Process analysis for the simultaneous production of aromatics and syngas from shale gas and CO2

https://doi.org/10.1016/j.enconman.2022.116480Get rights and content

Highlights

  • Optimization of non-oxidative and CO2 co-feeding BTX production process from shale gas.

  • Experiments-based surrogate model according to CO2 concentration and temperature.

  • The CO2 emission evaluation by lifecycle assessment.

  • The economic feasibility evaluation by technoeconomic analysis.

  • Carbon prices-based technoeconomic analysis of the proposed process.

Abstract

The production of benzene, toluene, and xylene (BTX) from shale-derived CH4, C2H6, and C3H8 is considered a promising alternative to their fossil-based counterparts. Herein, we evaluate a production technology for aromatics and hydrogen with non-oxidative and CO2 co-feeding reactions to predict the economics and environmental impact of each process. The optimal process for producing aromatics and hydrogen varies depending on the presence or absence of CO2, and we experimentally confirm the difference in the reaction systems. The operating conditions of both processes are optimized using an experiment-based surrogate model in terms of thermal energy demand and profit of products, which account for the highest portion of BTX production cost. Particularly, the CO2 emissions and economic feasibility of the proposed BTX production processes are compared with those of an ethane cracking center using shale gas and the naphtha reforming/Cyclar process to produce BTX. In the CO2 co-feeding case, the CO2 reduction effect is proven compared to other processes, and the economics was improved by the sale of H2 and CO compared to the non-oxidative BTX production. Furthermore, the BTX production cost reflecting the carbon price is comparable to that of commercial processes; as the carbon price increases, the economics has the potential to outperform existing processes. These findings present a platform for economic evaluation based on CO2 emissions as well as the economic benefits of processes that reduce CO2 emissions for energy-efficient conversion of shale gas into high value-added products.

Introduction

The production of benzene, toluene, and xylene (BTX) via the co-aromatization of shale gas, including CH4, C2H6, and C3H8 is a promising alternative for producing high-value-added chemicals from shale gas [1], [2], [3]. In our previous studies, an economic evaluation was performed by linking the shale gas-based BTX production and the subsequent CH4 utilization processes, and the effect of reducing CO2 was discussed [4]. Furthermore, CO2 co-feeding BTX production over Mo/HZSM-5 prevented carbon deposition on the catalyst [5], [6], resulting in a more stable process to dehydroaromatize hydrocarbons (HCs). Co-feeding CO2 in BTX production increases the H2 production rate by the dry reforming of HCs in shale gas and positively affects the process economics and environmental impact [7]. However, to improve the competitiveness of BTX production technology, economic benefits should be maximized by considering only BTX production without a subsequent CH4 utilization process.

Among the processes using shale gas, the most representative is the ethane cracking center (ECC) [8]. CH4, which is a major component of shale gas, is separated from C2H6 and C3H8 by cryogenic distillation and is mainly used to produce electricity, whereas C2H6 and C3H8 are used to produce C2H4 and C3H6 by steam cracking reaction [9], [10], [11]. The ECC accounts for approximately 55 % of ethylene production in the USA and is regarded as the most efficient commercial process for shale gas [12]. The non-catalytic steam-cracking reaction of ECC process has a high carbon yield of 70–80 % [13] and thus exhibits high economic benefits. Commercial processes for producing BTX include the traditional naphtha reforming process and the Cyclar process using light HCs (C3H8 or C4H10). Naphtha reforming accounts for approximately 54 % of global BTX production, and Pt-based Al2O3 catalyst is mainly used [14]. The BTX yield is approximately 60 %, which sufficiently guarantees the economic feasibility. In the Cyclar process [15], the feed source may include C3H8/C4H10 mixtures; the BTX yield is ∼ 60 %. Ga-based zeolite catalysts are mainly used in this process [16], and the process scheme is not significantly different from that of naphtha reforming. In the proposed BTX production process from shale gas, the BTX yield is considerably low (approximately 10 %) in the general shale gas composition [4]. However, compared with ECC, aromatics and additional by-product H2 are produced; this provides a different approach to using shale gas. Moreover, in contrast to other BTX processes, the economic efficiency can be improved by utilizing unreacted CH4 and linking it with subsequent processes. Therefore, the CO2 emissions and economic feasibility of the BTX production process from shale gas should be comprehensively compared with those of the aforementioned processes.

When producing BTX from shale gas, various intermediates or by-products are produced, which are highly dependent on temperature and feed composition. Experimentally verifying these variables is labor-intensive and time-consuming [17]. Consequently, several optimization methods have been developed based on surrogate models [18]. A surrogate model can effectively represent the target model and facilitate efficient model analyses and optimization [19]. A surrogate model can be regarded as a “regression” to a set of data, where the data are sets of input–output pairings obtained by evaluating a black-box model of the complex system. Yan and coworkers proposed a Bayesian migration methodology for Gaussian process regression (GPR) to promote rapid process modeling and optimization [20]. GPR-based modeling can effectively describe both practical chemical and simulated processes. In the Bayesian optimization algorithm, the objective function is evaluated sequentially and iteratively using a stochastic interpolation method [21], [22]. Shokry and coworkers proposed a machine learning-based methodology for a multiparametric solution of chemical process operations [23]. This methodology has a high prediction accuracy and the complexity of the optimization solution is significantly reduced in chemical process operations. Therefore, surrogate models based on experimental data with clear trends for manipulated variables can effectively predict data in ranges for which the experiment was not performed.

In this study, processes converting shale gas to aromatics and H2 with non-oxidative and CO2 co-feeding reactions are optimized to reduce the BTX production cost without additional processes to utilize CH4. The reactor performance for non-oxidative and CO2 co-feeding reactions is designed as a surrogate model using GPR, and experimental data for the corresponding operating conditions, including the CO2 concentration, temperature, and feed composition, are obtained. The hyperparameter of GPR is optimized using a Bayesian optimization algorithm, and the predicted results are compared with the experimental results. Based on the surrogate model, an optimizer is designed to minimize the thermal energy demand and maximize total profit of both processes. Furthermore, the CO2 emissions and economic feasibility of the proposed BTX production processes are compared with those of commercial BTX production processes (naphtha reforming and Cyclar processes) and ECC process using shale gas. To ascertain the economic impact of CO2 emissions on the process, the economic evaluation is performed again by applying the carbon price to the CO2 emission of the evaluated processes.

Section snippets

Process of converting shale gas to aromatics and H2

CH4, C2H6, and C3H8 are major components of shale gas; CH4 constitutes the largest proportion (85–90 %). In methane dehydro-aromatization (MDA), the initial conversion of CH4 is approximately 10–12 % at 973 K using the Mo-based ZSM-5 catalyst, and the corresponding BTX yield is approximately 5–6 % [24], [25]. BTX yield in shale gas aromatization can be higher than that in MDA owing to the presence of C2H6 and C3H8. However, it is not possible to convert all of the CH4 in shale gas. Therefore,

Experimental section

A 10 wt% Mo/HZSM-5 catalyst used in this study was prepared using commercial HZSM-5 and ammonium molybdate tetrahydrate ((NH4)6Mo7O24–4H2O). The detailed synthesis procedure is described in our previous study; additionally, the characteristics of the catalyst are presented [4]. The BTX yield, CH4 conversion, and H2 production ratio of non-oxidative and CO2 co-feeding BTX production when CH4/C2H6/C3H8 = 85/10/5% are specified in the Supporting Information. In this section, only the experiments

BTX yield according to the CH4 composition variation

The BTX formation rates of Sets 1 and 2 under the non-oxidative BTX production process are shown in Fig. 3a. The thermodynamic limitation is estimated using the Gibbs energy minimization method, assuming that coke and naphthalene are not produced [31]. In other words, it represents the maximum BTX formation rate under ideal conditions. The thermodynamic limitations of Set 1 with 85 % CH4 and Set 2 with 5 % CH4 are markedly different. This reflects that the probability of CH4 changing to BTX in

Conclusions

We optimized processes to convert shale gas to aromatics and H2 with two different configurations (i.e., non-oxidative and CO2 co-feeding BTX production processes). The proposed process configurations were determined by experimental results based on the CH4 content of the shale gas. In the non-oxidative BTX production process, the pre-rejection of CH4 enhanced the BTX yield of the reactor, thereby improving the process economics. However, in the CO2 co-feeding BTX production, CH4 was not

CRediT authorship contribution statement

Wonho Jung: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Resources, Writing – original draft, Writing – review & editing. Hyeona Kim: Validation, Investigation, Resources. Hae Won Ryu: Validation, Investigation, Resources. Yong Hyun Lim: Validation, Investigation, Resources. Do Heui Kim: Writing – review & editing, Supervision. Jinwon Lee: Writing – review & editing, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This research was supported by the C1 Gas Refinery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (2015M3D3A1A01064929) and (2021M3D3A1A01022109).

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