(474f) Heat Pump Assisted Configurations for Amine Based Natural Gas Sweetening Units

Natural gas, taken from gas wells, is a mixture of several hydrocarbons namely methane, ethane and slightly heavier hydrocarbons like propane and butane; amongst which, methane is the chief and the predominant component which constitutes almost 80-90 % of natural gas. Natural gas, from gas wells, also has impurities in the form of CO₂, H₂S, water, mercaptans etc. These impurities have to be removed for further usage of natural gas such as for pipeline distribution, liquefaction and/or fractionation. The acid gases, a mixture of CO₂ and H₂S, are very critical impurities and have to be removed as they can cause corrosion to the equipments used in the processing and distribution of natural gas. Even for natural gas distribution, the pipeline limits specified in United States are CO₂ < 2 mol% and H₂S < 4 ppm [1]. The pivotal and foremost step in the natural gas processing is the acid gas removal which removes acid gases from the natural gas. This process is also called natural gas sweetening. Among the different acid gas removal processes, amine based absorption is the most commonly used process because of its simplicity, flexibility in handling large quantity of feeds and easy retrofit ability [2]. A simplified flowsheet of an amine based natural gas sweetening system contains an absorber and a solvent regenerator (distillation column). The feed natural gas contacts with the lean amine based solvent in the absorber. Here an exothermic chemical reaction takes place between the solvent and the gas stream; the resultant sweet gas exits out from the same. The rich solvent (now loaded with acid gas) is then stripped from the acid gases in a regenerator. Large amounts of energy (reverse of the chemical reaction, thus endothermic), however, is required for the regeneration of the solvent. This makes this process highly energy intensive (3-4 MJ/kg of CO₂), which happens to possibly be a major drawback of this process [2, 3].

For reducing the energy consumption in the amine based natural gas sweetening systems, different studies have been carried which includes development of new and mixed amine solvents, optimizing the process and structural configuration of the system etc. [4-6]. An interesting development in this regard is the usage of heat pump in the acid gas removal system. The heat pump is for the solvent regeneration unit. The concept in a heat pump assisted scheme is to effectively heat integrate the condenser (rectification section) and reboiler (stripping section) thermal energy to reduce the heat to be supplied to the reboiler. Since the temperature of the rectification section is lesser than the stripping section, a mechanical means (compressor) is used where heat is moved from the heat source at a lower temperature to a heat sink at a higher temperature. This kind of a heat pump is called a mechanical vapor recompression (MVR). An interesting way to increase the energy efficiency of MVR is to utilize the available heat within the MVR to further heat other process streams in the process. This is called self-heat recuperation (SHR); the combination of MVR and SHR has been extensively used in many distillation based systems [7-10].

The objective of this work is to develop different process configurations of the solvent regeneration unit within an amine based natural gas sweetening system utilizing the concepts of MVR and SHR; and, to study the energy consumption of the same with respect to the conventional acid gas removal system. The process flowsheets were developed and simulation using Aspen Hysys V8.4 and the feed considered was 10 MMscfd of natural gas (82.5% methane, 10% ethane, 5% CO₂ and 2.5% H₂S; 824.7 psia and 37.8⁰C). 45 wt.% methyl diethanolamine (MDEA) was used as the amine solvent. Four different and new configurations were developed and the results were analyzed. It was observed that our new configurations showed at least 15-20% savings in the energy consumption in comparison to the conventional process for the same feed and almost the same acid gas recovery. Consequently, there is also reduction in the CO₂ emissions compared to the conventional acid gas removal units. Future work will seek to optimize these configurations within a simulation-optimization framework with respect to the cost of the process.

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