Ind. Eng. Chem. Res. 1999, 38, 391-395
391
Optimization of Dehydrochlorination of Waste 1,1,2-Trichloroethane to Vinylidene Chloride Eugeniusz Milchert* and Waldemar Paz´ dzioch Department of Organic Technology, Technical University of Szczecin, Pułaskiego 10, PL 70-322 Szczecin, Poland
The influence of changes in the reaction parameters on the vinylidene chloride yield in dehydrochlorination of waste 1,1,2-trichloroethane has been investigated. The following parameters have been examined: temperature, the molar ratio of NaOH to 1,1,2-trichloroethane, and a flow rate of the raw materials at a constant NaOH concentration (10 wt %). Introduction At the present time, in the industry, vinylidene chloride (VDC) is manufactured by dehydrochlorination of 1,1,2-trichloroethane (112TCE) in plants with an output of 10-40 000 t/year. However, the initial 112TCE is seldom produced for this purpose. This is formed in sufficient amounts during the production of 1,2-dichloroethane, which is the principal semiproduct in the synthesis of vinyl chloride.1 The development of new vinyl chloride factories with the 400 000 t/year output creates the possibility of the VDC production from the waste 112TCE. The processes produce about 10 kg of industrial-grade 112TCE/t of vinyl chloride after the distillation of the so-called “heavy fraction”. “Heavy fraction” with the following composition (wt %) is the distillation tails after distillation of 1,2-dichloroethane and the light components from the combined waste fraction from rectification of the products of chlorination and oxychlorination of ethylene to 1,2-dichloroethane: 1,2-dichloroethane, 16.2; 112TCE, 50.1; perchloroethylene, 7.5; 1,1,2-trichloroethylene, 1.5; 1,1,1-trichloroethane, 1.2; 1,1,1,2-tetrachloroethane, 8.6; 1,1,2,2-tetrachloroethane, 3.6; pentachloroethane, 1.8; heksachloroethane, 0.1; polymers and tars, 9.1. This fraction can be the source of technical 112TCE, as was indicated in the experimental part. In the currently working factories of vinyl chloride, it is utilized as a raw material in the chlorinolysis process or for combustion. Osiewicz et al.2 have shown that an additional amount of 112TCE can be obtained by the chlorination of the so-called “light fraction”. Both fractions are waste from distillation of 1,2-dichloroethane which was used for the vinyl chloride production. Dehydrochlorination is carried out with the use of the aqueous solutions of calcium or sodium hydroxide. These processes are carried out at pressures of 1.1-2.1 kG/ cm2 at temperatures of 60-110 °C, in reactors with agitators and heating. Thermal and catalytic dehydrochlorination is not sufficiently selective.3 Dehydrochlorination with the aqueous solution of calcium hydroxide causes the formation of wastewater mainly containing calcium chloride and requires about 50% excess of hydroxide in relation to stoichiometry. Simmrock (1976)4 and Milchert et al. (1994)5 stated that when sodium * To whom correspondence should be addressed. E-mail:
[email protected].
hydroxide is applied, it is possible to recycle the brine to electrolysis and thus complete the chlorine cycle. For these reasons hydrochlorination by means of NaOH is preferred. From our earlier research6 results dehydrochlorination of the pure 112TCE over a wide range of the parameters leads to the following mixture (wt %): VDC, 98.9; trans-1,2-dichloroethylene, 0.5; cis-1,2-dichloroethylene, 0.4; chloroacetylene, 0.1. They are obtained from the following reactions:
CHCl2-CH2Cl + NaOH f CCl2dCH2 + NaCl + H2O, ∆h°298 ) -132.95 kJ/mol (1) CHCl2-CH2Cl + NaOH f trans-CHCldCHCl + NaCl + H2O, ∆h°298 ) -130.02 kJ/mol (2) CHCl2-CH2Cl + NaOH f cis-CHCldCHCl + NaCl + H2O, ∆h°298 ) -131.70 kJ/mol (3) CCl2dCH2 + NaOH f CCltCH + NaCl + H2O, ∆h°298 ) -87.89 kJ/mol (4) The heats of these reactions at the optimal process temperature (82 °C) amounts consecutively (kJ/mol) to (1) ∆H355 ) -108.96, (2) ∆H355 ) -103.14, (3) ∆H355 ) -103.11, and (4) ∆H355 ) -113.07. The reaction rate increases after the interfacial surface development by a rapid agitation and the use of an emulsifier. No details have been found in the literature on the optimum parameters or the effect of the parameter variations on the VDC yield in dehydrochlorination of the aqueous solutions of sodium hydroxide. The behavior of particular components of industrial-grade 112TCE was presented in the previous work.6 Experimental Section Raw Materials. 112TCE of industrial grade was used with the following composition (wt %): 112TCE, 95.8; perchloroethylene, 2.6; 1,2-dichloroethane, 0.8; 1,1,1-trichloroethane, 0.5; 1,1,2-trichloroethylene, 0.3. This composition is laboratory distillation product of the “heavy fraction” from the vinyl chloride plant at ZA Włocławek (Poland). This fraction was subjected to a simple distillation in order to separate polymers, tars,
10.1021/ie9801709 CCC: $18.00 © 1999 American Chemical Society Published on Web 12/22/1998
392 Ind. Eng. Chem. Res., Vol. 38, No. 2, 1999
Figure 1. Scheme of the apparatus for vinylidene chloride synthesis: 1, reactor; 2, peristaltic pump; 3, dephlegmator; 4, distillation head; 5, reflux condenser; 6, receivers for vinylidene chloride; 7, brine receiver; 8, container for 1,1,2-trichloroethane; 9, container for a sodium hydroxide solution; 10, thermometer; 11, stirrer; 12, mixer.
and the components heavier than 112TCE. The distillate with the following composition (wt %) subsequently was subjected to a fractional distillation on a 20-plate Brunn column, resulting in the above-mentioned 112TCE, a raw material of dehydrochlorination: 112TCE, 73.0; 1,2-dichloroethane, 18.1; 1,1,1-trichloroethane, 1.6; 1,1,2-trichloroethylene, 1.8; perchloroethylene, 5.5. Pure NaOH was purchased from ZCh Os´wie¸ cim, Poland. Dehydrochlorination Method. Syntheses were carried out in the reactor with vigorous mixing and circulation of the reaction mixture. The mixing and circulation were caused by a helical or propeller agitator and a nitrogen stream. The apparatus presented in Figure 1 was blown with nitrogen, and its flow (0.61.0 N dm3/h) was continued during the synthesis. Before the experiment, the reactor (1) was filled with an aqueous solution containing 11.8 wt % NaCl and 0.08 wt % NaOH. In a continuous way with the use of peristaltic pumps (2) by a mixer (12) were introduced a 10 wt % sodium hydroxide solution and 112TCE. The temperature, anticipated in the experimental design, was kept constant by use of an ultrathermostat. The volume of the reaction mixture was 0.25 dm3. VDC formed, and byproducts and unchanged 112TCP were distilled from the reactor in the nitrogen stream. Distillate passed through the dephlegmator (3), distillation head (4), and reflux condenser (5) and flowed off into receivers (6), cooled to -30 °C (ultracryostat). These receivers contained VDC polymerization inhibitor (monomethyl ether of hydroquinone). The excess of an aqueous layer was passed through the siphon to the container (7). In effluent of the aqueous layer after cooling to an ambient temperature was evolved the
organic layer. The mechanical stirrer in the majority of experiments worked at 1100 rpm (revolutions per minute). The change of agitator revolutions in the range 800-1500 rpm did not cause changes in the yield of VDC. The reactor and mechanical stirrer ensured proper circulation and maintenance of the reaction mixture in the emulsion form. A mass balance was calculated for each experiment. When the decrement of the reaction mixture mass did not exceed 1 wt %, the analyses were performed and the VDC yield was calculated. Analytical Control. The composition of industrialgrade 112TCE and a raw VDC and the concentration of organic compounds in the reaction mixture were determined by gas chromatography. The determinations were performed using a Chrom 5 apparatus equipped with a 3 m × 4 mm steel column, filled with 15 wt % SE30 on Chromosorb W AW DMCS 80/100 mesh attached to a similar column filled with 10 wt % FFAP on Chromosorb W AW DMCS 80/100 mesh. The flame ionization detector was used, the carrier gas was nitrogen at 28 cm3/min, the temperature of the injection chamber and the detector was 200 °C, and the oven temperature was maintained at 80 °C for 10 min, then increased at 5 °C/min up to 160 °C, and kept at 160 °C for 22 min. The NaOH concentration in the reaction mixture flowing out from the reactor was determined by acidbase titration with a 0.1 M solution of hydrochloric acid. The analyses were performed every 10 min. On the basis of the analysis, results had changed the flow of a sodium hydroxide solution in such a manner as to maintain the assumed molar ratio of NaOH to 112TCE. Results and Discussion To maintain the reaction mixture in the state of emulsion, the yield of VDC is primarily dependent on the mole ratio of NaOH to 112TCE and the temperature. It depends to a lesser degree on the raw materials flow rate and concentration of NaOH solutions. The experiments have been performed using the NaOH solutions with concentrations 10, 20, and 30 wt % at a temperature of 80 °C. To maintain a constant NaOH to 112TCE molar ratio (optimal 1.17:1) and residence time of raw materials (112TCE + NaOH solution) in the reaction zone, with an increase of NaOH concentration, the flow rate of 112TCE was increased. At flow rates of 709.4, 592.5, and 497.1 cm3/h the NaOH solutions with concentrations of 10, 20, and 30 wt %, respectively, the flow rate of 112TCE amounted to 140.0, 256.9, and 352.3 cm3/h, respectively. Hence, the volume flow (space velocity) of the sum of raw materials was constant. Under these conditions the yield of VDC amounted to 96.0 mol % for a 10% NaOH solution, 91.7 mol % for a 20% NaOH solution, and 89.2 mol % for a 30% NaOH solution. The levels of values of examined factors and the range of the changes of temperature, sodium hydroxide concentration, the molar ratio of sodium hydroxide to 112TCE, and the flow rate of the raw materials were determined in the preliminary investigation.6 The results of these examinations showed that dehydrochlorination is a complicated process. Therefore, the statistical method of the experimental design was chosen for optimization and determination of the influence of the process parameters on dehydrochlorination.7,8 The effect of parameter variations on the yield of VDC related to
Ind. Eng. Chem. Res., Vol. 38, No. 2, 1999 393 Table 1. Levels of the Examined Factors molar ratio flow rate of coded temperature NaOH/112TCE 112TCE factor (°C) (z1) (mol/mol) (z2) (g/cm3 h) (z3)
level basic higher lower star higher star lower
0 +1 -1 +1.215 -1.215
70.00 85.00 55.00 88.23 51.77
1.100 1.220 0.980 1.246 0.954
200.00 300.00 100.00 321.54 78.46
Table 2. Design Matrix and Experimental Results experiment no.
z1 (°C)
z2 (mol/mol)
z3 (g/cm3 h)
Y (mol %)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
55.0 85.0 55.0 85.0 55.0 85.0 55.0 85.0 88.2 51.8 70.0 70.0 70.0 70.0 70.0 70.0 70.0 70.0 70.0 70.0
0.98 0.98 1.22 1.22 0.98 0.98 1.22 1.22 1.10 1.10 1.25 0.95 1.10 1.10 1.10 1.10 1.10 1.10 1.10 1.10
100.0 100.0 100.0 100.0 300.0 300.0 300.0 300.0 200.0 200.0 200.0 200.0 321.5 78.5 200.0 200.0 200.0 200.0 200.0 200.0
84.6 90.9 89.9 96.6 84.2 90.5 89.5 96.2 95.5 84.0 93.9 84.0 95.1 95.6 95.2 95.0 95.6 95.4 95.1 95.8
k
∑ i)1
k
bixi +
R ) 0.05 frep ) 5 fadeq ) 10 R(R) ) 0.99 n0 ) 6
∑ i
