Design and optimization of a separation process for butanediol dehydration for use as a biofuel.

Abstract

Ongoing research in incorporating renewable biofuels into the transport sector are fuels that can be used interchangeably with petroleum derived fuels. These fuels are termed “drop-in” fuels and can be used in the pure state or as a blending component. Diols such as butane-1,4-diol and butane-2,3-diol have been identified as appropriate drop-in fuels in various transport applications as they can improve octane numbers and heating values of the fuel blend. The butanediols are generally produced by the energy intensive process of chlorohydrination of butene with a subsequent hydrolysis step or hydrogenation and hydrolysis on the industrial scale. A potentially lower energy-impact process for the production of these diols is the biochemical process route which involves the fermentation of biomass (a renewable feed) by certain classes of bacteria. A low concentration aqueous mixture of the butanediols is produced, that must be dehydrated before use. Conventional distillation can be used for the dehydration and subsequent purification step, but the process is energy intensive as high-pressure steam must often be used as the heating medium, due to low concentrations of the butanediols and their high boiling points relative to water. Hence, there is merit in exploring lower-energy alternate separation schemes. The most promising options presented in the literature are hybrid techniques involving solvent extraction using butan-1-ol and recovery by distillation to first remove excess water and subsequently concentrate the butanediol product composition. However, those processes were designed based on model parameters extrapolated mostly from liquid-liquid equilibrium data only, and a limited set of vapour-liquid equilibrium (VLE) data. This yielded broadly qualitative designs in the literature. To improve this, in this work, novel isothermal VLE experimental data were measured for the binary systems of water/butan-1-ol in combination with the butanediol component species; butane-1,4-diol and butane-2,3-diol, utilizing a dynamic-analytical apparatus at sub-atmospheric conditions. For the binary systems of water (1) + butane-1,4-diol (2)/butane-2,3-diol (2), measurements were performed at temperatures ranging from 353 – 373 K. For the binary system of butan-1-ol (1) + butane-1,4-diol (2)/butane-2,3-diol (2), measurements were performed at temperatures ranging from 353 – 388 K. Temperature ranges were selected to maintain conditions up to atmospheric pressure which are commonly used in industry for these applications. For both sets of binary measurements, the P-T-x-y data was modelled using the γ-Φ approach. To account for the liquid-phase non-ideality, the Non-Random Two-Liquid and Universal Quasi-Chemical activity coefficient models were used while the Hayden and O’Connell correlation in the virial equation of state was used to account for the non-ideality in the vapour-phase. For all binary systems considered in this study, the experimental P-T-x-y data was concluded to be of good quality as thermodynamic consistency tests such as the area test and point test ii

were passed with tolerances of below 10 % and 0.01, respectively, and the root mean square deviations in pressure and the absolute average deviation values in the vapour-phase mole fraction was found to be within the experimental uncertainty in these measurements. The binary parameters regressed from the experimental VLE data were used to improve the simulated separation design to purify butane-1,4-diol and butane-2,3-diol from the aqueous mixtures that result from the biological process pathways proposed in the literature. This was executed by exploring the design potential of a hybrid extraction-assisted distillation separation process in comparison to conventional distillation. Separation techniques such as conventional distillation, heterogeneous azeotropic distillation and liquid-liquid extraction are utilized in the novel proposed separation process. To achieve the dehydration of the butanediol constituents, butan-1-ol was used as the solvent in the liquid-liquid extraction step. The design of the separation process was performed using Aspen Plus® and optimized using standard procedures to reduce duties and costs. The simulation was used to investigate the technical and economic feasibility of the process with further optimization of the design by considering heat-integration. Conventional distillation was found to be the most economically feasible process alternative for the butane-1,4-diol purification, with an estimated total annual cost in the range of $4,532,846.67 and $4,635,070.52 for a payback period of 3 years, while extraction assisted distillation with heat integration was found to be the economically viable option for butane-2,3-diol purification with total annual costs in the range of $2,997,204.58 and $3,988,868.70 for a payback period of 3 years.