Anaerobic digestion in wastewater treatment is used to reduce the volume of waste solids and recover energy in the form of methane, carbon dioxide and digestate. In the draft tube mixing system, the compressor is used to provide a steady supply of compressed gas to ensure consistent mixing that maintains a constant temperature throughout the vessel.
Although it requires more power, the all stainless steel liquid-ring compressor offers features and advantages that make it an ideal choice over piston, rotary lobe and sliding vane compressors. The significant benefits are:
- Steady performance under harsh, corrosive conditions. The single most substantial problem for pumps is handling what industry pros call "sour gas." Digester gases are composed of methane, hydrogen sulfide and carbon dioxide. The gases are dirty and corrosive, with entrained foam and soft semi-solids. Of the pump types used, the liquid ring is least affected. It can handle semi-solid carryover, and the stainless steel composition means there is minimal or no corrosion. The gases can break dow n the oil lubrication required for the other pump types, leading to high maintenance costs. A single-stage liquid-ring gas compressor can reliably provide gas flows to 1,800 standard cubic feet per minute at pressures up to 20 psi.
- The ability to handle incoming and discharging gas without cleaning or cooling. No filters are needed on the liquid ring to pretreat incoming gas. There is an added cost in cleaning the gas to tolerable levels before it enters the piston, rotary lobe and rotary vane compressors. As the use of biogas expands, there is a need for more stringent gas purification on discharge, which adds another expense to the other compressor types.
The incoming flow to the compressor is usually 95 degrees F. The liquid ring adds less than 20 degrees F across the compressor to the discharge gas without gas cooling equipment. This helps ensure a constant temperature for the anaerobic organisms, which work only within a narrow temperature range. Other compressor types require gas cooling measures.
- The liquid water seal can be either fresh water or plant effluent. The ability of the compressor to use properly strained and filtered plant effluent can result in considerable savings for the wastewater plant.
- Lower costs vs. other compressor types. The initial investment in a liquid-ring compressor is competitive with other compressors. Energy costs are higher with the liquid ring. However, the cost of maintaining the other types of compressors due to the severe environment is much higher. The extra cost to run the larger liquid-ring motor is easily offset by the maintenance cost of these other types.
The average minimum required maintenance on conventional, nonliquid-ring compressors can be calculated at $200 per horsepower (HP). For a 40-HP compressor, that is a minimum of $8,000 per year. The liquid-ring compressor is all stainless-steel construction with no metal-to-metal contact. This means less wear on the compressor, which translates to less maintenance. With sealed bearings and mechanical seals, the liquid ring has no maintenance required for at least two years, so cost savings start immediately upon commissioning.
- Delivery as a fully automated package. The typical liquid-ring compressor system is set up as a direct drive arrangement with explosion-proof motor, coupling and coupling guard on a structural steel base. The compressor inlet and discharge piping have check valves, temperature gauges, pressure switches and isolation valves. The compressor discharge has a vertical centrifugal- type gas/liquid separator built to ASME code standards and ASME stamped. The separator has a ball float valve that discharges excess water while maintaining a water seal to prevent gas from escaping the float drain. The discharge line has a pressure relief valve to vent excess pressure back to the compressor inlet. The compressor system includes a bypass proportional valve that serves as a flow control device during normal operation. The valve discharges gas back to the inlet when necessary to maintain pressure above the set point of the compressor low-suction pressure shutdown switch. The valve is sized to pass up to 100 percent of discharge flow if required.
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