(33b) Chill More with Less and Properly Control Your Mechanical Refrigeration Unit (MRU) via Dynamic Simulation

Use Dynamic Simulation to Validate Your Process Control of a MRU Gas Processing Plant

There are many misconceptions about the closed refrigerant loop widely used in Mechanical Refrigeration Unit (MRU) to extract Natural Gas Liquid (NGL) from natural gas. Two common mistakes are: 1) How can I control the refrigerant compressor discharge pressure? 2) Why can't I control the liquid level in the receiver? The best way to answer these kinds of questions is to model the MRU in a dynamic process simulator. The dynamic simulation allows users to operate the virtual MRU plant without worrying about shutting down the real plant. From the dynamic simulation, users quickly learn that the compressor discharge pressure is actually controlled by the ambient temperature and/or the fans speed. The reason why the liquid level can't be controlled is because, 1) the total mass (not mass flow) is fixed in the closed loop, 2) the amount of mass in liquid form is controlled in the chiller and the economizer if any; thus the liquid level in the receiver is dependent on the chiller and economizer level.

Chill More with Less by Optimizing Set Points

Further, Dynamic Simulation is able to help to extract more NGL with less energy consumption. Most operators operate the plants with the fixed setpoints given by the plant equipment provider. However, this dynamic simulation demonstrates that it is possible to achieve a lower cold separator temperature while using the same electricity consumption or less by optimizing the setpoints.

Summary

Dynamic Simulation can be used to validate the process control during the design phase and to optimize the plant output while under operation.