Optimized process configuration for CO2 recovery from crude synthesis gas via a rectisol wash process
Introduction
Fossil fuel combustion in industry and power plants is the main source of CO2 emitted to the atmosphere. These increasing anthropogenic CO2 emissions have resulted in global warming which is leading to changes in the world climate and is causing many serious environmental problems (Rao and Rubin, 2002). It is therefore imperative that immediate action to reduce CO2 emissions be taken in order to avoid long-term serious consequences (Adewole et al., 2013; Liang et al., 2015b).
In recent years, various approaches for reducing CO2 emissions have been proposed and investigated, including pre- and post-combustion CO2 capture, and oxy-fuel combustion with CO2 capture. Among these techniques, pre-combustion CO2 capture is favored for its low cost and huge development potential (Kanniche and Bouallou, 2007; Huang et al., 2008). In the pre-combustion capture process, CO2 is removed from crude synthesis gas that mainly consists of CO2 and H2 produced from the water gas shift reaction of CO and H2O from the partial oxidation of oil or coal (Kanniche et al., 2010). The typical physical solvents used in the pre-combustion capture are Selexol (dimethylether of polyethylene glycol), Rectisol (refrigerated methanol), and Fluor’s propylene carbonate (Allam et al., 2003). Selexol is preferable where the capture of deep sour gas (i.e. CO2 and H2S) is not required and Fluor’s propylene carbonate is mainly applicable for CO2 separation where small amounts of H2S are present or in the absence of H2S (Li and Smith, 2013). Rectisol wash with its high selectivity for sour components can be used to remove COS, H2S, and CO2 simultaneously, and also can separate out other impurities such as HCN and NH3. In addition, the methanol also shows high thermal stability and chemical stability at low operating temperatures, with lower viscosity, no degradation, and less corrosion (Burr and Lyddon, 2008). Thus, the rectisol wash process was chosen for investigation of its ability to capture CO2 and H2S from crude synthesis gas.
Rectisol wash is an efficient and economical physical absorption method for acid gas removal and was first jointly developed by Linde and Lurgi in the 1950s (Hochgesand, 1970; Wang et al., 2010). It has been successfully applied in treating various gases, such as methanol synthesis gas, ammonia synthesis gas, hydrogen, and fuel gas. This technology has been proven in over 100 plants worldwide, for example, the Sasol facilities in South Africa, ammonia plant in Ube, Japan, EL Paso Natural Gas Corporation in Schwechat, Austria (Kopyscinski et al., 2010), and the Integrated Gasification Combined Cycle (IGCC) power plants in Europe (Pruschek et al., 1995).
A conventional Rectisol wash process mainly consists of two sections, the sour gas removal section and the solvent regeneration section, as shown in Fig. 1 (Li and Smith, 2013). For the treatment of crude synthesis gas, most of the CO2 and the H2S/COS can be simultaneously removed in the sour gas removal (D101) section due to the fact that the solubilities of those gases in methanol are much higher than the other gases, according to Rousseau et al. (1981). It is well known that the loaded Rectisol solvents, sulfur-free and CO2-loaded solvent from the middle of D101 (L1) and CO2 and sulfur-loaded solvent from the bottom of the D101 (L2) are regenerated in four steps; 1) CO2 is desorbed from the L1 in a phase separator (S104) to the CO2 gas stream. 2) The CO2 remaining in L1 and L2 is stripped to the tail gas using N2, and the H2S in the solvent is concentrated in the H2S enrichment column (D102). 3) After that, the H2S rich solvent is fed to the hot-regeneration column (D103) from which the H2S is removed to the sour gas stream. 4) Finally, a part of the acid gas free Rectisol solvent is introduced to the CH3OH/water fractionating column (D104) to separate waste water and the already regenerated methanol that is recycled to the process.
However, the full recovery of by-product CO2 for further use is not considered to be important in the conventional process. According to Weiss (1988), the CO2 recovery is low because most of it is diluted by the stripping gas and then released to atmosphere. It has been indicated by Aresta (2010) that the recovered CO2 can be used as feedstock in many applications, i.e. chemicals, liquefaction, or the carbonation of beverages. In addition, it can be used as a flooding agent in Enhanced Oil Recovery (EOR) to increase natural gas or oil extractions (Morsi et al., 2004). In these applications, the high purity of the CO2 is of benefit. For instance, Nogueira and Mamora (2005) reported that higher purity CO2 applied in EOR could lower the compression costs and increase the oil recovery rates. It is well known that there are several existing large-scale industrial Rectisol wash processes in which the recovery of CO2 is only 65% or even lower. Therefore, there is a huge potential benefit to modify the Rectisol wash process to increase both the CO2 recovery and CO2 purity.
Various alternative process configurations have been proposed by a number of researchers to give better CO2 recovery from physical absorption processes (Ranke, 1977; Ranke and Weiss, 1982; Printers, 1997). Ranke and Weiss (1982) increased the CO2 recovery rate from a gaseous mixture by reducing the pressure on the two loaded scrubbing solvents causing the release of CO2. However, a part of the CO2 was still released to the atmosphere. Later, Ranke and Weiss (Printers, 1997) developed a process in which the two loaded solvent streams were expanded to release CO2 at two or more different temperatures. In addition, a 50,000 t per annum (tpa) CO2 capture project and by Yulin Coal Chemical Company has been completed, resulting in a low cost of CO2 capture of less than $18/tonne CO2 (Wang et al., 2017). The further increase of CO2 recovery from the scrubbing solvent was largely dependent on vacuum expansion, a process which has the potential for high energy consumption. In addition, the concentration of CO2 in the tail gas could also be as high as 70%.
There is also some work being done on the improvement of CO2 recovery from tail gas which contains mainly CO2 and N2 by the pressure swing adsorption (PSA) method (Alpay et al., 1994; Chou et al., 1994). In this work, although high CO2 recovery and purity were achieved, there is the potential of a corresponding significant increase in capital cost, which needs to be considered. Thus, although increases in CO2 recovery have been reported to be achieved in the literature, the energy consumption of those processes as well as the purity of CO2 production was not taken into consideration. The energy demands of the alternative processes could be higher than that of the traditional process and would cause extra greenhouse gas emissions from power plants, in addition to the environmental costs. Therefore in this work, an increase in CO2 recovery is accomplished by designing the two loaded solvent streams from the scrubbing column to be expanded and heated by recovering the heat from internal streams from the hot-regeneration column. Moreover, since it directly affects CO2 utilization, two additional flash separators are added in order to increase the CO2 purity of the CO2 gas stream.
In the present work, an improved Rectisol process is proposed in order to increase both the CO2 recovery and purity in the CO2 gas stream compared to the traditional process of a typical one-stage Rectisol wash configuration for a 200,000 t/y methanol production plant at Shanghai Coking and Chemical Corporation with a CO2 recovery rate of 35%. In the improved process, two phase separators (S104 and S105) were used to increase the CO2 purity through pressure reduction using expansion valves and the loaded solvents were expanded in the H2S enrichment column (D102) and withdrawn from D102 to be heated in order to increase the CO2 recovery. The simulations of both the traditional and improved process were performed in this work using ProMax3.2. Sensitivity analyses of important operating parameters were studied in order to reveal their effects on the compositions of the CO2 gas and sour gas streams and the total equivalent work of the improved process. In addition, the energy consumption of the two processes was investigated and compared.
Section snippets
Typical one-stage Rectisol wash process
Fig. 1 presents a typical one-stage Rectisol wash process of Linde technology. Some adjustments on the heat integration section have been made in order to simplify the process. Crude synthesis gas is first introduced and mixed with methanol to prevent ice formation. Then, it is cooled against several gas product streams to -19℃ in the heat exchanger E101, and the methanol solution is separated from the crude synthesis gas in the flash separator S101. Sulfur compounds and CO2 are removed from
A case study on purification of crude gas for methanol synthesis gas production
The original and improved Rectisol wash processes were developed in this work for the desulfurization and decarbonization of crude gas for a 200,000 t/y methanol production plant in Shanghai Coking and Chemical Corporation (Pruschek et al., 1995). The characteristics of crude gas and lean methanol are shown in Table 1.
Sensitivity analyses on the improved Rectisol wash process
In this section, sensitivity analyses were performed in order to reveal how the operational parameters affect CO2 purity and recovery, energy performance, and H2S concentration in the sour gas stream. It was also to investigate the roles of four separators (S104-S107) in the improved process. The effect of operational parameters, including the pressure of S104 (P1) ranging from 500 to 1200 kPa, the pressure of S105 (P2) ranging from 600 to 1200 kPa, the pressure of S106 (P3) ranging from 60 to
Conclusion
In this study, a typical one-stage Rectisol wash process for purification of coal-based methanol synthesis gas was compared to an improved process that was developed with the aim of increasing CO2 recovery and purity. The comparison was performed by simulation using the commercial simulation software ProMax3.2. The target of improving the CO2 recovery rate from the loaded solvent was achieved by means of expansion with internal heat in the improved Rectisol wash process. The CO2 recovery rate
Acknowledgements
The financial support from the National Natural Science Foundation of China (NSFC-Nos. 21536003, 21706057, 21606078 and 51521006), the Natural Science Foundation of Hunan Province in China (No. 2018JJ3033), the China Postdoctoral Science Foundation (Grant No. 2018M630899), and the China Outstanding Engineer Training Plan for Students of Chemical Engineering & Technology in Hunan University (MOE-No. 2011-40).
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