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Ebrahimzadeh E.,Brigham Young University | Baxter L.L.,Brigham Young University | Baxter L.L.,Sustainable Energy Solutions LLC
Applied Thermal Engineering | Year: 2016

Conventional distillation control processes use vapor distillate flowrate to control column pressure and condenser heat removal to control the reflux drum level. These intuitive control systems work well for isolated columns or columns with total condensers. However, these controls are not effective when columns with partial condensers occur in series. The pressure and reflux drum level interact in such systems in ways that defeat conventional control systems, rendering them unable to maintain product purities in the presence of large feed flowrate and composition disturbances. This investigation documents a plant-wide control structure that can address this issue by controlling pressure through reflux heat removal rate and reflux drum level by reflux flow rate. This control system demonstrates its capability to handle large disturbances in throughput and feed composition through a series of Aspen simulations. This alternative system is no more complicated than the conventional system and should work on distillation columns of nearly all designs, not just the coupled partial condenser designs for which it is essential. Common natural gas processing provides a specific example of this alternative control system. Natural gas commonly includes high concentrations of CO2 that must be removed prior to pipeline or LNG distribution. The existence of a minimum-boiling temperature azeotrope between ethane, virtually always present in natural gas, and carbon dioxide complicates the separation of CO2 from the hydrocarbons. This separation commonly employs extractive distillation with high-molecular-weight hydrocarbons. Our recent paper Ebrahimzadeh et al. (2016) discusses in detail the steady-state economic design of a new extractive distillation strategy for the CO2-ethane azeotrope separation with three columns. This strategy shows a 5% reduction in capital and 15% reduction in operating costs when compared to optimized versions of the conventional process. The new strategy also produces CO2 in a liquid rather than a vapor phase, which simplifies transport, storage, and handling. Two columns of the proposed design use partial condensers and are the focus of this investigation. © 2016 Elsevier Ltd. All rights reserved. Source


Ebrahimzadeh E.,Brigham Young University | Matagi J.,Brigham Young University | Fazlollahi F.,Brigham Young University | Baxter L.L.,Brigham Young University | Baxter L.L.,Sustainable Energy Solutions LLC
Applied Thermal Engineering | Year: 2016

CO2 is a common constituent of natural gas. Standards for its maximum concentration differ from about 2% for pipeline to 50 ppm for liquefaction. All natural gas constituents absorb CO2 to some degree when in the liquid phase, requiring multi-step natural gas treatment processes. The existence of a minimum-boiling temperature azeotrope between ethane and carbon dioxide particularly complicates CO2 separation. Extractive distillation with higher molecular weight hydrocarbons as the solvent represents the most competitive means for the separating CO2 from ethane. The conventional separation method involves two distillation columns in series and rather high amount of energy. This investigation proposes an efficient method for CO2-ethane separation that produces all products at high purity with less capital and operating costs in comparison with the conventional system. The new operating flowsheet includes three columns: a CO2 recovery column, a solvent recovery column, and a concentrator column. The proposed system requires 10% less total annual cost (TAC) and 16% less energy compared to the conventional system at the same purification. Additionally, unlike the conventional system, the proposed design separates CO2 in the form of a liquid product, which avoids the high amount of energy required for the liquefaction. Thus, this technology provides a useful alternative toward the less expensive CO2-ethane separation process. © 2015 Elsevier Ltd. All rights reserved. Source


Jensen M.J.,Brigham Young University | Russell C.S.,Brigham Young University | Bergeson D.,Brigham Young University | Hoeger C.D.,Sustainable Energy Solutions LLC | And 4 more authors.
International Journal of Greenhouse Gas Control | Year: 2015

Bench-scale experiments and Aspen Plus™ simulations document full-scale, steady-state performance of the external cooling loop cryogenic carbon capture (CCC-ECL) process for a 550MWe coal-fired power plant. The baseline CCC-ECL process achieves 90% CO2 capture, and has the potential to capture 99+ % of CO2, SO2, PM, NO2, Hg, and most other noxious species. The CCC-ECL process cools power plant flue gas to 175K, at which point solid CO2 particles desublimate as the flue gas further cools to 154K. Desublimating flue gas cools in a staged column in direct contact with a cryogenic liquid and produces a CO2-lean flue gas that warms against the incoming flue gas before venting. The CO2/contacting liquid slurry separates through a filter to produce a CO2 stream that warms to 233K and partially flashes to provide a CO2-rich product. The CO2-rich product (99.2%) liquefies under pressure to form a product for enhanced oil recovery (EOR) or sequestration. All contacting liquid streams cool and cycle back to the staged column. An internal CF4 refrigeration cycle transfers heat from melting CO2 to desublimating CO2 by cooling contact liquid. An external cooling loop of natural gas or other refrigerant provides the additional heat duty to operate the cryogenic process. The nominal parasitic power loss of operating CCC-ECL is 82.6MWe or about 15% of the coal-fired power plant's rated capacity. In different units, the energy penalty of CCC-ECL is 0.74MJe/kg CO2 captured and the resulting net power output is decreased to 467MWe. Lab- and skid-scale measurements validate the basic operation of the process along with the thermodynamics of CO2 solids formation. © 2015 Elsevier Ltd. Source

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