Analysis of Cryogenic Distillation for the Separation of Methane and Carbon Dioxide

David Belias

4/25/2014

ContentsExecutive Summary2Introduction2Distillation Analysis5Appendix A16

Executive SummaryThis report provides an analysis and evaluation of four separation unit operations. Cryogenic distillation, gas separation membranes, and absorption using water and MDEA were explored. Analysis for distillation utilized Aspen HYSYS, while the other three analyses utilized Wolfram Mathematica. Current market values for all prices were used in the cost analysis to find operational costs and revenues. All calculations can be found in the following discussions. The investigation found that absorption was the most profitable operation due to the highest purity product being sold. Specifically, absorption using water is the recommended option because it produced the greatest profit with the least uncertainty. Distillation had, by far, the greatest costs due to refrigeration and equipment prices. Gas separation membranes had much lower costs, but they were still triple those of absorption. The assumptions used in the analysis of absorption need to be explored further in subsequent examinations to more accurately examine the revenues and costs. In the future, changes in market prices and innovations in industry may yield different results in the capabilities of each unit operation. IntroductionThe separation of methane and carbon dioxide is a recurring industrial process that can be completed using numerous unit operations. In this analysis, Cryogenic distillation, gas separation membranes, and absorption using both water and methyl diethanolamine (MDEA) were investigated in great detail. The feed stream originated from a natural gas well and its properties are outlined in Table A below.Pressure (psia)Temperature (F)Methane Concentration (mol%)Carbon Dioxide Concentration (mol%)Feed Flow (MMSCFD)

494.777861435

Table A. Properties of the feedstock from the natural gas well.

The feed flow was converted to a molar flow using the ideal gas law at standard temperature and pressure (14.7 psia, 459.67 R). Equation A below shows how the molar flow was calculated.

Gases with varying energy levels could be sold for different prices, which are outlined in Table B below.ProductMethane Concentration (mol%)Price ($/MMBtu)Specifications

Waste Gas