The Net Present Value (NPV) is typically employed as the profitability criterion to determine investment decisions in chemical plants. Such an evaluation is performed on a plant by plant basis and does not consider the portfolio nature of the chemical process industry in that a company typically owns multiple plants to produce diverse sets of chemicals. Portfolio optimization, which is employed in other fields to guide investment decisions, has not been applied in the chemical engineering domain. To close the gap, this work introduces the optimization of process portfolios based on Modern Portfolio Theory (MPT). The customization of the MPT method to the chemical plant investment problem is presented. The application of the method is then illustrated with a case study considering six plants. Identified MPT portfolios are compared against portfolios constructed based on two commonly used profitability criteria, i.e. Net Present Value and Return on Investment (ROI). The effects of annual and monthly profit variabilities and of economies of scale on portfolio selection are also explored. The results indicate that portfolios developed with the proposed MPT methodology can outperform those portfolios selected based on NPV and ROI.

净现值（NPV）通常用作获利标准，以确定化工厂的投资决策。此类评估是按工厂进行的，并且不考虑化学过程工业的投资组合性质，因为公司通常拥有多个工厂来生产各种化学品。在其他领域中用于指导投资决策的投资组合优化尚未应用于化学工程领域。为了缩小差距，这项工作介绍了基于现代投资组合理论（MPT）的流程投资组合的优化。介绍了针对化工厂投资问题的MPT方法定制。然后通过考虑六家工厂的案例研究说明了该方法的应用。将已识别的MPT投资组合与基于两个常用获利标准（即净现值和投资回报率（ROI））构建的投资组合进行比较。还探讨了年度和每月利润变化以及规模经济对投资组合选择的影响。结果表明，使用建议的MPT方法开发的投资组合可以胜过基于NPV和ROI选择的投资组合。