IMPROVED PROCESS FOR REMOVAL AND RECOVERY OF VINYL CHLORIDE MONOMER FROM VENT GAS STREAM IN POLYVINYL CHLORIDE PLANT

Abstract: In polyvinyl chloride production plants air enters the process during production operations and must be vented to the atmosphere. The air becomes mixed with gaseous vinyl chloride monomer during the process and not all of the vinyl chloride monomer can be condensed therefrom as liquid in the normal monomer recovery operation. Consequently the vent gas stream contains appreciable proportions of vinyl chloride monomer along with air, and in many cases is saturated with watervapor. In order to remove and recover the vinyl chloride monomer from the vent gas before it is released to the atmosphere, the vent gas stream is treated counter-currently with a stream of liquid solvent which absorbs the vinyl chloride monomer and the vinyl chloride monomer is then stripped from the solvent by application of heat and condensed as liquid vinyl chloride while recycling the lean solvent to the absorption step. The solvent used as the absorbent is a branched chain saturated aliphatic hydrocarbon containing 8 carbon atoms and composed predominantely of 2,2,4-trimethyl pentane. This solvent enables the vinyl chloride content of the gases vented to the atmosphere to be reduced to less than 5 and even less than 1 part per million by volume. Its use assures that any solvent vapors emitted with the vent gas are controllable so as to be not harmful to the environment. Its use also assures that the normal steam supply at the plant will provide the heat necessary to strip the absorbed vinyl chloride monomer from the solvent.

Description: IMPROVED PROCESS FOR REMOVAL AND RECOVERY OF VINYL CHLORIDE MONOMER FROM VENT GAS STREAM IN POLYVINYL CHLORIDE PLANT BACKGROUND OF THE INVENTION - PRIOR ART Adverse human health effects from exposure to vapors of vinyl chloride monomer (VCM) have made it necessary that essentially no VCM be released from equipment used in VCM and polyvinyl chloride (PVC) pro- duction plants to the workplace environment or the outside atmosphere. Regulations were promulgated in 1974 by the United States Occupational Safety and Health Administration severely limiting the amount of VCM to which workers can be exposed in the workplace. These regulations have been met by the industry through the development and application of improved technological means for confining VCM inside production equipment and removing it from PVC as it leaves the production plant. At about the same time the United States Environmental Protection Agency (EPA) began development of a regulation restricting the release of VCM from VCM and PVC production plants to the outside environment. By late 1975 this regulatory effort had resulted in public distribution of a document, EPA-450/2-75-009 October, 1975, entitled "Standard Support and Environ¬ mental Impact Statement: Emission Standard For Vinyl Chloride", in which was described, in respect of PVC plants, the processes and equipment used in production of PVC from VCM by suspension polymerization, dispersion polymerization, and bulk polymerization; the sources of VCM emissions from the processes and equipment in each case, and the means which had then been used or which were proposed to be used for limiting VCM emissions from the various sources. The disclosures of this EPA document are incorporated herein by reference as repre¬ senting the best available prior art. In October, 1976, EPA promulgated regulations requiring that within two years, or by October, 1978, all United States PVC production plants limit VCM emissions such that, among other things, gas streams vented from emission sources in the flow of materials through the plant (except those "downstream" of the operation in which VCM is stripped from the PVC polymer after its formation in the polymerization reactors) contain no more than 10 parts per million (ppm) of VCM by volume. The* 1975 EPA document referred to indicates that the then state-of-the-art was such that technology to achieve this result could be developed within the alloted time through the use of "add-on" type control systems, such as sorbers (carbon adsorbers and solvent absorbers) refrigeration systems, or incinerators, or a combination thereof. The prior use of solvent absorbers and the particular solvents which had then been used are de¬ scribed in general terms in the EPA document. The EPA document also contains a description of a process in which vent. gases from a low temperature (-46° or -50°F) VCM condensing system are fed into the bottom of an adsorption column and passed upward countercurrent to liquid ethylene dichloride (EDO as solvent. The un- absorbed gases are cooled in a refrigerated vent conden¬ ser to remove solvent before being vented from the system. The EDC solvent with dissolved VCM is sent to a distillation column where VCM is stripped from the solvent and returned to the low temperature condensing system while the stripped solvent is cooled and returned to the absorption column for reuse. This system as operated prior to 1975 removed VCM from the vent gas to a level of 15 ppm but EPA in the document expresses the opinion that a solvent absorption unit of modern engin- eering design could meet the 1978 requirement of below 10 ppm. SUMMARY OF THE INVENTION This invention provides an improved solvent absorption/stripping process for removal and recovery of VCM from a gas stream in a PVC production plant where it is present with air prior to venting of the gas stream to the atmosphere. The process uses as the solvent for the absorption a branched chain saturated aliphatic hydrocarbon containing 8 carbon atoms and composed pre¬ dominately of 2,2,4-trimethyl pentane commonly called isooctane. Use of this solvent enables the VCM content of the vent gas stream to be reduced to well below 5 ppm by volume and even to as low as less than 1 ppm by volume, which is a substantially greater reduction than is required to meet the U.S. National Emission Standard for VCM. The process using this solvent does not re¬ quire the use of expensive refrigeration equipment to condense VCM from the gases prior to absorption, or to cool the gases after absorption to remove solvent prior to their venting. This solvent also is particularly adaptable to being stripped of the absorbed VCM at temperature and pressures compatible with use as the heat source for the stripping operation of ordinary plant steam. DESCRIPTION OF THE INVENTION The invention will be described through a detailed explanation of the vent gas streams in PVC plants used as the feed stream for the solvent absorption/ stripping operation and from which the VCM is removed and recovered; the system used in the solvent absorption/ stripping operation; the nature and characteristics of the particular solvent used and the conditions under which the solvent absorption/stripping system is operated. Feed Stream - Vent Gas Streams In PVC Plants: In the production of PVC by suspension polymerization or dis- persion polymerization, VCM is polymerized in an aqueous medium in a closed batch reactor to form an aqueous slurry or latex which is discharged from the reactor to a second vessel called a "stripper" where unreacted VCM is removed. The cycle starts with a clean empty reactor which is full of air. The amount of water required for the polymerization reaction is added to the reactor and a steam jet or vacuum pump is used to remove some of the air, the amount of air left depending on the absolute pressure after evacuation. The VCM in liquid form and other chemicals required for the polymerization are added to the closed reactor and the polymerization reaction occurs, at the completion of which the batch is discharged to the stripper. The VCM gas left in the reactor after the polymerization is removed from the batch by vacuum and transferred to a monomer recovery system. The VCM gas removed by the stripping operation is also recovered in a monomer recovery system. The gas stream going to the monomer recovery system contains the air which was initially left in the reactor plus air present in the stripper when the batch is transferred. Further, if the reactor is opened for cleaning, which is periodically required, the air left in the clean reactor in the next cycle also ends up in the monomer recovery system. All this air builds up in the monomer recovery system and not being condensable must be vented from the system to prevent pressure buildup. The monomer recovery system consists of compressors and refrigerated condensers to remove some of the VCM from the air stream before it is vented. The ratio of air to VCM in the stream is determined by temperature and pressure at the exit of the condenser. The effectiveness of the system to reduce the VCM con¬ tent of the vent gas increases with decreasing tempera- ture and increasing pressure. Reduction of VCM content is limited by high costs associated with cooling and OMPI .. W WilPFOO condensing the gases and also by freezing if water is present in the gas, which is, of course, the case since water is the medium for the polymerization reaction in the suspension and dispersion polymerization process. Accordingly usual design pressure and temperature of the monomer recovery system are not greater than 4 atmos¬ pheres or lower than 45°F. Under such conditions in the monomer recovery system, the gases leaving the system contain a ratio of VCM to inerts (the term "inerts" is used to include the components of air, i.e. oxygen, nitrogen and other gases, as well as water vapor) in the range of about 1:1 to 4:1. The stream containing the gases in this ratio is the vent gas stream to which the solvent absorption/stripping process of this invention is normally applied. It is possible, however, under certain specif¬ ic situations in PVC production plants and with specific VCM recovery systems that the vent gas stream be less rich in VCM. For example, if water is removed upstream of the vent condenser (as by absorption in methanol, or with certain streams from plants producing PVC by the bulk process, the conditions in the monomer recovery system are not limited by the possibility of water freezing and the vent gas stream of the condenser can contain a lower ratio of VCM to inerts than 1:1. In no event, however, is is possible to reduce the VCM content of the gas stream by refrigeration alone to as low as 10 ppm. It is.noted that gas streams off equipment used in the dispersion and suspension polymerization processes, when both are practiced at the same PVC plant, or from various other VCM sources such as VCM handling operations , containing varying ratios of VCM to inerts , may be combined and the combined stream will serve as the feed stream for the solvent absorption/ stripping process of this invention. S olvent Ab sorption/Stripping System - The system used in the solvent stripping process of this invention is outlined in the accompanying drawing in which the sole figure represents a simplified flow diagram. A vent gas stream from PVC production opera¬ tions as described above containing the VCM and the components of air is fed through line 10 to the bottom section of a packed absorber column, 11, while-a stream of solvent is fed through line 12 to the top of column 11. The solvent passes downward in the column counter- current to the vent gas stream flowing upward in the column, where the solvent absorbes the VCM in the gas stream and the air is vented overhead through line 13. The solvent containing absorbed VCM flows from the bottom of the column through line 14 to a heat exchanger where its temperature is elevated by the heat exchange medium and then flows through line 16 to a stripping column having a packed stripping section 17a and a packed rectification section 17b. The VCM-rich solvent from line 16 is fed to the top of section 17a where it is further heated and vaporized as it flows down the column by means of heat supplied through a reboiler 18 heated with steam. The heat strips the absorbed VCM from the solvent in section 17a converting it to VCM vapor which passes through rectification section 17b and overhead through line 19 where it is condensed to liquid in an overhead condenser and a portion of the liquid VCM returned for reflux through line 20 and the remainder goes to storage and reuse. The lean solvent from the bottom of stripper column section 17a is fed through line 21 back to heat exchanger 15 where it is cooled and flows through line 22 to a chiller 23 for further cooling before reentering the absorber through line 12 as previously described, so that the path of the solvent through the absorber/ stripper forms a complete "figure 8". Obviously pumps are used to circulate the solvent through the system but for simplification these are not shown on the drawing. Solvent for start up and for make-up if required, can be introduced into line 21, 22 or 12 through an additional line also not shown. Other modifications and variations in the system arrangement to meet particular conditions but without affecting the above described "figure 8" configuration will be apparent to engineers skilled in the art. Solvent Used In Absorption/Stripping - In accordance with this invention and as an essential feature thereof, the solvent which is used in operation of the system described is preferably 2,2,4-trimethyl pentane, common¬ ly called isooctane, which has a boiling point at at os- pheric pressure (760 mm of mercury) of 210-211°F. Another solvent which works equally as well is supplied by Phillips Petroleum Co. as "Soltrol 10" which has a boiling range of 203-218°F at atmospheric pressure, and which consists of a mixture of C-8 branched chain satur- ated aliphatic hydrocarbons with the predominant con¬ stituent being 2,2,4-trimethyl pentane. Either of these solvents is used in the absorption/stripping process described not because of any greater absorbing affinity for VCM than many other solvents but rather because of the excellent results secured by its use for the purpose intended in typical PVC plant operations, as will be indicated in the examples below. Conditions For Operation Of Absorption/Stripping System - The absorption/stripping system described, as de- signed for using the solvent described, can be operated over a wide range of temperatures and pressures and can handle feed streams with a wide range of VCM concen¬ tration flowing at rates well in excess of 250 pounds per hour, up to as high as 900 or even 1800 pounds per hour. It is estimated that in production plants of large capacity (designed to produce 150 mm pounds per year, or more, of PVC by the suspension process) where air vent rates normally exceed 250 pounds per hour, the absorption/stripping system of this invention may be easily operated in such a manner as to recover enough VCM monomer to pay for the cost of installing the absorption/stripping system in a period of about 1 year. The preferred operating conditions are those which do not require extensive refrigeration at any stage. Thus it is preferred that the system temperature be no lower than about 40-50°F, particularly if water is present in the vent gas, although the system can be designed to operate at lower temperatures. The highest temperature which can be used in the system is dependent on the steam pressure which is the source of heat to generate the reboil vapor in the stripper. Steam is typically available in PVC production plants at a pres¬ sure of 150 psig at a temperature of 366°F and the temperature in the stripping column should not be de¬ signed to operate above that temperature. The system can be designed at any convenient pressure. It is preferred to design the absorber to operate at a pressure below, but not necessarily more than about 1/6 to 1/3 below, the pressure at which the feed stream can be delivered to the absorber. It is also preferred to design the stripper at a high enough operating pressure to be able to return the recovered VCM for reuse. In operation of the absorber with a feed stream containing more VCM than air and with essentially complete removal of VCM from the feed stream, it is apparent that there is a stage at which the range of VCM concentration in air is from 3 to 33% by volume, which is the explosive range. However, the volume of gas in the explosive range is relatively small and any combus- tion hazard theoretically present is eliminated by the "heat sink" provided by the solvent in the absorber. _0 P EXAMPLES EXAMPLE I In this Example vent streams from VCM recovery sections of a plant producing PVC by both the suspension polymerization and dispersion polymerization processes are combined in a feed tank and the combined stream fed to an absorber/stripper system arranged as indicated in the drawing. The feed through line 10 is at a pressure of 55 psig and a temperature of 40°F and consists of VCM flowing at the rate of 620 pounds/hour mixed with air saturated with water vapor flowing at a rate of 280 pounds/hour for a total feed stream of 900 pounds/hour. The absorber column 11 is 18 inches in diameter and 20.5 feet high. The packing in the column is one-half inch "intalox" saddles. Commercial isooctane (2,2,4-trimethyl pentane) flows into the top of column 11 at a tempera¬ ture of 50°F, a pressure of 35 psig, and a rate of 5978 pounds/hou . The isooctane containing absorbed VCM leaving the absorber column through line 14 is at a temperature of 76°F and a pressure of 37 psig flowing at a rate of 6615 pounds/hour, including VCM at a rate of 620 pounds/ hour, as in the feed stream, air at a rate of 23 pounds/ hour and isooctane at a rate of 5972 pounds/hour. The vent stream leaving the top of the column through line 13 at a temperature of 51°F and 35 psig is essentially free of VCM, containing less than 1 ppm VCM by volume, and consists of 257 pounds/hour of inerts and only 7 pounds/ hour of isooctane vapors from the solvent. VCM rich isooctane in line 14 described above is pumped by a pump (not shown in the drawing) , in¬ creasing its pressure to 61 psig, to heat exchanger 15 to produce in line 16 a stream of liquid/vapor at a temperature of 235°F and a pressure of 41 psig. This stream is fed to a stripping column 17a, 17b which is a total of 50 feet high with stripping section 17a 30 inches in diameter and rectification section 17b 20 inches in diameter. VCM vapor leaving the rectification section 17b is at a temperature of 74°F and a pressure of 40 psig. Liquid VCM is returned to the rectification section 17b to provide a reflux ratio in the range of 1:1 to 3:1. Liquid VCM is recovered from the system at the rate of about 600 pounds/hour. The condensed VCM contains water present in the vent gas which is decanted and separated at intervals. Air through the system is recycled with uncondensed VCM. The solvent in stripping section 17a is heated in the reboiler 18 by steam at 150 psig and 366°F. The lean solvent from the stripping column in line 21 has a temperature of 310°F and a pressure of 42 psig as it goes to heat exchanger 15; in • line 22, after leaving the heat exchanger, it is cooled to 126°F and is at a pressure of 25 psig and in line 12 after leaving chiller 23 it is at a temperature of 50°F and a pressure of 35 psig and flows at essentially the rate above described, there being only 7 pounds/hour of isooctane lost from the system with the vent gas stream. This amount can of course be replenished with fresh solvent makeup as desired. In operation of the system as described, flow rates may vary up to 1/2 to 2 times those specifically set forth, but this does not affect the efficent opera¬ tion described. This example shows that with the use of the solvent described VCM monomer is reduced in the vent stream going to the atmosphere to below 1 ppm and that the VCM is recovered for reuse in essentially quanti¬ tative manner.- EXAMPLE II In this example the feed vent gas stream from another PVC plant contains approximately 86% VCM and 14% inerts. The solvent used in the absorption/stripping system is again commercial isooctane. The vent gas off the absorber in the absorption/stripping operation contains less than 5 ppm when the stripping column is operated at either the high pressure of 40 psig or the low pressure of 5.4 psig. In both cases the VCM is recovered for reuse in substantially quantitative amounts. EXAMPI-E III In this example representing a special plant situation, the vent gas feed to the absorber is free of water and is introduced to the absorber at a temperature of -25°F and a pressure of 2 psig while commercial isooctane is introduced into the absorber at 50°F and a pressure of 0.1 psig. Here again the vent gas contains less than 5 ppm VCM and the VCM absorbed in the isσ- ctane is recovered in substantially quantitative amounts. ADVANTAGES OF THE INVENTION The examples given describe highly successful operation of the process of this invention under diff- erent conditions and in different PVC production plants. These operations began less than 1 year before the date of this application and have continued until its date. During that time the VCM in the vent gas released to the atmosphere has never been above 5 ppm. The use of 2,2,4-trimethyl pentane (isooctane) as the solvent used in the process has enabled the stripping column to operate efficiently with 150 psig, 366°F reboiler steam. The amount of solvent vapor passing out of the system with the air vent after treatment has presented no problems from the environmental standpoint., but if it were desired to prevent this hydrocarbon in the amounts indicated from entering the atmosphere, this could be readily accomplished by burning it as fuel. In contrast the use of other solvents in absorption/stripping systems, or the use of other means, to reduce the VCM content of vent gas streams as by incineration or use of carbon adsorbers, is not as satisfactory as is the practice of this invention. If hydrocarbon solvents boiling higher than isooctane, such as kerosene or heavy petroleum oils or the like, or even hydrocarbons which boil only slightly higher than iso¬ octane such as normal octane or meta-xylene, are used for absorption/stripping as described the stripper cannot be supplied with sufficient heat by the use of a reboiler operating on 150 psig - 366°F steam to handle the potential load. On the other hand, if a lower boiling solvent is used a greater proportion of the solvent will be lost from the system with the treated vent gas and this is quite disadvantageous in case the solvent is an aromatic hydrocarbon, benzene for in- stance, because of the known toxicity of benzene, and in the case of a chlorinated solvent, EDC for instance, not only because of its possible toxicity but also because its incineration produces corrosive hydrogen chloride. . The use of other "add on" type systems for reducing the VCM content of the vent gas stream such as incineration or adsorption on carbon, is also disad¬ vantageous as compared with the solvent absorption/ stripping system of this invention. Incineration of the vent gas stream precludes recovery of VCM and produces such a quantity of corrosive hydrogen chloride that it must be scrubbed with alkali to convert it to salt. In- cincerators also have other disadvantages and their use in commercial plants is justified only if the vent stream is small and constant, or for use as the emer- gency backup for absorption or adsorption devices. The use of carbon adsorbers is disadvantageous because of their high capital cost and also because of the greater risk of explosion of a mixture of VCM in air as it passes through the explosive range. In summary, it is believed that the absorp¬ tion/stripping process described herein, using the solvent described herein, is the most advantageous method presently known for removing and recovery of VCM from vent gas in PVC plants. Because of its commercial success and its importance to PVC processes, the owner of this application is prepared to grant licenses on reasonable terms for use of the complete engineering and operational technology required including rights under the invention of this application.

Claims: CLAIMS 1. In a process for removal and recovery of vinyl chloride monomer from a gas stream containing air admixed with vinyl chloride monomer, which stream originates in a polyvinyl chloride production plant and is normally vented therefrom to the atmosphere, which process comprises the steps of passing the gas stream before its venting to the atmosphere countercurrent to a stream of absorbent solvent to absorb the vinyl chloride monomer in the solvent, stripping the solvent to remove the absorbed vinyl chloride monomer, recovering the stripped vinyl chloride monomer and returning the sol¬ vent after stripping to the absorption step, the im¬ provement which consists of using as the solvent a branched chain aliphatic hydrocarbon of 8 carbon atoms and composed predominately of 2,2,4-trimeth l pentane, whereby the concentration of vinyl chloride monomer in the gases vented to the atmosphere is reduced to below 5 parts per -million. 2. The process of Claim 1 further character¬ ized in that the starting gas stream contains a ratio of vinyl chloride monomer to other gases in the range of 1:1 to 3:1. 3. The process of the preceding claims further characterized in that the solvent is at a temperature greater than 40°F.

Inventors: KLEIN J
Applicants: GOODRICH CO B F, GOODRICH CO B
Application year: 1980
Main classification: B01D53
IPC codes: B01D53