 
               
               
                 3-Methyl Pentane 
                 0.38 
                 0.38 
                 n-Hexane 
                 1.59 
                 1.60 
               
               
                 Methylcyclopentane 
                 6.65 
                 6.68 
                 Benzene 
                 16.50 
                 16.58 
               
               
                 Cyclohexane 
                 9.23 
                 9.28 
                 2-Methyl Hexane 
                 0.32 
                 0.32 
               
               
                 Dimethylcyclo- 
                 1.59 
                 1.60 
                 3-Methyl Hexane 
                 0.50 
                 0.50 
               
               
                 pentanes 
                   
                   
                   
                   
                   
               
               
                 3-Ethyl Pentane 
                 0.32 
                 0.32 
                 n-Heptane 
                 3.13 
                 3.15 
               
               
                 Methylcyclohexane 
                 6.83 
                 6.86 
                 Dimehtylhexanes 
                 0.26 
                 0.26 
               
               
                 Ethylcyclopentane 
                 1.52 
                 1.52 
                 Trimethyl 
                 0.16 
                 0.16 
               
               
                   
                   
                   
                 Pentanes 
                   
                   
               
               
                 Sulfolane (Solvent) 
                 0.46 
                   
                 Unknown 
                   
                 4.25 
               
               
                   
               
               
                 *Composition on a solvent-free basis 
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 12 
               
             
             
               
                   
               
               
                 Composition of Extract Phase Extracted with 
               
               
                 the Sulfolane Solvent Under S/F of 4.0 
               
             
          
           
               
                 Component 
                 Wt % 
                 Wt %* 
                 Component 
                 Wt % 
                 Wt % 
               
               
                   
               
             
          
           
               
                 i-Pentane 
                 0.03 
                 0.13 
                 n-Pentane 
                 0.12 
                 0.53 
               
               
                 Cyclopentane 
                 4.05 
                 18.01 
                 2-Methyl Pentane 
                 0.00 
                 0.00 
               
               
                 3-Methyl Pentane 
                 0.01 
                 0.04 
                 n-Hexane 
                 0.05 
                 0.22 
               
               
                 Methylcyclopentane 
                 0.42 
                 1.87 
                 Benzene 
                 16.11 
                 71.63 
               
               
                 Cyclohexane 
                 0.63 
                 2.80 
                 Dimethylcyclo- 
                 0.06 
                 0.27 
               
               
                   
                   
                   
                 pentanes 
                   
                   
               
               
                 3-Ethylpentane 
                 0.08 
                 0.36 
                 n-Heptane 
                 0.08 
                 0.36 
               
               
                 Methylcyclohexane 
                 0.32 
                 1.42 
                 Ethylcyclopentane 
                 0.07 
                 0.31 
               
               
                 Sulfolane (Solvent) 
                 77.51 
                   
                 Unknown 
                   
                 2.05 
               
               
                   
               
               
                 *Composition on a solvent-free basis 
               
             
          
         
       
     
     From the compositions of the raffinate and extract phases shown in Tables 11 and 12, the separation factor (SF) between MCH and benzene under the extraction of sulfolane (with 0.8 wt % water) is determined to be 20.87. From the separation factor at a 4.0 solvent-to-feed weight ratio, it appears that the sulfolane solvent is more selective than the NFM solvent in recovering benzene from the mixture containing benzene and the cycloparaffins, including MCH. 
     The boiling point of sulfolane is significantly higher that of the NFM solvent. Stream stripping is normally used to separate the aromatic (benzene) product from the sulfolane solvent, in order to minimize the reboiler temperature in the solvent recovery column (SRC). An extraction process using the sulfolane solvent should be equipped with water circulation system to supply the stripping steam in SRC and to maintain the water content in the sulfolane solvent. 
     The foregoing has described the principles, preferred embodiments and modes of operation of the present invention. However, the invention should not be construed as being limited to the particular embodiments discussed. Thus, the above-described embodiments should be regarded as illustrative rather than restrictive, and it should be appreciated that variations may be made in those embodiments by workers skilled in the art without departing from the scope of the present invention as defined by the following claims.