Bio-separations

Modern biotechnology enables the use of engineered microorganisms such as E.coli, yeast and algae for the production of chemicals that are currently derived mainly from fossil fuel feedstocks. Processes that employ such biological routes (“bio-based chemicals”), as opposed to complex conversion steps from fossil fuel feedstocks, could in some cases be economically promising. Additional advantages of bio-processes include mild production conditions and selectivity toward a specific product. However, the effluent of bioreactors is dilute (containing less than 20 wt% product), and thus the downstream separation tends to be expensive (it usually accounts for 60–80 % of the total production cost). Past work on the synthesis of bio-separation processes have been mainly focused on specific products. There has been limited research towards the systematic treatment of the general process synthesis problem. To this end, we develop a general framework, based on superstructure optimization, for the synthesis of bio-separation processes (see Figure 1).

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Figure 1. General framework for the superstructure optimization based synthesis of bio-separation processes.

Specifically, based on general separation principles and insights obtained from industrial processes for specific products, we first identify four separation stages: Stage 1 – cell treatment, where cells are harvested and then disrupted to release intracellular products (present if the product is intracellular; bypassed if the product is extracellular); Stage 2 – product phase isolation, where the phase that contains the product is isolated; Stage 3 – concentration and purification, where water and impurities are removed; Stage 4 – refinement, where the product is further refined. Based on the four stages, we first perform a stage-wise analysis of general bio-separation processes. Then, for each stage, we systematically implement a set of connectivity rules to develop stage-superstructures, all of which are then integrated to generate a general superstructure (see Figure 2) that accounts for all types of chemicals produced using microorganisms. We further develop a superstructure reduction method to solve specific instances, based on product attributes, technology availability, case-specific considerations, and final product specifications (see an example in Figure 2). A general optimization model, including short-cut models for all types of units considered in the framework, is then formulated.

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Figure 2. The general bio-separation superstructure (including the “dimmed” parts), and the reduced superstructure (excluding the dimmed parts) for an example instance. The product in the initial product stream fed to the separation network is extracellular (EX), insoluble in water (NSL), light (LT, i.e., with density lower than that of water), non-volatile (NVL, i.e., with volatility lower than that of water), a liquid at normal condition (LQD), and a commodity chemical (CMD). The product is required to be completely colorless in its final product form, and all the technologies in the general superstructure are available except for filtration. The boxes represent units, and the labels in them denote the unit types, e.g., Dst (distillation), Mbr (membrane), Ext (extraction), and Ads (adsorption). Units that function together for a common major task are grouped into a module (represented by a dashed rounded rectangle), and the corresponding label denotes the product attributes that are applicable to the module, e.g., the “NSL LT” module is only applicable to products that are NSL and LT.