Sulfur tolerant shift catalyst

P. Frank, Sulfur tolerant shift catalyst — Dealing with the bottom of the barrel problem, Synetix, Johnson Matthey Group, 2003. 

Methanation as final purification for the raw gas from partial oxidation was proposed by Topsoe [739]. In this case the shift conversion is carried out in two stages with a special sulfur-tolerant shift catalyst followed by removal of hydrogen sulfide and carbon dioxide in an acid gas removal unit. Because of the potential danger of a sulfur break-through causing poisoning, the normal copper - zinc - alumina catalyst is usually not applied, which is surprising as the same risk exists in partial oxidation based methanol plants for the similarly composed methanol catalyst. 

These catalysts have to fulfil new safety requirements as they may be operated in the consumer s home, and also will experience a very different duty cycle than industrial catalysts. Another field that has attracted interest is sulfur tolerant shift catalysts. Catalysts have been developed for high sulfur concentrations in the feed gas since the 1960s, but have not found large-scale application up to the present. The fact is that most ammonia, methanol, and hydrogen plants are based on natural gas or naphtha feedstocks, which have relatively low content of sulfur-containing compounds, and therefore do not require these more expensive catalysts. HDS is typically used to reduce sulfur levels in the feed gas to ppm to ppb levels to protect the steam reforming and downstream catalysts. 

Sulfur-tolerant shift catalysts have been developed, but are not being used because of the development of other sulfur-sensitive technologies and their higher cost as compared to traditional catalysts. 

A highly active sulfur-tolerant shift-conversion catalyst has been developed for use with high sulfur concentration synthesis gas. This catalyst utilizes mixed metal sulfides and requires the presence of sulfur compounds in the gas to remain active (Nielsen and Hansen. 1981). The sulfur tolerant catalyst can operate over a wide range of temperatures (390°-890"F), and offers the advantage of allowing sulfur and carbon dioxide removal to be accomplished in one step (Haldor Topsde, Inc., 199IB). 

The rapid equilibration of the water gas shift reaction in the anode compartment provides an indirect source of H2 by the reaction of CO and H2O. If H2S poisons the active sites for the shift reaction, this equilibrium might not be established in the cell, and a lower H2 content than predicted would be expected. Fortunately, the evidence (77,78) indicates that the shift reaction is not significantly poisoned by H2S. In fact, Cr used in stabilized-Ni anodes appears to act as a sulfur tolerant catalyst for the water gas shift reaction (78). 

Advanced water-gas-shift reactors using sulfur-tolerant catalysts,... 

Sulfur-tolerant catalysts for water-gas shift reactors are necessary to lower syngas processing costs. 

FeS also catalyzes the shift reaction, but its activity is only half that of Fe,04 [592]-[594], In principle the catalyst can tolerate up to 500 or 1000 ppm H2S, but with a considerable loss of mechanical strength, which is additionally affected by other contaminants in the gas, such as soot and traces of formic acid. For this reason the so-called dirty shift catalyst is used in this case, which was originally introduced by BASF [639]. This cobalt-molybdenum-alumina catalyst [603], [630], [640]-[644] is present under reaction conditions in sulfidized form and requires for its performance a sulfur content in the gas in excess of 1 g S/m3. Reaction temperatures are between 230 and 500 °C. COS is not hydrolyzed on dirty shift catalysts, but may be removed in the subsequent sour-gas removal stage using the Rectisol process. Separate hydrolysis on alumina based catalysts is possible at temperatures below 200 °C [603],... 

Deactivation of Sulfur Tolerant Water-Gas Shift Catalysts... 

The eommereial HTS and LTS eatalysts require activation by careful pre-reduetion in situ and, once activated, lose aetivity very rapidly if they are exposed to air. Further, the HTS eatalyst is inactive at temperatures <300°C, while the LTS eatalyst degrades if heated to temperatures >250°C. The automotive application, because of its highly intermittent duty cycle, requires alternative water-gas shift catalysts that (1) eliminate the need to sequester the eatalyst during system shutdown (2) eliminate the need to aetivate the eatalyst in situ (3) inerease toleranee to temperature exeursions and (4) reduee the size and weight of the shift reactors. Another desirable property for an automotive WGS catalyst is tolerance to ppm levels of sulfur in the feed stream because sulfur species are present as contaminants or additives in conventional fuels (30 parts per million weight [ppmw] in future gasoline gets converted to 3 ppmv H2S in reformate). 

P. Hou, D. Meeker, H. Wise, Kinetic studies with a sulfur-tolerant water gas shift catalyst, J. Catal. 80 (1983) 280-285. 

I. Valsamakis, M. F. Stephanopoulos, Sulfur-tolerant lanthanide oxysulfide catalysts for the high-temperature water-gas shift reaction, Appl. Catal. B Environ. 106 (2011) 255-263. 

The shift reaction is carried out by commercially available sulfur-tolerant catalyst thus, the CO content of the gas is reduced to about 1%. The next step is removal of CO2 and H2S by a scrubbing process such as Rectisol or Selexol. The H2S is recovered separately eind sent to a Claus process unit for conversion to elemental sulfur. The CO2 is of adequate purity for urea production. 

Cobalt catalysts such as cobalt/manganese and cobalt/chromium show higher activity than iron/chromium catalysts at temperatures exceeding 300 °C and are highly sulfur tolerant [107]. However, their activity is certainly lower than that of the precious metal catalysts discussed below. Additionally, they are not suitable for low-temperature applications due to their low activity in this temperature range. Ruettinger et al. reported on proprietary base-metal water-gas shift catalyst development. The catalysts were claimed to have lower pyrophoricity than copper/zinc oxide catalysts, and to be stable towards air exposure at 150 ° C and even to liquid water [302]. 

In the late 1960s, with the development of gasification technology which uses heavy oil and coal as raw materials, high sulfur content in synthesis gas led to the deactivation of Fe-Cr high-temperature shift catalyst. Thus, sulfur tolerant Co-Mo shift catalysts have been developed and applied widely since then. The catalysts are active in the temperature range of 160°C-500°C, and also are called wide temperature-range shift catalysts. 

In some cases gas with a high sulfur content coming from partial oxidation or gasification units is passed directly to shift conversion. The high sulfur content can be tolerated by magnetic type catalysts, although the activity is reduced [221]. But more efficient sulfur tolerant catalysts based on cobalt-molybdenum sulfide have become available [169, 242-245]. With such catalysts... 

The reformate gas contains up to 12% CO for SR and 6 to 8% CO for ATR, which can be converted to H2 through the WGS reaction. The shift reactions are thermodynamically favored at low temperatures. The equilibrium CO conversion is 100% at temperatures below 200°C. However, the kinetics is very slow, requiring space velocities less than 2000 hr1. The commercial Fe-Cr high-temperature shift (HTS) and Cu-Zn low-temperature shift (LTS) catalysts are pyrophoric and therefore impractical and dangerous for fuel cell applications. A Cu/CeOz catalyst was demonstrated to have better thermal stability than the commercial Cu-Zn LTS catalyst [37], However, it had lower activity and had to be operated at higher temperature. New catalysts are needed that will have higher activity and tolerance to flooding and sulfur. 

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) ... 

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