(646a) Overcoming Heat Exchanger Issues within a Cryogenic CO2 Capture Process

Various methods are being developed to reduce CO2 emissions from energy and chemical industry processes. A novel concept being explored desublimates CO2 from a light gas stream such as flue gas by adjusting the lowest temperature in the system to determine the CO2 capture efficiency. Phase changes that occur at temperatures from 135 to 185 K at 1 bar are utilized in a manner which captures CO2 with greater efficiency than realized in alternative processes. This process is called Cryogenic Carbon Capture (CCC). A concern for the CCC process design involves the inability of commercial heat exchanger models to account for phase changes that will occur in the CCC heat exchangers. For processes which involve: no condensation, liquid condensation, and desublimation; commercial models have shown various capabilities in providing design specifications for estimating equipment sizing and energy requirements. For instance, should a desired temperature profile be made strictly linear and matched with available commercial software the change of temperature that would be modeled in a heat exchanger causes entropy estimates to skew results and thus causes refrigerant blend requirements to be under estimated. Enthalpy and entropy calculations for the system must account for the phase changes that can occur including the formation and handling of solids and be matched to the conditions provided by the other side of the heat exchanger. Of practical concern to the CCC process is fouling in heat exchangers from desublimation. Warm flue gas contacting cold refrigeration pipes will cause CO2 to desublimate on the surface of the pipes, greatly increasing the thermal resistance of the pipes to the point of ineffectiveness. Modeling the heat exchangers and properly accounting for the locations and amounts of desublimating components is essential to building a heat exchanger that can properly handle solids formation. To find the correct temperature profiles inside the heat exchangers a computer model must be written due to the inadequacy of commercially available software. With the thermodynamic properties of the flue gas programmed in, the flue gas cooling heat exchanger is modeled by a series of fully defined states equally spaced inside the heat exchanger with a constant enthalpy reduction between them. The inlet and outlet temperatures and states are known, but knowing only the temperature at each point inside the heat exchanger isn't sufficient due to multiple phase changes occurring, so each point through the heat exchanger must be fully defined. The molar fraction of each component and each phase is defined in an array with the molar flow rate of each phase in a separate variable. The inlet to the first heat exchanger is a flue gas composition of 78% N2, 16% CO2, 2% H2O, and 4% O2 at 25 degrees C. The gas flows through the heat exchanger counter current to the refrigerant, which will be designed to maintain a 10 degree C temperature difference across the length of the heat exchanger. At the exit of the heat exchanger the flue gas is cooled to -135 degrees C at which point over 99% of the CO2 and H2O have desublimated. The latent heat of desublimation has a strong effect on the temperature profiles in the heat exchanger, so accurately modeling this behavior is critical to making the proper refrigerant blends. The model is capable of showing the cooling temperature profile of a wide variety of flue gasses and can even take into account pollutants such as SO2, NO2, and Hg. These trace pollutants are too low in concentration to make any significant effect to the temperature profiles, but knowledge of where they condense inside the heat exchanger is critical to collecting them for removal.