(628e) A Control Technique Based On Compressor Characteristics With Applications to Carbon Capture and Storage Systems

Introduction

Compressors are vital pieces of equipment within the process industry and they are going to be important in the next few years for dealing with carbon dioxide from carbon capture and storage (CCS) systems.

Compressor characteristics (also called compressor maps) represent the operation of the machine in a graphical form. They are provided by the manufacturer of the compressor together with the machine and they are generated via experiments at reference conditions. The most common compressor characteristics represent the pressure ratio of the machine (i.e. the ratio between the output pressure and the input pressure) as a function of inlet flowrate and rotational shaft speed. For a single speed machine there is a single characteristic curve rather than a map (where instead the same function is plotted more times for different shaft speeds). While the inlet flowrate of the machine is generally a boundary condition of the compression system, the rotational shaft speed is very often the manipulated variable of the control system for a variable speed compressor.

State of the art and open questions

Steady state and dynamic simulations are routinely used by academics and practitioners to represent and analyse the behaviour of a compressor during different activities such as design, control and optimization of the machine.

In the literature there are many examples of compressor dynamic models (Botros et al., 1991, Venturini, 2005, Camporeale et al., 2006). Different techniques have been proposed for simulation and control applications.  However the models found in the literature do not rely much on the compressor characteristics. The reason for that is that they usually represent simple compressors i.e. single stage lab-size machine that can be tested in a lab to provide the parameters needed for the model calibration. This type of machine is closer to an ideal compressor than an industrial compressor. For this reason a simplified model cannot capture accurately the dynamic behaviour of an industrial compressor. Compressor characteristics should be used in order to achieve a more suitable representation and allow a fast and efficient control of the machine.

Professional tools (such as Aspen HYSYS® and CHEMCAD) are used in the process industries for dynamic simulation and control of compressors. Usually these tools allow only one modelling technique for the compressor characteristics i.e. lookup tables. They are industrially very important because for example they allow testing of a new control solution on a model rather than on the real compressor.

Contribution

The presentation will propose an advanced control technique that takes into account the compressor characteristics and the operational boundaries to improve the dynamic response of the CCS compressor under normal and surge operation conditions.

Tools for input/output simulation (I/O), also termed sequential modular simulation, are used to represent compressor characteristics using different modelling techniques such as polynomial fitting and interpolation. I/O simulation is very common and widely used, therefore the proposed scheme can be easily implemented inside advanced simulation and control tools.

The industrial case study is a multistage centrifugal compressors system for the compression of a mixture of carbon dioxide and light hydrocarbons in smaller percentage. This system is composed by four centrifugal compressors in series. The driver is an electric synchronous motor acting on a single shaft with intermediate gear boxes.

References

Botros, K. K., et al. (1991) Dynamic simulation of compressor station operation including centrifugal-compressor and gas-turbine. Journal of Engineering for Gas Turbines and Power, Transactions of the ASME, 113,300-311.

Camporeale, S. M., et al. (2006) A modular code for real time dynamic simulation of gas turbines in Simulink. Journal of Engineering for Gas Turbines and Power, Transactions of the ASME, 128,506-517.

Venturini, M. (2005) Development and experimental validation of a compressor dynamic model. Journal of Turbomachinery, Transactions of the ASME, 127,599-608.

Acknowledgements

Financial support from the Marie Curie FP7-ITN project "Energy savings from smart operation of electrical, process and mechanical equipment– ENERGY-SMARTOPS", Contract No: PITN-GA-2010-264940 and from the Marie Curie FP7-IAPP project “Using real-time measurements for monitoring and management of power transmission dynamics for the Smart Grid - REAL-SMART”, Contract No: PIAP-GA-2009-251304 is gratefully acknowledged.