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Apr-2014

Eliminating the claus furnace

A novel approach to the conversion of hydrogen sulphide to elemental sulphur is expected to extend the economic range for Claus plants.

MATT THUNDYIL, SAMEER PALLAVKAR, RAMIRO VAZQUEZ and DAVID SEEGER
GTC Technology US, LLC
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Article Summary
Fuel sources are becoming increasingly sour and society is increasing pressure on environmental regulatory agencies to improve emissions standards in a desire for a cleaner environment. As a result, producers and processors in the gas processing, refining, petrochemical and other industries have a growing focus on sulphur removal technologies that not only will improve the quality of their product streams but will also enable them to comply with the more stringent environmental regulations being imposed by governmental agencies. Since the price of sulphur is not sufficiently high for sulphur recovery processes to be profitable, the value of a sulphur recovery unit (SRU) is defined by the lost opportunity cost associated with downtime of the associated hydrocarbon processing unit as well as by its cost of operation. Then, for the producers and processors, it is important to understand the various sulphur removal and recovery technologies available so that the most reliable, lowest cost processes can be selected and used in their facilities.

Over the past decade, researchers at Phillips 66 developed a catalytic combustor that can be used to replace the burner and thermal reactor in Modified Claus units and improve the operations of the SRU.1-9 This new approach to the Claus sulphur recovery process was presented at the Laurance Reid Gas Conditioning Conference in 2009.10 Since that time, GTC Technology has acquired the exclusive licensing rights to the catalytic combustion technology that replaces the conventional Claus burner and thermal reactor and markets it under the name GT-CataFlame. GTC markets the complete process of GT-CataFlame integrated with the downstream Claus converters under the name GT-Spoc (Sulphur Partial Oxidation Catalysis). The ultimate design of GT-Spoc is a single vertical vessel that contains all the components of GT-CataFlame, followed by the Claus converters and sulphur condensers.

Background: conventional Claus

The Claus process was patented by Carl Friedrich Claus in 1883, and introduced in 1936. The Modified Claus process is the most successful commercial method for sulphur recovery. In this framework, the reaction of H2S with oxygen is separated into two stages: (1) a highly exothermic thermal stage where approximately 50% of the H2S is converted to elemental sulphur and, of the remaining H2S, a third of it is converted to SO2; and (2) a moderately exothermic catalytic stage where the remaining H2S in the gas stream reacts with the rest of the SO2 to produce more elemental sulphur. The reactions are reversible, and conversions are highly dependent on temperature, sulphur content and moisture content. To achieve sulphur conversions greater than 60-70%, the thermal stage is followed by sulphur condensation and separation which is followed by reheating upstream of a catalytic stage operated at temperatures higher than the sulphur dew point. Additional catalytic stages may be added to increase the efficiency of sulphur removal.

The flame stability of the combustion section is a critical parameter in Claus operations. At a H2S content of above 55%, the acid gas can be sent directly to the furnace. Between 30–55% H2S, the acid gas or combustion air (or both) may need to be preheated. At concentrations below 30% H2S, the Claus unit operates in a ‘split flow’ mode with preheat and, as the H2S content drops below 10%, fuel gas may need to be added.

A two-stage Claus unit can deliver 90–95% sulphur recovery efficiency, with a three-stage configuration delivering 95–98% recovery. The tail gas is generally sent to an incinerator if 96–97% sulphur recovery efficiency is acceptable. If sulphur recovery in the 99–99.5% range is required, tail gas operations based on a continuation of the Claus reaction under sub-dew point is generally undertaken either on a solid bed, or in the liquid phase. If sulphur recovery efficiencies of 99.9% are required, the sulphur in the tail gas is generally converted to H2S by hydrogenation and hydrolysis. Then the H2S is captured and recycled to the Claus unit.

Modified Claus units are challenging to operate reliably, and are particularly prone to problems during start-up and shutdown. For example, the furnace is usually started up and shut down using a fuel gas stream as the fuel, rather than the acid gas. When the feed shifts from acid gas to predominantly fuel gas, with an associated shift in oxidant ratio (air ratio), higher temperatures and soot formation often may result, among other undesirable consequences. The soot has a tendency to foul the Claus catalyst downstream of the furnace.

The operating cost associated with sulphur removal for a Claus unit with tail gas clean-up is in the $100/t range when considering utilities and maintenance costs. The sulphur that is recovered is generally bright yellow and preferred in the marketplace.

GT-Spoc: process description

GT-Spoc uses a patented, durable catalyst in a ‘short-contact-time’ reactor, GT-CataFlame, to achieve near-equilibrium H2S conversion and sulphur selectivity in one tenth of the volume used by a conventional Claus burner and reaction furnace.10,11 The catalyst also contains components that eliminate classic Claus catalyst deactivation mechanisms of sulphur poisoning and coke deposition of the catalyst in the first Claus converter during normal sulphur recovery operation and start-up/shutdown using fuel gas.

A simplified process flow diagram for GT-CataFlame integrated with the waste heat boiler (WHB), first Claus converter and sulphur condensers is shown in Figure 1. This diagram shows an expected application where the catalytic combustor is installed in place of a poorly performing Claus furnace.

In GT-CataFlame the air and the acid gas are preheated to approximately 220°C (428°F) before they are mixed. The gases are blended in a specially designed chamber to thoroughly mix the two gases upstream of the catalyst reactor bed. The gas contacts the front face of the reactor at a specific actual velocity, passes through the catalyst bed in under one second, and immediately contacts the WHB. In the reactor, approximately 70% of the H2S is converted to sulphur and H2 or H2O, 10% is converted to H2O and SO2, and most of the hydrocarbons are converted to H2, CO and H2O, and any ammonia is converted to N2 and H2. The gas exiting the WHB may be sent to a sulphur condenser where elemental sulphur is removed; after the condenser, the gas may be re-heated to pass into the first Claus converter. Alternately, the gas exiting the WHB may pass directly into the first Claus converter and the previously mentioned sulphur condenser and re-heater may be eliminated; passing directly from the WHB into the first sulphur condenser the process effectively ‘jumps’ the Gamson and Elkins curve and 90% sulphur recovery occurs after the first Claus converter.12 The process that includes sending the gas directly from the WHB into the first Claus converter is part of the patents that are included in GT-Spoc. The gas path after the first Claus converter is the same as that of a conventional Claus SRU. 
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