Intensified green process for synthesizing non-phosgene hexamethylene-1,6-dicarbamate

Hexamethylene diisocyanate (HDI) is an important chemical building block in the production of high value-added polyurethane because of its some excellent properties. Traditionally, the HDI synthesis route involves extremely toxic phosgene as an industrial scale reagent. Recently, thermal decomposition of hexamethylene-1,6-dicarbamate (HDC) has been considered as the most attractive non-phosgene process for the HDI synthesis because HDC can be synthesized by reacting 1,6-hexamethylene diamine (HDA) with a green compound, dimethyl carbonate (DMC), instead of toxic phosgene. Therefore, the intermediate HDC synthesis is crucial to the whole process. In the study, design of two plant-wide processes is explored for green HDC synthesis. In the first process, HDC is produced in two reactive distillation (RD) columns in series. Complete conversion of HDA is designed and excess DMC reactant is used to increase the reaction conversion. The mixture of un-reacted DMC and by-product methanol from RD column tops is separated by a series of conventional distillation columns containing a pre-concentrator column and an extractive distillation system comprising an extractive distillation column and a solvent recovery column. In the second process, RD is enhanced by vapor recompression (VR) to reduce energy consumption. The key cost saving from VR is attributed to fully take advantage of small temperature difference between RD column bottom and top, and higher RD column top temperature rendering condenser duty totally utilized by RD column bottom. Furthermore, additional latent heat of RD overhead vapor is released to the extractive distillation system by external heat integration, which can then achieve significant saving in the operation cost of this system. Compared to the RD intensified process, total annual cost of the RD + VR intensified process can be reduced by 35%.