Design of the air preparation system for SSF bioreactors

Abstract

All solid-state fermentation (SSF) bioreactor types potentially need an air preparation system. The need is most obvious for forcefully aerated bioreactors, since the conditions of the air at the inlet have a strong influence on the heat and mass transfer phenomena within the bed. However, even those bioreactor types that are not forcefully aerated can benefit from an air preparation system. For example, although it is possible to operate tray bioreactors by circulating air taken directly from the surroundings, it is likely that the process will operate better if conditioned air is circulated through the headspace. The considerations guiding decisions about the air preparation system are different for SSF and submerged liquid fermentation (SLF). In SLF the rate at which air is blown into the bioreactor is calculated based on the O2 demand of the microorganism; heat removal considerations do not influence decisions about the aeration rate. In other words, in SLF good temperature control can be attained by circulating hot or cold water through water jackets and cooling coils, without any need to supply or remove heat in the air stream. This is possible due to the diluted nature and favorable heat transfer conditions within the liquid fermentation medium: the medium is typically well agitated and has a high thermal diffusivity. In contrast, in SSF few alternatives are available for heating or cooling of the substrate bed other than manipulating the temperature, flow rate, and humidity of the inlet air. At first glance, it does not seem to be a difficult task, especially given the high heat removal capacity of evaporative cooling; however, we may be restricted in terms of the values that we can use for these two operating variables, even in a bioreactor operated in the intermittently-mixed mode. For example, if we supply air that is not saturated with water vapor to the bioreactor, this will improve evaporative heat removal but will also accelerate the drying of the bed, increasing the frequency with which water must be added. However, frequent water addition can be undesirable. If the bed is not mixed during the addition of water, then it will be almost impossible to ensure uniform distribution of the water, leading to flooded and dry regions within the bioreactor. Addition of water as a spray while the bed is being agitated may allow uniform distribution of water, however, frequent agitation will typically be deleterious to the performance of processes that involve fungi due to the mechanical damage caused to fungal hyphae by shear and impact forces within the bed. In order to minimize this damage, the frequency of mixing and water addition events should be minimized. However, this means that we should use saturated air at the air inlet in order to minimize the evaporation rate. Therefore the operating strategy must seek to find those conditions that give a reasonable rate of heat removal without causing undue damage to the microorganism. Note that even if saturated air is to be used, it is not necessarily easy to keep the air saturated if one intends to vary its temperature. In short, in SSF the air stream has a role that goes beyond the supply of O2. It plays a fundamental role in heat removal and the design of an adequate air conditioning system is essential for good operation and control of an SSF bioreactor. © Springer-Verlag Berlin Heidelberg 2006.