METHOD OF EXOTHERMICALLY AND ENDOTHERMICALLY REACTING

Abstract: Disclosed is a method of reacting a fluid that undergoes an exothermic reaction followed by an endothermic reaction or an endothermic reaction followed by an exothermic reaction. A container is divided into an endothermic compartment and an exothermic compartment by a heat transferring surface (6) and the two compartments are connected so that the fluid flows over one side of the surface (6), where either the endothermic or exothermic reaction occurs, and then over the opposite side of the surface (6), where the exothermic or endothermic occurs, then leaves the reactor. Heat from the exothermic reaction is used to promote the endothermic reaction.

Description: METHOD OF EXOTHERMICALLY AND ENDOTHERMICALLY REACTING Background of the Invention This invention relates to a method of exothermically and endothermically reacting a fluid, where heat from the exothermic reaction is transferred to the endothermic reaction. In particular, the invention relates to a method of making ethylene by exothermically reacting ethane and chlorine gases to form ethyl chloride and hydrogen chloride and using the heat from that reaction to endothermically dissociate the ethyl chloride into ethylene and hydrogen chloride. The production of ethylene and vinyl chloride from ethane and chlorine proceeds by way of two reactions in series. In the first exothermic reaction, ethane and chlorine react to produce ethyl chloride and vinyl chloride as well as a hydrogen chloride by-product: C 2 H 6 + Cl 2 > C 2 H 5 C1 + HC1 C 2 H 6 + 3C1 2 > C 2 H 3 C1 + 3HC1 In the second endothermic reaction, the ethyl chloride disassociates to form ethylene and hydrogen chloride: C 2 H 5 C1 > C 2 H 4 + HC1 (Subsequently, vinyl chloride is separated from the ethylene and hydrogen chloride. Oxygen is added to the ethylene and hydrogen chloride to make ethylene dichloride, and the ethylene dichloride is cracked to make more vinyl chloride.) The ethane-chlorine reaction has been performed in a single-pass tubular reactor. As ethane and chlorine flow through the inside of the tube, hot (about 1000°C) flue gas passes over the outside of the tube counter-currently. The flue gas provides the heat required to initiate the exothermic reaction and to maintain the desired overall reaction temperature. With the single-pass counter-current arrangement, considerable amounts of heat generated in the exothermic reaction are lost to the vented flue gas. In addition, serious coking occurs inside the tube where the exothermic reaction is initiated. Periodic shutdowns of the reactor are required in order to clean and remove the coke buildup. Because of the high temperatures and the extremely corrosive gases involved, a metal reactor tube lined with ceramic sleeve is used. However, the very hot flue gas warps the metal tube which cracks the ceramic liner. This results in the hydrogen chloride by-product penetrating the ceramic liner and attacking the metal reactor tube from the inside, eventually rupturing the metal tube. In addition, as the reactor tube temperature increases, the exothermic reaction tends to initiate prematurely before the reactants enter the tube. This results in the formation of coke upstream of the reactor, again requiring the shutdown of the process for cleaning. Summary of the Invention We have invented a method of reacting a fluid that undergoes an exothermic reaction followed by an endothermic reaction or an endothermic reaction followed by an exothermic reaction. In our method, the heat from the exothermic reaction is effectively utilized to sustain the endothermic reaction. This is accomplished by passing the reactants over both sides of a heat conducting surface in sequence. The heat generated by the exothermic reaction passes through the heat conducting surface and promotes the endothermic reaction on the opposite side of the heat conducting surface. In a particularly preferred embodiment of this invention, the reactant mixture passes through the inside of the tube then up over the outside of the tube, and the reaction heat is conducted through the tube surface. The tube can be cooled around the area where the reactants enter the tube to prevent any reaction from occurring prematurely and, because of the design of this reactor, scaling up the reactor to handle larger quantities of reactants can be easily accomplished by adding more tubes to the reactor. We have found that the preferred method of this invention, when used to make ethylene and vinyl chloride from ethane and chlorine, has very low coking, no premature initiation of the exothermic reaction, and does not crack or warp the tubes. We are able to use a lower flue gas temperature (900 to 950°C) since less heat is wasted in heating the flue gas as most of the heat from the exothermic reaction is used to promote the endothermic reaction. Surprisingly, we have found that the percent conversion of ethyl chloride to ethylene increased from about 24% using the single-pass tubular reactor to about 48% in the reaction method of this invention. (For both operations, an ethane/chlorine molar ratio of 2:1, a flue gas temperature of 900°C, a total throughput of 500 pounds/hour, and a reactor pressure of 85 psig was employed.) Brief Description of the Drawings Figure 1 is a side view in section showing a certain presently preferred embodiment of a reactor according to this invention. Figure 2 is an enlargement of a portion of Figure 1 showing the end of the tubes. Figure 3 is an enlargement of a portion of Figure 1 showing how the outer tube is held in position. Description of the Preferred Embodiments In Figure 1, reactor 1 consists of flue gas section 2, tube bundles 3, product outlet chamber 4, cooling chamber 5, process inlet/reaction tubes 6, and process gas inlet head 7. Flue gas section 2 of reactor 1 is comprised of a large diameter pipe or rolled plate 8 which is split in half and is flanged along the horizontal seams to form upper case section 9 and lower case section 10. Flue gas inlet nozzle 11 is installed in lower case section 10 and flue gas outlet nozzle 12 is installed in upper case section 9. Flanges 13 are installed on the flue gas inlet end of upper and lower case sections 9 and 10, and metal end cover 14 is provided to seal this end of the unit. For heat conservation, castable refractory 15 is applied along the inner walls of upper and lower case sections 9 and 10 and to inner wall of end cover 14. Referring especially to Figures 2 and 3, tube bundles 3 are welded to tube sheet 16. Tube bundles 3 are formed from metal outer tubes 17, which have fins 18 to enhance heat transfer. Into each metal outer tube 17 is inserted a silicon carbide inner tube 19. Silicon carbide inner tubes 19 are open-ended on one end and are closed on the other end and have a groove 20 around the circumference of the tube at the open end. These tubes are held in place by packing assembly 21. In Figure 3, packing assembly 21 consists of an inner split ring 22, which fits into groove 20 on silicon carbide inner tube 19. Keeper ring 23 holds split ring 22 in position and aligns silicon carbide inner tube 19 within metal outer tube 17. Grafoil gaskets 24 and 25, when compressed, seal the packing assembly to top plate 26, which is the upper half of packing assembly 21. Packing assembly 21 is bolted together to form the seal between tube sheet 16, metal outer tube 17, and tube bundle 3. Referring to Figure 2, graphite powder 27 is packed into the void between the inner wall of metal outer tube 17 and the outer wall of silicon carbide inner tube 19, and acts as the heat transfer media. Grafoil packing rings 28 are installed around the circumference of silicon carbide inner tubes 19 to provide a seal between silicon carbide inner tubes 19 and metal outer tubes 17. Metal plugs 29 are welded into the ends of metal outer tubes 17 to seal the tubes and hold grafoil packing rings 28 in position. Referring again to Figure 1, metal baffle plates 30 are placed in strategic positions along the length of tube bundles 3. Metal baffle plates 30 direct the flow of the flue gas across tube bundles 3 in flue gas section 2 of the reactor vessel 1. Grafoil gasket material 31 is placed around the circumference of tube sheet 16. For the reactor assembly, tube bundles 3 are placed into the lower case section 10 of flue gas section 2. The upper case section 9 of flue gas section 2 is bolted into position to form the entire flue gas section 2 and tube bundles 3 of reactor vessel 1. Product outlet chamber 4 is constructed from a large diameter pipe 32. Pipe 32 has flange 33 on one end which is bolted to tube sheet 16. Flange 34 of drilled front tube sheet 35 on the opposite end of pipe 32 is bolted to flange 36 of process inlet head 7. An intermediate tube sheet 37 is welded in position within the internal diameter of the body of pipe 32. Small diameter pipes 38 are installed between the front tube sheet 35 and intermediate tube sheet 37 and are seal welded to both tube sheets forming cooling chamber 5. Pipe couplings are welded to the lower and upper portions of cooling chamber 5 to provide water inlet port 39 and water outlet port 40. Castable refractory 41 is installed on both ends of product outlet chamber 4 for heat conservation. Product outer chamber 4 is bolted to tube sheet 16. Pipe nozzle 42 provides for product outlet. Process inlet/reaction tubes 6 are small diameter silicon carbide tubes which are inserted into front tube sheet 35 openings, pass through sealed pipes 38 of the cooling chamber, through product outlet chamber 4 and into the open ends of silicon carbide inner tubes 19 of tube bundles 3. The process inlet/reaction tubes 6 terminate near the bottom of the tube bundle inner tubes 19. Grafoil packing 43 is installed around the circumferences of process inlet/reaction tubes 6 for the length of cooling chamber 5 to seal the area between product outlet chamber 4 and the process inlet head 7. Packing gland assemblies 44, located on the front face of front tube sheet 35, hold the process inlet/reaction tubes in position and provide a seal at these areas. The process inlet head 7 is constructed from a large diameter pipe which has front tube sheet 35 on one end and has an elliptical metal head welded to the other end. Inlet nozzles 45 are installed into the body of process gas inlet head 7. Process inlet head 7 is bolted to front tube sheet 35. Steel support assemblies 46 support the reactor vessel when it is in service. In using the reactor to react ethane and chlorine, the ambient-temperature reactants enter reactor 1 through the process inlet nozzles 45, pass through process gas inlet head 7 into process inlet/reaction tubes 6. While flowing through cooling chamber 5, the reactants are cooled to prevent premature chlorination of ethane from occurring. As the reactants travel through the inlet/reaction tubes 6 they are preheated by the hot product gases in product outlet chamber 4. In tube bundles 3, heat carried by the flue gas flowing across metal outer tubes 17 is transferred through inner tubes 19 and through the walls of the process inlet/reaction tubes 6, causes an exothermic reaction of the ethane and chlorine mixture to occur within inlet/reaction tubes 6. The reactants and ethyl chloride formed continued to flow through and react within the confines of process inlet/reaction tubes 6, to the bottom of the tubes and thence into the void areas between the outer walls of the process inlet/reaction tubes 6 and the inner walls of the inner tubes 19 of tube bundles 3. As the process mixtures move out of inlet/reaction tubes 6 and into the void areas between the tubes, the heat propagated by the initial exothermic reaction, and the heat from the flow of flue gases across tube bundle 3, promotes an endothermic reaction of the ethyl chloride to form ethylene and hydrogen chloride in these sections of the reactor. The final products flow into product outlet chamber 4 and through product outlet nozzle 42 for further processing. The invention is applicable to any fluid (gas or liquid, including solutions of solids or slurries of solids) where components of the fluid first undergo an exothermic reaction and then an endothermic reaction, or an endothermic reaction and then an exothermic reaction. If the exothermic reaction occurs first, the fluid should pass through the inside of inlet/reaction tubes 6 first and then between inlet/reaction tubes 6 and inner tubes 19 in order to capture all of the heat generated by the exothermic reaction for the benefit of the endothermic reaction. However, if the endothermic reaction occurs first, the fluid should pass between inlet/reaction tubes 6 and inner tubes 19 first and then through inlet/reaction tubes 6 for the same reason. In either case, the endothermic reaction occurs between the tubes and the exothermic occurs inside the process inlet/reaction tubes 6. The invention also contemplates the addition of a reactant to the fluid to induce the occurrence of either the exothermic or the endothermic reaction. While it may not be possible to control the reactions so precisely that the interface between the endothermic and the exothermic reaction is exactly at the end of inlet/reaction tubes 6, one reaction should occur primarily within inlet/reaction tubes 6 and the other reaction should occur primarily in between the two tubes. The tubes can be cooled in cooling chamber 5 with any heat transfer medium, and water is preferred for the reaction of ethane and chlorine. The purpose of cooling is to prevent the exothermic reaction from occurring anywhere except in inlet/reaction tubes 6, especially near the ends of tubes 6, where the heat from the exothermic reaction can be used to promote the endothermic reaction. Similarly, media other than flue gas can be used as a source of heat (or cooling, if necessary) . In order to prevent back flashing from occurring inside the reactor, it is necessary for the linear velocity of the exothermic, flammable reactant system to exceed the velocity of the flame front. Fluid velocity can be increased by using inlet/reaction tubes 6 of small inside diameter. To provide a good mixing and an effective heat transfer within the tube, it is desirable to have the reactants flow in the turbulent regime, which is to have a Reynolds number of the reactants flow in excess of 2700. Knowing the amount of heat generated by the exothermic reaction and the amount of heat required by the endothermic reaction, one can calculate the amount of heat that must be provided by the flue gas, or, if the amount of heat generated by the exothermic reaction exceeds that necessary for the endothermic reaction, one can calculate the amount of cooling required by a medium used instead of the flue gas. Once the amount of heating or cooling as well as the heat transfer coefficient of the tube material are known, one can calculate the size and length of the tubes required to effect the necessary transfer of heat. Of course, the tubes should be long enough to achieve the necessary retention time required for the reaction to occur. Reactor processing volume can be increased by adding more tubes to the reactor. The materials out of which the tubes and the reactor are made will depend upon the intended reactor temperature and the compounds that are present within the reactor. For a reactor making ethylene, vinyl chloride, and hydrogen chloride from ethane and chlorine, ceramic materials are required because metals are attacked by the high-temperature chloride to form metal chlorides, which catalyze the decomposition of the hydrocarbons, resulting in coking. Various ceramics, such as, for example, silicon carbide, tungsten carbide, and tungsten nitride, can be used, but silicon carbide is preferred as it is readily available commercially. While the drawings illustrate a certain presently preferred embodiment employing tubes, it will be appreciated by those skilled in the art that other geometries are also suitable for this invention. For example, the tubes need not be round in cross-section, but can be square or rectangular. In addition, the reactor does not have to be mounted horizontally, but rather can be operated at any angle. It is only necessary that the exothermic reaction occur on one side of a heat conducting surface and the endothermic reaction occur on the opposite side and that the reacting fluid move from one side to the other side. The following examples further illustrate this invention. Example 1 The apparatus shown in the drawings was used. The reactor tube bundle consisted of 12 reaction tube assemblies. For each tube assembly, the metal outer tube was an 8'-l 1/2" long, 2 1/2" diameter, schedule 40, stainless steel 316 finned tube. The inner tube was an 8' long, 2" outside diameter, 1 5/8" inside diameter silicon carbide tube. The process inlet/reaction tube was an 11' long, 1" outside diameter, 3/4" inside diameter silicon carbide tube. The reaction tube assemblies were equally spaced on the tube sheet. The ethane/chlorine molar ratio, the flue gas temperature and the total flow rate were varied to improve ethyl chloride cracking. The initial reaction conditions included a molar feed ratio of ethane to chlorine of 2.5:1, a flue gas temperature of 900°C, and a feed flow of 500 pounds/hour. The reactor pressure was maintained at 10 psig throughout the test. The feed ratio was first lowered from 2.5:1 to 1.6:1 at 8-hour intervals. No sign of premature reaction was observed. At a feed ratio of 1.6:1, the total flow rate was decreased 10% at 8-hour intervals. No evidence of coking was noticed upon completion of operating at 50% of the design capacity. The ethyl chloride cracking improved from 21.3% to 48.3%, as the ratio was varied from 2.5:1 to 1.6:1. The cracking was further increased to 59.6%, as the total feed rate was decreased to 50% (or the retention time was doubled) . The flue gas temperature was then raised from 900°C to 975°C at 25°C increments. By doing so, an additional 17.8% increase of ethyl chloride cracking was realized. The final adjustments were made by lowering the ethane/chlorine ratio gradually from 1.6:1 to 1.27:1. The following table summarizes the ethyl chloride cracking achieved at the various operating conditions tested. At a feed ratio of 1.27:1, a flue gas temperature of 975°C, and a turndown of 50%, a maximum ethyl chloride conversion of 84% was accomplished. The εelectivities of vinyl chloride monomer and useful products were 23% and 98+%, while no acetylene was detected. After more than 150 hours, the equipment down stream of the reactor was found to contain only a light dusting of coke. In the reactor, only minor amounts of carbon were found in the probable reaction initiation zone located 18 inches from the inlet. Flue Gas % Ethyl Ethane/Chlorine Temperature Chloride Molar Ratio % Full Flow* (°C) Cracking 2.5:1 100 900 21.3 2.3:1 100 900 26.1 2.1:1 100 900 31.5 1.95:1 100 900 34.7 1.77:1 100 900 40.8 1.6:1 100 900 48.3 1.6:1 90 900 54.7 1.6:1 80 900 52.2 1.6:1 70 900 52.8 1.6:1 60 900 57.1 1.6:1 50 900 59.6 1.6:1 50 925 65.2 1.6:1 50 950 71.9 1.6:1 50 975 77.4 1.44:1 50 975 81 1.37:1 50 975 82.7 1.27:1 50 975 83.9 *Full Flow = 500 pounds/hour Example 2 Example 1 was repeated to demonstrate the operation of the reactor at elevated pressures. At a total throughput of 500 pounds/hour and reactor pressures up to 85 psig, the premixed adiabatic reaction was successfully carried out at ethane/chlorine feed ratios as low as 1.9:1. The following table summarizes the averaged results obtained for all of the pressures at a flue gas temperature of 950°C. No signs of feed line pressure increases or premature initiation were observed. Upon examination, the system downstream of the reactor was essentially free of powdery coke after over 175 hours. In the reactor, the coke buildup near the reaction initiation zone was insignificant and was easily removed by hydroblasting. Example 3 Tests were made to compare the performance of the single- pass tubular reactor and the reactor of this invention. The selected reaction conditions included an ethane/chlorine molar ratio of 2:1, a flue gas temperature of 900°C, a total throughput of 500 pounds/hour, and a reactor pressure of 85 psig. The single-pass tubular reactor consisted of 80 Incoloy tubes (3/4" in diameter) , and each was lined with a 1/2" outside diameter silicon carbide tube. Graphite powder was used to fill the annulus between the metal and the silicon carbide tubes. The tube bundle was secured into castable-lined headers. The reactor of this invention is shown in the drawings, and detailed dimensions are given in Example 1. After the tests, it was found that the percent conversion of ethyl chloride to ethylene increased from about 24% in the single-pass tubular reactor to about 48% in the reactor of this invention. After more than 120 hours, the reactors were examined by borescope. A ring of coke less than 1/8" in thickness was discovered 22 inches from the reactor entrance and extending approximately 1/4" in all the single-pass tubular reactor tubes. On the other hand, only insignificant amounts of coke were found 18" from the inlet in the reactor of this invention.

Claims: WE CLAIM : 1. A method of exothermically reacting and endothermically reacting a fluid comprising performing one of said reactions on one side of a thermally conductive surface, transporting said fluid to the opposite side of said thermally conductive surface, and performing the other of said reactions on said opposite side. 2. A method according to Claim 1 wherein said thermally conductive surface is a tube. 3. A method according to Claim 2 wherein said fluid is contained around said tube by a second tube. 4. A method according to Claim 1 wherein said surface is ceramic. 5. A method according to Claim 2 wherein said exothermic reaction occurs before said endothermic reaction and said fluid is transported through the inside of said tube and then over the outside of said tube. 6. A method according to Claim 2 wherein said endothermic reaction occurs before said exothermic reaction and said fluid is transported over the outside of said tube and then through the inside of said tube. 7. A method according to Claim 1 wherein said fluid comprises a mixture of ethane and chlorine gases. 8. A method according to Claim 7 wherein said surface is a tube and said mixture reacts exothermically inside said tube and endothermically on the outside of said tube. 9. A method of exothermically reacting then endothermically reacting, or endothermically reacting then exothermically reacting, the components of a fluid, comprising performing said exothermic reaction as said fluid passes through the inside of a heat conducting tube and performing said endothermic reaction as said fluid passes over the outside of said tube. 10. A method according to Claim 9 wherein said exothermic reaction occurs before said endothermic reaction. 11. A method according to Claim 9 wherein said endothermic reaction occurs before said exothermic reaction. 12. A method according to Claim 9 wherein said tube is contained within a second tube. 13. A method according to Claim 9 wherein said tube is made of ceramic. 14. A method according to Claim 9 wherein said fluid comprises a mixture of ethane and chlorine gases. , 1 15. A method of making ethylene comprising exothermically 2 reacting a first mixture which comprises ethane and 3 chlorine gases as said first mixture moves through the 4 inside a multiplicity of heat conducting tubes, whereby 5 a second mixture is formed which comprises ethyl 6 chloride, and endothermically reacting said ethyl 7 chloride to form said ethylene as said second mixture 8 moves over the outside of said multiplicity of heat 9 conducting tubes in the opposite direction of said first 10 mixture. 16. A method according to Claim 15 wherein said tubes are ceramic. 17. A method according to Claim 16 wherein said tubes are made of silicon carbide. 18. A method according to Claim 15 wherein said first mixture has a Reynolds number in excess of 2700. 19. A method according to Claim 15 wherein each of said tubes is enclosed within a second tube. 20. A method according to Claim 19 wherein a hot gas is passed over the outside of said second tubes.

Inventors: ACHEE GERALD F, CHEN HANG-CHANG BOBBY, ACHEE, GERALD, F., CHEN, HANG-CHANG, BOBBY
Applicants: OCCIDENTAL CHEM CO, OCCIDENTAL CHEMICAL CORPORATION
Application year: 1995
Main classification: B01J12
IPC codes: B01J12, B01J19, C07C1, C07C11