(484d) Design of Flexible Utility Systems

The design and optimisation of site utility systems is one of the most challenging topics in process engineering, as the complexity of equipment networks and choice of operating conditions present significant challenges to optimise utility systems in practice. Site utility systems are inevitably operated with different scenarios or operating conditions, due to variable conditions, such as different electric power tariffs, changing load in processes and variable ambient temperature. Therefore, utility systems should be flexible enough to change operating conditions (or scenarios) from one to another at minimum penalty and disturbance to the overall systems.

There exist transient periods in which operating conditions of utility systems are changed from one scenario to another. Operating costs in transient periods have been assumed in the past to contribute to only a small potion of total operation cost. Little transient research has been done for utility system design in the past. However, there is always spare capacity to meet changing demands, according to transient behaviour.

In this study, such transient behaviour is systematically investigated by using dynamic modelling and simulation of site utility systems, which enables flexible utility systems to be designed. Transient characteristics of utility systems may be analyzed by using unsteady calculations. Simplified dynamic and quasi-steady models have been developed in MATLAB/SIMULINK environment for transient analysis in the periods of operation changes. All the models are validated with published experimental data or more detailed models. Based on that, a novel methodology for the design and dynamic optimisation of site utility systems has been developed to improve operability and cost-effectiveness of utility systems. The proposed design procedure is summarized as follows: (1) the site configuration is set up, including economic and physical conditions, equipment properties, site steam and power demands, and safety constraints, etc.; (2) equipment network structure design and operating scenarios identification are carried out based on steady state methodology; (3) operating policies, such as boiler working loads, standby capacities and steam distribution paths, are optimised to minimize operating cost, while variation of steam conditions in scenario-transition period, simulated based on dynamic models, is kept within acceptable bounds. A case study is presented to illustrate significant benefits which can be gained from the modelling framework and transient analysis.

Transient analysis can be applied to check the feasibility of utility system design, analyze the operability and reliability of utility systems, and provide operating strategies in both steady-state operations and transient periods. Better understanding of transient phenomena can lead to cost savings as better use of equipment and/or less using standby capacity.

Keywords: flexible utility systems, transient analysis, dynamic models

Acknowledge The support from an EC Marie Curie Research Training Network ?Towards Knowledge-Based Processing Systems (PRISM)?; Contract No: MRTN-CT-2004-512233 is gratefully acknowledged.