HTS catalyst

HTS catalyst consists mainly of magnetite crystals stabilized using chromium oxide. Phosphoms, arsenic, and sulfur are poisons to the catalyst. Low reformer steam to carbon ratios give rise to conditions favoring the formation of iron carbides which catalyze the synthesis of hydrocarbons by the Fisher-Tropsch reaction. Modified iron and iron-free HTS catalysts have been developed to avoid these problems (49,50) and allow operation at steam to carbon ratios as low as 2.7. Kinetic and equiUbrium data for the water gas shift reaction are available in reference 51. 

J. H. Carstensen and P. S. Pedersen, "New Developments in HTS Catalyst Technology Solve the Fischer Tropsch Problem," AIChE Ammonia Safety Symposium, San Francisco, Calif., Nov. 1989, American Institute of Chemical Engineers, New York, 1990. 

In pathway B in Fig. 14.3, H2S is first removed by absorption and then enters a clean gas shift section. Catalysts used are standard HTS catalysts. The use of LTS catalysts is only possible when sulfur is removed to very low levels (<0.01 ppm), using a Rectisol (methanol) solvent [6], Downstream the WGS section C02 is removed by absorption. When partial pressures are high (>1.5 MPa, [6]) a physical solvent (e.g., methanol) can be used. Physical solvents can be easily regenerated by flashing at reduced pressures. At lower C02 partial pressures a chemical solvent [e.g., mono-ethyl amine (MEA)] is used, which requires energy input for steam regeneration of the solvent. 

Although in many WGS applications cases conventional HTS catalysts are used, for some applications water-gas shift catalysts may be needed that operate at temperatures above the current industrial standard. Also, stability in a steam atmosphere, during regeneration of a SEWGS reactor, is an issue. 

Scheme 5.9 shows a possible mechanism for this epoxidation reaction. First, H202 reacts with a base site on the HT surface to form a HOO species, which attacks a nitrile to generate peroxycarboximidic acid as an active intermediate oxidant. The oxygen of the peroxycarboximidic acid is transferred to an olefin molecule. Interestingly, the resulting amide can be further employed for the epoxidation of olefins in the presence of a HT catalyst [44b]. 

HT technology for catalysts-automated synthesis and testing appears to be reasonably adapted to date, but further improvements are expected for HT catalysts characterization, which is still restricted to costly and in general ex-situ spectroscopic techniques. These tools would provide the new catalyst descriptors needed to improve the ability to predict catalytic performances without testing. 

Figure 5 CH4 and CH3 are observed during heating the used, basic Pt/HT catalyst to 450°C in (ramp 5°C/min) in H2, indicating the desorption of CH4. The same desorption profile was observed for the other basic catalysts.

The Pt/LTL [. ], Pt/LTL [0.47, small], Pt/ASA and Pt/HT catalysts all have highly dispersed Pt particles. Based on the Htotai/Pt and Nptpt results, the average particle size for all these catalysts was estimated < 1 nm. The particle size for the Pt/ASA catalyst as revealed with HRTEM (1.5 nm) seems in contradiction with the other techniques. However, it has to be noted that with HRTEM the lower detection limit for Pt/ASA is approximately 8-10 A, and that on the HRTEM pictures taken only a small amount of particles was visible. In other words, with HRTEM the smallest particles, which make up the majority of all Pt in the Pt/ASA catalyst, are invisible. The relation between particle sizes as determined with HRTEM, H2 chemisorption and EXAFS was extensively described by de Graaf el a/35. 

A second batch of the hydrotalcite was activated, rehydrated, filtrated and then impregnated with an aqueous Pt(NH3)4(N03)2 solution. The resulting catalyst precursor was also dried in N2 at 80°C for 18 hours, followed by drying at 200°C (2°C/min) for 1 hour. The sample was directly reduced in H2 at 200°C for 1 hour, passivated and stored in air. The resulting catalyst is denoted as Pt/HT [IM], The resulting Pt particles in both Pt/HT catalysts will be located at the edges of the platelets, near the OH ions, and they will experience the support material as basic. 

The Pt/HT [ ] catalysts show a decrease in the Pt-Pt coordination number after evacuation. This indicates a change in morphology of the Pt particles after desorption of hydrogen. The decrease of Npt.pt can be explained by a flattening of the Pt particles. In contrast, the neutral catalyst, Pt/LTL [1.04] shows only minor changes in the morphology after evacuation and the catalysts Pt/LTL [0.82] and [1.53] show an increase in the Pt-Pt coordination number. 

The simplest solution to this problem is to modify the classic iron-based HTS catalyst by adding a moderate amount of copper to reduce the amounts of byproducts formed and to give a much higher catalyst activity for the shift reaction. A second and more radical solution, is the use of an iron-and chromium-free HTS catalyst that is copper-based73. 

Newsam JM, Design of catalysts and catalysts libraries Computational techniques in HT experimentation for catalysis, in Combinatorial Catalysis and HT Catalyst Design and Testing, Derouane et al., Eds., Kluwer Academic Publishers, The Netherlands, pp. 301-335, 2000. 

When comparing the intrinsic methanol synthesis activity of both catalysts (turnover frequency [TOF] per surface Cu site measured with N20-reactive frontal chromatography) at the space velocity of 20 mmol/gcatmin, the HT catalyst (0.54 min-1) appears twice as active as the MA catalyst (0.24 min-1). Although both catalysts are exposed to nearly the same reactant concentration, the MA catalyst operates at 11% CO2... 

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) ... 

The HTS catalysts are typically produced by precipitation of aqueous solution of iron sulfate and chromium sulfate with sodium carbonate or sodium hydroxide, followed by careful washing to remove essentially all of the residual sulfate. Washing is an important step in the catalyst manufacture, since the residual sulfate converts into hydrogen sulfide when the catalyst is reduced during the process startup, and hydrogen sulfide is a poison to the LTS catalyst located downstream of the HTS bed.t l... 

The second stage in WGS processes is LTS, which was introduced in the industry in the 1960s and is now widely used in hydrogen plants. Rapid success of the LTS process was due to the use of copper-based catalysts that are more active at lower temperatures than iron-chrome HTS catalysts and therefore enable low equilibrium CO concentrations in the gas exiting the reactor, which increases the yield of the hydrogen production process. 

Copper-based LTS catalysts are supplied to customers in their oxide form. Analogous to the iron-chrome HTS catalysts, LTS catalysts have to be activated by reduction prior to their use ... 

Other known chemical poisons for the HTS catalyst are halides, although under normal operating conditions they are not present in the feed at an appreciable concentration. Decay in the catalytic activity was also observed with the feed gas which contained minor amounts of unsaturated hydrocarbons, oxygen, and nitric oxides. Under the HTS conditions these compounds were converted into a heavy carbonaceous residue deposited on to the surface of the catalyst, blocking access of the reactants to the catalytic surface. 

Commercial HTS catalysts are mechanically quite strong. However, in an industrial process environment, the HTS catalyst can suffer from mechanical factors that deteriorate its performance, such as steam condensation leading to a gradual disintegration of the catalyst pellets, and deposition of foreign components (e.g. particulate matter from corrosion of the process equipment). Increase in the pressure drop across the catalyst bed resulting from these factors is yet another factor determining the catalyst lifetime. 

The HTS catalyst deactivated beyond an acceptable level has to be discharged from the reactor and... 

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