Ethanol Batch Fermentor

There has been considerable interest in recent years in the concept of producing ethanol from renewable biological raw materials for use as a supplement for fossil-fuel-based gasoline. New laws (as of 2000) even require its addition. A very small fraction of the world s transportation fuel is currently supplied by ethanol. 

Putting these important issues aside, the production of ethanol by batch fermentation is an important example of a batch reactor. The basic regulatory control of a batch ethanol fermentor is not a difficult problem because the heat removal requirements are modest and there is no need for very intense mixing. In this section we develop a very simple dynamic model and present the predicted time trajectories of the important variables such as the concentrations of the cells, ethanol, and glucose. The expert advice of Bjom Tyreus of DuPont is gratefully acknowledged. Sources of models and parameter values are taken from three publications.1 3 

The simple fermentor model has three state variables that change with time during the batch. The first is the concentration of cells X (grams of cells per liter of reactor liquid) that grow during the batch from some initial small value. This is provided by a seed fermentor that is itself a small batch fermentor in which a small number of cells are grown. 

Cysewski and C. R. Wilke, Process design and economic studies of alternative fermentation methods for the production of ethanol, Biotechnol. Bioeng., 20, 1421-1444 (1978). 

Nielsen, J. Villadsen, and G. Linden, Bioreaction Engineering Principles, 2nd ed., Springer, 2005. 

---

Bakers Yeast Production. Bakers yeast is grown aerobicaHy in fed-batch fermentors under conditions of carbohydrate limitation. This maximizes the yield of yeast biomass and minimizes the production of ethanol. Yeasts grown under these conditions have exceUent dough leavening capabHity and perform much better in the bakery than yeast grown under anaerobic conditions. 

Figure 6.35 Intracellular ethanol concentration versus time (Q experimental DPM for an experimental batch fermentor).

Park et al. [251, 252] used a mutant strain of R. eutropha capable of using alcohols as a carbon source for the production of PHB and PHBV in fed-batch fermentors. With phosphate limitation as inducing factor, ethanol was used for the production in 7 L of 46.6 g PHB L (74% of the CDM) in 50 h [251]. When 1-propanol was added to the medium, up to 15.1 mol% in 3HV units were incorporated to the polymer, and when propanol was the sole carbon source, the cells accumulated about 85% of their CDM in P(3HB-co-35.2-mol% 3HV). Both alcohols were completely consumed. In computer-controlled fermentations [252], switches between the alcohols or mixtures thereof led to improved copolymer yield from the substrates and production rates. 

The second type of semibatch reactor is when some material is removed from the reactor during the batch. The material is typically one of the products of the reaction. A common example is in fermentors producing ethanol in which the byproduct carbon dioxide is vented off during the batch cycle. 

Figure 4.37 Matlab program for batch ethanol fermentor.

A software sensor for on-line determination of substrate was developed based on a model for fed-batch alcoholic fermentation process and on-line measured signals of ethanol, biomass, and feed flow. The ethanol and biomass signals were obtained using a colorimetric biosensor and an optical sensor developed in previous works that permitted determination of ethanol at a concentration of 0-40 g/L and biomass of 0-60 g/L. The volume in the fermentor could be continuously calculated using the total measured signal of the feed flow. The results obtained show that the model used is adequate for the proposed software sensor and determines continuously the substrate concentration with efficiency and security during the fermentation process. 

A fed-batch reactor (Fig. 2) is one in which either nutrients only are fed to the reactor or products only are withdrawn. The feed rate or withdrawal rate may be discontinuous as well. One common example of a fed batch reactor is when cells are grown batch-wise until the late exponential phase, then a small amount of feed is added to the reactor continuously to provide just enough nutrients to allow for the production of secondary metabolites. These secondary products are then removed at given time intervals and purified. Other examples of fed-batch reactors include the discontinuous removal of an inhibitory end product such as ethanol from a fermentor, or the timed addition of an inducer to turn on production of a growth associated product after a cell culture has entered midexponential growth. 

Fig. 5. Fed-batch glucose fermentation with S. uvarum in an ALSA fermentor integrated with CO2 stripping of ethanol [18]...

Typically, the SSF process is carried out in a CSTR reactor in batch mode. Under these reaction conditions, the fermentation product, ethanol, exerts its effect not only on microbes but also on saccharification. To overcome this problem, and to improve the efficiency of ethanol production from cellulose, the continuous removal of end-product during ethanol production would have advantages. With this type of process application, the SSF process can be operated in a fed-batch mode. Fed-batch operation is similar to continuous operation except the fermentation broth is retained in the fermentor at all times whereas the solid substrate is continuously fed into the fermentor [73]. Another method is to continuously remove ethanol during the SSF process (see Sect. 2.1.3). 

Damiano, D., Wang, S. S. (1985). Improvements in ethanol concentration and fermentor ethanol productivity in yeast fermentations using whole soy flour in batch, and continuous recycle systems. Biotechnology Letters 7 135-140. 

Other continuous membrane fermentors show similar results. In the continuous production of ethanol from Zymomonas mobilis, the production rate was 120 g/hr/E compared to less than 8 g/hr/E for the batch system.77... 

FIGURE 183 Results of the two-subsirate batch culture by Candida twpicalis ATCC 13803 in a 3.5-L fermentor containing 1 L of fermentation medium (10 g/L yeast extract, 20 g/L bactopeptone, 105 g/L xylose, and 32 g/L glucose) at 30°C, pH 6,500 rpm of agitation speed, and 1 vvm of aeration rate., dry cell mass , glucose , xylose O, xylitol ethanol... 

For example, different fermentation schemes have been developed for the production of ethanol. Conventional batch, continuous, cell recycle and immobilized cell processes, as well as membrane, extraction and vacuum processes, which selectively remove ethanol from the fermentation medium as it is formed, were compared on identical bases using a consistent model for yeast metabolism (Maiorella et al., 1984). The continuous flow stirred tank reactor (CSTR) with cell recycle, tower and plug flow reactors all showed similar cost savings of about 10% compared to batch fermentation. Cell recycle increases cell density inside the fermentor, which is important in reducing fermentation cost. 

---