Adsorption Solutions, L.L.C.
Typical uses of Adsorbents in Process Industries
There are many uses for adsorbents in the process industries. The following is a summary of some of those applications.
Natural Gas Industry
Drying. When gas is cryogenically processed, such as for hydrocarbon recovery or liquefaction, the gas needs to be dried to extremely low water dew point. Molecular sieves can readily provide this dryness. However, the feed gas and/or the regeneration gas H2S and O2 content as well as the choice of the molecular sieve type will affect the dryness. During the drying cycle there is conversion of some of the H2S into COS which will effect the recovered propane composition and may make the propane unfit for sale. There is also hold-up of H2S which will result in an H2S and COS peak in the spent regeneration gas. You can get an H2S peak even when no H2S is present if there are mercaptans in the gas. This can be a plant operations problem when the drier spent regeneration gas is blended into the sales gas. Certain molecular sieves can also be used to dry gas containing high levels of H2S to meet pipeline transmission dryness.
In some gas plants the feed includes liquid condensate that can be dried with molecular sieves. However since the condensate contains some very high molecular weight components as well as solids, the aging rate will be greater than that of gas service and the design of the unit must reflect this.
CO2 Removal. Water and carbon dioxide removal is needed prior to liquifaction. This can easily be done with molecular sieves in the same adsorption unit. Because the adsorption capacity for carbon dioxide is much lower than that of water, the regeneration gas requirement is higher than that for drying alone. This makes it more difficult to dispose the spent regeneration gas. Specific adsorption cycles as well as the use of internal insulation will minimize the regeneration gas requirement. Extensive care must be used prior to and during the startup to avoid damage of the molecular sieve charge. In some cases some of the heavy components from the feed gas also need to be removed. This usually can be done by substitution of the type 4A LNG molecular sieve with a different type.
Occasionally there is need to remove CO2 from liquid ethane or propane. This can be done with molecular sieves.
Sulfur Compound Removal. All sulfur compounds, with the exception of CS2, can be removed from gases or liquid hydrocarbons. In gas service, a typical use of molecular sieve is to dry the gas and remove most of the H2S prior to hydrocarbon recovery so that the residue gas meets pipeline quality requirements. Because the natural gas always contains some CO2, there will always be some reaction of CO2 and H2S to form COS and H2O. The amount of COS in the product gas as well as the gas dryness will depend not only on the type of molecular sieve used, but also on the molecular sieve composition and the manufacturing process details. Unlike the H2S which fractionates predominantly with the ethane, COS will go with the recovered propane. So if the amount of the COS generation during the drying/H2S removal step is too great, the propane may fail sulfur specifications. Also, if the wrong molecular sieve is used, the required gas dryness for full hydrocarbon recovery may not be present. The spent regeneration gas containing the H2S needs to be treated in a liquid absorption unit for further concentration and sulfur recovery.
In some cases, molecular sieves are used to remove all sulfur compounds from pipeline quality natural gas prior to use as feed to reforming operation, such as ammonia production. The efficiency of molecular sieves for this purification depends on the CO2 content of the gas.
The typical applications in liquid service include propane and butane desulfurizers. In the case of propane, it is important to remove not only the H2S but also most of the COS to insure that the 1A copper strip test is consistently met. If any trace of water is present in the storage or in the transport, the COS will hydrolyze to H2S. A number of operators have experienced problems where the produced propane meets the specification, but the propane shipment fails when is reaches the customer.
As with all liquid units that are regenerated with gas, proper process cycle steps must be used. This includes not only the drain and refill steps, but may also require some purge steps to make sure that the product LPG is not contaminated with components present in the regeneration gas. After the drain step there is still some LPG in the molecular sieve bed that is then lost into the regeneration gas. There is technology available to greatly minimize this loss.
A common situation with some propane treaters is the presence of methanol. When methanol is used in the hydrocarbon recovery operations, it will fractionate and concentrate with the propane. Methanol containing propane can be treated with molecular sieves, but the design of the adsorption unit must account for it.
Petrochemical Industry Uses
- Drying of cracked gas and liquids in ethylene plants
- Purification of ethylene, propylene, butene, and hexene prior to polymerization
- Solvent purification
Chemical Plant Uses
- CO2 removal from ammonia synthesis gas
- CO2 removal from H2/CO mixtures
- NOx removal from gases
- CO2 and water removal from air in air separation plants
- Iso/normal separation
- HF alkylation feed drying
- Polar component removal from isomerization feeds
- Chloride removal from hydrogen
- Hydrogen drying