Abstract:[Objective] Hydrogen demands in refineries are increasing annually because of the growing processing of heavy crude oil, which necessitates optimizing the hydrogen network to improve hydrogen usage. The cost associated with hydrogen compressors is the second largest in a refinery hydrogen network, following the fresh hydrogen cost. Thus, the synthesis of hydrogen networks considering gas compression has been an active topic in process systems engineering. Previous work has been modeled on reciprocating compressors, focusing on reducing the number of compressors and/or compression power consumption. However, reciprocating and centrifugal compressors are widely used in refinery hydrogen networks. The advantage of centrifugal compressors is their suitability for large gas flow rates without too high exhaust pressures, while reciprocating compressors have high and stable exhaust pressures and are suitable for small gas flow rates.[Methods] This work presents a hydrogen network superstructure that considers the selection of multistage centrifugal and reciprocating compressors. Compressor selection is determined based on the characteristics of reciprocating and centrifugal compressors considering their inlet gas flow rates and exhaust pressures. A mixed integer nonlinear programming (MINLP) model is formulated to minimize the total annualized cost, comprising fresh hydrogen, compressor investment, and compression power. The developed MINLP model is examined based on a hydrogen network reported in the literature. It is coded in the general algebraic modeling system 35.2 and can be directly solved by the BARON solver.[Results] The results indicated that the optimal hydrogen network contained three centrifugal compressors and six reciprocating compressors, with one reciprocating compressor for two-stage compression and one for three-stage compression. The flow rates of the three centrifugal compressors were larger than the upper flow rate limit of the reciprocating compressors, while the outlet pressures were lower than the upper outlet pressure limit of the centrifugal compressors.[Conclusions] This phenomenon indicates that the flow rate constraint dominates the compressor selection in this hydrogen network. Since the cost correlation of the centrifugal compressor is smaller than that of the reciprocating compressor in this study, the centrifugal compressor is preferred when both types of compressors meet the compression demands. Hence, only hydrogen streams with small flow rates and large compression ratios are chosen for the reciprocating compressors. Compared with the previous work, although the numbers of compressors are identical, the optimal hydrogen network structures differ notably, and this study obtains small compression power consumption. This result is obtained because earlier studies neglected compressor selection, and the mathematical model in this study prefers the less expensive centrifugal compressor. Therefore, the flow rates of several hydrogen streams are enlarged to satisfy the flow rate constraint of centrifugal compressors, which is also more consistent with refinery practice. Finally, the computation time of the MINLP model is only 0.72 s, thereby demonstrating the usefulness and convenience of the proposed method.
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