Optimum Design Of Chloromethane Recovery Process For Exhaust Gases In Butyl Rubber Synthesis

As the key solvent in butyl rubber synthesis, chloromethane (CH3Cl) consumption can seriously affect the production cost. By experience, CH3Cl used in the synthesis would be wasted about 10% in the purification and dehydration processes. Therefore, CH3Cl recovery is very important. Because there are various vent gases differences in impurities, CH3CI concentration and stream pressure. In detail, the passivation gas composition of CH3Cl and dimethyl ether (DME) are similar to each other in boiling point; Rectifying column tower non-condensable gas has low concentration (CH3Cl=3.5 mol%) and high pressure; the drying tower regeneration vent gas has high concentration (CH3Cl=20.5 mol%) and low pressure. According to the feature of these, the passivation exhaust was alone recycle, other chloromethane vent gas recover according to the pressure and the concentration.In this paper, the main work is divided into two parts:(1) CH3Cl and DME are similar to each other in boiling point. It is very difficult to separate them with distillation. In industry, concentrated sulfuric acid is usually used to react with DME to purify CH3Cl, which is expensive and hazardous. According to the difference in forming hydrogen bonds with H+, a new method using water as the absorbent was proposed in this work to remove DME in CH3Cl. Thereby, an appropriate thermodynamic model is needed to accurately simulate and design the absorption system. UNIQUAC, NRTL and NRTL-RK models are tested with the binary phase equilibrium data of DME-CH3Cl. DME-H2O and CH3Cl-H2O. NRTL-RK prediction result can be more consistent with the experimental data, with a relative error less than 2.5%. Hence, NRTL-RK model is applied to estimate the ternary phase equilibrium data. The effects of system temperature and pressure on separation factor and solubility with liquid H2O are investigated intensively. This work supplied a theoretical basis for developing the optimal water absorption system to facilely remove DME from CH3C1.On the basis of thermodynamic model NRTL-RK, the water absorption-desorption process are designed to recover passivation exhaust’s chloromethane, the chemical process simulation software Aspen Plus was used to simulate and optimized the process, and analyses the influences of tray numbers, the pressure of absorber and desorption, absorbent temperature to the regeneration energy, DME recovery rate, CH3Cl loss rate. As a result,95.7% of CH3Cl in the vent gas can be recovered, and the specific consumption is 0.016 ￥·kg-1. In addition, the water absorption-desorption process did not need corrosive absorbent, so that it would be an clean and safe recovery process.(2) Rectifying column tower non-condensable gas has the characteristic of low concentration and high pressure, the drying tower regeneration vent gas has the characteristic of high concentration and low pressure, simple mixed can lead to back-mixing and energy consumption increased. The membrane separation-compression condensation-PSA integrated process were designed, it features membrane separation device enriching the low concentrations of CH3Cl, compression condensation enriching the high concentrations of CH3Cl, PSA removing the trace amounts of CH3Cl. the chemical process simulation software HYSYS was used to simulate and optimized the integrated process, which the operational conditions were determined. The optimum membrane area was chosen 80m2 and condensing temperature was chosen -35 ℃. Corresponding, the economic profit can be obtained 1.11 million RMB every year and make the after processing of exhaust gas less than 40 g/m3, what’s more, chloromethane can realize recovery rate 99.25%.