A.W. Gerritsen

Removal of H2S from fuel gases at high temperatures using MnO/γ-Al2O3

Abstract High temperature desulfurization of gases from coal gasification processes has important aspects for many industrial applications and electrical power generation plants. MnO, supported by γ-Al 2 O 3 (MnO/γ-Al 2 O 3 ), was used as a regenerable sorbent for high temperature removal of H 2 S from gases. The sorbent was prepared by wet impregnation and tested for successive sulfidation-regeneration cycles. Sulfidation was carried out at 600°C with an N 2 /H 2 /H 2 S mixture containing 1.41–4.48% H 2 S. Regeneration of the sorbent was performed with both N 2 /H 2 and N 2 /H 2 /steam mixtures at the same temperature. The sorbent can be completely regenerated with a gas-steam mixture, while only 25-20% of the sulfur can be removed by physical (N 2 /H 2 ) regeneration. The breakthrough and total capacities of the sorbent were found to be affected by the flow rate and the H 2 S concentration of the gas. Consumption of steam for regeneration increased slowly until 60–70% of the regeneration was completed, and then rose rapidly. The manganese conversion ranged from 15–19% at the breakthrough point, to 32–35% at the end of sulfidation (acceptation).

Regeneration of MnO/γ-Al2O3 used for high-temperature desulfurization of fuel gases

Abstract The regeneration of MnO/γ-Al 2 O 3 used as a sorbent to remove H 2 S from fuel gases was studied with a packed bed reactor. The sorbent was prepared by wet method in which MnO was impregnated onto a standard γ-Al 2 O 3 . The sorbent was regenerated at 600°C. The regenerating gas was a mixture of N 2 , H 2 and steam in various proportions. The N 2 -H 2 -steam mixture resulted in a complete regeneration of the sorbent. The composition of the reactor off-gas varied according to the regenerating gas composition. Under suitable conditions the concentration of H 2 S in the reactor off-gas was high enough for use to produce elemental sulfur.

Oxidation of ethylene to acetaldehyde over a heterogenized surface-vanadate Wacker catalyst in the absence of gaseous oxygen

Abstract The catalytic Wacker-oxidation of ethylene, 1 -butene and styrene to the corresponding aldehydes and ketones over heterogenized catalysts has been demonstrated recently by us. The heterogenized catalysts consist, for instance, of H 2 PdCl 4 adsorbed on γ-Al 2 O 3 or on TiO 2 (anatase), pre-covered with a monomolecular layer of vanadium pentoxide. Normally the feed of the reactor is composed of an alkene, water and oxygen. In the present article it is shown that the reaction can also be carried out in the absence of oxygen in the feed. In the last case the vanadate layer on the support is gradually reduced to a surface-hydrogen bronze of vanadium, H x V 2 O 5 , with x = 1.4 to 2 when the rate of reaction has fallen to zero. The catalytic activity can be reinstated by injection of an oxygen-containing gas in the feed. In view of the importance of the formation of the hydrogen bronze of vanadium in the reaction described above, a separate study has been made of hydrogen spillover from small palladium crystallites on monolayers of vanadium pentoxide supported on γ-alumina or an anatase. On V 2 O 5 /γ-Al 2 O 3 the diffusion coefficient of H atoms, D (373 K), was found to be about 10 −14 cm 2 s −1 , and the activation energy amounts to 87 kJ mol −1 . On the surface of the anatase-supported monolayer, however, the diffusion coefficient is appreciably higher. Here D ( 373 K) is about 2.10 −13 cm 2 s −1 .

Kinetics and mechanism of the gas-phase oxidation of 1-butene to butanone over a new heterogenized surface vanadate Wacker catalyst

Abstract A study has been made of the kinetics and mechanism of the oxidation of 1-butene to methyl ethyl ketone (MEK) over a new heterogeneous vana- date monolayer Wacker catalyst, in the temperature range of 363 to 408 K and at 0.11 MPa total pressure. The kinetics of reaction with respect to the dependency of the reaction rate on the partial pressures of 1-butene and of water, as well as on the surface Pd and NaCl concentrations, is logically related to the kinetics of the homogeneous Wacker process. At 10% con- version, the main product (MEK) is formed with 80 – 95% selectivity. The formation of byproducts, including crotonaldehyde, acetone, acrolein, acetaldehyde and acetic acid, has been studied and a reaction scheme developed. From an XPS/AES analysis of spent catalysts, it follows that deactivation is probably caused by a gradual decrease of the chlorine surface concentration. Compensation for the loss of chlorine by adding HCl to the deactivated catalyst leads to a partly regenerated catalyst. Replacing the PdCl 2 /NaCl sub-monolayer by PdSO 4 /H 2 SO 4 leads to a stable catalyst.

Attrition of an aluminate-based synthetic sorbent for regenerative sulphur capture from flue gas in a fluidised bed

Abstract Attrition tests have been performed on an aluminate-based synthetic sorbent intended for regenerative sulphur capture from flue gas in a fluidised bed coal combustor. A comparison with lime(stone) has also been made. Single-particle crushing strength tests have been used to investigate the role of breakage caused by static mechanical stress, while impact tests have been applied to study kinetic stress. Multi-particle fluidised bed tests have been used to examine attrition by thermal shock (thermal stress), coal combustion (thermal and chemical stress) and fluidisation (kinetic stress) independently. The attrition resistance of the synthetic sorbent is much higher than that of lime(stone). It appears however that the effect of coal combustion on sorbent attrition needs further research. The morphology of the sorbents is only slightly affected by the various tests mentioned above.

Mass transfer characteristics of parallel passage reactors

Abstract Relations were developed to describe mass transport phenomena in Parallel Passage Reactors (PPR), dustproof, low-pressure drop reactors consisting of shallow packed beds of catalyst particles confined between wire gauze screens. In a PPR, gas flows along the beds, and reactants are transferred through the screens to the catalyst particles by diffusion and dispersion. At low gas velocities along the beds, interparticle mass transfer in the beds is dominated by diffusion. At higher gas velocities it is significantly enhanced by dispersion, caused by a small gas flow through the catalyst beds. This flow, parallel to the bulk flow, results partly from the axial pressure gradient across the reactor. Up to a few particle diameters from the wire gauze screens, the gas flow through the beds is higher than would be expected from the pressure gradient. This is caused by the higher bed voidage near the wire gauze screen and by convective transfer of momentum from the gas channels, through the wire gauze screens, into the beds. The developed mass transfer relations were used to assess the feasibility of the PPR as a reactor for catalytic denoxing of industrial flue gases. At gas velocities normally encountered in industrial denoxing, dispersion in the catalyst beds of the PPR greatly enhances its efficiency. If the thickness of the catalyst slabs does not exceed six to ten catalyst particle diameters, the reactor performance is controlled more by intraparticle diffusion than by interparticle mass transfer. This holds true, except for very low gas velocities. If this rule of thumb is obeyed, the PPR is an attractive alternative to the Honeycomb Reactor for full Selective Catalytic Reduction of nitric oxide, especially at low temperatures.

The selective catalytic reduction of nitric oxide in the bead string reactor

For many gas/solid reactors pressure drop is a cost-determining factor. A striking example is flue gas treatment, with a financially tolerable pressure drop of only 10–20 mbar. This has led to the development of structured catalytic reactors like the Monolithic Reactor, the Parallel Passage Reactor and the Lateral Flow Reactor and more recently the Bead String Reactor. In the Bead String Reactor the catalyst is fixed on strings parallel to the flow. This new arrangement results not only in a low pressure drop but also a dustproof operation, an adjustable voidage (10%–100%) and lateral mixing. This paper presents the results of the selective catalytic reduction of nitric oxide with ammonia in the Bead String Reactor on a V/Ti/Si catalyst. The primary purpose of the experiments was the validation of mathematical models for the design of this novel reactor type for a potential industrial application. A comparison of the experimental results and the mathematical models is given for different arrangements of the strings. The experiments have shown that this flexible reactor configuration performs indeed well and that the present models predict both the pressure drop and the conversion of nitric oxide accurately. Though the Bead String Reactor might not be able to compete with monolithic reactors for exhaust gas treatment from vehicles, it is on the threshold of industrial application in power plants for flue gas treatment.

The Bead String Reactor: A Compact, Dust-Proof, Low Pressure Drop Reactor with Ordered Catalyst Material for Heterogeneous Catalysis

The Bead String Reactor (BSR) is proposed as a new type of reactor that combines a very low pressure drop (order of magnitude: 10 mbars) with dust-proof operation, effective heat and mass transfer, and radial mixing. The BSR is characterized by the fact that catalyst material is fixed on parallel strings that are ordered in the reactor, parallel to the flow. It is an interesting, compact alternative to the Honeycomb Reactor, Parallel Passage Reactor and Lateral Flow Reactor, e.g. as a reactor for Selective Catalytic Reduction (SCR) of NOx in industrial flue gases, using ammonia as a reductant. A special feature of the BSR is the possibility to choose the overall voidage between as low as 10 to 100%.