Process evaluation of novel CO2 capture processes for subsea application

• Main Author: Ommedal, Roy Sømme
• Format: Dissertation
• Language: English

With a subsea module for sweetening of natural gas, it is possible to extract natural gas from places with a too high concentration of CO2 is too remote or both. The separated gas containing a high amount of CO2 needs to be reinjected into the reservoir. In this work, it is given an overview of the different alternative of what can be implemented. Some membrane based process alternative are simulated with polymer membranes at different specification and parameters. To lower the CO2 content down to 8 mol%, with a feed flow of 2MSm3 that contains CO2 of 80-, 50- and 20 mol% and the rest are methane. The membrane area required for a crossflow model were 105 000m2, 188 000m2 and 203 000m2, and for a countercurrent model 94 000m2, 176 000m2 and 200 000m2, respectively. The crossflow model used are from an example in Aspen Custom Modeler implemented in Aspen Plus. The countercurrent model is from literature. For the same cases with multi components, and CO2 remains the same and other components such as C2, C3, C4 and water are added. The membrane area for a crossflow model was 93 000m2, 157 000m2 and 158 000m2. A case with a two stage membrane system to bring the retentate flow to natural gas specification (2mol% CO2) with feed content of 80mol% CO2. That case required a membrane area of 106 000m2 and 246 000m2 and a recycle compressor at 718kW. An advantage of this is that the natural gas reaches sales specs. However, this leads to more equipment used where the focus is to minimize equipment. For the case with 20mol% CO2 in the feed, an estimated subsea module would cost ~62 mill. USD with operational cost annually ~1.5 mill. USD. With a potential theoretical income of ~45 mill. USD with natural gas annually, this case seems promising. For the case with 80mol% CO2 in the feed, an estimated subsea module would cost ~146 mill. USD with operational cost annually ~5.8 mill. USD. With a potential theoretical income of ~10 mill. USD with natural gas annually, this is not economical in the view of natural gas income. However, if there is a marked for CO2 enhanced oil recovery in some nearby field, the CO2 rich gas could be sold.