In-situ product removal by membrane extraction

In bioproduction processes of chemicals and pharmaceuticals, downstream processing usually is a significant cost factor. The products require a high purity (especially biopharmaceutical products), therefore, the process usually contains a large number of separation steps. Moreover, the high costs in downstream processing are caused by the fact that the products are often produced in a dilute environment. Since high product concentrations can cause inhibition of biological growth and production, the product should be removed from the production medium at relatively low concentrations. The use of in-situ product removal (ISPR) is a useful strategy to overcome this problem. Integration of the first downstream process step with the bioreactor leads to direct removal of product during growth and production reactions, potentially increasing the productivity of the biocatalyst and thus the total yield of product. ISPR potentially decreases waste streams, fermentor volume and the stress on micro-organisms resulting from oxygen limitation and shear stress caused by the cycling of the fermentation broth. In addition, decreasing the number of steps in the downstream processing of the product potentially leads to a decrease in the total process costs and processing time. The aim of this thesis is to study the potential of integrated membrane extraction as a tool for ISPR for the removal of products from a fermentation broth. Membrane extraction (pertraction) enables a large contacting surface area between fermentation broth (aqueous phase) and solvent without the formation of an emulsion and is therefore a useful technique for ISPR. The production of phenol by Pseudomonas putida S12 was chosen as a model process to illustrate product inhibition and to demonstrate the effects of ISPR by extraction with 1-octanol. Phenol was chosen as a model component and is a typical example of a fine chemical. It serves as a good model for aromatics containing a hydroxyl group. Additionally, due to its toxicity, phenol can well illustrate the effects of product inhibition. An experimental study to illustrate product inhibition of phenol on the recombinant organism Pseudomonas putida S12 is described in chapter 2. It was demonstrated that the implementation of membrane extraction does not influence growth and phenol production. When phenol is removed from the fermentation broth by pertraction, a lower maximum aqueous phenol concentration is achieved, while the total phenol production increases to 132% as compared to the fermentation without pertraction. There are indications that the volumetric productivity increases slightly in the fermentations with in-situ pertraction as compared to the reference experiments. In chapter 3, detailed calculations on the production of phenol in a conceptual process design illustrate the benefits and disadvantages of ISPR with an implemented membrane extraction unit in a bioreactor as compared to ISPR with a membrane extraction unit outside the reactor. Results show that running the fermentation process at a lower product concentration results in a more efficient substrate utilization into biomass and phenol. The disadvantage of the integrated process is the need for large distillation columns and a high energy input for solvent regeneration due to the low product concentration in the solvent and the high solvent fluxes. Economic evaluations of the two processes show that to obtain a return of investment of 15%, the product cost price of the integrated process is a factor of three lower as compared to the non-integrated process. In chapter 4 mass transfer is studied for phenol in fermentation systems and single fiber modules. Additionally, an approach is given for a novel membrane extraction module design for implementation in a large scale bioreactor by combining experimental and theoretical results. Factors that were found to influence the overall mass transfer coefficient are the membrane wall thickness, solvent (partition coefficient), sterilization and fouling (negative effect). Furthermore, bottlenecks and strategies for improvement are discussed. The integration of an extra obstacle into the reactor can give rise to several bottlenecks for both the separation process and the biological growth and production processes, mainly caused by the altered mixing pattern. In chapters 5 and 6, the use of alternative solvents consisting of polymeric micelles solubilized in water are discussed and an alternative membrane extraction process evaluation is made. The micelles are formed of poly(ethylene oxide)–poly(propylene oxide) (PEO–PPO–PEO) block copolymers, commercially known as Pluronics. Pluronics are water-soluble, nonionic macromolecular surface active agents which are environmentally mild and hardly toxic to micro-organisms. The applicability of aqueous solutions of Pluronics for the removal of phenol in a separation and regeneration process is evaluated. Experimental results show that Pluronic micelles allow extraction of phenol from aqueous solutions at 30 °C (fermentation temperature). The phenol can be released due to the transition of the Pluronic micelles into unimers with a mild temperature switch from 30 to 8 °C. Ultrafiltration membranes provide a barrier between the aqueous Pluronic stripping solution and the aqueous solution in a (bio)reactor containing the desired product. Steady state model analysis and cost estimation show that the process costs are mainly determined by the required membrane area. In chapter 7 the potential of integrated membrane extraction as an in-situ product recovery tool for the removal of products from a fermentation broth is discussed. Furthermore, improvement of the mass transfer limitation at the reactor side by a discontinuous moving membrane module is discussed. Fouling of micro-organisms and medium components at the aqueous (shell) side of the membrane has a negative effect on the overall mass transfer coefficient by increasing the boundary layer thickness at reactor side at the membrane surface. To improve the shell-side mass transfer, the turbulence at the membrane surface can be increased by the use of alternative membrane modules which cause high surface shear rates along the membrane. The novel membrane module described in this chapter shows interesting possibilities in microfiltration to improve the flux by reducing the fouling at the membrane surface. Finally, it can be concluded that integrated membrane extraction shows potential as a tool for the removal of products from a fermentation broth. The benefits of an integrated process will pay off even more for very toxic and inhibiting products that do not allow for high concentrations in the (bio)reactor. The alternative process based on Pluronic micelles can be suited for products that allow for a higher critical concentration in the (bio)reactor as compared to phenol. The resulting higher driving force for membrane extraction will result in a decrease of the overall process costs. For products with a lower solubility in water, recovery is easy after regeneration of the micellar solvent.BiotechnologyApplied Science