Prico Process

Dynamic Modeling and Control of the PRICO© LNG process Arjun Singh and Morten Hovd Department of Engineering Cybernetics Norwegian University of Science and Technology Trondheim, Norway

1. INTRODUCTION For transportation of natural gas (NG), pipeline transportation is often used. However, when gas volumes are moderate, and/or transportation distances are large, the capital and operating costsfor pipeline transport become prohibitive. In such cases, transport of Liquefied Natural Gas (LNG) in tankers is often the preferred choice for bringing the gas to the market. In the liquefaction process the natural gas is cooled to around -160°C, and this requires significant amounts of energy. It is therefore important that the process can be operated safely, reliably and efficiently. To achievethis, good control is required. To understand LNG plant dynamics and to design a robust control system for its operation requires a dynamic model of the plant under consideration. Often the liquefaction unit of the plant is the critical unit which requires maximum attention. To develop a dynamic model for a liquefaction unit of an LNG plant is a challenging task and requires time and effort. Theacademic work on dynamic LNG plant simulation is limited (Hammer, 2004; Zaim, 2002; Melaaen, 1994). A significant part of these works focuses on modeling of a specific LNG plant. Development in process modeling tools such as Process Systems Enterprise’s gPROMS has made it easier to develop a dynamic model of typical chemical plants such as an LNG plant. This makes it easy to devote significanttime to study control aspects in details of such plants. We aim to do so in our work. The process considered in this work is a single mixed refrigerant process known as PRICO (poly Refrigerant Integrated Cycle Operations) process. (Stebbing and O’Brien, 1975). The PRICO process has been studied from optimization perspective in several publications. (Zaim, 2004, Lee et al., Del Nogal et al., 2005,Jensen and Skogestad, 2006). These works deals with steady state optimization and there is no literature available on dynamic modeling and control structure design for PRICO process. The focus of current paper is to use the model developed for PRICO process (Singh and Hovd, 2006) for control structure and controller design for the PRICO process. In addition to enabling the use of model based toolsfor control structure design, this allows testing the effects of common model simplifications, such as assuming constant temperature of the refrigerant at the condenser outlet, or ignoring the flash drum and refrigerant holdup. The effects of these model simplifications for model based

control structure development and controller tuning are described in the present work. 2. PROCESS DESCRIPTONFig 1 shows the flow sheet of the liquefaction unit of the PRICO process. Some features of the process are removed to make it simple.

Fig. 1: Flow sheet of liquefaction unit of PRICO process Natural gas enters the heat exchanger with a pressure of around 60 bars and temperature of about 12 C. Natural gas is composed of methane, ethane, propane, n-butane and nitrogen. A mix refrigerant having thesame components cools the natural gas in heat exchanger. When leaving the heat exchanger, the temperature of the natural gas has been reduced to around -155 C. The temperature is further lowered to around -163 C when pressure is lowered to near atmospheric. After compression, the mixed refrigerant is cooled in sea water cooled condenser before it enters the flash drum. After that it is furthercooled in the main heat exchanger. The high pressure (~ 30 Bar) sub-cooled refrigerant is throttled in a valve to produce a low temperature two-phase mixture which is vaporized in the main heat exchanger to cool the natural gas and high pressure hot refrigerant. The refrigerant needs to be superheated (by 5-10 C) before it enters the compressor to avoid damage to the compressor. 3. MODELING A...