Processing heavy hydrocarbon removal

NG processing is the largest industrial gas separation application for membranes. Membrane market share in 2008 was 5%, with interest in this application growing rapidly [2]. Membranes for CO2 and heavy hydrocarbon removal (C3+) from NG continue to be the two largest membrane gas separation applications, while N2 and H2S removal from NG is in its early stages of commercialization [12], Dehydration of NG using membranes is also attracting interest [13]. 

The removal of aromatics and relatively heavy hydrocarbons from gas streams with fixed beds of activated carbon is essentially the same process as. solvent recovery, and similar adsorbents and equipment are used. The principal differences are that in hydrocarbon recovery the feed is typically a natural gas or other combustible gas stream rather than air, and adsorption is usually (but not always) conducted at elevated pressure. The basic design approach for hydrocarbon recovery systems follows the same general logic as that described for solvent recovery systems. A brief outline of the key design steps is given in the Calgon Carbon Corporation bulletin, Heavy Hydrocarbon Removal or Recovery from Gas Streams (1987). 

Dehydration can be performed by a number of methods cooling, absorption and adsorption. Water removal by cooling is simply a condensation process at lower temperatures the gas can hold less water vapour. This method of dehydration is often used when gas has to be cooled to recover heavy hydrocarbons. Inhibitors such as glycol may have to be injected upstream of the chillers to prevent hydrate formation. 

The most common solvent employed is carbon dioxide gas, which can be injected between water spacers, a process known as WaterAlternating Gas (WAG). In most commercial schemes the gas is recovered and reinjected, sometimes with produced reservoir gas, after heavy hydrocarbons have been removed. Other solvents include nitrogen and methane. 

Of the four commercial processes for the purification of carbon monoxide two processes are based on the absorption of carbon monoxide by salt solutions, the third uses either low temperature condensation or fractionation, and the fourth method utilizes the adsorption of carbon monoxide on a soHd adsorbent material. AH four processes use similar techniques to remove minor impurities. Particulates are removed in cyclones or by scmbbing. Scmbbing also removes any tars or heavy hydrocarbon fractions. Acid gases are removed by absorption in monoethanolamine, hot potassium carbonate, or by other patented removal processes. The purified gas stream is then sent to a carbon monoxide recovery section for final purification and by-product recovery. 

Normally, treatment of coproduced groundwater during hydrocarbon recovery operations will include, as a minimum, oil-water separation and the removal of dissolved volatile hydrocarbon fractions (i.e., benzene, toluene, and total xylenes). In addition, removal of inorganic compounds and heavy metals (i.e., iron) is often required. Dissolved iron, a common dissolved constituent in groundwater, for example, may require treatment prior to downstream treatment processes to prevent fouling problems in air-stripping systems. Heavy metals removal is normally accomplished by chemical precipitation. 

The technology is best suited for the removal of water-soluble metals and organics. Organics that would most likely be mobilized by this process include aromatic compounds such as benzene, toluene, xylene, and phenolic compounds, as well as chlorinated solvents. Electroki-netic remediation is not a practical method of remediation for insoluble organics such as heavy hydrocarbons. 

The Sumitomo-BF PSA process uses carbon molecular sieves (CMS) as the selective adsorbent, CMS has a higher capacity of adsorption than zeolites for methane and oxygen, and it is considered to be advantageous for hydrogen purification. If dirty raw gases are fed to this process, minor amounts of heavy hydrocarbon components such as aromatics are likely to cause deterioration of the adsorbents. To remove the heavy hydrocarbons, prefilter columns that contain activated carbon are placed upstream of the main CMS adsorbent beds4. 

Carbon monoxide is first stripped off particulate matter in a cyclone separator or in a scrubber. Scrubbing also removes tar or heavy hydrocarbon fractions. Acidic gases, if present, are removed by absorption in monoethanol amine or in potassium carbonate. This pretreated gas is sent to the next section for further purification. Commercial processes for final purification are based on the absorption of carbon monoxide by salt solution, low-temperature condensation, or fractionation, or by pressure-swing adsorption using a solid material. 

Zeolites find major applications in catalysis. A form of the zeolite FAU is, for example, an active catalyst component in catalytic cracking of heavy hydrocarbons to produce motor gasoline and diesel. The catalyst activity arises from its Bronsted acidity, which in turn comes from the presence in the stmcture of protons attached to bridging oxygen atoms. Protons can be introduced by ion exchange of anunonium cations, followed by calcination to remove NH3 and generate the acid form of the zeolite. The process is more complex... 

The raw synthesis gases from partial oxidation of heavy hydrocarbons and coal differ mainly in two aspects from that produced from light hydrocarbons by steam reforming. First, depending on the feedstock composition, the gas may contain a rather high amount of sulfur compounds (mainly H2S with smaller quantities of COS) second, the CO content is much higher, in some cases in excess of 50%. The sulfur compounds (Section 4.3.1.4) can be removed ahead of the shift conversion to give a sulfur-free gas suitable for the classical iron HTS catalyst. In another process variant the sulfur compounds are removed after shift conversion at lower concentration because of dilution by C02. The standard iron catalyst can tolerate only a limited amount of sulfur compounds. With a sulfur concentration in the feed >100 ppm sulfur will be stored as iron sulfide (Eq. 87) ... 

There, the depolymerizate is hydrogenated under high pressure (about 10 MPa) at some 400-450°C, using a liquid phase reactor without internals. Separation yields a synthetic crude oil, which may be processed in any oil refinery. Light cracking products end up in the off-gas and are sent to a treatment section, for removal of ammonia and hydrogen sulphide. A hydrogenated bituminous residue comprises heavy hydrocarbons, still contaminated with ashes, metals and salts. It is blended with coal for coke production (2 wt%). 

Depending on the feed composition, Saipem offers different possible processing schemes. In a typical configuration, the feed is sent to the first column (1) where the heavy hydrocarbons (mainly n-butane and butene-2) are removed as the bottom stream. In the second column, (2) the butene-1 is recovered at the bottom and the light ends (mainly isobutane) are removed as overhead stream. 

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