Thomas L. Hall AWD Technologies, Inc. Houston Dow Center BIN 4A9 400 W. Sam Houston Parkway S. Houston, Texas edited by

Transcription

1 ~~ The SORBATHENE" Unit for Volatile Organic Vapor Recovery Thomas L. Hall AWD Technologies, Inc. Houston Dow Center BIN 4A9 400 W. Sam Houston Parkway S. Houston, Texas i edited by Larry Larrinaga The Dow Chemical Company Houston Dow Center presented at Mississippi Technical Assistance Program Southern States Annual Environmental Conference Biloxi, Mississippi October 28. Trademark of The Dow Chemical Company' 1

2 ORGANIC VAPOR ABATEMENT: AN INTRODUCTION New regulations requiring installation of an organic vapor abatement devices designed to remove or destroy a volatile organic component from carrier gas streams are expected to cost industry $2.3 to 5.8 billion annually through the year 2000'. The Dow Chemical Company has developed and patented the break-through SORBATHENE' Solvent Vapor Recovery Unit which meets the stringent emission requirements of the amendments to Clean Air Act of 1990 and returns value in the form of recovered chemicals to offset costs of installing the unit3. Designed and installed under The Dow Chemical Company belief that "Waste Reduction Always Pays," a SORBATHENE unit recovers thousands of pounds of chemicals that incineration processes would destroy as waste. The recovered chemicals are valuable to the company and the recycling option avoids the costs of downstream waste processing associated with thermal destruction. SORBATHENE TECHNOLOGY - NINETEEN INSTALLATIONS The Dow Chemical Company has developed the SORBATHENE pressure swing adsorption (PSAI process as an economical alternative for the recovery of volatile organic compounds IVOC'sl from storage, loading and process vent streams. Since The Dow Chemical Company has installed nineteen SORBATHENE units to collect hydrocarbons, chlorofluorocarbons (CFC's), chlorinated solvents, aromatics, and monomers. AWD Technologies, a subsidiary of The Dow Chemical Company, recently completed an installation of a SORBATHENE unit for control of vapors dispalced during loading at a marine terminal in Lake Charles. Units are now meeting air permit requirements for the abatement of VOC's such as ethylene dichloride, benzene, acetone. trichloroethylene, trichlorofluoromethane, carbon tetrachloride, and styrene monomer. The SORBATHENE process is a pressure swing adsorption process as distinguished from a temperature swing adsorption process. In operation, the SORBATHENE process conserves the heat generated during adsorption and utilizes this heat and depressurization with backpurge for batch regeneration of the adsorbent in a subsequent desorption step. The adsorption and dworption steps are batch processes that occur simultaneously in alternating twin beds to maintain steady state operation of the unit. The SORBATHENE unit is self-regenerating with no need for off-site regeneration or disposal of the adsorbent. Figure 1 shows an adsorption isotherm curve illustrating a pressure swing adsorption p'ocess. Adsorption is carried out at P2 (high pressure i.e. atmospheric pressure), where the equilibrium amount adsorbed is high. Reducing the organic partial pressure to P1 causes the amount of adsorbate equal to W2-Wl to desorb from the bed. MASS OF ADSORBATE LOADED ON ADSORBENT w2 --- P1 P2 ORGANIC PARTIAL PRESSURE L

3 Each operating SORBATHENE unit achieves greater than 99.9% VOC's removal from vent streams ranging in flow rate from 50 to 2000 CFM and feed concentrations between 1,000 and 160,000 ppm. The chemical components are recovered as a condensed liquid. The SORBATHENE unit requires electricity and a cooling medium for operation. No nitrogen 01 steam is required and no contaminated steam condensate is produced. SORBATHENE units have been designed to recover VOC's in process streams which range in concentration from 100 ppm to 500,000 ppm. The SORBATHENE unit is not sensitive to the surges and interruptions in concentration and flow rate which are common occurrence in loading and process operations. DIAGRAM OF A SORBATHENE UNIT Thepatented4 SORBATHENE unit is a pressure swing adsorption process utilizing two or more adsorption beds. The feed stream such as a vent or process gas stream containing organic vapor is passed through one bed under conditions at which adsorption will occur. Two or more adsorption beds are used to maintain continuous operation of the SORBATHENE unit. Control valves switch the beds over short cycle times to avoid an excessive temperature rise caused by the heat of adsorption. The energy from adsorption is stored in the bed to be used in the subsequent desorption step.6 Figure 2 shows the preferred apparatus of a SORBATHENE unit. The feed stream can be directed to either or both of two adsorption beds. The vacuum pump draws a vacuum on either bed for desorpt'cn. The desorbed vapor is condensed and recovered and the uncondensed vapor and backpurae gas are returned to the feed stream. Vacuum Pump Contaminated Vent Stream FIGURE 2 Recovered Liquld A SORBATHENE unit 3

4 A vacuum pump reduces the pressure in the desorbing bed so that on-site regeneration occurs. A slight backpurge gas carries the concentrated desorbed vapor through the vacuum pump to a condenser where the organic components are condensed and recovered. The remaining backpurge gas leaving the condenser is recycled into the feed stream. DETAILED DESCRIPTION OF SORBATHENE UNIT TECHNOLOGY A SORBATHENE unit is used to separate a solvent, monomer or hydrocarbon from a gas stream. The process is highly suited for recovering halogenated hydrocarbons and aromatic hydrocarbons For the abatement of higher value volatile organic compounds, the SORBATHENE unit will return significant value in the form of recovered chemicals. Additional processing equipment such as caustic scrubbers and halogen absorbers are not required for tail gas treatment as they are for incineration processes that destroy halogenated compounds. Some examples of vapors that are currently being separated from vent streams using a SORBATHENE unit are 1,l,l -trichloroethane. methylene chloride, trichlorofluoromethane (R-1 1 ), benzene, carbon tetrachloride, styrene, epichlorohydrin, butylene oxide, and acetone. The organic vapor is separated from the gas stream. The gas may be any gas that does not adversely affect the apparatus and which tends to adsorb upon the adsorbent material to a lesser extent than does the organic vapor being recovered. The gas preferably condenses at conditions much lower in temperature and much higher in pressure than does the organic vapor to be recovered. Examples of typical gas streams to be treated are air, oxygen, nitrogen, or argon. Most typically, organic vapors are separated and recovered from air. The volatile organic compound is removed from the feed gas stream by passing the stream through a bed of adsorbent which has a high capacity to remove the volatile organic compound from the feed stream in a manner which can be readily reversed by the reduction of pressure in the presence of a backpurge stream of gas. The proper choice of adsorbent material varies depending upon the nature and characteristics of the vapor to be recovered. Styreneldivinylbenzene microporous resin and activated carbon are preferred as the adsorption media in a SORBATHENE unit. To maintain a continuous operation and self-regeneration of the adsorbent, two or more adsorbent beds are used. Adsorption and desorption steps are alternated in each bed so that at least one bed is adsorbing at all times, providing continuous treatment of the vent or process feed stream. It is preferred that a SORBATHENE unit be designed and operated for removal of 99 percent of a volatile organic compound from the gas stream and most preferably, removal of 99.9 percent. The adsorption step is continued in each bed for a period of time short enough to retain the heat of adsorption in the bed for use in the desorption step. By operating the SORBATHENE unit on short cycle times, the continuous operation of the unit with onsite regeneration can occur in the presence of the retained heat and without the supply of additional heat. The cycle time for a SORBATHENE unit is determined by experimentation and is dependant on factors such as bed size, characteristics of the adsorbing vapor, the temperature of the bed, and the adsorption and desorption pressures. Preferably, a single adsorption step is continued for!'o more than 30 minutes and for no less than 5 minutes. The capacity of the adsorbent is r,x exhausted at the end of the adsorption step. Following each adsorption step, desorption occurs at a low enough pressure in the presence of the conserved heat of adsorption and the flow of backpurge gas to reduce the loading on the adsorbent material. The operating desorption pressure is dependant upon factors such as the characteristics of the adsorbent, the size of the bed, the concentration and nature of the adsorbing vapor, the flow and nature of the backpurge gas, and the temperature of the bed. Optimal desorption pressures are preferably selected between 50 mm Hg and 300 mm Hg (1 psia and 5.8 psial. During the desorption step, a slight stream of backpurge gas flows counter to the direction of feed flow so that the desorbed vapor is carried out of the bed. The backpurge gas may be any gas.. A

5 which does not adversely affect the SORBATHENE unit and which does not adsorb to a significant extent. Usually, the source of backpurge gas is a fraction of the clean gas leaving the adsorbing bed. By utilizing the clean gas leaving the unit, no additional sources of inert gas are required. The desorption step is continued for a period of time sufficient to remove the amount of solvent that was adsorbed. The length of time is short enough for desorption to occur using the heat conserved in the bed and without auxiliary heating. The desorption step is carried out for a time short enough that one bed can be brought to desorption pressure, desorbed to restore the previous capacity, and repressurized to adsorption pressure while the other bed is in the adsorption step. The concentrated stream of desorbed vapor and backpurge gas is drawn through the inlet of the vacuum system. The vacuum system compresses the inlet stream from the desorption pressure to a higher pressure. The outlet stream from the vacuum system typically passes to a condenser where the organic component condenses and is recovered as a liquid. The inert backpurge gas leaving the condenser carries an equilibrium concentration of the volatile organic compound and is recycled to the feed stream. By closing the loop on the vapor outlet from the condenser and feeding the outlet stream to the adsorbing bed, much higher recovery and abatement efficiencies of the organic vapor are achievable than are possible in a stand-alone vent condenser. A SORBATHENE unit is controlled with a programmable logic controller used to alternate the beds in adsorption and desorption modes. The sequencing strategy by which the flow valves switch the beds through the adsorption and desorption steps has been developed through experimentation to assure that the vent stream is treated continuously and the beds are evacuated and repressurized with minimal disturbance of the adsorbent in the bed. SORBATHENE UNITS: INSTALLATION AND OPERATING EXPERIENCE' Currentl,.r, there are nineteen SORBATHENE units in service throughout the United States and Canada. Eighteen of those units are operating in Dow facilities and one unit at an industrial marine terminal. The following examples highlight the typical application of this technology. Vapor Abatement for Rail Car Loading of Trichloroethylene During the loading of rail car, vapor is displaced at the rate 200 CFM. The concentration of solvents in the vapor phase is variable, increasing from a negligible level to 60,000 ppm at the end of the transfer. The permit requires the outlet concentration less than 100 ppm. The first commercial unit was installed in 1988 and has been in continuous operation meeting regulatory limits. This unit has been closely monitored to verify the basis of scale up from pilot plant research. Styrene Storage Facility Vents Styrene monomers are highly temperature sensitive and will polymerize. Two Dow facilities needed to install vapor control system to reduce emissions by 98.5%. Flares and oxidizers were rejected because of the proximity to facilities storing flammable chemicals. Carbon adsorption systems were rejected because either the accumulated heat of adsorption or the steam regeneration could trigger a polymerization reaction. Two units were installed to capture and separate styrene monomer from storage vents. The units were designed with short cycle time ~~~~~~ to ~ ~~~ limit ~ the temperature increase durjng the adsorption and the vacuum regeneration was employed as an alternative to high temperature regeneration. The monomer is recovered without polymerization. Acetone Co-Product Abatement. Vents from acetone storage needed to be controlled to meet permit requirements. Ignition sources were not acceptable within the tank farm. Carbon adsorption was rejected because of the history of fires caused by exothermic heat of adsorption and high flammability when adsorption occurs during a long cycle time'. 5

6 Three SORBATHENE units utilizing activated carbon with a short cycle time to limir temperature rise have been installed to recover acetone vapors. During the operation of these units it was observed that the recovered acetone was contaminated with diacetyl alcohol, a product of polymerization of acetone at ambient temperature on activated carbon. A styrene di-vinyl benzene polymeric bead adsorbent has shown to be effective as an adsorbent for acetone that does not catalyzes the low temperature polymerization reaction. Epichlorohydrin Vents from Epoxy Plant Vents from rhe storage tanks in the epichlorohydrin plant were captured in large balloon type devices designed to balance tank vapors during the dayinight breathing. These balloons have high maintenance cost and special buildings are required for enclosure and protection during the hurricane season. A SORBATHENE unit is an economical replacement for each balloon. The epichlorohydrin vapor is recovered and the real estate occupied by the balloon storage building becomes available. Maintenance of the SORBATHENE unit is minimal, limited only to the requirements of the vacuum pump manufacturer. CFC R-11 Vapor Control from Transfer Terminal State regulations require the control of vapor emissions of trichlorofluoromethane (R-11). During the initial economic study, this material was valued at more than $3.00 per pound. This value has since increased due to Federal taxes on CFC products. The chosen abatement device is intended to be used in the future when a CFC replacement enter the market. The vapor recovery unit was installed with an attractive return on investment. Because of the high value of the material recovered, the installation of this SORBATHENE unit was cost-effective. This technology should readily adapt to the recovery of the anticipated CFC replacement. Process Vents from Perchloroethylene plant The abatement of process vents from this chlorinated solvents plant presented a serious challenge due to the fluctuating nature of flow and composition. An on-line solution was needed to cope with upset production, shut-downs and start-ups as well as normal operation. Dow waste minimization guidelines were met by the recovery of chlorinated solvent vapors emitted by this production facility. The SORBATHENE unit installed at this site meets the 50 ppmv abatement requirement for Perchloroethylene. However, the unexpected presence of traces of a sublimating by-product lead to the formation of solid deposits on the adsorbent. A sacrificial bed preceding the SORBATHENE unit will be installed to capture the sublimating by-product to avoid plugging the unit. Containment of Process Solvent Vapors A production facility utilizing monochlorobenzene (MCB) as the process solvent required control of MCB vent losses from the inert purging unit to increase plant yield and reduce solvent costs. The SORBATHENE unit installed at the site returns significant value in the form of recovered monochlorobenzene solvent. The success of the first unit led to the installation of a second recovery unit at a sister plant. These units are tied-in to the plant control computer and were built as integral components of the production facilities rather than the more common independent skid mounted modular units. Pipeline and Barge Transfer to Benzene Storage Canadian environmental regulation required the control of benzene emission to less than 1 ppm. A vent control system needed to achieve six-nines ( %) abatement efficiency, to be on-line when needed, and minimize utility requirements. A SORBATHENE unit installed at this transfer operation collects the benzene vapors displaced as the storage tank is loaded. The benzene vapors are captured, concentrated. and condensed to 6

7 recycle to the storage tank. This unit meets the mandated limit of < 1 ppm for benzene emission. The only utility is electricity. Other General Applications of the SORBATHENE Unit Dow has applied this technology for the recovery of other organic chemicals such as: e Trichloroethane e Carbon tetrachloride Methanol lsopentane e Butylene oxide Ethylene diamine Sample performance data is shown in the following table': COMPOUND Trichloroethylene 1,1,1 -Trichloroethane Carbon Tetrachloride Perchloroethylene Monochlorobenzene Styrene Methanol lsopentane FEED CONCENTRATION (ppm) 59,320 5,000 80,000 40,000 4,500 5, , ,000 OUTLET RECOVERY CONCENTRATION EFFICIENCY (ppm) % , 1.o Ethylene Dichloride Marine Terminal The Occupational Safety and Health Administration (OSHA) worker exposure limits require the abatement of ethylene dichloride (EDC) vapor emitted during marine vessel loading. The U.S. Coast Guard safety regulations require third-party certification of vapor control systems installed at marine facilities. A major producer of ethylene dichloride located in Lake Charles, Louisiana, contracted with AWD Technologies, the Dow environmental subsidiary, for the installation of a SORBATHENE unit which meets the U.S. Coast Guard certification requirements and achieves an vapor abatement level to comply with OSHA worker exposure limits. This unit is designed to meet the 20 ppm emission limit for EDC. The EDC recovered is recycled to the customer's inventow. Figure 3 shows a three dimensional view of the SOREATHENE unit constructed for the customer to recover EDC at a marine vessel loading facility.

8 GENERAL DESIGN CONSIDERATIONS Materials of Construction The detailed engineering of each unit requires the correct selection of materials of construction for each piece of equipment based on the chemical characteristics and compositions of the vapors being treated. Although many of these units treat corrosive chlorinated organics, the adsorbers, piping, and valves can often be carbon steel in the vapor processing side of the unit. Dry chlorinated chemicals are usually not corrosive in the vapor phase. Following the discharge of the vacuum pump, material of construction become more important because this section of the unit will be in contact with corrosive liquid phase organics. This problem is compounded with the presence of moisture. Process Control Computer Origiiially, most of the SORBATHENE units were controlled by a simple programmable logic controller (PLC). Some applications integrate the control of these units into the same computer system controlling the rest of the process. The latest trend has been to evaluate the application of the CAMILE' data acquisition and control system' to replace the PLC. CAMILE is cost competitive and has the advantage of excess data handling capabilities to manage continuous emission monitoring (CEM) requirement now required in most new vapor control applications. Reactive Chemical Review Each commercial application must go through a formal reactive chemical review by an multidiscipline committee before start-up. The purpose of this review is to develop an understanding of the impact of the energy stored in the chemical system, the impact of changes to the system, and consequences of accidental energy releases. Considerations are given to the potential presence of oxygen.n the vapor phase and the increased reactivity of the chemicals on the catalytic surface of the adstxbent. The extent of temperature rises and pressure swings are critical parameters. The adsorptiim phenomena is exothermic and the safe temperature "window" must be determined and understtrod. Hazard 2 nd Oper8bility Review IHAZOP) A team :omposed of design engineers and operation personnel reviews each SORBATHENE unit based on HAZOP methodology. The purpose of this extensive review is to forecast any potential hazardous situation and eliminate the hazard. A secondary purpose of this review is to make the operating personnel aware and fully understand the constraints of the operating parameters. For example, each unit must go through a pre-start adsorbent conditioning stage. The purpose of the conditioning and the consequences of the lack of conditioning are extensively discussed during the HAZOP review. Compact Skid Mounted Modular System Most of tile vapor abatement SOREATHENE units are additions to existing production, storage or transfer facilities. For that reason, it has been preferred that the unit be fabricated off-site and delivered as a compact skid mounted modular unit. A few units has been installed integral to the process. The modular skid mounted concept requires careful consideration to maintenance requirements. Dow experience shows that maintenance has been limited to the practices recommended by the vacuum pump manufacturer and the maker of the high performance valves. For this reason, special attention is given to the location and accessibility of the vacuum pump and control valves. FURTHER DEVELOPMENT OF THE TECHNOLOGY The SORBATHENE unit was developed as an alternative technology to control the emission of chlorinated solvents from vent streams. This technology is based on recovery of the material and does not require the post-treatment of acidic tail gas from incineration devices or the associated treatment of wastewater from scrubbers or steam condensate from batch adsorption system. 8

9 . Based on the success with the recovery of chlorinated solvents, the application of this technology has been expanded to include other highly valued or temperature sensitive chemicals. Currently, the Dow Chemical Company is expanding its list of chemicals that can be recovered by adsorption processes. Cost containment will become particularly significant as the applicable flow rate ranges above 3000 CFM. Additionally, the process of modified pressure swing adsorption may find expanded use in industrial gas separations. * Trademark of The Dow Chemical Company REF ERE N C E S 1. The Dow Chemical Company (Delaware Corporation), U.S. Trademark 1,535,181 (Apr 18, 1989) ICF Resources Incorporated et al.. Business Oooortunities of the New Clean Air Act, January L. A. Robbins and T. L. Hall, "SORBATHENE Organic Vapor Recovery Unit - The Dow Chemical Company's Solution to Organic Vapor Emissions," Proceedings from the 1992 Air and Waste Management Association Annual Meeting, June, L. A. Robbins and T. C. Frank (to The Dow Chemical Company), U.S. Patent 4,857,084 (Aug ). C. W. Skarstrom, "Heatless Fractionation of Gases Over Solid Adsorbents," Recent DeveloDments in SeDaration Science, (2): 95 (1 975). A. L. Larrinaga and T. L. Hall, "Installationa and Operation of SORBATHENE Solvent Vapor Recovery Units to Recover and Recycle Volatile Organic Compounds at Operating Sites within The Dow Chemical Company," Proceedings from the 1993 Air and Waste Management Association Annual Meeting, June, Eastman Kodak Company, Rochester, N.Y., communication, T. C. Frank, The Dow Chemical Company, Midland, MI, personal communication, R. J. Pierce, The Dow Chemical Company - Camile Products, Midland, MI, personal communication,