Waste Heat Recovery and Recycling in Thermal Separation Processes: Distillation, Multi-Effect Evaporation (MEE) and Crystallization Processes

Abstract

Evaporation and crystallization are key thermal separation processes for concentrating and purifying inorganic and organic products with energy consumption over 1,000 trillion Btu/yr. This project focused on a challenging task of recovering low-temperature latent heat that can have a paradigm shift in the way thermal process units will be designed and operated to achieve high-energy efficiency and significantly reduce the carbon footprint as well as water footprint. Moreover, this project has evaluated the technical merits of waste-heat powered thermal heat pumps for recovery of latent heat from distillation, multi-effect evaporation (MEE), and crystallization processes and recycling into the process. The Project Team has estimated the potential energy, economics and environmental benefits with the focus on reduction in CO2 emissions that can be realized by 2020, assuming successful development and commercialization of the technology being developed. Specifically, with aggressive industry-wide applications of heat recovery and recycling with absorption heat pumps, energy savings of about 26.7 trillion Btu/yr have been estimated for distillation process. The direct environmental benefits of this project are the reduced emissions of combustible products. The estimated major reduction in environmental pollutants in the distillation processes is in CO2 emission equivalent to 3.5 billion lbs/year. Energy consumption associated withmore » water supply and treatments can vary between 1,900 kWh and 23,700 kWh per million-gallon water depending on sources of natural waters [US DOE, 2006]. Successful implementation of this technology would significantly reduce the demand for cooling-tower waters, and thereby the use and discharge of water treatment chemicals. The Project Team has also identified and characterized working fluid pairs for the moderate-temperature heat pump. For an MEE process, the two promising fluids are LiNO3+KNO3+NANO3 (53:28:19 ) and LiNO3+KNO3+NANO2(53:35:12). And for an H2O2 distillation process, the two promising fluids are Trifluoroethanol (TFE) + Triethylene Glycol Dimethyl ether (DMETEG) and Ammonia+ Water. Thermo-physical properties calculated by Aspen+ are reasonably accurate. Documentation of the installation of pilot-plants or full commercial units were not found in the literature for validating thermo-physical properties in an operating unit. Therefore, it is essential to install a pilot-scale unit to verify thermo-physical properties of working fluid pairs and validate the overall efficiency of the thermal heat pump at temperatures typical of distillation processes. For an HO2 process, the ammonia-water heat pump system is more compact and preferable than the TFE-DMETEG heat pump. The ammonia-water heat pump is therefore recommended for the H2O2 process. Based on the complex nature of the heat recovery system, we anticipated that capital costs could make investments financially unattractive where steam costs are low, especially where co-generation is involved. We believe that the enhanced heat transfer equipment has the potential to significantly improve the performance of TEE crystallizers, independent of the absorption heat-pump recovery system. Where steam costs are high, more detailed design/cost engineering will be required to verify the economic viability of the technology. Due to the long payback period estimated for the TEE open system, further studies on the TEE system are not warranted unless there are significant future improvements to heat pump technology. For the H2O2 distillation cycle heat pump waste heat recovery system, there were no significant process constraints and the estimated 5 years payback period is encouraging. We therefore recommend further developments of application of the thermal heat pump in the H2O2 distillation process with the focus on the technical and economic viability of heat exchangers equipped with the state-of-the-art enhancements. This will require additional funding for a prototype unit to validate enhanced thermal performances of heat transfer equipment, evaluate the fouling characteristics in field testing, and remove the uncertainty factors included in the estimated payback period for the H2O2 distillation system.« less