;)
;)
Biogas Upgrading Membrane
Biogas upgrading by means of a membrane separator
The process for this separation is based on the separative properties of the hollow fibre membrane modules. The relative dry and clean compressed gas flows through the membrane fibres in which the Carbon Dioxide, moisture and parts of the Oxygen and Hydrogen Sulphide is separated from the Methane.
The principle enabling this separation is the discrepancy in the permeation rate of various gases through the polymer membrane. Carbon Dioxide, Oxygen and water vapour permeate more rapidly than Methane and Nitrogen. The pressurised gas is fed into the fibres and the permeate will go through the membranes. The membrane housing operated at atmospheric pressure and the permeate goes through and open connection to the atmosphere/flare. The membrane housing therefore can not be pressurised and are no pressure parts. The driving force for the separation is mainly the pressure differential over the membrane fibres. Therefore compression of the gas is required.
Compression
In order to achieve the required gas separation the gas shall be pressurised. Before entering the compressor the liquid moisture is separated in a moisture trap (installed in the exciting installation). The gas is entering the compressor with a pressure of approx. 150 mbar overpressure and a temperature of approx. 35 deg C. At the discharge of the compressor the gas is approx. 7.5-8 barg. During compression the liquid in the gas is condensing and the temperature is increasing. In order to reduce the moisture content and to cool down the gas the compressor is cooled with cooling water in individually installed after coolers. Downstream the compressors separators are installed in order to separate the liquid water from the gas. The capacity is controlled with thyristor controllers and on/off control of the compressors. In case the capacity increase additional compressors can be started. Each compressor can be isolated with manual block valves at the in and outlet. The thyristor controller controls the capacity of the compressors. Cooling system The cooling water for cooling the compressors and the compressed gas is a closed loop cooling water system. A fin fan cooler is installed on the top of the container and a circulation pump is pumping the cooling water to the compressors. The cooling water control is in the compressor it self. The water cooled system has the advantage that the container does not require so much openings for cooling air (in case of air cooled compressors). Doing so the noise level to the environment can be kept low.
Filtration
Further filtration of the gas is achieved with a four (4) stage filter train. The first filter is a coalescent filter for removal of particles and droplets with a size of approx. 1 micron. The second stage is a coalescent filter for removal of particles and droplets with a size of 0.01 micron and larger. By means of the coalescing effect the moisture is separated to a relatively dry level (i.e.100% saturated). The third stage is an activated carbon trap. The trap is an adsorber vessel filled with Activated Carbon. The Activated Carbon has the capability to adsorb oil components coming from the lubrication system of the compressor and from the landfill. In the landfill gas parts of hydrocarbons will be present which will be separated in the activated carbon filter. The last stage is a dust filter. This filter is required for removal of dust particles that can come free from the activated carbon filter. These particles need to be removed in order to prevent the membranes for clogging. Each filter is installed with an automatic drain valve.
(Super)heating
The membrane fibres are sensitive for moisture and therefore the gas need to be superheated. The performance of the membranes is also sensitive for temperature effects. By superheating the gas and controlling the temperature of the membranes entering the membranes a constant performance can be achieved. For these reasons a heater is installed for heating the gas to approx. 55-60 deg C. At this temperature the gas is relatively dry and ready for entering the membranes.
Gas separation
The gas is separated to the acceptable level in a number of membrane bundles installed in carbon steel housings. The housings are design to withstand the pressure as generated in the compressors. The separation principles are already explained in the introduction.
Quality Control
The quality of the enriched gas at the outlet of the membranes are depending on feed gas capacity, feed gas pressure, feed gas temperature, membrane temperature and gas composition. This means a lot of parameters do have effect on the quality (CH4 content) of the product gas. Although most of the parameters are kept on a constant level it is impossible to control them all accurately. Especially the feed gas composition is out of our span of control. Therefore a control valve is installed for adjusting the flow over the membranes in order to achieve a constant CH4 content in the product gas, which is required for proper operation of the gas engines. The input signal for the control valve is the CH4 level measured with an analyser (not in our scope). The 4-20mA from the analyser is transferred in a control loop into a 4-20 mA signal the valve positioner. Off-gas In case the case is out of specification the gas is flared in the existing flare facility. This is to avoid CH4 emission to the atmosphere.