Process modes for a bioreactor

Process Modes for a Bioreactor 538 Binary Batch Distillation Column 490 Bubble Point Calculation for a Batch Distillation Column 504... 

The low volumetric productivities that characterize batch cultivation processes are a disadvantage for the use of this operation mode for production. However, a variant known as repeated batches is an interesting alternative. It consists of initially carrying out a batch cultivation for the time needed to attain the desired product concentration. At that moment, just a part of the bioreactor contents is harvested. The remaining cell suspension inside the bioreactor is then used as inoculum for a new batch, by filling the vessel with fresh medium. This procedure can be repeated several times, until a decrease in cell growth or product formation is observed. The use of repeated batches allows a decrease of the time the bioreactor is non-productive. This eliminates the time periods that would be necessary for cleaning and sterilization between each batch. 

The design of bioreactors for perfusion operation is more sophisticated, which makes the equipment more expensive. However, the productivity increases obtained by perfusion operation allow the use of much more compact systems than those operated under batch or fed-batch mode. In this way, perfusion bioreactors can be up to 10-fold smaller for a given production scale (Bibila and Robinson, 1995), decreasing the costs not only of the bioreactors themselves, but also of storage tanks and downstream processing equipment. 

Sterilized and introduced to a bioreactor or fermenter, is typically equipped with agitators, baffles, air spargers, and sensing devices for the control of the operating conditions. A pure strain of microorganisms is introduced into the vessel. The number of cells multiplies exponentially after a certain period of lag time and reaches a maximum cell concentration as the medium is depleted. The fermentation is then stopped and the contents are pumped out for the product recovery and purification. This process is operated either by batch or continuous mode. 

Batch bio reactors—This is the simplest bioreactor process. In this mode, all nutrients needed for cell growth are formulated into the basal medium and added to the production reactor with the cell inoculum at the initiation of the run. The cells grow for a finite period and are harvested after the nutrients become limited and the cell viability decreases to a predetermined level. Although simple to operate, batch mode is least likely to be used as other modes of operation yield higher product titers. 

The ability of Monod s empirical relation to fit kinetic data for biochemical reactions has its foundations in generalizations of two phenomena frequently observed for fermentation processes (1) nature places a cap on the quantity of microorganism that can be achieved during the exponential phase of growth in a bioreactor operating in a batch mode and (2) as the concentration of the limiting substrate approaches zero, the rate laws for biochemical reactions approach pseudo-first-order behavior with respect to that substrate. The cap indicated on the cell growth rate has been associated with the natural limit on the maximum rate at which replication of DNA can be achieved. 

The addition of parallel units can increase the efficiency of the plant considerably. Let us consider a simple biorefinery that is operated in zero-wait mode. Each batch is the same (1000 kg). In the first stage, the biomass is converted in a bioreactor to ethanol, which takes 12 h. In the second stage, a distillation is performed to pnrify the bioethanol. This separation step takes 3 h. The Gantt chart of the process is depicted in Figure 12.5. It is obvious that the CT for each batch is 12 h. 

Biological reactions that involve microorganisms and enzyme catalysts are pervasive and play a crucial role in the natural world. Without such bioreactions, plant and animal life as we know it simply could not exist. Bioreactions also provide the basis for production of a wide variety of pharmaceuticals and healthcare and food products. Other important industrial processes that involve bioreactions include fermentation and wastewater treatment. Chemical engineers are heavily involved with biochemical and biomedical processes. In this section we present a dynamic model for a representative process, a bioreactor operated in a semi-batch mode. Additional biochemical and biomedical applications appear in other chapters. 

A full set of bioreactors with pH and temperature controllers are shown in Figure 1.3. The complete set of a 25 litre fermenter with all the accessory controlling units creates a good opportunity to control suitable production of biochemical products with variation of process parameters. Pumping fresh nutrients and operating in batch, fed batch and continuous mode are easy and suitable for producing fine chemicals, amino acids, and even antibiotics. 

Recently, approaches to operate wave bioreactors in perfusion mode have been proposed. This turns this type of bioreactor into an attractive variant when there is a need for rapidly establishing a process, on a relatively small scale, and there are no robust facilities for the supply of auxiliary services. 

---