The use of biotechnology in one or more key process steps in chemical manufacturing is growing strongly and likely will account for a significant proportion of total chemicals output by 2025. Flowschemes typically comprise a series of linked process areas, each of which may include one or more unit operations (see Figure 1). The nature of these may vary from the relatively simple (e.g., medium preparation) to the more complex (e.g., fermentation, centrifugation and freeze drying).
Chemical manufacture using enzymes and microorganisms can involve a very different set of operating requirements and conditions from those of traditional methods of synthesis. So in this article, we'll review some of the major differences and discuss how these impact pumps, valves and mixer seals.
Where biomass is the raw material, preparation typically will involve crushing, chemical treatment to break down the cell structure, and conversion into a liquid or slurry to permit transfer to the bioreactor. The properties of the resulting liquid or slurry, the subsequent processing operations, and parameters (e.g., temperatures and pH) will determine the technology and materials to be used in the pumps, valves and seals. For example, with high-viscosity slurries, it may be more appropriate to use a progressing cavity pump, whereas a plastic-lined centrifugal pump may be preferable where solids' content is lower and pH is acidic.
A particular issue for second-generation feedstocks such as corn stover and other cellulosic materials is the often-abrasive quality of the biomass. When harvested, stover can include significant quantities of sand (up to 6%), which may not be removed completely at the separation stage. This can result in significant wear on the pump casing and impeller.
Therefore, many bio-based ethanol producers standardize on ASTM A995, grade CD4MCuN (DIN SEW 410) for pumps for performance and for inventory cost-reduction purposes. This duplex stainless steel is harder than austenitic varieties, offering improved wear resistance. It also has greater pitting and crevice corrosion resistance than austenitics and other duplex stainless steels. Suitable alternatives include: ASTM A995, grade CD4MCuN with hard coating; CR29 hard chrome cast iron (>500 Brinell); ASTM A744, grade CN7M (Alloy 20) (WN 1.4500); ASTM A744, grade CF8M (316 stainless steel) (DIN 17445); ASTM A395, grade 60-40-18 (DCI); and fluoropolymer.
In traditional chemical synthesis, temperatures, pressures and pH can vary significantly. Reaction temperatures can range from below 0°C (32°F) to more than 200°C (390°F), while pressures can extend from below atmospheric to several hundred bar.
By contrast, in biotechnology the conditions in the fermentation process tend to be within a much narrower range of temperature and pH, and with pressure at or near ambient. This is because enzyme activity is particularly sensitive to temperature and pH, and conditions outside of a limited range result in rapid enzyme degradation (denaturing).
It is at the fermentation stage where the operating conditions and requirements — and demands on pumps, valves and seals — can differ significantly from those of traditional chemical synthesis. For successful fermentation, sterilization of the process plant equipment, in addition to that of the nutrient media, fermenter additions (such as anti-foaming agents) and air (for aerobic processes), is essential. At current levels of output of bio-based chemicals, batch sterilization dominates. For large-scale fermentation in up to 10,000-L (2,650-gal) bioreactors, continuous sterilization is employed wherever possible, although semi-continuous/batch-fed operation may be more appropriate where nutrient delivery/culture removal is difficult to balance.
Table 1 summarizes some of conditions faced by pumps in various services in a bioethanol plant.
In conventional chemical production, pump casings tend to be cast and, depending upon the reactants and conditions, are made in a wide variety of materials, ranging from cast steel through stainless steel to materials such as zirconium and titanium. In terms of design, the emphasis is on efficiency rather than ease of cleaning. Pumps typically are attached to pipework via bolted flanges. However, for biotechnology applications, pump design focuses on cleanability. This limits material options, particularly in the fermentation stage where sanitary pump designs are almost exclusively austenitic stainless steel. Flow rates and heads tend to be smaller than those of their traditional chemistry counterparts; depending upon requirements, threaded, bolted or clamped connections to external pipework can be used.
For pumps, ease of cleaning influences the design of the volute (wetted part of pump) as well as the internal surface finish. In batch fermentation, it's essential that all associated process plant equipment is thoroughly cleaned at the end of the batch. Clean-in-place/sterilize-in-place (CIP/SIP) designs are important to ensure the desired level of cleanliness, to maintain sterility of the atmosphere within the pumping system while minimizing downtime.