(301c) Introduction of an Organophilic Membrane System in an Ethanol Production Process Based on Lignocellulosic Biomass

Ethanol production from lignocellulosic biomass is a promising path to produce this alcohol for the purposes of clean energy generation. However, the production of ethanol via this route represents a challenge in terms of energy consumption and achievable purity, especially in the separation stages of the process. The concentration of ethanol after the fermentation stage ranges from 5 to 10% w/w depending on the available substrate in the fermentation broth and the strain used ^[1]. Conventional separation techniques such as distillation are widely used to separate ethanol from the fermentation broth, but the achievable concentration of the distillate by using this separation method may not be high enough for biofuel standard purity (>99%). Membrane-based separation techniques have been widely investigated for industrial applications as they are less energy intensive ^[2]. One of the most widely employed techniques for the separation of alcohols is pervaporation, and in particular using hydrophilic membranes to dehydrate ethanol beyond the azeotropic point reached during the distillation ^[3].

This work aims to implement an organophilic membrane module between the simultaneous saccharification and co-fermentation (SSCF) stage and the separation stages (distillation combined with hydrophilic membrane separation) to intensify the production of ethanol by adding a new unit that will allow a reduction of the overall energy demand of the process. The new unit will also increase the productivity of ethanol during the fermentation stage by reducing the inhibitory effects of this alcohol and the associated impurities on the fermenting microorganisms. The combination of fermentation and hydrophilic membrane system has been widely studied showing promising results in terms of ethanol production, but the impact of this combination on the overall process has so far not been considered ^[4,5].

The overall process configuration consists of a SSCF reactor, an organophilic membrane module and a separation section consisting of either distillation and/or a hydrophilic membrane process. The permeate stream from the organophilic membrane is sent to the subsequent separation stage to purify the ethanol. The retentate stream (mainly reducing sugars and other large molecules that do not permeate) is recycled back to the SSCF reactor to further the ethanol production by using all the available substrate.

In order to consider the relative merits of the configuration, a dynamic model is developed of the overall process taking into account each individual processing step. Whilst distillation modelling is generally straight-forward, membrane modelling is not due to the intrinsic characteristics of the membrane properties. This relates in particular to the organophilic membrane as the impact of the numerous impurities from the fermentation broth (e.g glucose, xylose, acetic acid, glycerol and xylitol) on the performance of the membrane is not clearly understood, and hence very difficult to model.

This work introduces a mathematical model of an organophilic membrane PERVAP 4060, provided by Sulzer, which is to be used to remove ethanol from the fermentation broth taking into account the effects of the impurities on the performance of the membrane. The organophilic membrane used in this work is a composite membrane consisting of a very thin separation layer (0.5 – 5 µm) on top of a porous support, coated on a polymer fleece. A number of experiments have been conducted at different feed concentrations, temperatures and permeate pressures to determine the model parameters which have been estimated using gOPT in gPROMS.

The optimal design and operation of the overall process is then considered for a given lignocellulosic biomass feedstock based on different alternative plant configurations, including with and without the organophilic membrane and with and without a hybrid separation stage, showing that a) the introduction of the organophilic membrane improves the overall performance of the process in terms of reduced energy consumption and improved product yield, and b) the use of a hybrid separation stage also improves the overall performance.

References

[1]. Baia FW, Anderson WA, Moo-Young M. (2008).  Ethanol fermentation technologies from sugar and starch feedstocks. Biotechnology Advances, 26: 89–105.

 [2]. Kaewkannetra P,  Chutinatea N, Moonamarta S, Kamsanc T, Chiud TY. (2011). Separation of ethanol from ethanol–water mixture and fermented sweet sorghum juice using pervaporation membrane reactor. Desalination, 271. pp: 88–91.

[3]. Wei P, Cheng LH, Zhang L, Xu XH, Chen HL, Gao CJ. (2014). A review of membrane technology for bioethanol production. Renewable and Sustainable Energy Reviews, 30: 388–400.

[4]. Fan S, Xiao Z, Zhang Y, Tang X, Chen C, Li W, Deng Q, Yao P. (2014).  Enhanced ethanol fermentation in a pervaporation membrane bioreactor with the convenient permeate vapour recovery. Bioresource Technology, 155: 229–34.

[5]. Gaykawad SS, Zha Y, Punt PJ, van Groenestijn JW, van der Wielen LAM, Straathof AJJ. (2013). Pervaporation of ethanol from lignocellulosic fermentation broth. Bioresource Technology, 129: 469–76.