Plant simulation and operation optimisation of SMR plant with different adjustment methods under part-load conditions

Highlights

• We set up a plant simulation model considering the performance variations of devices.

• We conducted an operational optimisation for SMR plant under part-load conditions.

• We obtained an operation strategy for SMR plant under part-load conditions.

Abstract

A plant simulation model of SMR (single mixed refrigerant cycle) plant which considers the performance variations of devices under different working conditions is established. It could be used to predict the performance variations of an SMR plant under part-load, and other working conditions. The operational optimisation based on plant simulation is conducted for SMR plant with different load adjustment facilities. The constraint condition uses a penalty function method and the Box method is applied for solution optimisation. The results show that the frequency conversion method could achieve the best adjustment performance and the composition optimisation is significant for the frequency conversion method, especially when the load is below 40%. The operational strategy giving the lowest energy consumption for SMR plant with different adjustment methods under part-load conditions was discussed and the efficiency of the MR compressor is regarded as the prime influence thereon.

Introduction

Small-scale LNG plants play an important role in gas resources utilisation in remote gas fields, offshore gas resources, and unconventional natural gas resources (Remeljeja and Hoadley, 2006). In recent years, the development of small-scale LNG plants has been rapid in many countries, especially in developing countries (Remeljeja and Hoadley, 2006, Wang, 2009, Lin et al., 2010). A mixed refrigerant cycle (MRC) is the most popular process in LNG plants: among these, a single mixed refrigerant cycle (SMR cycle) is particularly suitable for small-scale LNG plants due the simplicity of the process, the use of fewer devices, and its high energy efficiency. Small-scale LNG plants cannot always work under the designed NG flow rate for many reasons, such as seasonal fluctuations in gas supply or other commercial reasons. Thus, the ability to use load adjustment is important for such LNG plants. There are two main adjustment methods: the throttle method and a variable-frequency MR compressor. The throttle method refers to operations where a throttle valve is added at the entrance to the refrigerant compressor to reduce the suction gas density in the compressor by throttling. In this way, a power consumption reduction and load adjustment can be realised. A variable-frequency MR compressor was used to reduce power consumption and adjust the load by changing the rotational speed of the MR compressor to match the imposed load. The variable-frequency effect can be acquired by using a variable-frequency motor or a gas turbine. Some small-scale SMR plants have no such facilities. It is necessary to evaluate the performance of each adjustment method by simulation for determination of load adjustment facility.

Many process simulations and optimisations of MRC cycles have been reported (Remeljeja and Hoadley, 2006, Wang, 2009, Lin et al., 2010). A comprehensive study of a C3MR cycle used in a large-scale LNG plant is available (Alabdulkarem et al., 2011). Aspelund et al. developed a gradient free optimisation–simulation method for processes modelled with HYSYS software, which could be used to find the total refrigerant flow rate, composition, and the refrigerant suction and condenser pressures that minimise the energy requirement of a PRICO^® process (Aspelund et al., 2010). Danahe and Truls compared two classes of energy efficiency definitions and discussed their applications in the liquefaction process (Danahe and Truls, 2012). Xu et al. programmed a genetic algorithm method coupling the process simulation software Aspen Plus with a linear regression on the MR composition to derive a set of functions (Xu et al., 2012). The work can be used to determine the performance of the plant and be useful when searching for optimal MR compositions. However, the literature did not involve plant simulation in conjunction with load adjustment problems which differed from process simulations which have been widely studied from both an academic and industrial perspective. In process simulation, the process parameters and device performance are free to be selected. However, the devices and plant are fixed in load adjustment problems, so the simulation of these processes should be coupled with simulation of the device's performance. This type of simulation is designated “plant simulation”, which forms the foundation of this study of load adjustment.

Another important plant operation problem is the minimisation of energy consumption. The literature involving the operation of LNG plants is relatively rare (Jórgen and Sigurd, 2006, Jørgen and Sigurd, 2009a, Jørgen and Sigurd, 2009b). Arjun and Morten developed a simplified model applicable to control structure design without compromise (Arjun and Morten, 2007). Wilco et al. discussed the potential application of modern control technology in LNG production (Wilco et al., 2009). Hasan et al. presented a generalised model for compressor operations in large-scale AP-XTM LNG processing (Hasan et al., 2009). Sun et al. have implemented a model simulation study into the working conditions and how they change when refrigerant is charged into the plant or leaked (Sun et al., 2012). These studies do not involve the operational problems of load adjustment. The operational optimisation of a fixed LNG plant should be based on plant simulations, which differs from the more widely-used design optimisation. Hatcher and Abbas represented the concepts of design optimisation and operational optimisation (Hatcher et al., 2012). However, their studies focussed on the effects of the heat exchanger, the performance variations of the MR compressor – which is the key to understanding load adjustment problems – is neglected. Thus, another aim of the research is to develop an operational optimisation model and study the optimised operation of an LNG plant under changed loads.