\ I / United States Environmental Protection Agency Hazardous Waste Engineering Research Laboratory Cincinnati OH 45268 Research and Development EPA/600/S2-86/113 March 1987 &EPA Project Summary Catalytic Dehydrohalogenation: A Chemical Destruction Method for Halogenated Organics Dehydrohalogenation shows poten- tial as a means for converting certain halogenated organics in wastes to inor- ganic salts and gaseous aliphatic com- pounds. Dehydrohalogenation is a dehalo- genation/elimination reaction that is initiated by a strong base. The resulting products are the halide salt, water, and an elimination compound. A novel reagent, sodium or potas- sium hydroxide mixed with a polyethylene glycol, is a very effective dehydrohalogenation agent. This reagent is shown to dehalogenate six organic compounds that are represen- tative of low molecular weight com- pounds encountered in hazardous wastes: CCI4, CHCI3, CH2CI2, C2H4Br2, and CCI3NO2. Kinetics data for the reac- tions of this reagent with the six com- pounds is given to allow reactor design and calculation of destruction effi- ciency. This Project Summary was devel- oped by EPA's Hazardous Waste Engi- neering Research Laboratory, Cincin- nati, OH, to announce key findings of the research project that is full docu- mented in a separate report of the same title (see Project Report ordering infor- mation at back). Introduction Background Halogenated organic compounds ac- count for a major portion of toxic and persistent hazardous wastes. Various techniques have been proposed for de- stroying these constituents. Incinera- tion is a widely used destruction tech- niques, but has limitations. There have been other methods pro- posed and used for destroying halo- genated species, such as ultraviolet (UV) and UV/ozone degradation, biotreatment with specially adapted mi- croorganisms, and sodium metal reduc- tion. Each of these techniques has shown promise for destruction (or transformation) of the halogenated spe- cies, and each of these technologies has certain applications wherein it is the method of choice. Preliminary work has been initiated toward development of an alternate method for the destruction of halo- genated organics that is based on a classical organic chemistry technique for dehalogenation. It involves reaction of the halogenated species with caustic to produce an elimination reaction. The products of the reaction are the halide salts, water, and a multiple bond on the organic molecule at the site of the de- halogenation. H H I I R-C-C-X + NaOH-> i I H H H H \ NaX + H20 H This reaction mechanism is referred to as dehydrohalogenation. ------- Typical dehydrohalogenation reac- tions are carried out in an initially anhy- drous system in the presence of solid caustic, or a small amount of water may be used as a catalyst. These conditions lead to very vigorous, and even uncon- trollable, reactions. In order to make this technology useful for the treatment of hazardous waste, the following criteria must be met: 1. The reaction system must accom- modate wastes containing water. 2. The reaction must be smooth and controlled. 3. The methodology must provide treatment efficiency equivalent to or exceeding existing treatment techniques. The full report documents an experi- mental program to validate the treat- ment of halogenated waste compounds by means of a novel dehydrohalogena- tion reagent that meets the above listed criteria. This reagent, composed of caustic mixed with a relatively small quantity of polyethylene glycol, has been shown to be effective in rapid yet controlled dehydrohalogenation in or- ganic syntheses. The polyethylene gly- col acts as a catalyst in the reaction. In this program, the reagent mixture cho- sen for experimentation was potassium hydroxide (KOH) and tetraethylene gly- col (TEG). This reagent will be referred to subsequently as KTEG. Objectives There were four objectives in this pro- gram: 1. Validation of the efficacy of KTEG to destroy ethylene dibromide, 2. General, qualitative observation of the reactivity of KTEG with a series of halogenated and nonhalo- genated compounds to determine those showing promise for treat- ment with KTEG, 3. Determination of reaction kinetics data to allow design of treatment systems and calculation of treat- ment efficiencies, and 4. Conceptual design of a possible treatment system. Experimental Program The experiments for both the qualita- tive observations and the kinetic studies were carried out in a bench scale reactor system (Figure 1). The reactor vessel was a 1 L three-neck flask. Attached to the flask were a thermometer, a chilled reflux condenser, a sample withdrawal system, and a mechanical stirrer. The mechanical stirrer was used instead of a Thermometer Gas Sampling Port Syringe Sample Vial Inverted Graduatet Cylinder Thermometer Temperature Control Bath High Torque Stirrer Figure 1. Experimental set-up. magnetic stirrer because the density of the reaction mixture increased with time; if a magnetic bar were used it would become stalled in the slurry. A typical run is outlined below: 280 g of potassium hydroxide (KOH) was dis- solved in 180 ml water in the 500 ml 3-neck flask. A 5 ml portion of n-nonane was added to the flask as an internal standard. A 5 mL portion of ethylene dibromide (EDB) was added, the system was closed, and stirring was initiated. The system was observed closely for several minutes to assure that no reac- tion was taking place. An initial sample was taken from the reaction vessel, and then 10 ml of tetraethylene glycol (TEG) was added to the vessel. Reaction began immediately, as evi- denced by vigorous bubbling, and gas displacement versus time was plotted. Gas samples and liquid samples were taken at several intermediate reaction times and at the end of the reaction. After the gas evolution had stopped, an additional 5 mL portion of EDB was added to the reaction mixture, and the reaction resumed. The sampling and gas displacement readings were taken as before. The entire process was re- peated until the mixture became too thick to stir and the reaction showed signs of slowing. The reflux condenser was chilled to 0°C (±5°C) to prevent escape of the moderate boiling point halogenated species. Gases generated in the reac- tion passed through the reflux con- denser and into the inverted graduatec cylinder by means of a \" ID flexible tube. Gas samples were taken at the toj of the reflux column through a septum closed sample port by means of a gas tight syringe. Liquid samples were removed fron the reactor through the suction mecha nism shown in Figure 2. The empt^ sample vial was retained in place soleh to prevent system pressure loss. Thre< mL of water was placed in a 5 mL sam pie vial and frozen. To remove a sampU from the reacting mixture, the empt> vial was removed and the syringe wai filled with approximately 5 mL of air Static pressure within the sample tube would usually prevent any leakage o reactor contents during the sample via transfer. The vial containing the deion ized ice was transferred to the syringe assembly, the j mL of air pushec through to clear the sample line, and the syringe withdrawn 1 mL to create a suc- tion and draw sample from the reaction flask into the ice vial. The sudden cole and dilution from the melting ice haltec the reaction. This facilitated a reason able delay in actually getting the sample to analysis. The samples were then re frozen for preservation. Such sample; taken at intervals during the reactior provided process "snapshots" and en abled detailed determination of the re action kinetics. Analyses of the gas anc liquid samples were performed by G FID. \ ------- I Empty Vial in Place to Prevent Loss of Pressure in the Reactor Sample 3 mL Ice To Reactor Collecting Sample from the Reactor Figure 2. Liquid sample collection. Results and Conclusions General and Qualitative Observations A total of seven compounds were ested in the dehydrohalogenation sys- tem. These compounds are listed in Table 1 along with qualitative descrip- tions of the results of the dehydrohalo- genation. It should be noted that several chemicals appeared to react with the te- traethylene glycol (TEG) catalyst under the reaction conditions. The mecha- nisms for these non-catalytic TEG reac- tions are not known at this time, but such reactions always resulted in an overall slowdown of the desired dehy- drohalogenation. Carbon disulfide, for example, a nonhalogenated test com- pound, stoichiometrically reacted with the TEG to form a sticky brown sludge; this reaction prevented all further reac- tion of the reagent with any halo- genated compounds. Carbon tetrachlo- ride and chloropicrin both showed evidence of slight consumption of the catalyst, but this did not seriously im- pair their dehalogenation. The effects of such compounds in real world situa- tions can be overcome by addition of excess reagent. Chloroform showed an unusual reac- tion with the TEG. The chloroform re- acted vigorously when added to the sys- tem, and it evolved a large volume of ^os very quickly. If a large volume of ^•oroform was added to the reactor, a gray-colored foam formed, and this foam solidified into a stable open-celled spongy material. A portion of this foam was cleaned with a series of solvents and analyzed by infrared spectroscopy (IR). From this cursory analysis, the foam appeared to be a substituted form of the TEG. Chloroform could be added in small portions to the reactor without foam for- mation. However, under these condi- tions, the chloroform reacted smoothly and consumed a small amount of the TEG in a similar manner to CCI4 and chloropicrin. The reactions with ethylene dibro- mide and ethylene dichloride were fast, controlled, and showed no consump- tion of the TEG. The reaction products consisted of vinyl bromide or vinyl chlo- ride, acetylene, and the halogen salt as a reactor residue. Determination of Reaction Kinetics The reaction rate constants for dehy- dorhalogenations by the KTEG reagent vary by temperature. Table 2 shows rate constants for six compounds over the temperature range of 21°-50°C. These rate constants may be used to design reactors and to determine the re- action efficiencies for KTEG systems. Since this particular dehydrohalogena- tion reaction is a first order (or pseudo first order), nonreversible reaction, the level of destruction is dependent only upon time of contact of the halogenated compound and the KTEG reagent. This means that any desired level of destruc- tion may be accomplished by designing the reaction system for proper resi- dence times. The values of ka in Table 2 may be easily converted to other units as needed. For example, the rate constant for ED6 at 21°C on a molar basis is: 90.2 mLEDB 2.179gEDBx mole TEG • min ml EDB 1 mole EDB 187.87 g EDB = 1.05 mole EDB mole TEG • min Reactor Design Conceptual designs of reactors for treatment of halogenated wastes were developed by utilizing the kinetics data described above and the parameters listed below: WASTE COMPOSITION - 95% CCI4, 5% EDB (V/V) REACTOR TYPE - (A) Plug flow, tubular (B) Batch OPERATION TEMPERATURE - 50°C FEED RATE - (A) 1 gal/min (B) 60 gal/hr DESTRUCTION EFFICIENCY - 99.999% of the EDB Table 3 shows the necessary lengths for a tubular, plug-flow reactor accord- ing to the tubular inner diameter. Alternately, a batch reactor sized to handle a feed rate of 60 gallons per hour would have a volume of 550 to 600 gal- lons. The batch reactor could operate at atmospheric pressure, and it could be fitted with manual or automatic valves for filling and discharging the reactor contents. A practical design for any dehydro- halogenation waste treatment reactor must be based on realistic consider- ations of material costs and reactor size as well as its capability for achieving the desired treatment efficiency. The ulti- mate cost of treatment of a unit volume of waste is determined by adding all as- sociated costs (system construction, op- eration, waste disposal, etc.), and divid- ing that sum by the volume of waste treated. Thus, even though operating costs of a continuous feed process are generally lower than those of a batch feed process, the high initial cost of the tublar reactor might never be offset, if there were relatively small volumes of waste to be treated. ------- Table 1. Compounds Investigated Chemical Results Carbon disulfide (CS2) Methylene chloride (CH2CI2) Chloroform (CHCI3) Carbon tetrachloride (CCIJ Quantitatively reacts with the TEC to form a brown sludge. Reacts slowly. Consumes some TEG. Rapid, uncontrollable reaction if added too quickly. Forms a stable, open-celled foam. Slow addition pro- duces a controlled reaction without foam formation. Consumes some TEG. Moderate reaction rate. Consumes some TEG. Table 3. Representative Lengths Diameter (in.) 2 4 6 8 W 12 Tubular Reactc Length (ft 1,056 264 117 86 42 29 Cl Cl I \ I Ethylene dichloride \ —C—C- V i i Br Br Ethylene dibromide\ — C— C- V i i Cl Chloropicrin \ Cl—C—NO2 Cl Fast but controlled reaction. No evidence of reaction with TEG. Slightly faster rate than with EDC, but still controlled. No TEG consumption. Moderate reaction rate. Consumes some TEG. Table 2. Reaction Rate Constants by Temperature ka (mL Constituent/mole TEG • min) Compound 21"C 30°C 40°C 50°C Ethylene dibromide (EDB) 90.2 153 267 449 Ethylene dichloride (EDC) 102 163 265 419 Carbon tetrachloride 16.0 22.3 31.6 43.7 Chloroform 9.8 21.3 48.0 103 Methylene chloride 0.10 0.36 1.36 4.78 Chloropicrin 5.72 N/D N/D N/D N/D = no data. Modification of key design parame ters, such as temperature of operatic or feed rate, could change the size r< quirements of the tubular reactor suff ciently to make it more cost effective a a treatment system design. The desig parameters of a dehydrohalogenatio reactor system must be derived on case-by-case basis for the specifi wastes to be treated. This may be don by using techniques similar to those de scribed in the full report. Recommendations Dehydrohalogenation technology i to be developed and evaluated as a pos sible alternative treatment of halocai bon wastes. In determining the relativ advantages and disadvantages of thi technique, it is necessary to compare to existing treatment methods. The potential advantages of dehydrc halogenation include cost, energy sa\ ings, materials recovery, and the nor production of harmful by-products Dehydrohalogenation by-products ten to be hydrolysis products of the parer molecule; it is highly unlikely that corr plex aromatic species will be produce' in these reactions. The potential disac vantages of dehydrohalogenation in elude the production of waste gases, 01 ganics, and brine that may requir disposal. Future research with this techniqu should explore its practicality for treat ment of contaminated soils and fo scrubbing halocarbons from gaseou streams. The technique should b tested on actual wastes, such as distilla tion bottoms from chlorinated solver recycle, to determine the effects of com plex waste matrices on the treatmen efficiency of the technique. ------- |