Process of Natural Gas Treatment

Usually, natural gas treatment on the basis of thermal process engineering takes place in three steps (see Fig. 7.9). The first step that may consist of partial steps just like all other subsequent steps, serves the preparation of the crude gas for its processing. Here, for example, acid-forming gas components, such CO2, H2S and other sulphuric compounds are removed. Usually, chemical scrubbing with amines (MEA, DEA, MDEA) is applied in which the adsorbent is being regenerated. Then the natural gas is dried. In case of moderate water dew point requirements, glycol is used as wash liquor. The lowest water contents ( 1 ppm) are achieved with the application of zeolitic molecular sieves. Finally, mercury is removed in case aluminium will be used as material of construction for equipment. Mercury in contact with aluminium may lead to catastrophic corrosion. 

In the central process step, the pre-treated natural gas is separated into a light and a heavy fraction. As a rule, this separation takes place by means of partial condensation below ambient temperature. 

The light fraction always contains methane and nitrogen, sometimes even lighter hydrocarbons. For further use it is either compressed to pipeline pressure or liquefied and used as LNG. Beginning with ethane, the heavy fraction can contain all higher hydrocarbons that may be isolated, if required, by means of fractioning and then be marketed in technically pure quality. 

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It will be recalled that before and after most reaction stages, separation steps are necessary. Also certain processes have no reaction stages and are just a scries of physical unit operations, for example, milk processing or natural gas treatment. The raw material may need purifying and with regard to the Case Studies (CS) the following are important examples for CSl, filtration of molten sulphur and drying of the combustion air for CS2, removal of sulphurous compounds from the feed to the primary reformers for CS.3, air purification is often important and for CS4, water and silt, which are produced simultaneously with the oil and gas, are removed from the hydrocarbons as soon as possible. 

Because of the wide variation in the composition of natural gas as it is recovered at the wellhead and because natural gas can be used over a wide range of hydrocarbon contents, any specification for natural gas is usually broadly defined. However, the natural gas obtained at the wellhead usually undergoes some type of treatment or processing prior to its use for safety, economic, or system and material compatibiUty reasons. 

FoUowiag Monsanto s success, several companies produced membrane systems to treat natural gas streams, particularly the separation of carbon dioxide from methane. The goal is to produce a stream containing less than 2% carbon dioxide to be sent to the national pipeline and a permeate enriched ia carbon dioxide to be flared or reinjected into the ground. CeUulose acetate is the most widely used membrane material for this separation, but because its carbon dioxide—methane selectivity is only 15—20, two-stage systems are often required to achieve a sufficient separation. The membrane process is generally best suited to relatively small streams, but the economics have slowly improved over the years and more than 100 natural gas treatment plants have been installed. 

One of the principal aspects of refinery gas cleanup is the removal of acid gas constituents, ie, carbon dioxide, CO2, and hydrogen sulfide, H2S. Treatment of natural gas to remove the acid gas constituents is most often accompHshed by contacting the natural gas with an alkaline solution. The most commonly used treating solutions are aqueous solutions of the ethanolamines or alkah carbonates. There are several hydrogen sulfide removal processes (29), most of which are followed by a Claus plant that produces elemental sulfur from the hydrogen sulfide. 

Ethanolamines are important absorbents of acid gases in natural gas treatment processes. Another major use of ethanolamines is the production of surfactants. The reaction between ethanolamines and fatty acids... 

Alcohols used in the manufacture of alcohol and alcohol ether sulfates are obtained either by chemical treatment of fats and oils or by petrochemical processes from natural gas or crude oil. In either case the hydrocarbon chain ranging from 8 to 18 carbon atoms corresponding to the composition of coconut oil is the most desirable. 

S. Jullian, M. Thomas, and A. Rojey. Process for complete treatment of natural gas at a storage site (procede de traitement global de gaz naturel sur un site de stockage). Patent EP 781832,1997. 

Chapter 8 presents problems of natural gas production, transportation, and processing which are related to hydrates. Because a standard kinetic treatment method has progressed past the fledgling state in the second edition (1998), the state-of-the-art in flow assurance is turning away from thermodynamic properties which encourage hydrate avoidance, to kinetic properties which encourage a new philosophy in flow assurance—that of risk management. 

Gas Purification. The best-known purification process using 4A is that of natural gas this involves two benefits, namely, peak-shaving whereby the calorific value of fhe gas is adjusted by CO2 (and water) removal, and sweetening by H2S removal. The latter purification also is used in LPG treatment. Another example is the trapping of hydrocarbons in gas and vacuum circuits. 

There are many methods for the desulfurization of nature gas, which can be classified into dry desulfurization, wet desulfurization, and catalytic adsorption. In the dry desulfurization, some solid sorbents, such as iron oxide, zinc oxide, activated carbon (AC), zeolites, and molecular sieves, are used. In wet desulfurization method, liquid-phase chemical/physical solvent absorption systems are usually used for scrubbing H2S amine-based processes are subject to equipment corrosion, foaming, amine-solution degradation, and evaporation, and require extensive wastewater treatment. As a result, this sulfur removal technology is complex and capital intensive,44 although the processes are still employed widely in the industry. The desulfurization of coal gasification gas will be reviewed in detail in Section 5.5. In the catalytic-adsorption method, the sulfur compounds are transformed into H2S by catalytic HDS or into elemental sulfur or SOx by selective catalytic oxidation (SCO), and then, the reformed H2S and SOx are removed by the subsequent adsorption. 

Physical solvent processes give some, but not all, ofthe above qualities. The Selexol process has several industrial applications, most of them for synthesis gas deacidification and some for natural gas treatment [6-8]. A methanol-based refrigerated solvent process such as the Ifpex-2 process from the Ifpexol technology matrix ofIFP is also a good contender [9]. However, physical solvents have a high affinity for hydrocarbons and the separated acid gas stream contains large quantities of valuable hydrocarbon products. 

Depending on the purity of the gas feedstock, there may be pre-treatment of natural gas to remove impurities such as sulphur compounds that will poison catalysts used in subsequent processes. There are three common technologies in use to convert natural gas into syngas steam methane reforming (SMR), partial oxidation (POX) and auto-thermal reforming (ATR). 

Efficient transportation of natural gas either upstream or downstream is an important step in the natural gas industry that affects all the consumers. Here, upstream is defined as the portion in which the crude natural gas as obtained from the gas reserve is transported to the refineries for treatment. Downstream is the portion wherein the processed natural gas is transported to the ultimate destinations (e.g., households, industries, etc.). 

The escape of natural gas from the gas distribution networks also exerts a considerable effect on physical, chemical and biological processes in the soil. The extent of the gas zone depends greatly on the rate of its escape, depth of the groundwater level, type of pipeline, the nature and moisture of the soil, treatment of its surface, etc. The composition of the gaseous phase in the soil is affected by the microbiological oxidation of methane. The rate of this depends on the soil temperature, presence of oxygen and the content of nutrients. At low temperatures, this microbial process is restricted, which also restricts the anaerobic zone. 

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