Heat Transfer in Fermentors

At the start of batch fermentor operation, the broth must be heated to the fermentation temperature, which is usually in the range of 30-37 °C, by passing steam or warm water through the coil or the outer jacket. 

The rates of heat transfer between the fermentation broth and the heat-transfer fluid (such as steam or cooling water flowing through the external jacket or the coil) can be estimated from the data provided in Chapter 5. For example, the film coefficient of heat transfer to or from the broth contained in a jacketed or coiled stirred tank fermentor can be estimated using Equation 5.13. In the case of non-Newtonian liquids, the apparent viscosity, as defined by Equation 2.6, should be used. 

It should be mentioned here that the cooling coil can also be used for in situ media sterilization, by passing steam through its structure. 

A fermentation broth contained in a batch-operated, stirred-tank fermentor, with 2.4 m inside diameter D, is equipped with a paddle-type stirrer of diameter (I) of 0.8 m that rotates at a speed N — 4 s 1. The broth temperature is maintained at 30 °C with cooling water at 15 °C, which flows through a stainless steel helical coil that has a 50 mm outside diameter and is 5 mm thick. The maximum rate of heat evolution by biochemical reactions, plus dissipation of mechanical energy input by the stirrer, is 51 000 kcal h1, although this rate varies with time. The physical properties of the broth at 30°C were density p = 1000 kg m viscosity p = 0.013Pas, specific heat cp = 0.90 kcal kg 1 C thermal conductivity k — 0.49 kcal h1 m 1 °C 1 — 0.000136 kcal s 1 m 1 °C 1. 

Calculate the total length of the stainless steel helical coil that should be installed in the fermentor. 

---

Details of heat transfer in fermentors are provided in Chapter 5. 

The raw materials for solid-state fermentation should be sterilized at 121 °C. The sterilization is carried out in situ or in vitro dependent on the fermentor type and scale. In order to enhance the heat transfer in the solid material, the medium pile should not be too tight and direct steam is generally employed. The heating time is in the range 30 - 60 min. For in vitro sterilization, a fixed bed, fluidized bed, rotating drum or belt sterilizer, etc. can be used. 

Coils or other internals may be inserted into the bubble column to promote heat transfer. In addition, the columns may be sectionalized by a baffle system or perforated plates to inhibit liquid phase backmixing or bubble coalescence. Simplicity of operation, lack of moving parts, ease of removal of heat and low operating costs are major advantages of bubble columns as reactors or fermentors. 

In chemical engineering, the terms transfer of heat, mass, and momentum are referred to as the transport phenomena. The heating or cooling of fluids is a case of heat transfer, a good example of mass transfer being the transfer of oxygen from air into the culture media in an aerobic fermentor. When a fluid flows through a conduit, its pressure drops because of friction due to transfer of momentum, as shown later. 

If the rate of heat transfer to or from the broth is important, then the heat transfer area per unit volume of broth should be considered. As the surface area and the liquid volume will vary in proportion to the square and cube of the representative length of vessels, respectively, the heat transfer area of jacketed vessels may become insufficient with larger vessels. Thus, the use of internal coils, or perhaps an external heat exchanger, may become necessary with larger fermentors. 

As this trend levels off with larger columns, it is recommended that values estimated for a 60 cm column are used. If heat transfer is a problem, then heat transfer coils within the column, or even an external heat exchanger, may become necessary when operating a large, industrial bubble column-type fermentor. Scale-up of an internal loop airlift-type fermentor can be achieved in the same way as for bubble column-type fermentors for external loop airhfts see Section 7.7. 

Most industrial fermentors incorporate heat-transfer surfaces, which include (i) an external jacket or external coil (ii) an internal coil immersed in the liquid and (iii) an external heat exchanger, through which the liquid is recirculated by a pump. With small-scale fermentors, approach (i) is common, whereas approach (iii) is sometimes used with large-scale fermentors. These heat transfer surfaces are used for... 

Another recent trend in fermentation is the use of disposable bioreactors instead of stainless steel reactors for process development, especially in pharmaceutical manufacturing (Hanson et al., 2(X)9). Disposable bioreactors are typically plastic devices, such as microtiter plates, T-flasks, shake flasks and wave reactors, with different sizes. Disposable bioreactors can be used as a seed fermentor or as a production fermentor for products on a small scale (Mikola et al, 2007). The main advantages of disposable reactors include more flexibility in operation and use for different products, elimination of CTOss-contamination, less time needed to set up because the reactor is ready to use, low cost and less labor needed. However, disposable reactor sizes do not exceed 2,000L because of physical limitations, stabihty issues and heat and mass transfer limitations, as these reactors do not have impellers for mixing. In addition, disposable plastic reactors may leach chemical components into the media that could negatively impact the quahty of the final product (Hanson et al, 2009). 

---