Objective
This project was understood as a supplement to previous projects, which are dealing with liquefaction of natural gas and associated gas on offshore sites. With said previous projects conventional gas scrubbing with alkanol amine solution followed by adsorptive gas drying were provided for the purification of feed gas. The aim of this project was to ascertain, whether there is a reduction in investment and operating costs, if the conventional gas purification is substituted by one with gas separating membranes. An incentive for this investigation was the fact, that in the case of offshore process plants, the comparatively little space requirement of a membrane battery and the simplicity of its operation are of a greater influence upon the overall economy of LNG production, than in the case of land based process plants.
Summary: also for offshore process plants, the performance of up to now commercially available membrane moduls is not sufficient and their price too high to justify their use for the purification of LNG-plant feed. For the case 1 plant (on TLP) surplus costs of approx. 700,000 DM p.a. were calculated, compared with the annual costs of the original LNG-plant with conventional gas purification. Only for the case 2 plant (in subsea housing) which of course represents a very special application, savings of approx. 250,000 DM p.a. were calculated.
DETAILS ABOUT CASE 1 (TLP):
The so called superstructure deck (13 m above the main deck) could be abandoned. The membrane modules are housed in a room below deck, thus leaving the space on the main deck, which was previously occupied by the amine contacting plant, for other equipment. With the original plant a greater part of the subdeck rooms could not be utilized. The overall dimensions of the platform remained unchanged, the reduction of the mass of steel is not significant.
DETAILS ABOUT CASE 2 (subsea concrete housing):
The diameter of the spherical housing was reduced by 2 m, resulting in a reduction of concrete mass from 10,100 m3 to 8,100 m3. Main reason: the membrane battery, which can be composed of modules of different length, can be fitted into wedge shaped spaces near the spherical wall of the casing, which cannot be used for other equipment.
The annual energy costs of the non plant are about 500,000 DM less than those of the conventional plant, due to the process heat needed for the regeneration of amine solution. In the case of the TLP-plant this saving does not count, because process heat is recovered from the gas turbine exhausts, i.e. by waste heat. The subsea plant has only steam as source of energy, so all extra energy needs extra fuel.
Evaluation of laboratory state membranes: the permeability data of one such membrane, as stated by its developer, were used to optimize a combined permeation/adsorption plant as described above. Accordingly the performance of this membrane (separation factor CO2/CH4 about 7 times, permeation rate for CO2 nearly 2 times the corresponding values of marketed membranes) would be well above the breakeven point, provided its price would be of comparable magnitude as that of current prices of gas separating membranes.
The said previous projects, forming the background of this project, are:
1. a LNG production plant, resting on a tension leg platform
2. a LNG production plant, housed by a spherical concrete casing, which rests on the sea bottom.
The process data which were considered for the 2 cases were respectively:
case 1 - feed rate 71,000 Kg/hr; 93.63% CH4; 1.79% C2 H6
case 2 - feed rate 83,000 Kg/hr; 79.60% CH4; 10.20% C2 H6
Level of purity which has been set for cryogenic section design: 1 mol-ppm H2O; 6 mol-ppm H2S; 150 mol-ppm CO2.
Since the required purity cannot be achieved by permeation separation alone, a combination of membrane batteries and absorbers was applied, the membranes as a first stage and the adsorbers for the final purification to the above specification. The regenerating gas leaving the adsorbers is fractionated in an extra membrane battery. The residue out of this is recycled to the feed, the permeate is used as fuel gas.
A multitude of possible configurations of membrane batteries, adsorbers and regenerating gas cycles were evaluated for optimization. Optimizable parameters are among others: interface concentration of impurities (between the permeation and the adsorption stage) permeate pressure(s), number and operating sequence of adsorbers, length of time of the operating cycle of the adsorbers, respectively mass of their beds. A restriction is, that the quantity of hydrocarbon contained in the produced waste gas must not exceed the fuel gas demand of the liquefaction process. Otherwise energy would be lost through the flare.
In the main part of the study only commercially marketed membranes are considered. Some of such membranes, with improved qualities, which are described in public or company literature but not yet used in an industrial scale, are dealt within an extra part of the study.
The optimized permeation/adsorption plants are incorporated into the LNG production plants and rearranged layouts of the overall plants were prepared, inorder to evaluate the possible reduction of space requirement for each of the 2 cases. Then the offshore structures - TLP in case 1, subsea concrete housing in case 2 - were adapted to the new plant layouts, savings in steel respectively concrete mass evaluated.
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Coordinator
1000 Berlin
Germany
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