Natural gas treatment

Some chemical production processes use the municipal gas supply as chemical feed stock. In some cases the gas has to be treated to meet the production specifications. Pressure swing adsorption is often used to remove higher hydrocarbons. One drawback of this application is the Umited lifetime of the adsorber in the presence of higher hydrocarbons and the costs for replacement and trans- 

In the case of recycling no methane losses occur but the permeate has to be recompressed and a condensation unit has to be installed to separate the higher hydrocarbon from the enriched permeate stream. The realized system consists of a membrane stage and the permeate was used as boiler fuel (Fig. 1.17). 

Process and off-gas streams in refineries and the petrochemical industry often contain hydrogen and hydrocarbons. The hydrogen stream has to be purified before it can be reused. Large volume streams are treated by cryogenic condensation and fractional distillation and in the case of smaller streams by pressure 

Membrane systems can also be integrated with traditional units. The design of a hybrid membrane separation system depends on several aspects, such as membrane permeance and selectivity, CO2 concentration of the inlet gas and the target reqnired, the gas value (per ca. 30 Nm, the price of gas in 2007 was 6- 7 in the United States, whereas in Nigeria, which is far from being as well-developed a gas market, it may be as low as 0.50 if the gas can be nsed at all) and the location of the plant (on an offshore platform, the weight, footprint, and simplicity of operation are critical onshore, total cost is more significant) [51]. 

Fignre 14.6 reports the block diagrams of two typical carbon dioxide membrane systems that treat natural gas with low CO2 concentration, as proposed by Baker [51]. Both systems are designed to treat a feed stream with 10% of carbon dioxide. One-stage systems are preferred for very small gas flows. In such plants, methane loss to the permeate is often 10-15%. If there is no fuel use for this gas, it must be flared, which represents 

Water is a common impurity in natural gas that must be removed to prevent hydrate formation. This is a good opportunity for the apphcation of membrane technology, but to be competitive, the membrane system must minimize the loss of methane with the permeate water [5], This loss can be reduced, on the one hand, by choosing membranes with desired selectivity, but, on the other hand, by the process parameters, because this separation is pressure ratio-limited [5]. 

Several natural gas reserves are considered sub-quality because of the high nitrogen content. The gas pipeline specifications for inert gases, in fact, fixes fhe nitrogen content to a 4% limit [56]. Currently, cryogenic distillation is used for this separation however. 

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In the petroleum refining and natural gas treatment industries, mixtures of hydrocarbons are more often separated into their components or into narrower mixtures by chemical engineering operations that make use of phase equilibria between liquid and gas phases such as those mentioned below ... 

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. 

Primary Raw Materials for Petrochemicals 3 NATURAL GAS TREATMENT PROCESSES... 

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... 

Prognosis of natural gas treatment volume andNOx emissions (Bashkin etal., 2002). 

Not selective High-pressure absorption may require intermediate flashing Commercial - developed for high-pressure natural gas treatment 1,2,3,5... 

Hydrogen sulfide, water/methane Natural gas treatment Niche applications, difficult for membranes to compete with existing technology... 

As mentioned earlier, hydrogen sulfide is present in some natural gases as a non-combustible impurity. In addition, hydrogen sulfide is toxic. Therefore, the purpose of natural gas treatment which is best performed at the wellhead is to remove hydrogen sulfide along with carbon dioxide. 

The membrane contactor for CO2 removal deserves special attention. It can be used for natural gas treatment, dehydration, and removal of CO2 from flue gas (see Section 4.4.4). A contactor (see Figure 4.22) patented and developed for this purpose by Aker Kvaerner— pUots have been installed and tested both in Norway (at Karstp) and at a gas terminal in Scotland. This module is based on PTFE membranes. A different commercial contactor based on polyimide membranes was recently installed at Santos Gas Plant in Queensland, Australia (December 2003). Santos is the largest gas producer in Australia. 

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. 

Emissions of sulphur oxides, particularly SOj, present a serious problem in the industry of the petroleum and natural gas treatment. The SO3 concentrations in the atmosphere depend on the sulphur content in the raw... 

Gritsenko A. 1., Aleksandrov 1. A., Galanin 1. A., Physical bases of natural gas treatment and utilization, Nedra, Moscow, 1981 (in Russian). 

A system study for bio-gas upgrading has shown that the specific relative primary energy use can be reduced from 33% to 20% under TSA conditions. For natural gas treatment, various operational conditions are being examined. Current materials research at ECN is focusing on the influence of sulfur containing impurities like H2S, COS, CS2, and CH3SH, as well as measures to increase the cyclic capacity for CO2. 

Stable membrane gas desorption (MGD) using a thin-film composite membranes made of hydrophobic glassy pofymer poly[l-(trinietliylsyhl)-l )ropyne] on a flexible metal-ceramic microfilter was achieved based on MDEA as COrabsoiption liquid. MGD can be used as a novel methodology for desorption of amine solution for natural gas treatment In particular, MGD can be useful for obtaining CO2 with methane pipeline specification... 

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