Hydrocarbon Engineering

August

2019

HYDROCARBON

ENGINEERING

Even minute amounts of liquids contamination or

condensable hydrocarbons in the liquefaction stage can

cause fouling or freezing in the MCHE, leading to reduced

heat transfer and, eventually, plant shutdown. The potential

for hydrocarbon condensation must be considered

carefully, and removal of any solids and free or aerosolised

liquids in the gas must be done with very high efficiencies.

LNG contaminants

Contaminants are most usefully classified according to their

source, as the source is the best location to implement a

solution for removal or mitigation. The most common

sources of contamination in liquefaction units are the feed

gas, process solvents, process materials and chemical

additives, and process-generated contaminants.

Feed gas contamination levels are generally lower in

LNG feed gas compared to conventional natural gas plant

feed gas because of the pipeline gas quality used, but it can

still significantly affect the LNG process. Contracts for feed

gas supply to LNG facilities often have stricter requirements

for gas quality and composition, but not all contaminants

have specifications or are even considered. In some

respects, the feed gas to LNG facilities can be just as

contaminated as unprocessed natural gas. Table 2

demonstrates some of the most commonly observed

contaminants in gas feeds to LNG facilities. Some of the

contaminants in the table, such as acid gases and mercury,

are accounted for with process technologies at each plant.

Other contaminants such as surfactants and chemical

additives have no dedicated solution that is effective for

their removal or mitigation, so special precautions must be

taken to prevent their ingression or mitigate any

detrimental effects.

Any carryover of process solvents into the liquefaction

unit will cause problems including fouling, corrosion,

reduced heat transfer, and missed product specifications. In

addition to the solvent itself, build-up of contaminants

from other sources within the process may also be present.

It should be noted that water is perhaps the most

important solvent used for LNG processing and also the

most susceptible to contamination. The most common

problem with water is the presence of metals, foam,

suspended solids, and emulsified hydrocarbons.

Materials used within the LNG process must be fully

compatible with the process streams they contact.

Incompatible materials can be considered contaminants

because they are undesired components introduced to the

system that cause a detrimental effect. The detrimental

effect may be a release of some material into a process

stream, or a reaction or degradation of the material leading

to new contaminants or reduced performance of the

related system. Molecular sieve dust, for example, may carry

over from the bed past the dust filter to the downstream

liquefaction stage and foul the MCHE. Filters and

adsorbents that were not properly cleaned, use

incompatible materials, or do not have the proper

efficiency, may not remove contaminants effectively and

may also release residues that cause downstream problems.

Contaminants that are process-generated are

components that were created as a result of reactions that

occurred within the unit itself. Corrosion products are one

of the most commonly referenced process-generated

contaminants. Eliminating the source of corrosion is the

best method of prevention and requires knowledge of

low-risk process conditions and corrosive contaminants.

Contaminated recycle streams feeding back to certain units

at an LNG facility may also be considered

process-generated and can cause unexpected issues when

not considered. Gas used to regenerate molecular sieve

beds is frequently recycled to the front end of the beds

and can contain elevated concentrations of sulfur species

and a number of other contaminants desorbed

from the molecular sieve beds. Products of

unwanted and often unnoticed reactions that

occur in the process are also a significant

concern in many plants, such as hydrogen

sulfide (H

S) formation from mercaptans when

regenerating molecular sieve beds.

Liquefaction unit

contamination control

The liquefaction process itself usually has

little to no means of contaminant removal, as

LNG plants are designed to remove all

contamination upstream. HRUs remove

Table 1.

Sales specifications for natural gas and LNG

Parameter

Sales gas specification LNG specification

Methane

>75 mol%

>96 mol%

Ethane

<10 mol%

<4 mol%

Propane

<5 mol%

<2 mol%

Butanes

<2 mol%

Pentanes+

<0.5 mol%

<0.01 mol%

Hexanes+

<10 ppmv

Benzene

<1 ppmv

Carbon dioxide

<2 – 3 mol%

<50 ppmv

Hydrogen sulfide <4 ppmv

<1-4 ppmv

Total sulfur

<40 – 80 ppmv

<20 ppmv

Water vapour

4 – 7 lb/million ft

<0.1 – 1.0 ppmv

Nitrogen and inerts <3 mol%

<1 mol%

Mercury

<0.01 mg/Nm

Oxygen

<1 mol%

Table 2.

Common contaminants in LNG feed gas streams

Solids

Liquids

Gases

Corrosion products

Compressor lubrication oil

Acid gases

Calcium/magnesium

scale

Hydrocarbon condensate

Condensable

hydrocarbons

Natural gas formation

solids

Water and dissolved salt/metals

Mercury

Waxes, paraffins, and

asphaltenes

Chemical additives and byproducts Oxygen

Chemical additives

and byproducts

Organic acids and surfactants

Surfactants and

alcohols