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Successful ammonia destruction

Sulphur

It is well known that the processing of more than a few per cent of ammonia in the total feedgases to a modified Claus unit can lead to operational difficulties unless special design featureshave been incorporated into the plant. Lisa Connock reviews the chemistry ofammoniadestruction and the different approaches adopted in refineries to achieve destruction tothe required level of less than 50 ppm.

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Ammonia (N[H.sub.3]) is a byproduct from the hydrotreating of crude oils containing nitrogencompounds. The quantity of nitrogen contained in the crude is generally larger if the crude is lowAPI, low quality. Although ammonia is a valuable commodity, transportation of it from therefinery to market is often precluded by either economic or regulatory factors. In addition, due toits harmful properties, it cannot be disposed of directly to the atmosphere. Consequently,several methods have been developed in which a combined stream of [H.sub.2]S andN[H.sub.3] is processed in a Claus furnace with the aim of producing [N.sub.2] as the solenitrogen-containing product. The individual steps for conversion of N[H.sub.3] to [N.sub.2] willinvolve a sequence of radical intermediates but the elementary reactions can be summarised asshown in Fig. 1. (1)

The ammonia from the hydrotreater is gathered as sour water containing [H.sub.2]S andhydrocarbons and, after flash, the water is stripped with steam to produce an acid gascontaining typically 30-33 vol-% N[H.sub.3], 30-33 vol-% [H.sub.2]S and [H.sub.2]O as balance.Ammonia is also present in significant amounts in the sour water stripper (SWS) acid gas feedfrom coker units.

The SWS gas is co-fired with amine acid gas (AAG) in the thermal reactor stage of the Clausplant to recover sulphur. Ammonia must be destroyed adequately in the sulphur recovery unit(SRU) reaction furnace to ensure trouble-free operation of the SRU.

Increased need to process ammonia

The need to destruct increased quantities of ammonia in Claus SRUs has acceleratedsignificantly in the past 15-20 years. Previously, a refinery SRU might receive a mixed acid gasfeed containing 5-10 mol-% N[H.sub.3]. Today, it is usual to require an SRU to process mixedacid gas feeds containing 15-20 mol-% N[H.sub.3] or more.

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