Natural Gas Purification Levels

Written by:

El sayed Amer

1. Gas purification

Natural gas used by consumers is significantly different from the natural gas that is brought up from the wellhead. The processing of natural gas, in many respects, is less complicated than the processing of crude oil but is equally important before it is used by the end users.

Gas purification, involves the removal of vapor-phase impurities from gas streams. The processes which have been developed to accomplish gas purification vary from simple once-through wash operations to complex multiple-step recycle systems. In many cases, the process complexities arise from the need for recovery of the impurity or reuse of the material employed to remove it. The primary operation of gas purification processes generally falls into one of the following five categories:

1. Absorption into a liquid
2. Adsorption on a solid
3. Permeation through a membrane
4. Chemical conversion to another compound
5. Condensation

2. Sulfur Contaminants

These Contaminant can be:

• Hydrogen sulfide (H2S)
• Hydrogen cyanide (HCN)
• Carbon dioxide (CO2)
• Carbonyl sulfide (COS)
• Carbon disulfide (CS2)
• Mercaptan (RSH)
• Sulfur dioxide (SO2)
• Elemental sulfur

3. Sweetening Process levels

Sulfur exists in natural gas as hydrogen sulfide (H2S), and the gas is usually considered sour if the hydrogen sulfide content exceeds 5.7 mg of H2S per cubic meter of natural gas. The process for removing hydrogen sulfide and carbon dioxide from a natural gas stream is referred to as “sweetening” the gas.

Absorption: A separation process involving the transfer of a substance from a gaseous phase to a liquid phase through the phase boundary.

Adsorption: The process by which gaseous components adhere to solids because of their molecular attraction to the solid surface.

Membrane permeation: is a relatively new technology in the field of gas purification. In this process, polymeric membranes separate gases by selective permeation of one or more gaseous components from one side of a membrane barrier to the other side.

Chemical conversion: is the principal operation in a wide variety of processes, including catalytic and noncatalytic gas phase reactions and the reaction of gas phase components with solids.

Condensation: as a means of gas purification is of interest primarily for the removal of volatile organic compounds (VOCs) from exhaust gases. The process consists of simply cooling the gas stream to a temperate at which the Organic compound has a suitably low vapor pressure and collecting the condensate.

4.Process Selection

Selecting the optimum process for removing one or combination of the impurities is not easy.

In many cases, the desired gas purification can be accomplished by several different processes. Determining which is best for a particular set of conditions ultimately requires a detailed cost and performance analysis.

However, a preliminary screening can be made for the most commonly encountered impurities by using the following generalized guidelines.

Both absorption in alkaline solution (e.g., aqueous diethanolamine) and absorption in a physical solvent (e.g., polyethylene glycol dimethyl ether) are suitable process techniques for treating high-volume gas streams containing hydrogen sulfide and/or carbon dioxide. However, physical absorption processes are not economically competitive when the acid gas partial pressure is low because the capacity of physical solvents is a strong function of partial pressure.

When hydrogen sulfide and carbon dioxide are absorbed in alkaline solutions or physical solvents, they are normally evolved during regeneration without undergoing a chemical change.

There are many factors to be considered in the selection of a given sweetening process. These include the following:

• Type of impurities to be removed (H2S, mercaptans, CO2)
• Inlet and outlet acid gas concentrations
• Gas flow rate, temperature, and pressure
• Feasibility of sulfur recovery
• Acid gas selectivity required
• Presence of heavy aromatic in the gas
• Well location
• Environmental consideration
• Relative economics

5. Most Common Sweetening processes

   1. CHEMICAL SOLVENT PROCESSES

Utilize an aqueous solution of a weak base to chemically react with and absorb the acid gases in the natural gas stream. Absorption occurs as result of the partial pressure

differential between the gas and the liquid phases

Alkanolamines:  are bases used in Gas Plants and Refineries to remove H2S, Mercaptans and/or C02 contaminants from natural gas, reformed gas or refinery gases and LPG by a chemical absorption mechanism.

* NH2 Group: responsible of the acid gas absorption.

* OH Group: responsible of the amine solubility in water.

Types of Alkanolamines

• Primary Amine: MEA and DGA (the most reactive amines).
• Secondary Amine: DEA and DIPA amines.
• Tertiary Amine: MDEA and TEA amines.

Amine solvent

H2S + Amin⁡e ⇒ Amin⁡e Sulfide + H2O

B – Solid BED PROCESSES

Fixed bed of solid particles can be used to remove acid gases either through chemical reactions or through ionic bonding.

Gas stream flows through a fixed bed of solid particles which removes the acid gases and holds them in the bed.

When the bed is spent, the vessel must be removed from service and the bed regenerated or replaced.

Several types of adsorbents have been used during time. The most common types are:

• Activated carbon
• Molecular sieves/ zeolites
• Synthetic polymers
• Silica gel
• Activated alumina
• Iron Sponge.

6. Acid Gas disposal

H2S should be converted to non-toxic and useful elemental sulfur that can be used in fertilizer industry.

The options can be selected for safe disposing and converting of hydrogen sulfides are:

1. Re-inject the acid gas into a formation.
2. Burn the acid gas,
3. Converting the H2S into SO2. Release the SO2 to the atmosphere.
4. Convert the H2S into elemental Sulphur (S). Sell the Sulphur into the world market.

The most common method of converting H2S into S is the Claus process.

Clause Process: a process in which 1/3 of the H2S in the acid gas feed is burned to SO2 which is then reacted with the remaining H2S to produce sulfur.

The modified Claus process, developed by London chemist Carl Friedrich Claus in 1883.