319953 Thermo-Economic Optimization of a Novel Zero Carbon Emission Polygeneration Plant Based On Methane Oxidative Coupling of Shale Gas and Nickel-Oxide Combustion Processes
Thermo-economic optimization of a novel zero carbon emission polygeneration plant based on methane oxidative coupling of shale gas and nickel-oxide combustion processes
Yaser Khojasteh Salkuyeh, Thomas A. Adams II
Department of Chemical Engineering, McMaster University, 1280 Main St W, Hamilton, Ontario, L8S 4L7, Canada.
Despite serious challenges on crude oil supply and price, the demand for petrochemical products is growing fast. This concern provides an incentive for petrochemical complexes to find new resources as feedstock, such as new approaches for production of olefins (petrochemical feeds) from natural gas. On the other side, the increasing natural gas supply due to remarkable growth in shale gas reservoirs in developed countries has made some significant shocks in the difference between the natural gas and oil prices. This drop in gas price indicates that new adaptations for the worldwide energy supply chains are required. In this work, a novel process for ethylene and ethane production is developed by integrating an oxidative coupling of methane (OCM) process for olefin production with a Nickel-oxide (NiO) chemical looping combustion (CLC) power generation process with zero carbon emissions. A simplified process diagram of this system is shown in figure 1. The aim of this system is to increase the adoptability of natural gas-based processes by connecting them to petrochemical complexes. The major limitations of OCM process are its low olefin yield and selectivity with high product separation difficulty, which lead to a significant ratio of unreacted gas. Therefore, the strategy of this polygeneration plant is to recover and use this unreacted gas efficiently. The CLC approach is investigated for electricity generation using portions of the unreacted gas. It is a promising alternative for electricity generation without any greenhouse gas emission that uses NiO as oxygen carrier. NiO is reduced to Ni by combusting the inlet gas in the reducer reactor, and then regenerated in the oxidizer with fresh air to complete the cycle [1]. The fully heat-integrated process is modeled in the Aspen Plus simulation program.
Fig. 1. Co-production of power, ethylene and ethane with 100% carbon capture
The OCM reactor is a catalytic (La2O3/CaO) reactor that operates isothermally at about 800°C and atmospheric pressure and converts methane to ethylene. The reactor simulation utilizes a reduced model derived from experimentally observed data provided by Godini et al. [2] as a function of the inlet CH4 to oxygen ratio. This ratio, along with the recycle ratio of unreacted gas from the product recovery section, are the two major decision variables that affect the thermal efficiency and net present value (NPV) of the plant. Diglycol-Amine (DGA) is used for CO2 removal of outlet gas. Its energy requirement is less than previous work that used MDEA [2]. In addition, a detailed model for gas turbine is built to calculate the cooling requirement of blade surfaces. This model is a significant improvement over the classic turbine models in Aspen Plus because the details of how the cooling management system is implemented can significantly impact the actual performance of the power generation section. This model was developed in Visual Basic and connected to Aspen Plus via the Microsoft Excel interface. Furthermore, economic evaluation is performed by using Aspen Icarus Process Evaluator for direct capital cost and updated values of economic parameters for 2000 tonne/day natural gas and 30 years lifetime.
An integrable particle swarm optimization (PSO) model [3] is established in Matlab for the thermo-economic optimization of process. The highest obtainable thermal efficiency is about 40.9% LHV (37.5% HHV) with 3.97 for the optimal ratio of inlet shale gas to oxygen, and no recycle of unreacted gas to the OCM reactor. However, from an economical point of view, about 30.6% improvement on NPV and 24% reduction on thermal efficiency will be achieved compared to thermal optimum point, when the objective function is switched to NPV. The optimal recycle ratio is changed to the maximum allowable ratio in this case with NG/O2=3.75. In addition, a sensitivity analysis on optimum ratio is performed to define the match point of the operating conditions of both scenarios. The final results show that they match when ethylene and ethane prices are about 40% of their base case values. This means the price of liquid products are currently high enough to sacrifice thermal efficiency in order to achieve a higher NPV.
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