Oxidation of ammonia on woven and knitted platinoid gauzes

Abstract

The results from pilot tests of catalytic systems equipped with 7–12 woven or 5–14 knitted platinoid gauzes in the oxidation of ammonia are considered. OAO Yekaterinburg Non-Ferrous Metal Processing Plant produces gauzes of the two types: Pt092 0.092-mm wire woven and Pt076 0.076-mm wire loosely knitted (large-mesh) gauzes. Both types of gauzes are made of PdRdRu 15-3.5-0.5 alloy (referred to as alloy no. 5) and have the following elemental composition: Pt, 81%; Pd, 15%; Rh, 3.5%; Ru, 0.5% (STO (Branch Standard) 00195200-013-2007 and STO (Branch Standard) 00195200-014-2007). The third type is 0.076-mm tightly knitted (small-mesh) gauzes (Uncore, Germany), which differ from the Russian knitted Pt076 gauze by their greater mass and external surface area per unit reactor cross section. The effect of the mass, external surface area, and free volume of woven and knitted platinoid gauzes on the yield of nitrogen oxide is studied in this work, and the residual concentration of ammonia downstream of a stack of gauzes is determined in dependence on their number. The efficiency of the unit surface of knitted gauzes depends on the influence exerted by transfer phenomena and the direct homogeneous ammonia reduction of nitrogen oxide in the free volume of a platinoid catalyst stack. Loosely-knitted Pt076 gauzes are less efficient than woven Pt092 gauzes within a ≥91.5% range of nitrogen oxide yields. Tightly knitted gauzes are suitable for use within a ≤94.5% range of nitrogen oxide yields. Yields equivalent to those obtained on woven gauzes cannot be attained on tightly knitted gauzes even after their external surface area is enlarged considerably. Maximum nitrogen oxide yields (≈95%) are attained on Pt092 woven gauzes. From the results of our tests of loosely and tightly knitted platinoid gauzes, it follows that the latter (with less free volume in a stack) are preferable for use. The free volume of gauzes and the fraction of ammonia for nontargeted conversion can be minimized by placing a smaller number of loosely knitted platinoid gauzes on a regular honeycomb block catalyst or packing bed.