In a new field such as Biochemical Engineering, considerable confusion often arises over the definition of a number of terms. Biotechnology is usually taken to mean the use of sophisticated genetic techniques outside the cell to result in the production of useful products. However, many people broaden the term to include applied biology, or even engineering. Strictly speaking, biochemical engineering is usually defined as the extension of chemical engineering principles to systems using a biological catalyst to bring about desired chemical transformations. It is often subdivided into reactor design and downstream separation.
Despite the appearance that it is a new discipline, biochemical engineering as such dates back to the turn of the century, when certain rudimentary principles were used in the biological treatment of wastewater and in the production of acetone and butanol for explosives. Furthermore, fermentation ethanol was one of the basic raw materials for the chemical industry until the Second World War. The modern genesis of the discipline dates back to the early 1940s when penicillin was required in vast quantities.
The recent development of biochemical engineering is the direct result of advances in molecular biology (e.g., recombinant DNA, tissue culture, protoplast fusion, monoclonal antibodies) and protein engineering. Biochemical engineers work in a wide cross-section of industry including; pharmaceuticals, food, fine chemicals, wastewater treatment, mining and energy. Hence, biochemical products vary from high volume, low coast to very low volume and extremely high cost (e.g., Factor 8, the blood clotting protein costs around $108/kg!). Figure 1 details this wide variation in cost.
Figure 1. Concentration in the broth versus final selling price. (Reprinted by permission from J. L. Dwyer, “Scaling Up Bioproduct Separation with High Performance Liquid Chromatography,” Bio/Technology, vol. 2, p. 957, 1984.)
Biochemical engineering involves the use of a catalyst, either an enzyme or whole cell, which is either freely suspended in an aqueous medium or “immobilized” in a gel or attached to a solid surface. In contrast to an enzyme, whose activity decreases over time, cells are autocatalytic (dx/dt = μx, where x = cell concentration and μ = growth rate, t–1) and can grow exponentially with excess nutrients. Reactor conditions are typically: a pH of 6–8; a temperature of 15–16°C; atmospheric pressure; and a cell concentration of 2–40 g/l. Reactors can be operated in either batch mode (most common in the pharmaceutical industry), typically over 24–48 hours, or continuously (e.g., wastewater treatment). Reactors commonly used vary from CSTRs (continually-stirred tank reactors), through plug flow (whose efficiency is similar to CSTRs due to back mixing), to specialized reactors which immobilize the cells or enzymes to prevent them from being washed out of the reactor due to their small size and density (SG = 1.04). These types of reactors essentially “de-link” the hydraulic retention time from the cell retention time, thereby enabling process intensification to occur.
The downstream separation of biological products is one of the major challenges in biochemical engineering. These products are often present in very low concentrations (100 mg/l), and are extremely labile to mechanical shear and extremes in pH and temperature. The difficulty of the separation process depends very strongly on the product itself and whether it is the biomass, an extracellular product excreted by the cell, or an intracellular protein or inclusion body. Since most fermentation broths are extremely complex mixtures of a variety of organic and inorganic constituents and most products have to meet extremely stringent standards in terms of purity — especially the therapeutics — recovery and concentration can often involve 10–15 unit operations. This can lead to quite low overall recoveries (10–20%); hence, separation can constitute as mach as 70% of the overall product cost. The separation techniques used include those based on size, diffusivity, charge, surface activity, density and polarity, and often include techniques rarely seen in the chemical process industry (see Figure 2). (See also Liquid-Solid Separation.)
Figure 2. Ranges of applications of various unit operation. (After B. Atkinson and F. Mavituna, Biochemical and Biotechnology Handbook, Macmillan Publ. Ltd., Surrey. England, 1983, p. 935.)
The modeling of most biochemical engineering processes is still fraught with difficulties, although simple enzymatic reactions can be represented by the theoretically derived Michaelis-Menten relationship.
ν = rate of reaction, t–1;
νmax = maximum rate of reaction, t–1;
Km = Michaelis-Menten, or half rate constant, ML–3;
c = concentration of substrate, ML–3.
However, the modeling of whole cell reactions is far more complex since cell metabolism consists of many sequential reactions (103–104), any one of which may be rate limiting at any point in time. In addition, the release of product may be kinetically associated with cell growth (“growth associated”), or occur after growth has virtually ceased (“secondary metabolite”), or be a complex mixture of both. In modeling growth kinetics, it is possible to take two approaches: using an “unstructured” model, which takes an uncritical “black box” stance; or developing a “structured” model, which is based more on the basics of cell metabolism. Due to its simplicity and relative success, the most commonly-used cell growth model is the unstructured one developed empirically by Monod:
μ = rate of cell growth, t–1;
μmax = maximum rate of cell growth, t–1;
Ks = half rate constant, ML–3;
c = concentration of rate limiting substrate, ML–3.
While empirically derived, the equation has obvious similarities to the Michaelis-Menten equation for enzyme kinetics.
Bailey, J. E. and Ollis, D. F. (1986) Biochemical Engineering Fundamentals, 2nd edition. McGraw-Hill.
Atkinson, B. and Mavituna, F. (1991) Biochemical Engineering and Biotechnology Handbook, 2nd edition. Macmillan Publishers.
Kennedy, J. F. and Cabral, J. M. S. (1993) Recovery Processes for Biological Materials. John Wiley and Sons.
Moo-Young, M. (1985) Comprehensive Biotechnology. Pergamon Press.