Apr-2015

Oxygen enriched sulphur recovery

Revamping its SRU to oxygen enriched operation increased a refiner’s sulphur processing capacity and significantly reduced operating costs.

ZHIFANG TANG, ZHONGFANG NIE, WUSHENG HUANG, XIAOCHEN JIN, JIANPENG WU, JIANWEI WANG and 
KENG H CHUNG,
Liaoning Huajin Tongda Chemicals

Article Summary

Liaoning Huajin Tongda Chemicals Co. Ltd. (Huajin), a subsidiary of China North Industries Group Corp. (Norinco), operates an integrated petrochemical complex at Panjin City in Liaoning Province, China. The plant has a 160 000 b/d crude oil processing capacity and produces 700 000 t/y ethylene, 1 million t/y synthetic resin, 200 000 t/y acrylonitrile butadiene styrene (ABS), 900 000 t/y synthetic ammonia, and 1 million t/y asphalt.

In August 2012, Huajin completed a series of revamps and retrofits to its refinery to comply with clean fuel regulations that require the production of lower sulphur content fuel products. This article provides an overview of the sulphur recovery unit (SRU) revamp project that involved the addition of an oxygen enrichment process to increase the sulphur processing capacity from 60 000 t/y to 95 000 t/y. In addition, the technical background and design considerations of oxygen enrichment technology, benefits realised from the implementation of an oxygen enrichment process at the facility, and observations during unit commissioning and start-up are provided.

Project overview
The 60 000 t/y SRU at Huajin refinery consists of two trains of Claus units and a common tail gas treating unit (TGTU) originally designed for air-only operation (see Figure 1).

As a result of more stringent environmental legislation, Huajin added hydrogenation capacity to produce cleaner transportation fuels. Consequently, an additional 35000 t/y sulphur processing capacity was needed to align with debottlenecking of the hydrogenation capacity.

Huajin considered the use of an oxygen enrichment process for revamping its existing SRU/TGTU system because of:
• Lack of space within the refinery for a new SRU/TGTU installation
• A ready supply of oxygen from two Huajin affiliate facilities nearby consisting of by-product oxygen from an air separation unit at the Huajin fertilizer plant and a pressure swing adsorption unit at the Huajin petrochemical plant
• The length of time required to construct and start up a new SRU/TGTU installation as well as the additional congestion within the refinery during the major turnaround
• Lower capital cost; $5 million for revamping versus $14 million for a new installation.

Technical background and design considerations
The basis of oxygen enrichment technology lies in removing all or part of the nitrogen typically carried in the combustion air and replacing it with oxygen. The principal reactions of the modified Claus process are as follows:

H2S + 3/2 O2 → SO2 + H2O              (1)
2 H2S + SO2 → 3 S + 2 H2O              (2)
3 H2S + 3/2 O2 → 3 S + 3 H2O (overall)                          (3)

In the modified Claus process, approximately one-third of the H2S in the incoming acid gas is combusted in the reaction furnace to form SO2 (Eq. 1). The resulting SO2 reacts with some of the remaining H2S via the Claus reaction (Eq. 2) to form elemental sulphur and water vapour. About 60-70% of the total sulphur conversion occurs in the reaction furnace, while the remainder is accomplished in downstream catalytic reactors.

The stoichiometry of the overall Claus reaction (Eq. 3) shows that one mole of oxygen is required for every two moles of H2S converted to elemental sulphur. If oxygen is provided by atmospheric air, nearly four moles of nitrogen accompany each mole of oxygen. The overall reaction with air is as follows:

H2S + 1/2 O2 + 2 N2 → S + H2O + 2 N2                          (4)

Although nitrogen does not participate in the reaction, its presence is detrimental to the operation of the SRU and TGTU. Firstly, nitrogen increases the volumetric flow rate through the entire unit, resulting in larger (and thus more expensive) equipment, as well as increased pressure drop. Secondly, some of the thermal energy generated in the combustion reaction (Eq. 1) is consumed to heat the nitrogen in the process gas, thereby lowering the bulk temperature in the furnace. Decreased reaction furnace temperatures are known to inhibit contaminant destruction.
By reducing or eliminating the amount of nitrogen present, oxygen enrichment technology successfully circumvents the undesirable operating parameters discussed above. At 100% oxygen enrichment, there is one mole of gas present in the tail gas as opposed to the three moles of gas present during air-only operation. For a revamp unit, this newly available volume can theoretically be used to increase the sulphur processing capacity of the unit by up to 200% (via replacement of nitrogen with acid gas). However, when processing a real-world acid gas in which the H2S concentration is less than 100%, the capacity increase is typically limited to approximately 150%. Even though the oxygen enrichment process is a proven technology that was first commercialised in 1985, experience from technology implementation, especially revamping an existing SRU, is case dependent. Table 1 shows the theoretical sulphur processing capacity increase as a function of oxygen enrichment level.

Historically, oxygen enrichment has been classified at one of three different levels. Low-level oxygen enrichment (<28 mol% O2) enables a capacity increase in sulphur processing of up to 25% by injecting oxygen into the main air supply line. Usually, no equipment modifications are required beyond providing a tie-in point for oxygen into the combustion air line. Medium-level oxygen enrichment (28-45 mol% O2) allows for a capacity increase in sulphur processing up to 70%, although several modifications to equipment are required. Combustion air piping must be upgraded, and a specially designed oxygen burner must be installed. High-level oxygen enrichment 
(>45 mol% O2) can increase sulphur processing capacity by up to 150%, but necessitates major modifications to equipment for moderating reaction furnace temperatures.

For this project, medium-level oxygen enrichment was selected for revamping the existing SRU/TGTU to achieve a 60% increase in sulphur processing capacity. After conducting process evaluation and due diligence studies, the patented COPE oxygen enrichment process licensed by Fluor/GAA was chosen.¹

Revamping
In the revamp project, the design overall hydrogen sulphide (H2S) concentration in the total acid gas feeds was 90%. This required 29% oxygen enrichment to stay within the 1400°C limit of the refractory in the reaction furnace using the process.