Hydrocarbon Engineering

August

2019

HYDROCARBON

ENGINEERING

he process of treating and liquefying natural gas

into LNG is a complex and often delicate

operation. The profitability of a plant depends on

throughput, which is maximised by maintaining

stable operation, consistently meeting product

specifications, and managing a number of process

contaminants present in the feed gas to the facility or

produced within the process.

The specifications involved with natural gas liquefaction

are stringent and often difficult to maintain, as shown in

Table 1. The gas purity required for liquefaction is higher

than that for sales gas in several respects such as heavy

hydrocarbon content, carbon dioxide (CO

), oxygen, and

water. Additional restrictions are present as well for mercury

and benzene that are not of major concern in conventional

gas plants.

The liquefaction process is the back end of the LNG

plant and is downstream of all pre-treatment processes. In

the most common process, the gas stream is cooled by

propane and then further cooled and liquefied by mixed

refrigerant in the main cryogenic heat exchanger (MCHE).

Plant upsets caused by contaminants that often lead to

shutdowns or reduced throughput can usually be remedied

quickly in conventional gas plants, and the plant can return

to normal flow rates shortly thereafter. LNG facilities,

however, cannot recover as quickly from process upsets.

Due to the design of liquefaction units, start-up can take

several hours and sometimes days to get back down to the

low process temperatures required to liquefy gas.

Variations in feed gas composition can have a significant

effect on the liquefaction process. The amount of high

molecular weight hydrocarbons separated from the feed

gas and used at a heavy hydrocarbon recovery unit (HRU)

can affect the overall removal efficiency of the unit. A feed

gas that is leaner than the anticipated design can actually

result in more heavy hydrocarbons entering the liquefaction

stage and causing a series of fouling problems.

Ingression of mercaptans and other sulfur species can

also cause fouling in the cold plant section and affect the

specifications for total sulfur.

David Engel, Cody Ridge, and Scott Williams, Nexo Solutions,

Amine Optimization Company Division, USA,

analyse cryogenic

heat exchanger fouling troubleshooting at LNG facilities.