Fundamentals of Biochemical Engineering, Part 1 - Bailey, J., & Ollis, D. 1989

Transport Phenomena in Biotechnological Systems

While studying the material of the previous chapters, we gradually moved from relatively simple (molecular) systems to increasingly complex ones—first to individual Cells, and then to cultures containing millions or billions of cells per milliliter. The physical Separation of system components (nutrients, cells, metabolic products) increases the likelihood that the overall rate of conversion of substances dissolved (or suspended) in the reaction mass will be influenced less by The rate of Chemical Reactions than by various physical transport processes. Indeed, according to Weisz's data, cells and their constituent catalytic complexes are characterized by Thiele moduli approximately equal to unity [1]; in other words, cells and subcellular structures function at maximum rates with practically no diffusion limitations. Therefore, if we, for example, increase The amount of carbon-containing nutrients in a microbiological process, aerobic cells will be able to fully assimilate them only if a sufficiently high oxygen concentration is maintained in the immediate vicinity of the cells. Given that the oxygen solubility in aqueous media is very low, it seems quite likely that this condition can be met only by increasing the mass transfer efficiency in the gas-liquid system, thereby ensuring an accelerated supply of oxygen to the cells.

Obviously, the boundary separating regions dominated by aerobic or anaerobic processes is determined by the local oxygen concentration, the diffusion coefficient of O₂, and local Respiration rates in the aerobic region. In the case of obligate aerobes, such as Molds in mycelial pellets or tissue cells in malignant tumors, this boundary simultaneously separates viable cells from dying ones; it also determines the depth of the region dominated by aerobic processes in the surface layers of lakes and separates coexisting aerobic colonies from anaerobic bacterial populations in soil particles. The beginnings of modern work on The Role of oxygen mass transfer in biological processes can be attributed to the 1940s, when industrial penicillin production was unfolding at the height of World War II. It has now been established that oxygen mass transfer also plays a major role in numerous natural processes, including food spoilage caused by oxidative processes and lake eutrophication associated either with inadequate aeration of the system by natural oxygen sources or with elevated concentrations of certain substances, such as phosphates or nitrates.

Other sparingly soluble gases can also play a major role in microbiological processes. For instance, in the PRODUCTION OF SINGLE-Cell Proteins, methane and other Hydrocarbons are used as substrates. In this important process, the culture broth must be continuously saturated with both oxygen and methane at a rate sufficient to meet the needs of the microorganisms. The removal of methane from solution is an important step in anaerobic waste Treatment; The final stage of this process is the decarboxylation of carboxylic acids (primarily acetic acid) to the corresponding alkanes.

Carbon dioxide is generated in almost every microbiological process. Despite the high solubility of CO₂, the mass-transfer-coupled interconversions between gaseous CO₂ and Various Forms of dissolved carbon dioxide (CO₂, H₂CO₃, HCO₃^-, CO₃^2-) significantly affect pH; this fact is of no small importance when regulating pH in processes involving acid-sensitive anaerobic organisms that release both CO₂ and CH₄ simultaneously. We will consider such processes in Chapter 14.

Liquid–liquid mass transfer plays a major role in The production of single-cell proteins from liquid hydrocarbons, as well as in the isolation of microbiological process products, such as in the extraction of culture broths or their filtrates with organic Solvents in pharmaceutical manufacturing (Chapter 11).

The biotransformation of renewable natural raw Materials (e.g., agricultural and wood-chemical Processing wastes such as cellulosic, hemicellulosic, and Lignin fractions) used as feedstock in various microbiological processes typically involves stages (biomass solubilization, liquefaction, Hydrolysis) whose rate is limited by the solid substrate surface area available to the Reagents and the diffusion rates of dissolved solutes. In addition, mass transfer between the liquid and solid phases plays a specific role in various sorption and chromatographic Methods for the Isolation and Purification of substances, as well as in The transport of dissolved oxygen to mold pellets or to films and pellets of immobilized cells.

At high cell population densities, mass transfer can become the rate-determining factor for the entire process. Such a situation is characteristic of A wide variety of processes—from laboratory shake flasks to massive industrial reactors intended for the production of penicillin or extracellular Biopolymers (such as xanthan) and activated sludge wastewater treatment plants. It follows that when developing and designing biological reactors, the engineer must clearly understand whether the process rate is determined by transport phenomena or by The kinetics of biological transformations.

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FIG. 8.1. MAIN STAGES OF oxygen transfer from a gas bubble to a cell cluster.

Often, dissolved solute diffusion and momentum transport phenomena are closely intertwined; in an even more complex variant, these phenomena are also coupled with chemical reaction kinetics. Structure/149.html">The problem of the interrelationship between diffusion and reaction was examined by us in Chapter 4; from this perspective, enzyme and cell activity (expressed, for example, as the effectiveness factor) can be fully described using two parameters—the Thiele modulus and the saturation parameter K_s/s₀. Unfortunately, in the vast majority of cases, mass transfer phenomena are intimately linked with momentum transfer, i.e., fluid mechanics; indeed, this topic occupies a major portion of the chemical engineering literature. Here we will limit ourselves to outlining the Basic Concepts and providing tables of formulas used to calculate or estimate solute Mass transfer coefficients.

In the concluding brief section of this chapter, we will examine Examples of processes in which The behavior of biological systems is significantly influenced (via non-uniform Temperature distribution) by another important transport phenomenon—Heat transfer. Such examples include relatively exothermic microbiological processes, such as wine vinegar production in a plug-flow Reactor, wastewater treatment, self-heating of compost piles (as well as municipal and other wastes), and other processes involving a solid phase.