Motivated by reliability/availability concerns in chemical plants, this paper proposes MINLP models to determine the optimal selection of parallel units considering the trade-off between availability and cost. Assuming an underlying serial structure for availability, we consider first a case where the system transitions between available and unavailable states, and second the case with an intermediate state at half capacity. Two non-convex MINLP models maximizing net profit are introduced for the two cases. In addition, a bi-criterion MINLP model is proposed to maximize availability and to minimize cost for the first case. It is shown that the corresponding epsilon-constrained model, where the availability is maximized subject to parametrically varying upper bound of the cost, can be reformulated as a convex MINLP. Availability is also incorporated in the superstructure optimization of process flowsheets. The performances of the proposed models are illustrated with a methanol synthesis and a toluene hydrodealkylation process.

出于对化工厂中可靠性/可用性问题的考虑，本文提出了MINLP模型，以考虑可用性和成本之间的折衷来确定并联装置的最佳选择。假设可用性具有底层的串行结构，我们首先考虑系统在可用状态和不可用状态之间转换的情况，其次考虑具有半容量的中间状态的情况。针对这两种情况，引入了两种使净利润最大化的非凸型MINLP模型。另外，提出了一种双准则MINLP模型，以在第一种情况下最大化可用性并最小化成本。结果表明，相应的ε约束模型可以重新构造为凸MINLP，在该模型中，可用性随参数上限的变化而最大化。在工艺流程图的上层结构优化中也包含了可用性。提出的模型的性能通过甲醇合成和甲苯加氢脱烷基化工艺进行了说明。