Optimal Operation of Parallel Heat Exchanger Networks

Abstract

Optimal operation of parallel heat exchanger networks is desirable for many processesaiming to achieve increased supply and potentially higher profit. The aimis to control the final outlet temperature within a certain range, which in manycases includes a trade off between maximum outlet temperature and minimumoperating costs.The goal with this study has been to investigate the performance of the selfoptimizingJäschke temperature control variable, proposed by post doctor JohannesJäschke. The Jäschke temperature approach seeks to achieve near optimaloperation of parallel heat exchanger networks, exclusively by manipulation of thebypass selection - only based on simple temperature measurements. The methodhas been demonstrated for several different cases and investigated both at steadystate and dynamically.For balanced heat exchanger networks, with evenly distributed hot stream heatcapacities throughout the network, the Jäschke temperature showed good performancefor all cases studied. The simulations revealed satisfactory disturbancerejection and very close to optimal operation. For cases suffering a more unevenheat capacity distribution, the method did not give near optimal operation. Also,exposed to major, non-realistic disturbances the Jäschke temperature control configurationgave poor performance due to singularities in the control variable whencertain temperatures achieved equal values. In the presence of such incidents, amodified control variable was implemented by re-writing the expression controllingthe Jäschke temperatures to a denomiator-free form. This gave slightly betterperformance and was concluded to operate the system satisfactory.