Oct-2012

Valuable liquids from refinery gases

With appropriate treatment schemes, refinery off-gases can be valuable sources of olefins and hydrogen

ZAHEER MALIK and JOHN SLACK
Linde Process Plants

Article Summary

Refinery off-gas comes from a variety of units within the refinery. The quality and quantity of the off-gas are dependent on the type of crude and the complexity of the refinery. The majority of the fuel gas stream is generated in the fluidised catalytic cracking (FCC) unit, where the long-chain hydrocarbon molecules are broken into lower molecular weight chains or lighter hydrocarbon components. Significant amounts of olefins are also produced during hydrotreating. These olefins are valuable and can increase revenue. Often, the off-gas also contains a large amount of hydrogen that, if recovered, can provide economic value. Hydrogen can be recovered by utilising pressure swing adsorption (PSA) or a cryogenic process.

Table 1 shows the range of compositions of off-gas components. Table 2 reflects the refinery gas’s contaminants. Several of the contaminants have to be removed from the feed gases, either to meet product specifications (see Table 3) or to mitigate operational hazards. After removal of the hydrocarbon liquids, the residual gases are an excellent feedstock for a hydrogen plant utilising steam methane reforming technology.

In order to prepare the gases for ethylene liquids recovery, the following contaminants must be removed to meet the product specifications listed in Tables 3-7:
•  Hydrogen sulphide (H2S) and carbon dioxide (CO2)
•  Ammonia
•  Chlorides
•  Water
•  Mercury
•  Carbonyl sulphide (COS )
•  Arsine
•  Acetylene
•  NO, NO2, NOx (nitro gum)
•  Dienes (1-3 butadiene, acetylene, propadiene and so on).

The composition of the off-gas depends on the type of crude, the cracking severity and type of catalyst used for cracking. A typical range of components in the FCC gas is listed in Table 1; the trace components are listed in Table 2.

Before a treating system can be designed, several trace components must be properly analysed (see Table 2). The concentration of the components shown in Table 2 is dependent on refinery complexity and type of crude.

The prospective products of the refinery liquid recovery units are:
•  Ethylene
•  Ethane
•  Ethylene/ethane mix
•  Propylene
•  Propane
•  Gasoline or C5+
•  Residue gas or fuel gas.

The product specification is dependent on the market.

Table 3 lists typical product specifications for chemical- and polymer-grade ethylene.

Ethane sold as an ethylene cracker feedstock must meet certain purity requirements. Table 4 lists typical specifications for the ethane feedstock.

Propylene may be sold as chemical, refining or polymer grade. Propylene is separated from propane using C3 splitter fractionation. Table 5 lists chemical- and refining-grade specifications, while Table 6 gives a polymer-grade product specification. Table 7 shows a HD-5-grade propane specification.

Contaminant removal Mercury removal
Mercury is present in natural gas, natural gas-associated condensates and in refinery off-gases as organometallic and inorganic compounds, and in the elemental (metallic) form, depending on the gas source. The elemental form can be found in either the vapour or liquid phase. The organometallic (typically dimethyl mercury, methyl ethyl mercury, or diethyl mercury) and inorganic forms (such as HgCl2) of mercury condense into the liquid phase in any hydrocarbon fractionation column. Vapour-phase elemental mercury is a primary source of corrosion in aluminium heat exchangers.

Elemental mercury that leaves the plant with the hydrocarbon liquid streams is a primary source of corrosion for the aluminium equipment in olefins liquid recovery plants. Mercury also poisons the selective hydrogenation catalysts in olefin plants and can pose inhalation hazards to workers.

The organometallic and inorganic forms of mercury usually end up in the condensate stream from the natural gas or refinery liquid recovery plant. These compounds are important environmental toxins that are easily absorbed and accumulated by biological organisms. The presence of these compounds in gas condensate streams leads to waste disposal problems and safety hazards to workers.

Although relatively high levels of elemental mercury were discovered in the Groningen (Holland) field as early as 1969, the first recorded cold box failure attributed to mercury corrosion was in the aluminium spiral-wound heat exchanger of the LNG plant at Skikda, Algeria,1 in 1974. Since this time, mercury in natural gas has become a major concern in cryogenic gas processing industries. These industries, processing refinery gases including liquefied natural gas (LNG), liquefied petroleum gases (LPG) and olefins recovery, often use brazed aluminium heat exchangers in their cold boxes. Mercury corrosion of aluminium exchangers has led to several additional failures since the problems at Skikda. In addition, mercury accumulation can lead to poisoning of catalysts used in olefin processes, personnel safety hazards and waste disposal difficulties

Mercury sources
Elemental mercury is a natural contaminant present in the produced natural gas in various concentrations at certain geographic locations. The concentration of elemental mercury in the gas stream is often expressed in μg/Nm3, which is a very small unit of measure. Generally speaking, elemental mercury levels have been found to be the highest in Southeast Asian gases (up to 400 μg/Nm3 in the vapour phase) and lowest in US Gulf Coast gases (as low as 0.02 μg/Nm3), although wide variation is known to occur even within regions. However, even at the very lowest natural concentrations, it is still desirable to reduce the amount of mercury before any cryogenic processing, either in refinery or natural gas streams. The mercury can accumulate to higher concentrations over time.