W3V21 - Gas Treatment - Handout

Oil & Gas From exploration to distribution Week 3 V21 Gas Treatment Olivier Bernaert

W3V21 - Gas Treatment p. 1 IFPEN - IFP School 2015 / TOTAL SA 2015 / IFP Training 2015

At the outlet of the field: the first step is the separation of the liquid condensate, water and gas. In this lecture, we will discuss the treatment of the gas recovered from the stabilization process. The treatments and the process units that need to be installed depend on the final use of the gas and the type of transportation: by pipelines in gas form or by tankers in liquid form. Gas Consumers You have already seen the first two uses of the gas available at the outlet of the stabilization process. It can be utilized directly on the production field. Remember the gas re-injection in the well for reservoir pressure management, or activation of the production, by gas lift.

The other gas consumers are:

The power sector, to produce electricity, is by far the largest user of natural gas with around 40% of global gas consumption.

Industry uses roughly 24% of total gas consumption, including the gas used for refinery furnaces.

Natural gas is traditionally consumed in the residential sector, mostly for heating (22%).

In the industrial sector natural gas is used as a raw material, for instance natural gas is a typical feed used in the petrochemical industry.

A more recent sector is the transportation sector which today represents only 3% of the global consumption, more especially for public transportation in some countries like Argentina, Brazil, China or India.


Gas Composition Gas is mainly composed of methane and ethane. In addition to these two major compounds, you can find some impurities such as heavier hydrocarbons, like propane and butane, acid compounds like CO2 and H2S, but also water and mercury. The level of impurities will mainly depend on the origin of the gas field production. It is important to eliminate these impurities because:

heavy hydrocarbon can condensate in the gas depending on pressure and temperature conditions,

acid compounds are corrosive. It is important to eliminate

water especially when you transport the gas in liquid form at a very low temperature, otherwise you will have a problem of plugging because of the formation of hydrates and ice.

Mercury is really corrosive especially for aluminium metallurgy used in the liquefaction unit.

In the following parts, we will detail the treatment units used to eliminate all these impurities. Of course, the treatment processes to be installed will depend on the types and quantities of impurities in the gas at the inlet of the plant.

Gas Tretment Processes LPG Extraction Here you can see the normal boiling temperature of light compounds.


From 36 degrees Celsius for pentane to -161 degrees Celsius for methane. We use these differences of temperature from C1 to C5 to extract the heavy hydrocarbons present in the natural gas. First we eliminate the condensate with a carbon number higher than C5. Then we eliminate the propane and butane, called LPG for liquified petroleum gas. The processes of removing LPG is done by adjusting the dew point, by cooling and separation, in combination with a pressure drop. Finally we obtain the Liquified natural gas called LNG. Amine Unit The two main acid gas compounds in Natural gas are Hydrogen Sulfide (called H2S) and Carbon dioxide (called CO2) To eliminate these 2 acids, the main process used is amine absorption.

The gas containing H2S and CO2 is put in contact with a chemical liquid absorbent. For H2S and CO2 removal, the solvent is an amine, such as diethanol amine called DEA. A chemical reaction occurs between the H2S and the DEA. The sour gas containing CO2 and H2S enters at the bottom of an absorber column. The DEA enters at the top of the column. The sweet gas, without H2S and CO2, is recovered at the top of the absorber. The DEA, with the H2S, flows to a regenerator column. The DEA and the H2S goes inside the column where the H2S is stripped from the DEA by steam generated at the bottom of the column by a reboiler. The H2S is stripped out of the DEA solution at the top of the regenerator. The regenerated DEA goes to the absorber again. Because H2S is a poison, H2S and CO2 are treated in an additional unit called a Claus unit.


Claus Unit This unit transforms H2S into liquid or solid sulfur used by chemical industries. The first equipment of the Claus unit is a furnace where H2S is mixed and burnt with air, under controlled operating conditions, to give sulfur and water. The sulfur is condensed and recovered in liquid form.

Because this reaction is not complete, we add 2 or more catalytic stages to the process. The reactors containing the catalyst are operated at around 250 degrees at 1 bar. Liquid sulfur is again recovered at the outlet of each reactor. Gas Dehydration The water comes from the gas itself. Water in Natural Gas can create problems during transportation and processing, like hydrates or ice formation which may block the instrumentation, equipment or pipe lines. A hydrate is a solid formed from the combination of water and low-molecular weight molecules, such as CH4. There are 3 common processes of dehydration:

Physical absorption using Glycols like Triethylene glycol (called TEG). This process is the most frequently used for transportation by pipeline. This process is really similar to an amine unit used to eliminate H2S and CO2 where the DEA is replaced by TEG to eliminate water from the natural gas.

The second process is based on adsorption on solids such as a molecular sieve, activated alumina or silica gel. This process is used when you need a very low level of water, like when producing LNG , which requires a very low temperature and thus a level of water below 1 ppm volume.

The third dehydration process is membrane permeation.


Mercury adsorption The chemical reaction takes place again in a reactor full of another type of solid. The mercury is adsorbed and stays on the adsorbent. From time to time, it is necessary to change the adsorbent when is full of mercury. The natural gas flows, free of mercury, to the bottom of the reactor. Liquefaction Unit To transport the gas in liquid form, it is necessary to add a liquefaction unit. Then, the LNG is transported by tanker at -160C and then re-vaporized for the different consumers. The natural gas is cooled down to -160C to condensate methane and obtain LNG at the outlet of the liquefaction unit. To decrease the temperature, step by step, the gas passes through a series of cooling systems.

The first one is a cooling system full of propane. In this condition, we obtain a temperature around -35 degrees Celsius. Then a second loop is full of ethylene to obtain a temperature around -98C. The last cooling loop is with Methane to reach the outlet of the unit at a temperature around -155C. The pressure at the inlet of the unit is around 60 bars. At the outlet, the LNG is stored at a pressure of 1 bar and at a temperature of -160C. To conclude, at the outlet of the gas treatment plant, we have two options.

1. the gas is exported in gas form by pipeline after compression to around 150 bars. 2. after the liquefaction unit, the LNG is exported in liquid form by tanker.


Summary Process Flow Diagram After the stabilization to eliminate the condensates, the gas is treated in an amine unit to eliminate H2S and CO2. H2S is further treated in a Claus unit to transform the H2S into liquid or solid sulfur. Then, the gas is dehydrated to eliminate water. The gas then flows to a mercury adsorber, to eliminate mercury, and protect the aluminium exchangers used in the following liquefaction unit. The LPG extraction unit eliminates heavy hydrocarbons like LPG. Then the gas can be exported by pipeline after compression or treated in a liquefaction unit. At the outlet, the LNG is stored at -160C ready for transportation by tanker.


W3V21 - Gas Treatment - Handout

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