Amoco Cold Bed Adsorption Process
The conventional Claus sulfur recovery process is limited by reaction equilibrium considerations to sulfur recoveries in the range of 94-97%. For applications requiring an intermediate level (97.5-99.5%) of sulfur recovery, the so-called “sub-dewpoint” Claus process can be used. This process extends the capability of the Claus process by operating the Claus reaction at a lower temperature, so that the sulfur produced by the reaction condenses. Since the Claus reaction occurs in the gas phase, this liquid sulfur does not inhibit the reaction like sulfur vapor does, resulting in a favorable shift in the reaction equilibrium and higher sulfur conversion.
Amoco Corporation developed and licenses the most widely used sub-dewpoint Claus process, the Cold Bed Adsorption (CBA) sulfur recovery process. The CBA process is normally employed downstream of a conventional Modified Claus Process front-end processing section. This conventional Claus section contains a reactor furnace to oxidize one-third of the hydrogen sulfide (H2S) in the acid gas feeds:
(1) H2S + 3/2 O2 ⇌ SO2+H2O
The conventional Claus section also contains one or two (typically) catalytic reaction stages to react the sulfur dioxide (SO2) formed with the remaining H2S to form sulfur:
(2) 2 H2S + SO2 ⇌ 3/nSn + H2O
The conventional Claus section will typically convert 80-90% of the sulfur compounds contained in the feed streams to elemental sulfur and recover it as a molten product. Adding more conventional Claus catalytic stages beyond this point would not add much sulfur recovery because the Claus reaction, equation (2) above, is an equilibrium reaction, and becomes limited by the concentrations of water and sulfur vapor in the gases flowing through the plant. The CBA portion of the sulfur plant overcomes this limitation through the use of “sub-dewpoint” conversion stages.
Although catalytic conversion of H2S and SO2 is higher at lower reactor temperatures, conventional Claus reactors must be operated at temperatures sufficiently high to keep the sulfur produced from condensing. Sulfur catalyst will adsorb liquid sulfur into its pores, which blocks the active sites where the Claus reaction occurs. If the Claus reactor temperature is too low, the sulfur concentration in the vapor will exceed its dewpoint concentration, causing liquid sulfur to form and adsorb on the catalyst. Over time, this liquid sulfur will block all of the active sites in the catalyst and render the catalyst bed almost completely inactive.
A CBA reactor is operated in a cyclic fashion to avoid complete catalyst deactivation from liquid sulfur blocking the active sites. The CBA reactor is operated at low temperature (250-300° F) initially so that it is below the sulfur dewpoint of the reaction products (i.e., “sub-dewpoint”) and the sulfur formed is condensed and adsorbed on the catalyst. After operating in this manner for a period of time, the CBA reactor is “regenerated” by flowing hot gas through the reactor to vaporize the adsorbed liquid sulfur, which is then condensed and removed in a downstream sulfur condenser. This process is analogous to the processing steps used when dehydrating gas streams with molecular sieves. There are normally two or more CBA reactors in series so that at least one can be operating sub-dewpoint while the other is being regenerated.
Not only does a CBA reactor benefit from a more favorable Claus reaction constant at its lower operating temperature, it also has the advantage of shifting the Claus reaction equilibrium. The Claus reaction is a vapor-phase reaction, so condensing the sulfur product removes it from the vapor, forcing the equilibrium in reaction (2) farther to the right, toward higher conversion. These two factors allow much higher sulfur conversion than in a conventional Claus reactor, resulting in overall sulfur recovery efficiencies in excess of 98-99% for CBA plants.
The process is described below:
1 – The conventional Modified Claus Process section contains an acid gas burner, reactor furnace, and one or more catalytic reactors to react the majority of the H2S and SO2 and recover the molten elemental sulfur formed. The last sulfur condenser in this section operates at low temperature (250-300°F).
2 – During the adsorption mode, the feed gas to the first CBA reactor is not heated.
3 – Each CBA reactor contains a 36″-48″ deep bed of sulfur conversion catalyst, usually alumina-based. The H2S and SO2 in the gas will react via reaction (2) to form sulfur, which condenses due to the low operating temperature and is adsorbed on the catalyst.
4 – The reactor effluent is cooled to 250-275°F by generating low pressure (15-20 PSIG) steam in the sulfur condenser. No sulfur is condensed at this point because nearly all the sulfur has been adsorbed on the catalyst in the CBA reactor.
5 – The gas leaving the sulfur condenser flows directly to the second CBA reactor where much of the remaining H2S and SO2 is converted to sulfur, which also condenses and is adsorbed on the catalyst.
6 – The tailgas from the second CBA reactor is routed to a Tailgas Thermal Oxidizer to incinerate all of the sulfur compounds to SO2 before dispersing the effluent to the atmosphere.
7 – After operating in this manner for a period of time, the first CBA reactor is regenerated to remove its adsorbed sulfur. During the regeneration mode, the gas from the conventional Claus section is heated to 550-650°F in the heater before entering the first CBA reactor. Gas/gas exchangers, indirect fired heaters, in-line fired heaters, and electric heaters have all been successfully employed in this service.
8 – The hot gas vaporizes the liquid sulfur held on the catalyst bed, which leaves the reactor in the gas as sulfur vapor.
9 – The sulfur vapor is condensed in the sulfur condenser and removed.
10 – The remaining gas flows to the second CBA reactor which continues to operate in adsorption mode, converting most of the entering H2S and SO2 and adsorbing the sulfur produced.
11 – The gas leaving the second CBA reactor flows to the Tailgas Thermal Oxidizer as before.
Once the first CBA reactor has been regenerated, it is then cooled by sending the gas from the conventional Claus section to the reactor without heating. After the reactor has been cooled, it is placed into the second position by changing the positions of the appropriate switching valves. What was the second CBA reactor in “cleanup” position now becomes the first bed in “adsorption” position, and vice versa. By operating in this manner, the final bed always has the lowest sulfur content and keeps the sulfur concentration in the sulfur plant tailgas as low as possible.
The Cold Bed Adsorption process is generally capable of sulfur recovery efficiencies in the range of 97.5-99.5%, depending on the H2S concentration in the acid gas and the number of catalytic stages used. For recoveries in excess of 99.5%, the Modified Claus Process with Tailgas Cleanup is normally used. Due to the cyclic nature of the CBA process, the CBA switching valves are subjected to very demanding sulfur vapor service that has caused significant operation and maintenance problems in many of the CBA plants designed by others. In vivid contrast, Ortloff’s Proprietary Sulfur Vapor Valve Assemblies have functioned flawlessly without operational or leakage problems in the CBA plants designed by Ortloff.