CO2 capture using liquid crystals: Selectivity analysis for CO2 capture from syngas

The current need to reduce CO2 emission from power plant flue gas demands development of new and more energy efficient separation processes. Liquid crystals are a new class of solvents for CO2 absorption; making use of a solubility switch between two phases. Solubility of CO2 is higher in the isotropic liquid phase than in the structured liquid phase. Phase transition enthalpy between both phases is low and therefore CO2 capture with liquid crystals has the potential to consume less energy in an absorption/desorption cycle than conventional CO2 capture processes. Liquid crystals enable precombustion CO2 capture at high pressures, hence the capture process can be used for CO2 capture from syngas; gas mainly consisting of CO2 and H2. At this stage research is focused on ideal selectivity and especially on the solubility of CO2 in liquid crystals. Literature study is conducted on experimental phase behaviour of liquid crystal like structures to validate the Predictive Soave-Redlich Kwong equation of state. The Predictive Soave-Redlich Kwong equation of state is used to predict phase behaviour of different liquid crystals with H2 and CO2 to identify the most promising molecular structures, resulting in a selectivity analysis. In the Thermodynamics Laboratory (Process & Energy department, TU Delft) the binary mixtures of pentyl cyanobipenyl, heptyloxy cyanobiphenyl, ethyl propyl bicyclohexyl, propyl butyl bicyclohexyl, hexyloxybenzylidene aminobenzonitrile and phenyl cyclohexyl with CO2, and the binary mixtures of ethyl propyl bicyclohexyl, phenyl cyclohexyl and heptyloxy cyanobiphenyl with H2 are measured using a Cailletet setup. Henry coefficients obtained from the experimental data indicate the solubility of the different structures. Accuracy of the bubblepoint curve prediction by the Predictive Soave-Redlich Kwong equation of state is higher for small molecular structures at relatively low temperatures. The vapour liquid equilibrium prediction has larger deviations for binary mixtures with acetate structures in combination with CO2 and for all binary mixtures with H2; however the bubblepoint curve order is maintained in the vapour liquid equilibrium prediction. The Predictive Soave-Redlich Kwong equation of state simulation predicts that highest solubility for both CO2 and H2 with bicyclohexyl based structures and the lowest with biphenyl based structures. From the Predictive Soave-Redlich Kwong equation of state simulation it is found that more polar structures increase the solubility of both CO2 and H2. The experimental results measured in the Cailletet setup underline the predictions made with the PSRK simulation and demonstrate that weakly polar structures (PCH-type) have a higher solubility of CO2. Highly polar structures (7OCB) are also demonstrating high solubility of CO2 in the Predictive Soave-Redlich Kwong equation of state simulation and experiments. Simulation of the experimental liquid crystals with the Predictive Soave-Redlich Kwong equation of state shows prediction of bicyclohexyl structures phase behaviour is more accurate than predicting phase behaviour of biphenyl and cyclohexylbenzene based structures.