One of the important chemicals is propylene glycol. Propylene glycol is used in various sectors of the national economy, including the production of saturated polyester resins, in the preparation of pharmaceutical and cosmetic preparations, in the manufacture of paints, in the food industry, etc. In this regard, the development of algorithms to ensure the technological process of production of propylene glycol in optimal regimes is an urgent problem for the development of the chemical and petrochemical industry. The aim of this work is to calculate the optimal working regime of the hydrator for technological process of propylene glycol production based on mathematical models, including the Lagrangian function. The process of obtaining propylene glycol is carried out in accordance with the given temperature and pressure in the hydrator. As for the quantitative and qualitative indicators of the production of propylene glycol, the necessary conditions are set. For example, the density of commercial propylene glycol produced should be 0.981–1.036 g/cm³. Quantity and quality of the propylene glycol obtained in the process depends on the predetermined amount of propylene oxide, temperature and pressure in the hydrator. When solving the problem of optimizing the static regime of the technological process, such parameters of propylene glycol quality as quantity and density were taken into account. These indicators were taken into account in accordance with coordinates of states and constraint conditions as nonlinear functions. Taking into account the above, problem of optimal control of the parameters in the hydrator was formulated taking into account the real limitations imposed on the input and control parameters in the technological scheme (propylene glycol consumption, propylene glycol density, pressure in the hydrator, temperature in the hydrator, amount of steam and propylene oxide supplied in the mixer). To solve this problem, an algorithm was developed based on a model including the Lagrangian function. The optimum operating regime for the hydrator of the process for the production of propylene glycol was calculated on the basis of nonlinear programming. To solve the optimization problem, the coordinates of the saddle point of the Lagrangian function were determined. The optimum values of temperature, pressure and water-oxypropylene ratio in the hydrator, consumption and density of propylene glycol are found. For the obtained optimal data, it is recommended to use the nonlinear programming method, taking into account the obtained coordinates of the saddle point of the Lagrangian function.