Airlift reactor

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Airlift reactors , also called airlift fermenters, are tower-shaped bioreactors in which biotechnological, mostly aerobic reactions take place under optimized conditions . The most common areas of application are biotechnological and environmental biotechnological processes in which microorganisms have to be cultivated in large systems (> 500m 3 ). In order to create uniform conditions within the reactor, the ingredients are circulated exclusively by blowing in air according to the mammoth pump principle .

construction

Airlift reactors do not have mechanical agitators (see agitated tanks ). The circulation is achieved through a controlled introduction of air within a structurally defined loop. The occurrence of nutrient or oxygen gradients, which would lead to a reduced rate of product formation, should be prevented. Airlift reactors are gassed (at the bottom), the gas escapes again at the top. The resulting hydrostatic pressure difference results in a pump effect that generates a flow of liquid in the fumigated zone.

A distinction is made between airlift reactors with internal and external circulation. In the former, the air is introduced into a vertical core pipe (flow guide pipe) and conveyed upwards via the space enclosed by the pipe. While the air escapes upwards, there is a backflow at the head end via the annular space, which transports the gas-free liquid to the bottom of the reactor. The result is a continuous flow. Optionally, improved degassing can be achieved through a conical extension at the head end.

A further distinction is made between airlift reactors with external circulation, which, depending on the reactor geometry, generate a circulation flow by means of a bubble column.

Applications

  • Production of citric acid : Citric acid is one of the most important biotechnologically manufactured products and is used in the cosmetics industry , for derusting and cleaning metal surfaces or in the food sector. It is produced using molds ( Aspergillus niger ) or yeasts ( Candida spp. Hansenula spp. ). The latter have a shorter process time (approx. 4 hours), while the production by mold takes 6–10 hours. In ( aerobic ) fermentation , beet or cane sugar molasses is primarily used as a source of energy and carbon.
  • Single Cell Protein (SCP) : A SCP refers to proteins that are produced by single-cell microorganisms such as bacteria , yeast or algae . The term encompasses both recombinantly produced and naturally produced proteins by the organisms. Airlift reactors are primarily associated with applications in the food industry. All fermentable carbon compounds that are inexpensive and available can be used as the source of nutrients for the microorganisms in question. E.g .: carbohydrates , lignins , lower alcohols , fatty acids and long-chain n- paraffins , methane or ethane .
  • Wastewater technology : activated sludge process

Further developments

  • Pressure Cycle Reactor : In the course of time, several types of reactors that work on the airlift principle have been developed. The pressure cycle reactor is characterized by a very high oxygen uptake. With a larger scale, a higher circulation capacity is achieved. Pressure cycle reactors are usually around 35 m high and are also popular in wastewater treatment.
  • Deep Shaft Reactor : This type of reactor, which is mainly used in aerobic wastewater treatment , consists of a shaft up to 200 m deep into which the air is blown from above through a vertically arranged flow guide tube. The wastewater flows upwards through the annulus, resulting in a continuous flow. The air required for ventilation is only introduced into the upper third of the reactor and is entrained by the downward flowing liquid. The deep shaft reactor represents the largest possible type of bioreactor, as the biological limits are reached through its size. The high pressure at the bottom of the reactor (approx. 11 bar ) inhibits the growth of the microorganisms involved. Large reactors of this type can have several 1000 m 3 and are primarily used for cleaning chemical waste water ( BASF , Bayer AG ). This enormous size allows a cost reduction of 60–70%.
  • Deep jet fermenter : With this type of construction, the oxygen is introduced via a nozzle which is arranged above the liquid to be gassed. The pumped liquid, which is previously gassed with oxygen, hits the surface of the liquid at high speed (8–12 m / s). The nozzle is either immersed in the liquid (immersion jet) or is located just above the surface (free jet). The fermenter is characterized by a high oxygen input (14 kg / m 3 h) and can have a capacity of several 1000 m 3 . Areas of application here are also wastewater treatment or the production of animal feed (1–2 million tons / year)

literature

Specialist literature

  • D. Wang, W. Liu, B. Han, R. Xu: The bioreactor: a powerful tool for large-scale culture of animal cells. In: Curr Pharm Biotechnol . 6 (5), Oct 2005, pp. 397-403. Review.
  • Y. Martin, P. Vermette: Bioreactors for tissue mass culture: design, characterization, and recent advances. In: Biomaterials. 26 (35), Dec 2005, pp. 7481-7503. Review.

Textbooks

  • Winfried Storhas: Bioreactors and peripheral devices. (= Vieweg textbook biotechnology ). Vieweg, Braunschweig / Wiesbaden 1994, ISBN 3-540-67054-8 .
  • Karl Schügerl: Bioreaction technology: Bioprocesses with microorganisms and cells. Birkhäuser, Basel / Boston / Berlin 1997, ISBN 3-7643-5682-0 .