Turndown of a Split-flow Mode Modified-Claus Plant
Authors: Downey Christopher I. 
Published: March 2010

Abstract
A split-flow mode modified-Claus plant currently in operation was originally capable of processing only a twofold range of acid gas flows. In light of a predicted decline in the inlet flow, a turndown in excess of the existing limit was desired. The process changes to the Claus plant which have decreased the minimum processing capacity are described. Results are presented which indicate that these changes also have derived a more efficient process in regard to heat conservation. Solutions to difficulties which may arise in other Claus-type sulphur recovery units of the split-flow kind, when processing acid gas at flows much reduced from design, are also discussed.

Introduction
Inherent to a plant processing a non-renewable resource is the inevitable requirement of handling flows much reduced from design. Such was the case, recently, with an existing split-flow mode modified-Claus plant. In fact, the situation was soon going to occur where the facility would have been operating close to the minimum processing limit (this limit is often expressed in terms of the turndown, ratio-defined as the ratio of maximum to minimum flow(1)). This occurrence was not desired because the process would have become very inefficient and extremely unreliable. As a result, steps were taken to prevent this from happening by reducing the minimum processing limit of this plant.
The work reported in this paper will outline the procedure which, was followed to identify the minimum processing capacity of this existing split-flow mode sulphur plant, detail the modifications which were required to reduce the minimum flow limit, and present solutions to additional problems which may occur when other modified Claus plants of the split-flow type are processing flows much reduced from design.

The Modified-Claus Process
In the modified-Claus process(2), hydrogen sulphide is converted to elemental sulphur through a two-step process. Firstly, one-third of the hydrogen sulphide in the inlet stream is oxidized in the combustion zone of a reaction furnace to form sulphur dioxide. Then, the unoxidized hydrogen sulphide reacts with the sulphur dioxide to form elemental sulphur. The major chemical reactions involved in the process are:
(Equation in full Paper)
The temperature in the combustion zone at which Reaction (I) usually proceeds to completion is in the order of ll00°C. However, at this temperature, Reaction (2) does not proceed to a high per cent of completion. As a result, the combustion products are cooled to condense the sulphur vapour, then the liquid sulphur is separated from the gaseous products which are subsequently fed to a catalyst reactor in order to permit Reaction (2) to proceed further toward completion.
Following each catalytic reactor, the products are cooled in order to remove sulphur product as liquid. Generally, two or three catalytic condensing stages are required to obtain the desired conversion. It is important that the temperature of the stream to each catalytic reactor is maintained at a value above the sulphur dew point to prevent poor conversion and catalyst deactivation due to sulphur deposition.