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Case study: Tail-Gas treating catalyst unlocks value
Sinopec selects a catalyst that helps to cut energy costs and to meet emission mandates.
The challenges facing China's largest refiner, China Petroleum & Chemical Corporation (Sinopec), in its first greenfield upstream exploration project, are typical of those for many operators trying to bring new resources into production. The gas is highly contaminated with hydrogen sulphide and the project must meet increasingly stringent environmental regulations while being as cost-effective and efficient as possible.
In addition, the project is a highly ambitious venture on a massive scale that needs state-of-the-art technologies to cope with its challenges. The giant Puguang gas field in Sichuan Province is China's largest, with proven gas reserves of 114 billion cubic metres.
To remove the contaminants cost-effectively, Sinopec built the Zhongyuan Puguang natural gas purification plant. This features 12 sulphur recovery units with a combined sulphur production capacity of 2.4 million tonnes a year. The trains are identical and each includes one high-temperature and two low-temperature Claus conversion processes.
However, the sulphur recovery of such units is limited to about 97%, which leaves small quantities of sulphur as unconverted hydrogen sulphide and sulphur dioxide. In addition, impurities in the Claus feed react during the process to form carbonyl sulphide and carbon disulphide. If the facility does not treat those compounds they will be incinerated and escape to atmosphere as sulphur dioxide, and the site may fail to comply with emission regulations.
C-234 has found wide acceptance at refineries and gas plants around the world because operating at lower reactor inlet temperatures cuts energy costs and reduces carbon dioxide emissions.
Sinopec, therefore, installed tail-gas treating units to further enhance the overall sulphur recovery by converting the sulphur dioxide to hydrogen sulphide and hydrolysing the carbonyl sulphide and carbon disulphide to hydrogen sulphide. The hydrogen sulphide that leaves the tail-gas treating unit is then recycled to the sulphur recovery unit, which yields elemental sulphur. This process, when properly operated, permits the recovery of about 99.9% of the sulphur fed to the Claus unit.
After a rigorous evaluation (see below), Sinopec selected Criterion’s tailgas treating catalyst C-234, a cobalt–molybdenum trilobal catalyst. This was primarily because it converts sulphides to hydrogen sulphide at lower temperatures (240°C) than the other catalysts that were available at the time and has considerable stability under normal operating temperatures (200–400°C).
It also has a low sock-loading density (450 kg/m3), which helps to reduce the cost. Criterion's extensive experience was also a contributing factor: it is the market leader in tail-gas applications, with more than 90% of the installed catalyst capacity, and a global market share of over 80%.
For even lower operating temperatures, alternative catalysts are available. For instance, C-734, the latest-generation catalyst, offers higher activity, lower pressure drop and higher sulphur conversion than C-234. Consequently, its operating temperature can be as low as 220°C, which enables low-pressure steam to be used for greater energy efficiency.
"Conventionally, tail-gas treating units operate at reactor inlet temperatures of 280°C, but catalyst C-234 is designed to perform at 240°C," says Bhasker Nair, Technical Consultant, Criterion. "It has found wide acceptance at refineries and gas plants around the world because operating at lower reactor inlet temperatures cuts energy costs and reduces carbon dioxide emissions."
"The tail-gas treating catalyst C-234 started up very successfully in Puguang," adds Liu Wei, Technical Manager, Sinopec Zhongyuan Oilfield Company, Puguang Branch Company. "It is operating with very high activity and stability at low temperature, and meeting our production requirements."
Aspects to consider when selecting a tail-gas catalyst are its:
- activity: High activity is necessary to convert the sulphur dioxide, carbonyl sulphide and carbon disulphide in the tail gas into hydrogen sulphide, but high activity at 240°C or less offers energy-saving benefits.
- stability: Because tail gas contains large amounts of steam (typically more than 30%), the catalyst must be highly stable in such an environment. Tail-gas treating catalysts can have cycle lengths of up to 10 years.
- pressure drop: To enable the tail-gas unit to operate for extended periods without a catalyst charge, minimum pressure drop is essential, as this process operates just upstream of the stack and the lowest operating pressure is advantageous for optimal process conditions.
- operating temperature: A catalyst that operates at a lower temperature can enhance energy efficiency and may offer lower carbon dioxide emissions.
- cost: Low loading density is beneficial because less weight is required to fill a reactor, which reduces costs.