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Biogas and Vapor recovery Biogas operation



In waste water treatment Anaerobic Digestion is the most common method used to break down biodegradable material by means of microorganisms in the absence of oxygen. One of the end products is Methane gas.

The digestion process begins with bacterial hydrolysis of the input material in order to break down insoluble organic polymers such as carbohydrates and make them available for other bacteria. Acidogenic bacteria then convert the sugars and amino acids into carbon dioxide, hydrogen, ammonia and organic acids. Acetogenic bacteria then convert these resulting organic acids into acetic acid, along with additional ammonia, hydrogen and carbon dioxide. Finally, methanogens convert these products to methane and carbon dioxide.

The process, wherein digestion is conducted under several atmospheres of pressure, involves the application of Henry’s Law. A recirculation loop that features pressure release degassing of carbon dioxide. Degassed sludge is then pumped under pressure back into the digester. The recirculated rate designed to maintain sludge in the digester in an unsaturated state with regard to carbon dioxide solubility. This keeps the carbon dioxide to precipitating out of the sludge within the digestion tank and results in high purity methane.

Using a liquid ring compressor is a low cost option and it is a more effective stirring method. The bubbling of the methane gas prevents channeling in a digester designed with internal membranes. The methane gas also has elements of the bacteria entrained in the gas stream which is drawn from the top of the digester and pumped in at the bottom to stir the sludge. This will result in higher methane production.

Why use a liquid ring compressor for this application?

With a liquid ring compressor no upstream knock out pots and filters are needed to remove water, water vapor and other small particles from the inlet gas stream. No oil or oil vapor filters are required in the discharge gas stream. Oil contamination could be detrimental in the digester.

Process Vacuum has supplied a fully automated biogas liquid ring compressor package to a local EPC and was installed at a petro chemical plant in South Africa. We conformed to very strict international petro chemical, fabrication and safety standards.

Vapor recovery and well heads

Vapor recovery is the process where vapor is removed from storage tanks through a closed venting system through a vapor recovery system which will recover the vapors for return to the process or destroy them. Vapor recovery is done to reduce noxious and potentially explosive fumes and pollution. These vapors are either environmentally hazardous or valuable. Vapor recovery are also used in the oil and gas industry to provide flash gas recovery at near atmospheric pressure without the potential of oxygen ingress from the top of the storage tanks.

Vapor recovery units are also more commonly used in the oil and gas industry as a means to recover natural gas vapor and converting it to profitable products. There are other liquids that can be used in liquid ring compressors instead of water.

On natural gas heads Dry compressor systems are not capable to handle hydrocarbon liquids or free water due to the incompressibility of any liquids. This could lead to mechanical damage when hydrates, liquids and/or particles enter into the compressor. Natural gas well heads with variable feed composition that could include heavy hydrocarbon tails through-out the life of the field, may have dew point and compressor damage issues on a dry compressor. The possibility of oil contamination from the dry compressor on the discharge is a real possibility. Knock out pots in the suction line of a dry compressor will typically be used to remove and prevent liquids and free water from entering and damaging the compressor. However if the liquids are in a fine mist form the demisters in the vessel is too large to force gravity droplet separation. Gravity settlement is dependent on the droplet diameter fine mists tend to entrain with the gas stream. Outlet demisters may or may not be functional to agglomerate fine mists into larger droplets for liquid mist removal. Particles in the feed could also clog the demister and starve the dry compressor feed.

When wet feeds are expected through the life of the field, dew point fluctuations may be present or condensation is likely, it is therefore advisable to use a liquid ring compressor. Even if the feed contains up to 50% mole fraction of water the liquid ring compressor can accommodate it without concern and can facilitate water condensation and removal. The saturated vapor phase water will condense under pressure and will be removed in the phase separation discharge vessel.

The liquid ring compressor acts as a condenser unit, whilst at the same time compressing the gas. The seal water will also act as a particle trap within the liquid ring compressor and the outlet gas should be free of particles. Liquid ring compressors packages do not require any feed conditioning, feed filters or liquid knock-out pots in the suction line. No oil mist filters and oil traps are require in the discharge line of the liquid ring compressor.

For low pressure well head gas that needs to be pressurized from 100kPag to 300kPag, that contains wet or condensing feed gas the liquid ring will be better suited when compared to other technologies.

Process Vacuum have supplied a Hydrogen liquid ring compressor package on GTL project. This project was also completed to strict international petroleum, Fabrication and safety standards. Process Vacuum has a good track record in the vacuum industry and with our previous experience in Vapor recovery, chemical and petro chemical projects we can supply the equipment to the relative specifications and standards.   Process Vacuum use Cross EPC as engineering partners on large projects. With collaboration of Cross EPC Process Vacuum is capable to deliver engineered systems to the highest quality and integrity across the globe.

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