(444b) Dynamic Modeling and Control Structure Design for A Liquefied Natural Gas Process

Dynamic Modeling and Control structure design for a Liquefied Natural Gas Process

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 costs for 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.  It is quite common to have a heavy upfront investment in large industrial plants for producing liquefied natural gas (LNG) since cost per unit of gas volume will be relatively low over the plant lifetime. Multi-component refrigerants have been commonly used in such plant to achieve low temperature for LNG (-160 C at near atmospheric pressure).

However, there is a growing need for liquefaction of natural gas at places where it is not possible or economically acceptable to have a heavy investment. This includes local distribution of natural gas in small markets, where plant needs to be arranged at a gas pipe, while the LNG is transported by trucks and small ships. For such plants low investment costs have priority over optimal energy utilization. Traditionally the relative investment cost for small-scale LNG liquefaction plants increases almost exponentially with decreased production capacity from about 50,000 tones per annum and below[1]. SINTEF has developed a low capacity plant which requires low investment cost and is easy to construct at desired sites. The plant design has been patented by SINTEF [2]

This work is the continuation of the authors work [3] which has been acecpted for publication at American Control Conference(ACC) to be held in
New York, US in July 2007.  ACC paper discusses development of a  dynamic mathematical model for this plant and design of a control structure for the plant to ensure stability and ease of operation. Fundamental limitations on performance of this plant are also analyzed.   

However, heat exchanger model used in the above study neglects mass hold up of refrigerant in heat exchanger. Due to this assumption, in the developed plant model it is sufficient to control only liquid level in one of the flash drums. But it is known that approximately half of the refrigerant is not in drums and lies in the heat exchangers and in the pipe connecting different components in plant. So it is necessary to include this hold up in the model before a accurate control structure can be proposed and implemented. This work deals with modifying heat exchanger model to include refrigerant mass hold up in the heat exchanger and study the effect of this change on control strucuture design of the plant. A complete description of the process can be found at [2]. Here a brief process description is given.  Fig 1 represents the simplified flow sheet of the SINTEF LNG plant. Some features of the process are removed for clarity. In Fig.1 sub-components of plants are numbered and referred as units. Units 13, 14, 15, 16, 17 and 18 are heat exchangers. Heat Exchanger (HX) numbered 13, 15 and 17 are called the ?Refrigerant HX' and HX number 14, 16 and 18 are called ?LNG HX'. Units 3 and 5 are separators and units 4, 6, 7, 8, 9 and 12 are valves. Units 10 and 11 are ejectors and units 1, 2 and 19 represent the condenser, cooling water stream, and the compressor, respectively.

The refrigerant is compressed in compressor (unit 19). After the compressor, the refrigerant is partly condensed primarily by water cooling in unit 1. The vapor from unit 3 is further cooled and partly condensed in unit 13 while the liquid from unit 3 is mixed with two refrigerant streams from unit 15 and 16. The vapor from unit 5 is further cooled and condensed in unit 15 and sub-cooled in unit 17 before it is flashed to a pressure of about 2-4 bar, giving the cold refrigerant for unit 17 and unit 18 before mixing it with the liquid from unit 5. After  mixing, the refrigerant flow is divided and distributed to unit 15 and 16. The natural gas is cooled in unit 14, condensed in unit 16 and sub cooled in unit 18.

                                                                                                            Fig 1: SINTEF Plant

References:

[1] O.B. Neeraas, E. Brendeng, Aa. Wallentinsen and M. Mangersnes ?A new concept for small scale LNG production? AIChE Spring National Meeting,
Houston, April 22-26 2001

[2] ?Method and device for small scale liquefaction of a product gas?, US Patent No.
US 6,751,984 B2

[3] Singh, A and Hovd, M. ?Dynamic Modeling and Control structure design for a Liquefied Natural Gas Process? Accepted for ACC , to be held in New York, USA in July 2007.