Sulphur acids, catalysts

In a typical process, 100 parts of poly(vinyl acetate) are added to a mixture of 200 parts acetic acid and 70 parts water, which has been warmed to about 70°C, and stirred to complete solution. Sixty parts of 40% formalin and 4 parts sulphuric acid (catalyst) are added and reaction is carried out for 24 hours at 1Q°C. Water is added to the mixture with rapid agitation to precipitate the granules, which are then washed free from acid and dried. 

The production of sulphuric acid by the contact process, introduced in about 1875, was the first process of industrial significance to utilize heterogeneous catalysts. In this process, SO2 was oxidized on a platinum catalyst to S03, which was subsequently absorbed in aqueous sulphuric acid. Later, the platinum catalyst was superseded by a catalyst containing vanadium oxide and alkali-metal sulphates on a silica carrier, which was cheaper and less prone to poisoning. Further development of the vanadium catalysts over the last decades has led to highly optimized modem sulphuric acid catalysts, which are all based on the vanadium-alkali sulphate system. 

One of the main drivers for the development of new sulphuric acid catalysts over the last decades has been the desire to reduce S02 emissions from sulphuric acid plants without costly tail gas cleaning or an additional interbed... 

This chapter describes the background, the strategy and the methods used at Haldor Topsoe in the 1990ies to develop and design a new commercial low-temperature sulphuric acid catalyst called VK69. 

Figure 3. The five variants of size and shape of sulphuric acid catalysts from Haldor Topsoe.

The background for the development of VK69 was a need for reduction of S02 emissions from double-absorption plants by installing a more active catalyst at low temperature downstream from the intermediate absorption tower. Clearly, the catalytic solution should be more competitive than the alternatives, e.g. tail gas scrubbing or triple-absorption layout, in terms of capital and operating costs. In the following, the required technical performance of the catalyst with respect to S02 oxidation activity, mechanical strength and pressure drop is discussed, and input from the literature and from practical experience in the field is presented. Reviews of the extensive literature published on sulphuric acid catalysts can be found in [2-5],... 

Sulphuric acid catalysts are not truly heterogeneous catalysts but so-called supported liquid phase (SLP) catalysts, where the oxidation of S02 takes place as a homogeneous reaction in a liquid film covering the internal surface of the support material [2], This was proposed already in 1940 by Frazer and Kirkpatrick [6], who found that the promoting action of the common alkali metals was due to their ability to form relatively low-melting pyrosulphates, which dissolve vanadium oxides, e.g. for potassium... 

The silica carrier of a sulphuric acid catalyst, which has a relatively low surface area, serves as an inert support for the melt. It must be chemically resistant to the very corrosive pyrosulphate melt and the pore structure of the carrier should be designed for optimum melt distribution and minimum pore diffusion restriction. Diatomaceous earth or synthetic silica may be used as the silica raw material for carrier production. The diatomaceous earth, which is also referred to as diatomite or kieselguhr, is a siliceous, sedimentary rock consisting principally of the fossilised skeletal remains of the diatom, which is a unicellular aquatic plant related to the algae. The supports made from diatomaceous earth, which may be pretreated by calcination or flux-calcination, exhibit bimodal pore size distributions due to the microstructure of the skeletons, cf. Fig. 5. 

Figure 5. Scanning Electron Micrograph of the interior of a sulphuric acid catalyst. Diatom skeletal fractions more or less covered with solidified catalytic melt are discernible.

The development and design of the new sulphuric acid catalyst involves numerous disciplines, work tasks and methods from the field of practical catalysis and chemical engineering. The most important steps of the development process are briefly described in this section but for a deeper technical approach to the theory and methods, the reader is referred to text books within the field [1, 13-14],... 

Figure 7. Strategy for design of a new sulphuric acid catalyst.

Figure 8. Simplified diagram of unit operation in the production of sulphuric acid catalyst...

Because the composition and nano-scale structure of a catalyst depend on the temperature and the gas environment, it is important to measure catalyst activity under conditions experienced in the industrial converters. For the sulphuric acid catalysts, the activity for conversion of S02 to S03 was measured in the set-up shown in Fig. 9. 

For a more detailed analysis of measured transport restrictions and reaction kinetics, a more complex reactor simulation tool developed at Haldor Topsoe was used. The model used for sulphuric acid catalyst assumes plug flow and integrates differential mass and heat balances through the reactor length [16], The bulk effectiveness factor for the catalyst pellets is determined by solution of differential equations for catalytic reaction coupled with mass and heat transport through the porous catalyst pellet and with a film model for external transport restrictions. The model was used both for optimization of particle size and development of intrinsic rate expressions. Even more complex models including radial profiles or dynamic terms may also be used when appropriate. 

The evaluation of carriers and catalyst compositions showed that significantly higher SO2 oxidation activity could be achieved with Cs as a promoter under the operating conditions downstream the intermediate absorption tower as demonstrated by the results in Table 1, where the activity compared to the standard product is increased by more than a factor 2. This was clearly sufficient for the introduction of VK69 to the market as a new sulphuric acid catalyst. The activity results for different melt compositions were used to optimise the vanadium content and the molar ratios of K/V, Na/V. and Cs/V. However, the choice of Cs/V was not only a question of maximum activity, because of the significant influence of the Cs content on the raw material costs (the price of caesium is 50-100 times the price of potassium on a molar basis). Here, the economic benefits obtained by the sulphuric acid producer by the marginal activity improvement at high Cs content also had to be taken into account. 

The design of the new sulphuric acid catalyst VK69 by Haldor Topsoe was based on experience from existing products, market knowledge, and an innovative attitude especially in the first stages of development. The... 

Cognate preparations sulphuric acid catalyst. Ethyl butanoate. Use a mixture of 88 g (92 ml, 1 mol) of butanoic acid, 23 g (29 ml, 0.5 mol) of ethanol and 9g (5 ml) of concentrated sulphuric acid. Reflux for 14 hours. Pour into excess of water, wash several times with water, followed by saturated sodium hydrogen carbonate solution until all the acid is removed, and finally with water. Dry with anhydrous sodium sulphate, and distil. The ethyl butanoate passes over at 119.5-120.5 °C. Yield 40g (69%). An improved yield can be obtained by distilling the reaction mixture through an efficient fractionating column until the temperature rises to 125 °C, and purifying the crude ester as detailed above under methyl acetate. 

The results demonstrate that hydrogensulfate and tetrakis(hydrogensulfato)borate ionic liquids are highly interesting additives to mineral acids to form new, highly Bronsted-acidic catalysts. For example, it was found that a mixtures of sulphuric acid with only 2.2 mol% of [0MIM][B(HS04)4] ionic liquid yielded 90% more monoalkylbenzene product than the neat sulphuric acid catalyst under identical reaction conditions. This and related results are explained by an interplay of solubility and acidity effects caused by the ionic liquid additive. 

Hansen, L. (2004) Topsoe s sulphuric acid catalysts VK-series. Paper distributed at Sulphur 2004 conference, Barcelona, October 24-27, 2004. www.haldortopsoe.com... 

Topsoe (2004) VK series sulphuric acid catalysts for today and for the future, Haider Topsoe A/S brochure, 2004 www.haldortopsoe.com... 

Peracetic acid is the most widely used organic peroxy acid. It is a strong oxidizer, which could he used in disinfection and bleaching agent. Peracetic acid can be synthesized from acetic acid and hydrogen peroxide. The formation of peracetic acid takes place in the equilibrium reaction (1). In order to accelerate the reaction rate, acid catalyst is needed (Swem, D., 1970). Conventionally homogeneous sulphuric acid catalyst is used. The reaction scheme is shown in Eq. (1)... 

Conventionally peracetic acid is produced in a tank reactor in the presence of homogeneous acid catalyst. In the process, sulfuric acid catalyst is first charged into the reactor, after which acetic acid and hydrogen peroxide are fed into the reactor. The mixture is heated up and equilibrium reaction (1) takes place. When homogeneous acid catalyst is used, separation of it from equilibrium mixture is carried out in a distillation column. When equilibrium is reached, sub-atmospheric pressure is drawn in the reactor. Vaporization of the reaction mixture begins. In the distillation column acetic acid, hydrogen peroxide and peracetic acid are separated from sulphuric acid catalyst (Swem, D., 1970). The simplified scheme of the conventional process is illustrated in Figure 3. 

Independently of the work on the chemical composition of sulphuric acid catalysts, attempts to develop kinetic equations to describe the rate of reaction both for mechanistic analysis and reactor design purposes have been numerous. There is much evidence to suggest that in common with its catalytically oxidative behaviour in other environments, valency states of vanadium of V " and V " are involved. Mars and Maessen in 1964 developed a rate expression based on a simple two-step redox mechanism ... 

Of more interest mechanistically though is the liquid nature of the molten phase present in the interstices of the inert porous support. It is now recognized that, deliberately or accidentally, a number of heterogeneously catalysed processes involve a supported liquid phase (SLP) rather than a solid catalyst. SLP catalysts are reviewed by Villadsen and Livbjerg. These include the vanadium-based sulphuric acid catalysts and, of comparable antiquity, the Deacon catalysts for oxidizing hydrogen chloride, where mixtures of copper and potassium chlorides can form melts in the catalyst support under reaction conditions. Thus in addition to diflfusional restrictions arising from pellet pore structure any... 

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