Studies conducted in the oil refining industry have shown that a priori choice of a model from a particular class without identifying an object class to satisfy a given criterion when modeling technological processes and solving optimal control problems often leads to failures. Therefore, when creating an optimal control system for any technological process, first of all, it is necessary to determine the nature of changes in both external and internal material flows that characterize this process. As a result, a mathematical model of the object is built and the problem of its analysis is solved. The article is delivered the solution of the issue of building a mathematical model of a gas fractionation plant. The propane cylinder of a gas fractionation plant is taken as the object of research.