On Improving the Technology for Forming Catalyst Supports

V. M. Vitogin, I. I. Lozbina, V. P. Koshintsev, E. P. Vole

(Presented by the scientific-methodological seminar of the Department of General Chemical Technology)

At present, in order to obtain catalyst supports, the following series of technological operations is used in most cases: preparation of the starting components, moistening and mixing them in Z-type mixers, extrusion of the mass in screw presses of the die type in the form of a cylindrical strand with simultaneous cutting into cylinders of a specified height, curing in the open air, drying, and calcination of the support in shaft-type furnaces.

The main disadvantages of such a scheme are the periodic nature and duration of the process at the stages of curing, drying, and calcination, caused by the imperfection of the scheme and of the equipment used; the difficulty of obtaining a product with a uniform granulometric composition (with respect to the length of the shaped pieces); and the relatively rapid wear of the press-forming tool. In addition, the resulting support product, as a rule, has a nonuniform structure and a comparatively small specific surface area with low gas permeability.

To increase the activity of a catalyst, it is desirable that its surface be not only as large as possible, but also accessible in all parts for adsorption of the reacting molecules. In this connection, in order to eliminate the above-listed shortcomings and to improve the technology for forming catalyst supports, we propose a fundamentally different method—wet granulation using an inclined pan granulator, followed by drying and calcination of the catalyst support in “fluidized-bed” reactors. Granulators of this type are simple in design and operation, more productive, reliable in service, and make it possible to obtain a homogeneous product of spherical form with specified porosity and specific-surface-area characteristics.

The specific aim of the present study is to improve the technology for producing catalyst supports based on alumina and zinc oxide. At the first stage, the objective of the study included determining the possibility of replacing the shaping process by granulation. The following types of catalyst supports were studied: GIAP-3 based on alumina and 20% nitric acid; 481-Zn based on zinc oxide, kaolin, and ammonia water.

To determine the optimal granulation conditions, the following were determined (according to a method developed at the Department of General Chemical Technology): the indices of maximum molecular and maximum capillary moisture capacity and the calculated indices of wettability of the ma-

materials, as well as the optimal values of the working moisture content of the materials before granulation, according to formulas (1) and (2) [1, 2, 3]

\[ K=\frac{W_{\mathrm{MMV}}}{W_{\mathrm{MKV}}-W_{\mathrm{MMV}}}; \tag{1} \]

\[ W^{p}_{\mathrm{opt.}}=\frac{W_{\mathrm{MMV}}}{K}, \tag{2} \]

where \(K\) is the lumping index;
\(\quad W^{p}_{\mathrm{opt.}}\) is the optimal working moisture content, %;
\(\quad W_{\mathrm{MMV}}\) is the maximum molecular moisture capacity, %;
\(\quad W_{\mathrm{MKV}}\) is the maximum capillary moisture capacity, %.

The values obtained are presented in Table 1.

The starting material for the GIALG-3 catalyst support is mixed, before granulation, with a certain amount of nitric acid corresponding to the optimal working moisture content, and is held for 2–3 days. After this period, granulation was carried out. The granulation was performed on a plate-type laboratory granulator with a diameter of 500 mm.

The lumping process for this charge proceeds satisfactorily, but the resulting raw granules have insufficient strength. This is explained by the low plasticity of the alumina gel itself. To strengthen the granules, the use of a plasticizing additive is necessary [4]. We used bentonite as the plasticizer in amounts from 1 to 5%. The lumping coefficient of the charge with the bentonite additive increases; accordingly, \(W^{p}_{\mathrm{opt.}}\) also changes (Table 1). The granules obtained from this charge have increased strength compared with the granules from the first charge, but it is insufficient for their rapid transport. At the same time, work was carried out on replacing nitric acid with water with the addition, as a catalyst, of freshly prepared colloidal aluminum hydroxide in an amount of 5, 10, 15%. The charge was thoroughly mixed and granulated immediately. In this case, a considerable increase in the strength of the freshly obtained granules was observed.

A somewhat different picture is obtained in the granulation of the 481-Zn catalyst (Table 1). As is evident from Table 1, the MKV and \(W^{p}_{\mathrm{opt.}}\) for zinc oxide are very high, since zinc oxide is fine-grained. A charge prepared according to the plant formulation for this support does not granulate.

Table 1

Data on moisture capacities, lumping, and optimal working moisture content

The use of a plasticizing additive does not give a positive effect. Consequently, for normal granulation it is necessary to introduce thinning coarse-grained additives. In practice, this problem can be solved by introducing into the raw charge a relatively coarse return after calcination of the granules. We prepared a charge consisting of 30% coarse return (calcined granules ground to 0.25 mm) and 70% fine-grained fraction. The MKV and \(W^{p}_{\mathrm{opt.}}\) values for this charge are given in Table 1. Granulation carried out at \(W^{p}_{\mathrm{opt.}} = 42\%\) gave positive results.

Conclusions

1. The strongest raw granules for GIAP-3 supports are obtained when colloidal aluminum oxide hydrate is added in an amount of 15%.

2. The charge for the 481-Zn catalyst support is satisfactorily granulated when lean coarse-grained materials are introduced in an amount of 30%.

Literature

1. A. S. Bogma. Study of the process of wet aggregation of dispersed materials. Candidate dissertation, Tomsk, 1967.

2. V. M. Vityugin, A. S. Bogma. Study of the process of drying iron-ore pellets, No. 12, Moscow, 1965.

3. V. M. Vityugin, P. N. Dokuchaev. On the question of choosing a rational technology for pelletizing Sokolovsko-Sarbai concentrates. Mining Industry of Kazakhstan, No. 3, Alma-Ata, 1961.

4. P. N. Dokuchaev. Study of the effect of bentonite and lime additions on the pelletizing of fine-grained iron-ore concentrates. Candidate dissertation, Tomsk—Rudny, 1967.