Membrane contactor replaces adsorption in process technology
A membrane gas/liquid contactor has been developed that processes and separates components in gas streams as well as exhaust gases. The developers are Kværner Oil & Gas and W.L. Gore & Associates GmbH (Gore), which have begun commercialization of the process.
Conventionally, a widely used technology for processing gas streams is the absorption process, which uses a liquid as the solvent to absorb and remove undesired components. Physical and chemical solvents are used for gas treatment. The largest part of the absorption process is the absorber, where gas is brought into contact with the liquid and components from the gas are absorbed by the liquid.
In order to minimize size and weight of the absorber component and reduce costs in these processes, Kværner and Gore developed the membrane gas/liquid contactor. Lab testing began in 1993 and plant trials on real natural gas and exhaust gas streams began in 1998. The next step is a full scale commercial installation this year.
Technology
The alkanolamines are the most generally accepted and widely used of the available solvents for removal of carbon dioxide. Sour gas enters the bottom of the absorber column and flows up through the absorber in counter-current contact with the aqueous amine solution. The amine absorbs the carbon dioxide on the way down.
The amine is pressure released, degassed, and routed through a heat exchanger to the desorption tower, where the solvent is heated by vapor from the reboiler. The reboiler vapor strips the carbon dioxide from the rich solution. The carbon dioxide and the steam leave the top of the desorber and pass overhead through a condenser, and the carbon dioxide is separated in a separator and vented to atmosphere or sent to compression or to processing. The lean amine is pumped through the heat exchanger and back to the absorber.
The design of a conventional contactor is as follows: The diameter of the contactor is dictated by the maximum allowable gas velocity before the gas starts to entrain the liquid, or the gas pressure drop is too large. A certain liquid area is needed to obtain the required purity of the gas leaving at the top.
The absorber column contains a packing material. The packing is designed to ensure distribution of the liquid, and at the same time provide a high contact surface (packing density, liquid area/volume). The height of the column is in principle dictated by the needed volume of packing to obtain the needed liquid area and the needed "empty" volume in the column (usually about 1/3 of the volume). By introducing the membrane gas/liquid contactor some of the main restrictions in the design of contactors are changed:
- The diameter is reduced by allowing higher gas velocity; no contact between the gas and the liquid, which means no entrainment and lower pressure drop. The height is reduced.
- The volume needed to obtain the required liquid area is reduced due to a very large packing density (tower 100-250 sq meters/cu meter; membrane contactor 500-1500 sq meters/cu meter). The "empty" volume in a conventional column is eliminated in the membrane contactor.
Membrane gas/liquid contactor
Kværner and Gore say the essential element in the membrane gas/liquid contactor is the microporous membrane. The membrane wall keeps the gas phase and the absorption liquid from each other. The gas stream is fed along one side of the membrane. The components to be removed from the gas stream will diffuse through the membrane. On the other side of the membrane, they will be absorbed by the liquid.
The absorption in the liquid phase is determined either by physical absorption or by chemical reaction. According to the developers, the use of a gas absorption membrane has several advantages over conventional contacting equipment (columns):
- High packing density: 500-1,500 sq meters/cu meter; column: 100-250 sq meters/cu meter)
- High flexibility with respect to flow rates and solvent selection
- No foaming, channeling, entrainment, or flooding
- High turndown ratio
- Unit is insensitive to motion
- Flexibility with respect to orientation of the unit(s)
- Savings in weight.
Natural gas treatment
For natural gas treatment (gas sweetening and dehydration), the membrane technology has been tested in two field pilot plants. The performance of the membrane absorber technology is proven in the pilot plants, both for gas sweetening and gas dehydration. The developers say the results confirm previous theoretical and laboratory results, and a scale-up shows a potential of 75-85% reduction in vessel size and weight for gas sweetening and 60-80% reduction for gas dehydration.
Ongoing long-term duration tests show no decrease in performance, and the same membrane has been in operation for 5,000 hours in parallel to a conventional gas sweetening column, the developers point out. There is no additional processing or cleaning of the gas prior to entering the contractor, and it is therefore the same quality as the gas entering the treatment column.
Field testing
The first field test location was a gas terminal north of Aberdeen, Scotland. There, the test unit was fed with a slip-stream of sour gas and lean amine from one of the plant's existing gas treatment trains. The gas was processed in the membrane test unit, and then returned to the plant. The main design parameters were: pressure - 88 barg; gas flow - 5000 Ncu meters/hour; liquid flow - 5 cu meters/hour.
Field testing also was carried out at the Shell Fandango gas field in Zapata, Texas during the spring of 1999. The scope of the testing at Fandango was to verify: gas sweetening, using a physical solvent (NFM-NAM); gas dehydration using glycol (TEG); and the performance and the reliability of the membrane protection system.
Exhaust gas treatment
For exhaust gas treatment, the membrane technology was tested in a number of laboratory units and in one field pilot plant. The results of the field test were very good, say the developers, and substantiated the target weight and size reduction.
A larger scale pilot unit for exhaust gas treatment from a gas engine (520 kW) has been in operation at the Statoil Gas Terminal at Karstø, beginning in the autumn of 1998. The pilot unit is a complete stand-alone amine process incorporating both conventional columns and membrane contactors absorber and desorber. The exhaust gas flow treated is 2,610 kg/hour, and 85% (195 kg/hour) of the carbon dioxide is separated from the exhaust gas.
