Review
Process operation performance of PDMS membrane pervaporation coupled with fermentation for efficient bioethanol production

https://doi.org/10.1016/j.cjche.2018.12.005Get rights and content

Abstract

There would be strong product inhibition on ethanol fermentation process if ethanol is not removed in situ from broth. PDMS membrane pervaporation coupled with fermentation is a promising process for efficient bioethanol production since ethanol inhibition is relieved or eliminated. From the perspective of process operation, membrane separation performance, ethanol fermentation performance and the subsequent processing on membrane downstream are the three key issues. This review aims at contributing a comprehensive overview on the operation performance of the integrated process. The state-of-the-art of the three key issues related to the operation performance is focused. Finally, the tentative perspective on the possible future prospects of the integrated process is briefly presented.

Introduction

The transportation sector worldwide is almost entirely dependent on petroleum-based fuels and it is responsible for over than 50% of the world oil consumption [1]. It is predicted that motor vehicles account for more than 19% of global carbon dioxide emissions and 70% of global carbon monoxide emissions [1], [2]. Bioethanol with carbon neutral, derived from a renewable biomass, is considered an important way of progress for improving air quality, limiting greenhouse gas emissions, and finding new energetic resources. As a transportation fuel, ethanol has a higher octane number, higher flame speeds, broader flammability limits, and higher heats of vaporization [1], [2]. Ethanol could be combined and blended with petrol within unmodified spark-ignition engines or burned in its pure form within modified spark-ignition engines [3].

Batch, fed-batch and continuous operations are three fermentation modes [4]. Batch fermentation is the simplest operation mode. During the process, nothing is added after inoculation except possibly acid or alkali for culture pH control. The yeast cells works in initial high substrate concentration and final high ethanol concentration. Yeast cells work at low substrate concentration with an increasing ethanol concentration during the course of fed-batch fermentation process. The major advantage of fed-batch over batch fermentation is the ability to prolong culture life time, reach the maximum viable cell concentration and allow a higher concentration of accumulative fermentation product. Continuous fermentation can be performed in different kinds of bioreactors with stirred tank reactors in single or series. In the process, feed containing substrate, culture medium and other required nutrients, is pumped continuously into an agitated vessel. Continuous fermentation can give a higher productivity than batch fermentation and the highest productivities can be achieved at low dilution rates.

During the above fermentation processes, the accumulation of ethanol in the broth inhibits the cell growth and fermentation, leading to low ethanol productivity, low ethanol concentration in the broth, and large amount of wastewater treatment. In the subsequent process, a lot of energy is consumed for product recovery from the broth. Ethanol fermentation is a biochemical reaction process controlled by product concentration, and ethanol accumulated in the broth has a serious inhibition effect on fermentation. During conventional fermentation process without ethanol in situ removal from broth, the cell metabolic activity would be weaken leading to the decrease of ethanol productivity with ethanol accumulation. The inhibition of ethanol on yeast began to appear, if ethanol concentration in the broth reaching 60 g·L−1–80 g·L−1. The final ethanol concentration can hardly reach 140 g·L−1 during batch fermentation [5]. Several separation technologies, including gas stripping, extraction, adsorption, distillation and pervaporation, have been explored for ethanol removal in situ from the broth during fermentation to meet the requirement of reducing the cost of ethanol recovery [6], [7].

Pervaporation is one of the most promising approaches for the recovery of alcohols from fermentation broths [8]. It is simple, nontoxic to cells, and potentially less energy consuming than distillation [9]. The feed or fermentation broth is forced flowing on one side of the membrane, and a gaseous phase permeate would be released. In order to supply a driven force between the membrane upstream and downstream sides, a vacuum or sweep gas less common is exerted at the other side of the membrane. Polydimethylsiloxane (PDMS) is the current benchmark hydrophobic material for ethanol pervaporation, since it has good stability and nice tolerance to the organic solvent [10], [11]. Ethanol in situ removal by pervaporation can reduce ethanol concentration in the broth, relieve inhibition, prolong culture time and enhance fermentation process. In recent years, several reviews have provided the state-of-the-art of the membrane material and fabrication for bioalcohol pervaporation [8], [12]. The operation performance of the integrated process is of great importance, since it could provide significant information for process design and regulation. However, so far there is no review focused on the operation performance of the integrated process. The main objective of this review is to provide the recent insights about the operation performance of PDMS membrane pervaporaiton coupled with ethanol fermentation for bioethanol production, which includes the separation performance of PDMS membrane, ethanol fermentation performance with in situ removal and subsequent processing on the downstream of the membrane.

Section snippets

Mass transfer steps and model

The mass transfer of ethanol from the upstream of the membrane to the downstream of the membrane during pervaporation could be included four steps, as shown in Fig. 1: (1) transferred through a liquid boundary layer by convective mass transfer, and ethanol concentration in liquid bulk (cb) was decreased to the concentration in liquid boundary layer (cbm) at interface of liquid boundary layer/membrane; (2) sorpted into the upstream surface of the membrane, and ethanol concentration was increased

Enhanced fermentation performance

The schematic diagram of fermentation coupled with PDMS membrane pervaporation is illustrated in Fig. 3. Compared with conventional batch fermentation or continuous stirring tank reactor (CSTR) operation, there are five advantages for ethanol fermentation coupled with pervaporation: increase of ethanol productivity, increase of cell density, continuous operation for a long time, reduction of the amount of wastewater treatment and energy consumption required for subsequent purification [36], [62]

Purification for fuel grade ethanol

In general, ethanol concentration on the downstream of the PDMS membrane is in the range of 20 wt% and 40 wt% [41]. Ethanol concentration on membrane downstream could not meet the requirement of fuel grade ethanol, since ethanol concentration must be over 99.5 wt% for fuel grade ethanol. Distillation would be the common approach to increase ethanol concentration to the azeotropic point (95.5 wt%). However, distillation is energy extensive since liquid evaporation and vapor condensation are

Conclusions and Perspectives

PDMS membrane pervaporation coupled with fermentation is a promising process for bioethanol production. Solution-diffusion model is the most common theory for the description of ethanol mass transfer from the upstream to the downstream of the membrane. In order to increase the mechanical strength of the PDMS membrane, several polymer and inorganic materials, such as Cellulose acetate (CA), Polyamide (PA), Polysulfone (PSF), Polyetherimide (PEI) and ceramics can be used as the supported layer

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    Supported by the National Natural Science Foundation of China (Nos. 20176030, 20276041, 20776088, 21808144), China Postdoctoral Science Foundation (No. 2016M592710), Fundamental Research Funds for the Central Universities (No. 20822041B4013) and Key Laboratory of Development and Application of Rural Renewable Energy, MOA, China (No. 18H0491).

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