Bioreactors

While bioreactor is a general term for any reactor that uses a biological process catalyzed by microbes to produce a desired product, the term typically refers to biological applications of continuous stirred tank reactors or chemostats, plug flow reactors, or fixed film reactors. For more information on these types of reactors please refer to the CSTR PFR or FFR pages of the encyclopedia. A fermenter, often used to convert sugars to acids or alcohols, is the most common type of bioreactor due to its simplicity.

(Copyright GEA Process Engineering Inc., Columbia, MD)

Fermenters

The fundamental function of a fermenter is to provide a suitable environment in which microorganisms can efficiently produce target products such as ethanol through the metabolization of sugars by yeast.

JVNW Fermentation Tanks.png

(Copyright JVNW, Canby, OR)

General Information

Fermenters can be run in either batch or continuous modes, depending on the application, and can be agitated either mechanically , with an impeller, or pneumatically , using injected gas or fluid. Some of the most common types of fermenters are listed below.

  • Airlift fermenters forego standard mechanical mixing systems for a stream of gas injected into or outside of a riser tube at the bottom of the tank. The rising action from the gas causes the contents of the tank to circulate around and through the riser tube, thus mixing the system.

Diagram of a riser tube within an airlift fermenter

  • Stirred tanks are the most basic fermenters, consisting of a vessel with a 3:1 aspect ratio and a mixing system that incorporates an impeller driven through the base or top. The top also has ports for the addition of reactants, as well as instrumentation.
  • Tower fermenters are characterized by large height to diameter ratios, upwards of 15:1, and aeration occurs by the introduction of gas streams at the bottom of the tower
  • Bubble columns are continuous tower fermenters. Gas is continuously sparged in from the bottom and acts to agitate and react with the downward flowing liquid stream

Equipment Design

A typical fermenter, as shown below,  consists of a stainless steel housing containing an impeller or agitator for uniform mixing of microorganisms and nutrients. Inlets are located at the top of the body, a sparger is present at the bottom to introduce gas streams into the liquid if needed, and probes measure and control various process parameters. Often times a jacket will surround  the body to allows process engineers to facilitate heating or cooling of the slurry contained within.

Diagram of a typical stirred tank fermenter

Usage Examples

Fermenters are used in a variety of industries that require biological manufacturing, such as the pharmaceutical and food industries. In the pharmaceutical industry they are used to grow cells and bacteria for drugs such as antibiotics or penicillin. They can also be used for alcoholic fermentation in breweries or even to grow yeast for bread-making.

( Pictures courtesy New Brunswick Scientific Co., Inc. , Edison, NJ)

Advantages

Disadvantages

  • Simple, inexpensive batch design
  • Process parameters can be easily varied
  • Can be used for a variety of biological reactions
  • Agitation from aeration as cost-saving means
  • Can operate at lower temperatures compared to reactors using non-biological reagents and processes
  • Long retention times
  • Constant monitoring
  • Difficult sampling
  • As process parameters change with time, microbes can begin to produce unwanted products
  • Contamination of microbes can ruin batches and result in large costs
  • Higher mixing levels can damage microbes

Other Bioreactors

In addition to fermenters, some types of  bioreactors use more complex processes to achieve an efficient mixing to increase reactor conversion. These reactors rely on fluidization, membrane filtration, and the recycling of moving bed particles, among other methods.

General Information

Chemical processes in bioreactors can be aerobic, anaerobic, or a hybrid of the two. Bioreactors typically involve the flowing of a liquid-phase through a medium containing microbes that catalyze the process. These microbes are typically affixed to the surface of particles such as granular activated carbon (GAC) or larger industrially-produced plastic pieces. Later on, the media is filtered out and the liquid product is recovered . A significant portion of operating costs goes toward controlling fouling, the unwanted buildup of filtrate. Three of the most commonly used bioreactors are :

  • Fluidized Bed Bioreactors, in which gas or fluid is bubbled through a bed of biological media. This action causes the bed to rise slightly and behave as a fluid. This allows for a longer retention time of reactant in the bed, as well as better contact between reactants, and therefore a higher conversion.
  • Membrane Bioreactors, where a biological reaction occurs in a medium and then a membrane is used to filter out the product based on molecule size. Membrane pore sizes can range from 0.4-0.01µm. Over time the pores can be fouled , which can raise costs due to constant cleaning.
  • Moving Bed Biofilm Reactors, such as the one pictured below, which allow carrier pieces, to which biofilms are affixed, to flow as a bed along with the reactant fluid through the system. As the carriers and liquid flow, the reaction occurs. The carriers are then filtered out using a sieve at the exit and recycled back to the reactor entrance .

bioprocessH2O_RI_MBBR Media Tank.JPG

(Copyright BioprocessH2O, Portsmouth, RI)

Equipment Design

The design of bioreactors varies upon the type of bioreactor used but there are still commonalities between the different units. Commonly, gases are sparged in from the bottom plate of a unit, and reactants flow in through the top. Often bioreactors will incorporate some carrier, such as those pictured below, that has microbes fixed to its surface and acts as the site of the reaction. These carriers are often made from high density polyethylene, as it s density is very similar to water.

(Copyright PEWE LLC, Camas,WA)

Usage Examples

Bioreactors can be used to create biodiesel from algae. In the tubular algae bioreactor shown below, algae within the tubes grow with the assistance of a light source and carbon dioxide inputs. Once enough algae has grown, it is scraped from the tube and used for its natural oils.

(Copyright W2 Energy, Inc., Carson City, NV)

Advantages

Disadvantages

  • Can be designed for many different processes
  • Can operate at lower temperatures compared to reactors using non-biological reagents and processes
  • Continuous operation more feasible than in fermenters
  • Require constant monitoring
  • Difficult to pull samples
  • More complex than fermenters thus more difficult to control
  • Microbes can produce unwanted products as process parameters change

Acknowledgements

GEA Process Engineering Inc. , Columbia, MD

New Brunswick Scientific Co., Inc. , Edison, NJ

W2 Energy, Inc. , Carson City, NV

Process Engineered Water Equipment LLC , Camas, WA

BioprocessH2O , Portsmouth RI

Mott Corp. Farmington, CT

References

Banz, Gregory. "Piloting Bioreactors for Agitation Scale-Up." Chemical Engineering Progress. 104.2 (Feb. 2008): 32-34. Print.

Daigger, Glenn T. Membrane Bioreactors. Alexandria, Virginia: Water Environment Federation, 2011.

McNeil, B. and Harvey, L. Practical Fermentation Technology. Chichester, England: Wiley, 2008.

Stanbury, Peter F. Principles of Fermentation Technology . Tarrytown, New York, USA: Elsevier Science inc. 1984.

Winkler, M. A. Chemical Engineering Problems in Biotechnology. Essex, England: Elsevier Science inc. 1990.

Developers

Sam Catalano

Kelsey Kaplan

Thomas Plegue

Amani Ramli

Joel Holland