Dynamic Modeling and Optimization of a Fixed-Bed Reactor for the Partial Water–Gas Shift Reaction

Abstract

This work presents an approach for the model-based design of a load-flexible fixed-bed reactor for performing the partial water–gas shift (WGS) reaction in the gas cleanup section of a coal-to-methanol plant. Three optimization problems with various combinations of objective functions (H2/CO ratio, pressure drop, and operating costs) are formulated and solved using the nondominated sorting genetic algorithm (NSGA-II) to obtain the Pareto-optimal fronts, and the selected solutions are subjected to dynamic stability analysis. The optimum values of decision variables such as the weight of the catalyst, catalyst particle size, ratio of the reactor length and diameter, feed gas temperature, and steam to CO ratio are estimated and are found to be dependent on the choice of the objectives and tolerable deviation from their required values. The dependence of the H2/CO ratio on the inlet gas temperature varies with the choice and combination of the objectives. In contrast, higher values of the steam to CO ratio (S/C) are associated with a lower deviation from the desired H2/CO ratio for all three cases. However, optimum values of S/C are found to be below 1:1 because of its effect on the pressure drop and operating costs. The solutions obtained in the equilibrium-limited regime are found to be more stable in the presence of fluctuations in the feed flow rates but show intermediate stability with fluctuations in the inlet gas temperature. Four designs are recommended as the feasible designs, and the criteria of their application are discussed. Even though the work is related to a WGS reactor, the methodology used can be applied for the design of any other reactor system operating under variable feed conditions.

A critical review on the practical use of lithium cycle as an upfront nitrogen removal technology from natural gas

“…26 Although the molten salt mixture of LiCl-KCl/LiOH-LiCl has lithium hydroxide as a component, an 88.5% current efficiency was achieved for the lithium yield, while the lithium hydroxide electrolysis was held between 400°C and 450°C. 92 This occurred because the experimental setup designed to effectively separate the lithium product from other components in the electrolysis cell to avoid potential side reactions. The molten salt mixture used for electrolysis that does not contain lithium hydroxide can mitigate undesirable side reactions and produce a relatively high current efficiency.…”

“…A more detailed technoeconomic analysis of using the Li‐Cy was recently conducted by Pal et al 90 and their findings are summarized in Table 8, which includes the profits calculated for the UNR in the range 12.5%–87.5% compared to the basic scenario where the stream is directly fed to the liquefaction cycle. The authors developed and the detailed cost analysis of the Li‐Cy based on the process parameters extracted from Li and colleagues 16,26,42,44,91‐93 . Although Pal et al 90 resultsdo show that employing the Li‐Cy on the LNG plant is feasible, several comments can be observed.…”

“…This goes against the fundamental idea behined how nitrogen from NG reacts with lithium to generate lithium nitride. Kherdekar et al 92 determined the optimum L / D ratio for a catalytic water gas shift reaction bed, while Afzal and Sharma 91 assume the bed porosity to be 0.5 for a Ti bed for hydrogen storage, which is more forgivable for the lithium nitridation reactor due both being metals despite the differences in properties. In addition, the process parameters obtained from Li 16 such as ammonia prices and electricity costs are not based on the value reflected in Qatar, which would require modifications as previously stated.…”

“…In comparison, a cathode current efficiency of 84%–86% was obtained using LiCl–41 mol% KCl or LiCl–17 mol% KCl–26 mol% CsCl as the molten salt, which has a high decomposition potential for the electrolysis of lithium hydroxide at 548–673 K (274.85–399.85°C) 26 . Although the molten salt mixture of LiCl–KCl/LiOH–LiCl has lithium hydroxide as a component, an 88.5% current efficiency was achieved for the lithium yield, while the lithium hydroxide electrolysis was held between 400°C and 450°C 92 . This occurred because the experimental setup was designed to effectively separate the lithium product from other components in the electrolysis cell to avoid potential side reactions.…”

“…The authors developed and the detailed cost analysis of the Li-Cy based on the process parameters extracted from Li and colleagues. 16,26,42,44,[91][92][93] Although Pal et al 90 resultsdo show that employing the Li-Cy on the LNG plant is feasible, several comments can be observed. First, some of the process parameters, such as the lithium bed L/D ratio and porosity, were assumed to be equal to beds from other studies that do not involve lithium.…”