Thermochemical treatment of zeolitic cracking catalysts containing rare-earth elements

Abstract

Thermochemlcal treatment of hydrogel beads is an important stage in the manufacture of bead and microbead cracking catalysts, as it is in this stage that the catalyst acquires the optimal cation composition and structure. For the replacement of Na + by other cations, the activating solutions are pumped through series-connected columns charged with freshly molded zeolitic hydrogel beads. By varYing the composition and concentration of the activating solutions, the temperature, and the sequence and duration of the treatment, catalysts of various qualities may be obtained. It had been shown previously [i, 2] that the LnS+ and NH4 + cations differ in their selectivity in replacing Na + cations in fauJaslte-type zeolites and in the alumlnosilicate matrix. With this difference as the starting point, basic principles were formulated for the technology of manufacturing Tseokar-2 and KMTsR catalysts [3-5]. However, under commercial production conditions, because Of the different scale of the equipment and different velocities of movement of the ion exchange solutions (generally characterized by the region of external diffusion retardation), mass transfer processes become considerably more complicated. In various manufacturing operations, the column volume may be 50-70 m s , and the height of the bed of hydrogel beads may be 4-6 m. In commercial units, the conditions of thermochemical treatment required in order to give uniform ion exchange of the entire mass of beads may not always be maintained, and this often results in the production of catalysts with poor service characteristics in cat crackers. These factors were taken into account in the Process designed for thermochemlcal treatment of the catalyst KMTsR-N, which differs from the KMTsR catalyst in having a higher content of aluminum hydroxide [4~. Here we are reporting on optimlzation of process conditions in the thermochemical treatment in commercial production of the catalyst. Freshly molded zeolitic hydrogel beads, with or without specially introduced aluminum hydroxide, were treated successively with a nitrate solution of rare-earth elements (REE) (2-3 g/liter) and an ammonium nitrate solution (16-20 g/liter) (scheme i). A simplified flow plan for the thermochemlcal treatment is shown in Fig. i. Series-connected columns were filled in turn with freshly molded hydrogel beads, and the activating solutions were pumped through the columns. The beads were subjected to ion exchange and washed with desalted water, which passed successively through columns 2 and 3 and then into the tank 14. Next, this water was line-mlxed with the ammonium nitrate solution, and the resulting mixture was fed to the second activation of the hydrogel beads and columns 4-6. The second spent activating solution passed into tank 13, from which, after fortification with the REE solution, it was directed to the first activation of the hydrogen beads in columns 7-12. The duration of a single cycle was 6 h. The total duration of the thermochemlcal treatment was regulated by switching a different number of columns to each operation. Samples of the activating solutions from the columns were drawn when steady-state conditions were established in the treatment (2-4 times in the course of a cycle). The solutions were analyzed to determine the contents of Na +, NH4 +, and Ln a+ cations, and also the pH of the medium. Beads were drawn for analysis from the upper layer (one time only) before switching the columns, and also at different vertical levels in the bed in the course of unloading from the columns. These bead samples were washed free of salts with distilled water and analyzed to determine the contents of Na and REE oxides. In Figs. 2 and 3 we show the dynamics of the variation and concentration of the treating solutions and the composition of the hydrogen beads during thermochemical treatment in accordance with the scheme we have described (without additional introduction of aluminum