Control and operation of centrifugal gas compressors, with an emphasis on CO2 compression

Abstract

The PhD project focuses on operation and control of centrifugal gas compressors for carbon dioxide application. The analysis of the industrial background and the review of the academic literature have highlighted some research needs consisting in: - Improvement of the performance of the pressure controller of the compressor - Integration of pressure and surge control - Analysis of the effect of the recycle configuration on the operation of the compressor. Control configurations have been proposed in order to: - Reduce the interaction between control loops - Guarantee the stable and safe operation of the control system - Deal with partial and full recycle operation in supercritical carbon dioxide applications. Dynamic modelling and simulations have been employed in order to analyse the effect of boundary disturbances on the operation of the open and closed loop compression system. The performance of the system has been evaluated by means of graphical representation of controlled and manipulated variables and also by means of proposed performance indicators. These indicators focus on specific aspects of the operation of a compressor such as control performance, power consumption and amount of gas recycled. The results of the thesis demonstrate that better pressure control reduces the risk of surge. Moreover the interaction between pressure controller and surge controller can be avoided by employing a linear model predictive controller. The controller has been tuned according to various control purposes such as energy saving, surge avoidance or pressure control performance. The results also demonstrate that the recycle configuration influences surge occurrence, control stability and power consumption of the compression station. This phenomenon is emphasised when dealing with supercritical carbon dioxide because of the high density of the fluid.