Analysis of multi-stream heat exchanger in cryogenic air separation plants producing high pressure oxygen.

For streams having a wide variation in specific heat and change of phase, pinch poses a problem to maintain reasonable size of heat exchanger in an air separation process producing pressurized gases through vaporization of pumped liquids. In case the air streams are divided into different pressures, both pressure and flow rates of each stream become important parameters to keep the size of heat exchanger reasonable. We have attempted to predict the required pressure of air through simple calculations and evaluated the accuracy of results through detailed simulation. Pressures of air required for vaporization of pumped liquid oxygen (LOX) were very accurately predicted when LOX pressure is equal to or below 15 bara. However, for pressures of LOX between 20-35 bara, our calculations predicted 1-2 bara higher. Similarly, for pressures of LOX between 35-45 bara, we predicted 1-2 bara lower pressures of air compared to the pressures obtained from Aspen Hysys®™ 8.6.