Chilled water optimization at Beek INEOS PVC Plant: ammonia cycle

In BEEK INEOS PVC plant, polymerization takes place in a suspension process in twenty reactors in five lines. As the reaction is exothermic, a 17MW chilled water unit (CWU) removing heat from reactors which are producing different grades in batch processes. The objective of the project was to minimize energy consumption in compressors of the chilled water unit. In order to understand the importance of energy saving in the CWU, it is good to know that the CWU energy cost in 2010 was about 750.000 euros. The Coefficient of performance (COP) was chosen as optimization parameter. Based on this parameter, it is needed to find the optimum pressure and temperatures in chilled water unit which result in minimum COP at different input heat loads and cooling tower temperatures. In order to find the optimum operating points, a model for ammonia cycle needed to be developed to predict the COP of the system at different operating variables of system which are pressure in the evaporators, condensers and economizer. The historic process data such as pressures, temperatures, and flows in different parts of the system over two years were extracted from process history database (PHD) by using Honeywell PHD software. These data are processed to find a correlation to predict the actual work of compressors and the heat transfer coefficients in the evaporators and condensers. With the validated model, the COP can be predicted as a function of system variables, it was possible to change the variables to find the optimum operating point which results in minimum COP. The pressure ratio has the most important influence on the COP of system. Although it was expected that the optimum operating point is the built in pressure ratio of the screw compressors in which the compressors work at maximum isentropic efficiency, but the increase in the efficiency proved to be not sufficient to compensate the amount of increase in work which results of discharging ammonia to higher pressure. So it was noticed that COP has the highest value when pressure ratio decreases to the feasible lowest value which by compressors manufacturer it is necessary to have a minimum pressure ratio of 1.7 to run the compressors, but in most of the cases, this pressure ratio was not reachable due to size of condensers and cooling water temperature. Based on average production rate which is related to the heat load and cooling water temperature in 2010 from PHD, the model showed that it is possible to save up to 9% in energy cost which results in 70000 euros per year.