Abstract:
A continuous online process for rejuvenating whole stream of contaminated lean sulfolane in an extraction system is provided. A rejuvenator is installed in the solvent: circulation loop to remove the contaminants continuously to keep the solvent clean, effective and less corrosive. The rejuvenator includes a high pressure vessel with a removable cover and a round rack with vertical stainless steel tubes fitted in the high pressure vessel. A magnetic bar is placed in each stainless steel tube. A screen cylinder is installed outside the ring of stainless steel tubes. As the contaminated sulfolane is passed through the rejuvenator, the rejuvenator picks up contaminants. The rejuvenator can be dissembled to remove the contaminants periodically. The rejuvenator is simple in construction, reliable in operation, and low in operation and maintenance costs. With this rejuvenator, the extraction system operates at high efficiency and high capacity without the dreaded corrosion.

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. patent application Ser. No. 13/345,359 that was filed on Jan. 6, 2012, now U.S. Pat. No. 8,506,820, which is a divisional application of U.S. patent application Ser. No. 12/112,623 that was filed on Apr. 30, 2008, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a process for on line rejuvenation of contaminated sulfolane solvent in an aromatic extraction system continuously to maintain the solvent quality at the desired level. More particularly, it relates to a process employing a novel, rejuvenator in the circulation loop to remove the sulfolane degradation and corrosion products, including acids, polymers and small solid particles of iron and sand to maintain the solvent at a high quality level, and in turn, a high operation efficiency and a low corrosion extraction system. Moreover, by removing these contaminants, the rate of sulfolane degradation is greatly reduced, leading to minimum consumption of sulfolane. The present invention is characterized by high efficiency for contaminants removal, simple construction, and low operation and maintenance costs. 
     BACKGROUND OF THE INVENTION 
     Sulfolane, C 4 H 8 O 2 S is also known as tetrahydrothiophene-1,1-dioxide and tetramethylene sulfone. Sulfolane is used as an extraction and a reaction solvent. It is used to separate aromatic hydrocarbons (benzene, toluene and xylenes) from non-aromatics, such as paraffins and naphthenes. It is used to separate n-propyl alcohol and see-butyl alcohol. It is used to purify natural gas streams and for fractionation of fatty acids into saturated and unsaturated components. 
     A sulfolane unit is usually incorporated within an aromatics complex to recover high-purity benzene and toluene products from reformate and pyrolysis gasoline. In a modern aromatics complex, the sulfolane unit is located downstream of the reformate splitter column. 
     In sulfolane extraction column, aromatics are dissolved in sulfolane and this mixture is the extract phase (bottom), while the remaining non-aromatics counter-currently contacted and efficiency of stage can be improved by mixing with rotating baffles such as rotating disk column, RDC. 
     Sulfolane is recovered from raffinate first in a coalescer and then in a water wash column. The final raffinate is pumped to Naphtha Blending as gasoline components. 
     The extract is stripped and enriched in aromatics after recontacting with additional sulfolane. The solvent is recovered under vacuum distillation from the extract; while the extract is separated from water by cooling and clay treated to eliminate olefins and diolefins, and sent to Aromatics Fractionation. Unit to separate benzene, toluene, 
     In the sulfolane extraction system, there are often severe corrosion problems. The locations most often corroded by contaminated sulfolane include the rich solvent line to the stripper, extractive stripper column, reboilers, transfer lines and solvent recovery column. As the corrosion rate increases, pinholes can develop, leading to safety and hazard concerns. 
     The root cause of the corrosion is mostly the accumulation of acidic material in the circulating solvent. The contaminated sulfolane turns from colorless to black, becomes more viscous and gooey, acidic (to a pH level as low as 4) and corrosive. In general the following four items are seen as the main causes for corrosion/erosion problems in the sulfolane extraction unit.
         Oxygen in the plant (ingress or via incoming flows)   Chlorine in circulating solvent   Accumulation of degradation and corrosion products in the plant   High temperature in reboilers       

     The degradation and corrosion products are gooey, gummy, tacky, smelly, and difficult and unpleasant to handle. Over the years, degradation and corrosion products will have accumulated in the plant. These degradation and corrosion products will deposit in the system to plug, the fillers, carbon adsorption bed to increase the pressure drop and most importantly to reduce the heat transfer coefficient of the heat exchangers, leading to poor operation efficiency. In addition, the dirt accumulated in the plant is usually corrosive material and/or act as a kind of catalyst to make more degradation products. An expedient way to control corrosion is to neutralize the acid by addition of monoethanol amine (MEA), leading to additional contamination of the sulfolane solvent. The experience from the operation indicates that one in a While a very thorough cleaning of the, plant is required. In a commercial sulfolane extraction unit, about 10-15 m 3  of dirt was taken out of the unit. It is indeed, a hard job to clean the sulfolane extraction unit thoroughly. 
     There are many ways to keep the sulfolane in the circulation unit clean, including:
         1) A filter bed or cartridge is placed in the lean sulfolane transfer line to catch the degradation and corrosion products. However, the filter or cartridge can be overcome by the large quantity of the degradation and corrosion products in short time. Most of the commercial units are equipped with such filters but the filters are mostly bypassed because of operational difficulties.   2) Twin adsorption towers filled with adsorbent such as activated carbon to remove the degradation and corrosion products. Commercial experience indicates that such adsorption tower is impractical because it can be plugged up quickly and it is difficult to clean and change the adsorbent.   3) Reclamation of sulfolane: A small split stream of sulfolane laden with the degradation and corrosion products is withdrawn from the circulation system and charged into a vacuum distillation unit. The overhead is clean sulfolane for recycling to the circulation system, while the bottom is the degradation and corrosion products. Which are disposed as waste. The operation is inefficient because only a small fraction of the solvent in the system is reclaimed. In addition, there is significant amount of hazardous waste generated for disposal.   4) Use of cationic and/anionic resins to remove the acidic contaminants, including sulfonic acid, and small amount of carboxylic acid and hydrochloric acid. U.S. Pat. No. 5,053,137 by Lal and Bhat taught purification or regeneration of contaminated or spent process sulfolanes by passing a split stream of it through two columns in series. The first column is :filled with cationic exchanger while the second column is filled with anionic exchanger. Liu and Li showed that the acid in the degradation and corrosion products could be effectively ion exchanged with cationic resin [Liu lingchi and Li Dong-sheng, Petrochemical Design, 2002, 19(2) 31-33], leading to cleaner sulfolane solvent. The acid laden cationic resin is regenerated with NaOH solution. In the process, only a split stream is ion exchanged, so that the efficacy in improving sulfolane quality is limited. The process is messy and produces a great quantity of hazardous waste for disposal.       

     To control the corrosion rate, the acid can be neutralized by use of amines, such as monoethanol amine (MEA). However, the degradation and corrosion products remain in the circulation system to foul up the heat exchangers and to cause plugging and increase in pressure drop. In addition, the accumulated degradation and corrosion products catalyze the degradation of sulfolane due to autocatalysis and accelerate the corrosion and accumulation of degradation and corrosion products leading to a vicious cycle. Thus, a significant amount of purge is required to keep the quality of sulfolane at an acceptable level. 
     The prior art on controlling sulfolane corrosion and rejuvenating sulfolane in the circulation loop suffers from a number of disadvantages including:
         1) High corrosion rate of the system because the accumulated degradation and corrosion products catalyze the degradation of sulfolane and accelerate the corrosion and accumulation of degradation and corrosion products, leading to a vicious cycle.   2) Accelerated rate of accumulation of the degradation and corrosion products due to their autocatalysis.   3) Tedious, dangerous and expensive operation in replacing the filters and/or adsorbents.   4) Decreased operation capacity and efficiency when the quality of sulfolane is not maintained at or higher than the desired level.   5) Inefficient in rejuvenation of sulfolane solvent, due to split stream operation.   6) High operation and maintenance costs.       

     Clearly, there is a need in the industry for an efficient, safe and economic method to rejuvenate the contaminated sulfolane to operate the aromatic extraction system safely, efficiently with high capacity, and low operation and maintenance costs. 
     SUMMARY OF THE INVENTION 
     The present invention provides a continuous process for rejuvenating contaminated sulfolane in the circulation loop of an aromatic extraction system to a quality substantially the same as the fresh sulfolane. In particular, the process employs a novel rejuvenator to rejuvenate the contaminated sulfolane solvent by removing the degradation and corrosion products from the total stream in the circuit to keep the capacity and operation efficiency of the aromatic extraction system high. 
     A novel rejuvenator for removing the degradation and corrosion products is placed in the contaminated sulfolane solvent system to treat the total stream in circulation. The preferred locations for the rejuvenator is in the lean sulfolane section., and the coolest location after the cooler and before entering the extraction column is most preferred. Two rejuvenators are placed in parallel so that the rejuvenator online can be switched over to the spare rejuvenator for discharging the collected degradation and corrosion products. Specifically, the rejuvenator comprises a high pressure vessel with a removable cover, and a round rack with vertical stainless steel tubes fitted. In the high pressure vessel. A magnetic bar is placed in each stainless steel tube. A ring of stainless steel tubes is installed: inside the screen cylinder. The rejuvenator picks up the contaminants as the contaminated sari:thine is passed through the rejuvenator. The rejuvenator can he dissembled for removing, the contaminants periodically. 
     As the contaminated sulfolane enters the rejuvenator, the degradation and corrosion products are attracted and adhere to the vertical stainless steel tubes filled with powerful magnetic bars, and removed from the solvent system. The removal of the degradation a id corrosion products by the magnetic bar is enhanced by the presence of the inner screen cylinder. This enhancement becomes crucial when the level of residual degradation and corrosion products is to be kept low. Through the inner screen cylinder, the rejuvenated, clean sulfolane leaves the rejuvenator and is recycled back to the extraction column. After on stream for a certain period, say two weeks, the rejuvenator is loaded with the degradation and corrosion products and the pressure drop across the rejuvenator increases. The stream is switched to the spare rejuvenator and the degradation and corrosion products loaded rejuvenator is opened to remove the round tray along with the magnetic bars. The round tray with the magnetic bar in the vertical tube is placed in a container and the magnet is removed from the tube. Upon removal of the magnetic bars, the degradation and corrosion products fail from the vertical due to removal of the magnetic bars and loss of the attractive force. The system is characterized by high efficiency for contaminants removal, simple construction, and low operation and maintenance costs. 
     Accordingly, several objectives and advantages of the present invention are as follows:
         1) Low corrosion rate due to removal of corrosive acids, the degradation and corrosion products.   2) Low accumulation of the degradation and corrosion products due to removal of the autocatalysts.)   3) No tedious, dangerous and expensive operation to clean the system and replace the filters and/or adsorbents.   4) :Stable operation Capacity and efficiency by maintaining the quality of sulfolane at nearly the fresh level.   5) Efficient rejuvenation of sulfolane solvent due to whole stream rejuvenation of sulfolane in the circulation circuit.   6) Low operation and maintenance costs.       

     Further objectives and advantages will become apparent from a consideration of the ensuing description and drawing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view showing the vertical section of the rejuvenator in a preferred embodiment according to the present invention. 
         FIG. 2  is a schematic view showing the structural decomposition of the rejuvenator in a preferred embodiment according to the present invention. 
         FIG. 3  is a schematic view showing the 3-3 section in the  FIG. 1 . 
         FIG. 4  is a schematic flow diagram of an extraction system containing the rejuvenator of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In accordance with the present invention, the whole stream of the contaminated lean sulfolane solvent is passed continuously through the rejuvenator of the present invention to remove acids, and the degradation and corrosion products to maintain the sulfolane solvent quality essentially the same as the fresh one. The novel rejuvenator is the heart of the present invention. The installation a id operation of the rejuvenator are critical as well. 
     The rejuvenator 
     The rejuvenator is the heart of the present invention. The invention and design of the rejuvenator is based on the discovery of the nature of the degradation and corrosion products. The accumulated degradation and corrosion products are generally considered to be:
         1. The acid produced from oxidation of sulfolane, such as sulfonic acid and sulfuric acids   2. Acid from chlorine such as hydrochloric acid   3. Oligomers of sulfolane produced from oxidative condensation of sulfolane and its degradation products       

     These products are acidic, low in pH black, gooey and tend to deposit in the system, such as the lines, filters and heat exchangers, causing losses in operation capacity and efficiency. These products are hard to remove from the solvent circuit and the process equipment. Removing these products from the solvent system and cleaning the system equipment are the subjects of research in the petroleum and petrochemical industries throughout the world. 
     In our research, we discovered that the gooey product contains fine but gritty black particles. Further characterization of the products revealed that these products are paramagnetic in nature and can adhere to a magnet. We were surprised that, by use of a magnet, these products were removed and the contaminated sulfolane solvent became clear and its property restored. It is believed that, in the process operation, the equipment is corroded by the acidic byproduct and Fe ion is released. The iron ion reacts further with sulfur, oxygen and water to form fine paramagnetic particles of FeS, FeO and Fe(OH) 2 , etc. It is these line black particles that make the sulfolane solvent black. These fine paramagnetic particles are then occluded in the organic degradation products and make the whole mass of the degradation and corrosion products paramagnetic. Thus, by use of a magnet, the whole mass of degradation and corrosion products can be miraculously removed from the contaminated solvent to rejuvenate to the used solvent. Through experimentation and testing, a novel rejuvenator was designed and its efficacy was proved. 
     The schematic views of the rejuvenator are shown in  FIG. 1  to  FIG. 3 , respectively. The rejuvenator  30  comprises a high pressure vessel  37  with a removable cap  36  to withstand the pressure of the aromatic extraction system. The rejuvenator  30  is equipped with inlet  31  and outlet  32 . Inside the high pressure vessel  37 , a screen cylinder  80  is fitted. The mesh size of the screen can range from 1 to 200 (wires per inch), and 1.0 to 100 (wires per inch) is preferred. Inside screen cylinder  80 , three round racks  33 ,  34 ,  35  are spaced from the top to the bottom. To the round racks  33 ,  34 ,  35 , plural stainless tubes  40  are attached. The number of stainless steel tubes  40  can range from 2 to 30 or more. In this :embodiment, there are 12 stainless tubes attached to the round racks  33 ,  34 ,  35 . Into each of the stainless tubes  40 , magnetic bars  60  are inserted. 
     Process Configuration 
     A schematic flow diagram of a typical aromatic extraction system is shown in  FIG. 4 . Feed  11  is introduced into extractor  10 . The lean solvent  22  is introduced from the top of extractor  10 . The raffinate phase  13  is removed from the top of extractor  10  to recover non-aromatics, while the extract phase  12  is withdrawn from the bottom and charged to recovery tower  20  to recover aromatics as the overhead  21  and recover the lean solvent  22  as the bottom for recycling to extractor  10 . The lean solvent  22  is cooled with heat exchanger or air cooler  23 , and passed through the rejuvenator  30  before returning to extractor  10 . 
     It is important to make the rejuvenator  30  effective and efficient. To accomplish this objective, we found that the best way is to treat the whole stream rather than a split stream. Thus, the regenerator  30  is placed in the circulation loop to treat the whole stream. 
     To make rejuvenation truly continuous, two rejuvenators  30 A,  30 B are installed in parallel so that one is on rejuvenation while the other is on cleaning. 
     Please refer to  FIG. 1  and  FIG. 4 . The lean solvent  22  is introduced into the rejuvenator  30  through inlet  31  and flows down along the stainless tubes  40  containing the magnetic bars  60  and finally exits through outlet  32 . The screen cylinder  80  is intended to minimize the short-circuiting of the lean solvent  22  inside rejuvenator  30 . 
     As the on-stream time of rejuvenator  30 A,  30 B increases, the degradation and corrosion products continue to gather and accumulate around the stainless tubes  40  containing magnetic bars  60 . The pressure across rejuvenator  30  increases and has to be cleaned. Depending on the cleanliness of the sulfolane system, the time period for rejuvenator  30 A,  30 B cleaning can range from 2 to 30 days or longer. 
     To clean rejuvenator  30 A, lean solvent  22  is switched to the rejuvenator  30 B to continue operation. The top cap  36  is removed, and then the ring rack along with the stainless tubes  40  and magnetic bars  60  are removed from rejuvenator  30 . The surface and the spaces between the stainless tubes  40  are now full of the accumulated degradation and corrosion products. When the magnetic bars  60  are removed from the stainless tubes  40 , the accumulated degradation and corrosion products drop off cleanly by gravity. The magnetic bars  60  are put back into the stainless tubes  40 , and the round rack  33 ,  34 ,  35  is put back into rejuvenator  30 , and finally, the top cap  36  is put on to complete the cleaning procedure. Rejuvenator  30  A is now ready for service again. 
     Process Conditions 
     The rejuvenator  30  can be operated at the temperatures found in typical extraction systems. However, a too high temperature can lead to loss in magnetic field, while a too low temperature can lead to high efficiency due to increased tendency for the degradation and corrosion products to adhere to the magnets. The operation temperature can range from 10 to 200° C. and 20 to 150° C. is preferred. Thus, the preferred location of the rejuvenator  30  is after the heat exchanger  23  and before the extractor  10  in the lean solvent  22  circuit. 
     For efficient operation, the flow velocity through the rejuvenator  30  in term of superficial velocity can be 10 to 10,000 v/v/Hr and 50 to 5,000 v/v/Hr is preferred. 
     The pressure drop across the rejuvenator  30  is an indicator of the cleanliness or the remaining capacity of the rejuvenator  30 . When the pressure drop (delta P) across the rejuvenator  30  is 1 to 5 Kg Cm 2  or higher, the rejuvenator  30  should be cleaned according to the procedure described above. 
     EXAMPLES 
     Example 1 
     Two rejuvenators were constructed. The dimensions were 33 Cm in diameter and 65 Cm in height, with a volume of 55 Liter. Magnetic bars were inserted in the stainless steel tubes on the round racks. The size of the screen cylinder was 80 meshes. The rejuvenators were installed side by side in the lean sulfolane circuit below the cooler and before the extractor. 
     Example 2 
     The rejuvenators were tested in an aromatic extraction system in a refinery. This extractor had been in operation for 3 years and the lean sulfolane solvent was dirty, corrosive and full of gooey degradation and corrosion products. The extractor was 3,600 mm ID×42,000 mm T-T. The feed to the extractor is 42,531 Kg/Hr of reformate. The circulation rate of the contaminated lean sulfolane through the rejuvenator and to the extractor was 138,602 Kg/Hr. Thus, solvent to feed ratio for this test was about 3 . The rejuvenator was cleaned as the pressure drop across the rejuvenator reached 2 Kg/Cm 2 . At the start of the test, the lean sulfolane solution was so dirty, the rejuvenator has to be cleaned at the end of 1 week. However, as the lean sulfolane solution continued to he cleaned up by the rejuvenator, the periods of rejuvenator cleaning increased from 1 week to 2 weeks, to 4 weeks and 3 months. When the rack was removed from the rejuvenator, the surface and the spaces between the stainless steel tubes were full of black, gooey degradation and corrosion product. As the magnetic bars were removed, the black, gooey degradation and corrosion products fell off, and the stainless tube became clean for reuse. 
     The samples before and 3 months after installation of the rejuvenator were characterized to show the efficacy of the, rejuvenator of the present invention. The properties characterized were:
         1) The quantity of the residue left from distilling the lean sulfolane solvent to 288° C. Since sulfolane boils at 285° C., the residue must he the degradation and corrosion product. Thus, the lower the residue content, the cleaner the lean sulfolane solvent.   2) The color of the lean sulfolane solution: Since the degradation and corrosion products are highly colored, the lighter the color, the cleaner the sulfolane solution,   3) The rate of amine (monoethanol amine): Since acid is generated in the process of sulfolane degradation, the lower the addition rate of amine to maintain a constant pH, say 8, the cleaner the sulfolane solution.   4) The rate of sulfolane make-up rate: Since sulfolane degrades in the process, certain amount of lean sulfolane solvent has to he purged off to maintain the quality of the sulfolane quality. Therefore, fresh sulfolane has to be added to make-up the volume. Thus, the lower the fresh sulfolane make-up rate, the cleaner the sulfolane solution.       

     The test results are shown in Table 1. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Before 
                 3 months of 
               
               
                   
                 Sample 
                 Rejuvenation 
                 rejuvenation 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 1. Quantity of residue, % 
                 1.0 
                 0.4 
               
               
                   
                 2. Color, Number 
                 3 
                 0.5 
               
               
                   
                 3. MEA addition, Kg/Y 
                 200 
                 50 
               
               
                   
                 4. Sulfolane make up, Kg/Mo 
                 1000 
                 50 
               
               
                   
                   
               
             
          
         
       
     
     The quantities of residue based on ASTM method 86 were 1.0 and 0.4% for the samples before and after 3 months of rejuvenation, respectively. It was noted that the extraction system was very dirty with a large quantity of degradation and corrosion products. Thus, the sample taken for this test was not representative and the real residue content would have been much higher than the 1% shown. hi contrast, the extraction system after 3 months of rejuvenation was surprisingly clean and the residue content of 0.4% was a good representation of the lean sulfolane in the system. 
     The colors of the samples before and after 3 months of rejuvenation were strikingly different. This observation is consistent with the results of the color number based on ASTM D1500 of 3 and 0.5. 
     The above results clearly show that the rejuvenator of the present invention is effective in rejuvenating the contaminated or spent sulfolane solution. 
     Those described above are the preferred embodiments to exemplify the present invention. However, it is not intended to limit the scope of the present invention. Any equivalent modification or variation according to the spirit of the present invention is to be also included within the scope of the present invention.