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Scrub Column Pressure Control Rev 2

IP.com Disclosure Number: IPCOM000248567D
Publication Date: 2016-Dec-19
Document File: 3 page(s) / 215K

Publishing Venue

The IP.com Prior Art Database

Abstract

In liquefaction of natural gas (LNG) plants, scrub columns integrated with the liquefaction unit are frequently operated close to the mixture critical point. This maximizes the efficiency of the liquefaction unit as the natural gas is liquefied at high pressure. For stable operation, it is important to operate the scrub column reflux drum at constant pressure and maintain an adequate margin away from the critical point. The scrub column performance will degrade significantly if the conditions in the column get too close to the critical phase region, leading to an upset of the liquefaction unit. Therefore, the scrub column reflux drum pressure is the targeted control parameter. However, feed pressure is typically controlled in upstream units.

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Scrub Column Control Scheme

In liquefaction of natural gas (LNG) plants, scrub columns integrated with the liquefaction unit are frequently operated close to the mixture critical point.  This maximizes the efficiency of the liquefaction unit as the natural gas is liquefied at high pressure.  For stable operation, it is important to operate the scrub column reflux drum at constant pressure and maintain an adequate margin away from the critical point.  The scrub column performance will degrade significantly if the conditions in the column get too close to the critical phase region, leading to an upset of the liquefaction unit.  Therefore, the scrub column reflux drum pressure is the targeted control parameter. However, feed pressure is typically controlled in upstream units.  See Figure 1 below.

Figure 1:  Typical Feed Pressure Arrangement

Frictional pressure drop in equipment upstream of the scrub column reflux drum, such as an Acid Gas Removal Unit (AGRU), Dehydration, pre-cooling, and warm bundle of the Main Cryogenic Heat Exchanger (MCHE), is a significant percentage of the operating pressure (approximately 10% to 15%) and may change as operating conditions such as feed rate are adjusted.  This poses a challenge as the feed pressure in the upstream units must be changed by the plant operator to ensure stable scrub column operation.  Failure to do so in timely manner may result in a plant upset.

In order to simplify the control arrangement and maintain a constant pressure in the scrub column reflux drum, the control arrangement can be adjusted as shown in Figure 2.

 

Figure 2:...