Polyvinyl chloride (PVC) is one of the most mass producing thermoplastics in chemical industries in the world. As a feed stock of PVC production, 1,2-dichloroethane (EDC) is mainly manufactured by either the direct chlorination or oxychlorination of ethylene. However, conventional processes have some problems such as sublimation of catalyst, byproducts and explosion. Therefore, conventional reaction should be divided into two reactions such as ethylene chlorination by reduction of cupric chloride and re-oxidation of cuprous chloride by hydrogen chloride and oxygen.
The hydrodynamic properties and reaction characteristics of the two-step oxychlorination have been determined in a circulating fluidized bed reactor.
For the hydrodynamic properties study, the flow regimes in the reactor as a function of gas velocity have been determined. Also, the effects of gas velocity ($U_g$) and solid circulation rate ($G_s$) on solid phase holdup were determined. The minimum fluidized velocity ($U_{mf}$) and particle terminal velocity ($U_t$) decrease with increasing temperature but the maximum pressure fluctuation in a bubbling bed ($U_c$) and the transport velocity ($U_{tr}$) between slugging to turbulent fluidization flow regimes have reverse trends.
The catalyst was prepared from the commercial product of cupric chloride that was immersed in γ-alumina powders (MEDC/B, SUD-CHEMIE Inc.). Ther mean particle diameter is 70μm with particle density of 2000 $kg/m^3$. The amount of copper loaded on the alumina carrier is 4.12 wt% determinied by ICP (Inductively coupled plasma) analysis. The mixture of catalyst and the inert material (Zirconia) was packed in the reactor tp prevent particle agglomeration, A CFB reactor was consisted of a bubbling bed (0.076 m-ID × 1.5 m-high) and a riser (0.025 m-ID × 3.0 m-high). Hydrogen chlorine, oxygen and nitrogen as the reactant gases for oxidation in the bed. The ethylene conversion and EDC selectivity were determined from analyzing the off-gas from the riser by gas chromatograph. Manometers were connecter to pressure taps along the riser height to measure pressure fluctuation in the riser. From the pressure drop measurement, solid holdups ($ε_s$) at different heights in the riser were determined.
The optimum reaction condition for the two-step oxychlorination have been determined with variations of reaction temperature, ethylene concentration, gas velocity and solid circulation rate in the riser.
With variations of reaction temperature and ethylene concentration at the reaction temperature above 200℃ and the ethylene supply with 20% excess of the stoichiometric requirement, the maximum ethylene conversion is 90% and EDC selectivity is 60%. The ethylene conversion and EDC selectivity increase with increasing solid circulating rate. On the other hand, the ethylene conversion and EDC selectivity decrease with increasing gas velocity in the riser. In the bubbling flow regime, the resulted low EDC selectivity was caused by short gas residence time in the bed so that more than 10 s reaction time is needed for high EDC selectivity. Therefore, the optimum conditions for two-step oxychlorination in a circulating fluidized bed were chosen at the reaction temperature above 200℃ under 20% excess of stoichiometric requirement of ethylene with high solid circulation rate and more than 10 s of reactant gas residence time in the riser.
폴리염화비닐(PVC)은 5대 열가소성 플라스틱의 하나로 그 수요가 날로 증가할 것으로 예상되는 소재 중 하나이다. PVC 생산의 공급 원료 중 하나인 1,2-디클로로에탄(EDC)은 직접 염소화법과 에틸렌의 옥시염소화법에 의해 주로 제조된다. 그러나, 일반적인 과정은 촉매 소실, 부산물 그리고 폭발의 위험성과 같은 몇몇 문제를 가지고 있으므로 염화수소와 산소에 의한 염화제이구리의 환원과 염화제일구리의 산화(촉매 재생공정)에 의한 2단계 옥시염소화 공정의 개발이 대두되었다.
본 연구에서는 2단계 옥시염소화 공정의 구현을 위한 순환유동층 반응기(bed: 0.076 m-ID × 1.5 m-high ; riser: 0.025 m-ID × 3.0 m-high)에서의 수력학적 특성을 연구하여 반응 실험에서의 기초자료를 획득한 뒤 반응실험을 수행하였다.
촉매로는 γ-alumina 를 지지체로 사용하는 cupper 계열의 촉매(MEDC/B, SUD-CHEMIE Inc.)를 사용하였으며 실험변수로는 상승관에서의 기상유속, 고체순환속도, 반응기체인 에틸렌의 농도, 반응 온도를 설정하였다.
그 결과 순환유동층 반응기에서는 200 ℃ 이상의 반응 온도와 양론비 보다 20 % 이하의 에틸렌 농도 공급, 20 kg/ m2s의 고체순환속도, 2.5초 이상의 기체 체류시간을 만족할 때 90 % 이상의 에틸렌 전환율과60 % 이상의 EDC 선택도를 가짐을 알 수 있었다. 또한 기포유동층과 난류유동층에서의 반응실험을 비교해 본 결과 기체 체류시간이 10초 이상이 될 때 EDC 선택도가 90% 이상이 되는 것을 알 수 있었다. 따라서 보다 높은 EDC 선택도를 위해서는 순환유동층 조업 시 상승관의 높이를 증가시키는 것이 바람직 할 것으로 예상된다.
