The purification of gas from Hydrogen Sulfide (H2S) and Carbon Dioxide (CO2) is one of the most pressing problems in the biomethane power plants and in the entire oil production and refining industry.
Today, there are the following main methods to purify biogas: the method of liquid (wet) and solid (dry) chemical absorption of impurities (absorption and adsorption) and the method of membrane separation and freezing (cryogenic method).
In this case, the high initial investment (average cost of the adsorbent), the complexity of technological schemes, the amount of gas to be treated, as well as high management costs (for example, solvent recovery) and maintenance, lead us to the choice of a membrane system.
There are several advantages of using the membrane technology:
– Provides high quality gas
– Needs minimal control and maintenance
– Very small investment
– Low energy consumption
– High reliability
– Service life of membranes from 5 to 8 years
– No moving parts
– Doesn’t need to use any chemicals or water
– Modifiable system by adding or reducing the number of modules
– Simple, automated and remotely controlled process
– Easily customizable
Thanks to this technology, ATP was able to develop a system with CAPEX and OPEX smaller than competing technologies, allowing the combined removal of H2S and H2O in one equipment (membrane) instead of two units in series, and a significant costs and consumption reduction.
The Cleaning Process
The cleaning process takes place in 4 stages.
In the first step, before entering the electric heater, the feed gas is pretreated to remove liquid fractions and solid particles using a coalescing filter equipped with a cartridge-type filter element. Particles larger than 0.1 µm must be removed to avoid condensation or membrane fouling. When passing through the cartridge, solid particles are captured, while water-containing compounds agglomerate into larger droplets and fall to the bottom of the filter.
The second step in the process is heating the gas to be treated. Heating the fuel gas is necessary to optimize the efficiency of the membrane separation, since the separation efficiency depends not only on the pressure, but also on the temperature of the fuel gas at the inlet to the membrane module. The set temperature for heating the air is 45 ° C.
At the third step, filtered and heated fuel gas enters the fiber membranes. At this stage, the gas is purified of hydrogen sulfide. Supplying gas at high pressure is introduced from the side of the shell of the hollow fiber, and impurities are removed as low-pressure gas from the holes. Due to its molecular size, the methane remains inside the fibers and is collected at the outlet of the membrane module, while H2S/H2O and other impurities penetrate the outer fibers and are ejected through the outlet hole of the membrane permeate.
The fourth step serves to control the purity of the fuel gas. The maximum permissible of H2S content is 150 ppm (rpm).
Membrane separation of gas mixtures is based on the use of membranes that allow a selective permeability of the gas mixture components, which penetrate through the membrane at different rates. During gas preparation in the membrane module, the initial feed stream is divided into two streams: a low-pressure one (permeate) that penetrate through the membrane, and a residual one (retentate). The driving force of the process is the difference between the partial pressures of the gas components in the high (HPP) and low (LPP) pressure cavities. This inverse selective behavior has been used to design successful systems that allow the separation between different gases.
The efficiency of membrane technology application depends on the task and conditions of the site and, as a rule, reaches the APG utilization rate of more than 95%, and in some tasks of natural gas drying by water – up to 100% (excluding liquid phase).
In general, it can be notice that once again the main advantage of a membrane technology-based filtration unit, is the ability to solve a set of problems, whether they are associated with the preparation of APG, or with the regulation of its components, or even related to its utilization.