Increase the Flexibility of Your Claus Unit

By Gary J. Nagl (Vice President, Merichem)

Introduction

With the introduction of sophisticated ratio control, O2 enrichment, improve burner design, etc. , great strides have been made in the ability of Claus units to adjust to rapidly changing feed conditions, which are experienced in both petroleum refineries and natural gas processing facilities. However, even with these advancements, some process applications still challenged Claus units with their ability to adjust to large turndown ratios while maintaining high removal and on-stream efficiencies. By incorporating a liquid redox process, such as the LO-CAT® process, together with a Claus unit, turndown ratios of 100%, removal efficiencies of 99.9+% and on-stream efficiencies approaching 100% can be achieved. In addition for certain applications, the typical Claus incinerator can be eliminated, which will greatly reduce CO2 and SO2 emissions and the energy requirements to operate the incinerator. All of this can be accomplished without having to recycle gas back to the Claus.

Sulfur Recovery Via Liquid Redox Processing

Liquid redox processes employ aqueous-based solutions containing metal ions, usually iron, which are capable of transferring electrons in reduction-oxidation (redox) reactions. Currently, the redox process of choice is the LO-CAT® process, which is licensed by the Gas Technology Products division of Merichem Chemical & Refinery Services LLC. In this process, a non-toxic, chelated iron catalyst is employed to accelerate the reaction between H2S and oxygen to form elemental sulfur.

(1) H2S + ½ O2 So + H2O
    (Fe)  

As implied by its generic name, liquid redox, all of the reactions in the LO-CAT® process occur in the liquid phase in spite of the fact that Equation (1) is a vapor phase reaction. In the process, the sour gas is contacted in an absorber with the aqueous, chelated iron solution where the H2S is absorbed into the solution and ionizes into sulfide and hydrogen ions as follows.

(2) H2S + H2O 2H+ + S=

The ionization reaction is very fast while the mass transfer is relatively slow.

The dissolved sulfide ions then react with chelated, ferric ions to form elemental sulfur as follows.

(3) S= + 2Fe+++ SO + 2Fe++

This reaction is very fast and is not equilibrium limited. In addition, since the reactions are occurring at ambient temperatures, the sulfur is formed as a solid.

The solution is then contacted with air in an oxidizer where oxygen is absorbed into the solution and the ferrous ions are converted back to the active ferric state as follows.

(4) ½ O2+ H2O + 2 Fe++ 2 Fe+++ + 2 OH-

Again, the oxidation of the ferrous ions is very fast and the mass transfer of the oxygen into the solution is relatively slow.

Adding Equations 2, 3, and 4 yields Equation 1.

Liquid Redox Processing Schemes

Depending on the type of gas being treated, different processing streams are available for optimizing a liquid redox operation. As illustrated in Figure 1, Autocirculation type LO-CAT units are used when treating acid gas streams and streams, which can be mixed with air without creating a safety problem or contaminating a product gas. In this type of unit, the absorber where equations 1 through 3 occur and the oxidizer where equation 4 occurs are contained in one vessel separated by baffles. Due to the large differences in aerated densities between the liquids in the absorber and the oxidizer sections, large circulation rates are achieved between the various compartments of the vessel without having to employ pumps. The acid gas enters the absorber section of the vessel (centerwell) where it is contacted with oxidized LO-CAT solution and where the H2S is absorbed and converted to elemental sulfur. The partially reduced solution then circulates to the oxidizer section where it is contacted with air, which reoxidizes the iron in accordance with equation 4. The exhaust air from the oxidizer and the sweet acid gas from the absorber are combined and are generally exhausted to the atmosphere.

In the conical portion of the vessel, the sulfur settles into a slurry of approximately 10% to 15 wt% solids. A small stream is withdrawn from the cone and sent to a vacuum belt filter where the sulfur is further concentrated to an approximately 65 wt% sulfur cake. Some units stop at this stage and sell the sulfur cake as a fertilizer. Drier sulfur cake can be formed by employing pressure filters. If molten sulfur is required, the cake is reslurried and melted.



Removal efficiencies of greater than 99.99 % and turndowns of 100% can easily be achieved with an Autocirculation unit.

Coupling a Liquid Redox With a Claus Unit

There are two approaches to coupling a liquid redox process to a Claus unit. The first method involves processing the Claus tail gas through a cooler and then directly into a liquid redox unit. The second method involves processing the Claus tail gas through a hydrogenation/hydrolysis reactor, which will convert all of the sulfur vapor, SO2, CS2 and COS to H2S followed by a cooler and a liquid redox unit. Both methods will result in removal efficiencies exceeding 99.9%. The only difference will be the operating cost of the first approach will be greater than that of the second.

When considering liquid redox to treat Claus tail gas without the inclusion of a hydrogenation/hydrolysis reactor, the amount of SO2 in the tail gas is an important operating parameter. Since liquid redox units operate at alkaline pH’s in the range of 8 to 9, any SO2 in the tail gas will be easily absorbed, and form sulfates and sulfites in accordance with Reaction 5 and 6.

(5) SO2 + 2 NaOH + ½ O2 Na2SO4 + H2O
(6) NaOH + SO2 NaHSO3

It is important to note that SO2 does not interfere with the liquid redox chemistry and consequently, does not affect the H2S removal efficiency of the process. However, Reactions 5 and 6 do affect the operating cost of the process in two ways. First, caustic is consumed for each mole of SO2 absorbed, which increases the operating cost of the unit. Secondly, the resultant sulfate/sulfite product will accumulate in the liquid redox solution, and eventually a continuous blowdown will be required resulting in loss of catalyst solution, which must be replaced, again, increasing operating cost even further. Consequently, if this process configuration is to be employed, it is advantageous to minimize the formation of SO2 in the Claus unit by operating the Claus unit with sub-stoichiometric quantities of oxygen, thus increasing the H2S:SO2 ratio in the tail gas.

A flow diagram of a typical LO-CAT liquid redox unit for treating Claus tail gas directly is shown in Figure 2. Since the liquid redox system is aqueous-based, elevated temperatures will cause water balance problems; consequently, the tail gas is first passed through a quench tower where the gas temperature is reduced from approximately 135°C to 50°C. A portion of the sour condensate produced in the cooling operation may be employed as makeup water in the liquid redox unit; however, some of it will need to be sent to a sour water stripper with the vapor being routed back to the liquid redox unit.

For direct treatment of Claus tail gas, the LO-CAT process would employ a venturi absorber followed by a proprietary Mobile Bed Absorber (MBA). The venturi not only supplies much needed draft to the system, but it also provides a fair amount of H2S removal. The MBA employs hollow, ping-pong-like spheres as the contacting media which, when fluidized, are self-cleaning



This mode of operation (Figure 2) will still yield overall H2S removal efficiencies of 99.99+%. In addition, the effluent gases from the liquid redox unit will probably not require incineration since the tail gas will contain only a very small amount of H2S and no SO2, and it will be diluted with the effluent air from the Oxidizer. It is also important to note that there is no recycle stream back to the Claus unit; hence the capacity of the Claus unit will not be reduced by the addition of a liquid redox system to treat the tail gas.

In the indirect processing scheme, all sulfur compounds in a Claus tail gas are converted to H2S by passing the tail gas through a hydrogenation/hydrolysis, catalytic reactor at elevated temperatures. Reactions 7 and 8 (hydrogenation) and Reactions 9 and 10 (hydrolysis) represent the major reactions, which occur in the reactor.

SO2 + 3H2 H2S + 2H2O (7)
S2 + 2H2 2H2S (8)
CS2 + 2H2O CO2 + 2H2S (9)
COS + H2O CO2 + H2S (10)

In this processing scheme (Figure 3), a fuel gas is subjected to partial oxidation, which not only generates sufficient heat to raise the tail gas to reaction temperatures but also generates sufficient hydrogen to satisfy the requirement of Reactions 7 and 8.



After passing through the reactor, the effluent gas must again be cooled to approximately 50°C, which will generate sour condensate. Again, a portion of the sour condensate may be used as makeup water for the liquid redox unit; however, some of it will need to be sent to a sour water stripper with the vapor being routed back to the liquid redox unit. The operating scheme of the LO-CAT unit will be identical to those described for the indirect treat treating case.

Achieving 100% Turndown

Operating a Claus unit in a turndown mode will generally result in operating problems as the amount of turndown increases. By installing a LO-CAT unit in parallel with a Claus unit these operating problems can be eliminated by base loading the Claus unit at a condition, which results in acceptable Claus operation. Any variation below the base load will be diverted to the LO-CAT unit, and the Claus unit will be put into standby mode. If the LO-CAT unit is also to be employed as the tail gas unit, the operation will be as illustrated in Figure 4 for a direct treat tail gas unit. To accommodate this operating scheme, the operating capacity of the Autocirculation vessel will need to be equivalent to that of the turndown capacity of the Claus unit.

All of these flow schemes will yield overall H2S removal efficiencies of 99.99+% and turndowns of 100%. In addition, the effluent gases from the LO-CAT unit will not contain any SO2 and only a very small amount of H2S; consequently, incineration may not be required, which will save energy and will not increase green house gas emissions. It is also important to note that there is no recycle stream back to the Claus unit; hence the capacity of the Claus unit will not be reduced by the addition of a liquid redox system to treat the tail gas.