Jul-2010

Designing for sulphur removal and storage: part II

Selection of technologies for the sulphur block must serve legislative demands and the efficient operation of upstream processes

Shamim Gandhi, Wayne Chung and Krish Nangia
Fluor Corporation

Article Summary

The sulphur block is an essential part of any new or revamp refinery project and its cost contributes significantly to the overall cost of a project. A typical sulphur block includes facilities for hydrogen sulphide removal, amine regeneration, sour water treating, sulphur recovery, tail gas treating, sulphur degassing, sulphur granulation and sulphur storage. The major criteria in selecting the sulphur block’s design include the project’s environmental regulatory constraints, process design requirements, operating philosophy, plant reliability, and the associated capital and operating costs.

Issues discussed in the first part of the article (see PTQ, Q2 2010) included unit capacity, amine and sour water system design and licensor selection. This second part of the article addresses design and selection issues regarding the sulphur recovery unit (SRU), tail gas treating unit (TGTU), and sulphur degassing and storage.

Sulphur recovery unit
The acid gas lines from the amine regeneration unit (ARU) and the sour water treating unit (SWTU) are routed separately to the SRU. The SRU converts hydrogen sulphide (H2S) to liquid sulphur and ammonia (NH3) to nitrogen.

Figures 1 and 2 show simplified process flow diagrams. The SRU typically consists of a Claus thermal section and at least two Claus catalytic stages. It also includes facilities for liquid sulphur collection, liquid sulphur degassing and liquid sulphur storage. The two-stage Claus recovers about 95% of the sulphur from the acid gas feed streams and produces liquid sulphur. The remaining sulphur is removed in the downstream TGTU. The SRU also destroys NH3 by conversion to nitrogen gas (N2) in the furnace.

The composition of the acid gas feed, mainly H2S, CO2, NH3, benzene, toluene and xylene, essentially determines the configuration of the Claus thermal section. The required sulphur recovery and tail gas effluent standards determine the selection of the Claus catalytic section and the TGTU.

Feed basis
Shortcut methods are available to define the SRU feed basis in the early phase of the project, when minimal information is available from the upstream process units. These methods are based on sulphur and nitrogen in the crude oil, along with yield data from similar process units. As rich amine and sour water design data are developed for the upstream units, the feed basis for the ARU and SWTU can be established. The ARU and SWTU can then be modelled using simulation programs to develop the acid gas feed compositions to the SRU.

Critical contaminants such as CO2, hydrocarbons, NH3, cyanides, chlorides and mercaptans need to be identified in acid gas from the upstream units. These contaminants have a big effect on the design of the SRU. Different cases should be evaluated to ensure the SRU is designed with enough flexibility to handle all potential contaminants and trace contaminants, feed cases and operating scenarios.

CO2 in the amine acid gas feed is based on the gas composition of the feed to the upstream gas-treating contactors, along with the selectivity of the amine selected. Use of a selective amine such as methyl diethanol amine (MDEA) would reduce the CO2 content to the SRU, while use of a non-selective solvent such as monoethanol amine (MEA) or diethanol amine (DEA) would result in higher amounts of CO2 in the SRU acid gas feed. Higher levels of CO2 lower the reaction furnace temperature, affecting hydrocarbon and NH3 destruction. In addition, some CO2 converts to carbonyl sulphide (COS) and CS2, which affects the design of the SRU.

NH3 is produced in upstream process units from organic nitrogen in crude oil. Wash water is injected to remove NH3 from the hydrocarbon phase. NH3 is dissolved in the aqueous phase to form sour water. The NH3 is subsequently stripped from the sour water in the SWTU and routed to the SRU.
 
Turndown criteria
The turndown requirement for the SRU is established by evaluating various operating scenarios, during which the load to the sulphur block is reduced. For the ARU, a lean amine recycle to the rich amine flash drum can be provided to achieve a very low turndown. A similar recycle can be provided for the SWTU. For the SRU, a typical turndown is 25%; however, in special cases, a turndown as low as 10% of SRU design capacity can be achieved.

A 10% turndown for the SRU can be achieved by co-firing natural gas in the main burner, with proper design and operating procedures in place. Due to limitations of measurable ranges, dual instrumentation may be required to enable a 10% turndown. Operating at 10% turndown for a short period is acceptable; however, long-term operation in this mode is not advised. Co-firing natural gas forms COS and CS2, results in less favourable Claus equilibrium and reduces sulphur recovery. Also, greater operator action is required to ensure that coke formation does not occur.

Air or oxygen enrichment
New SRUs are typically designed for air-only operating mode. There is, however, a minimal capital cost implication if operation in oxygen enrichment mode is implemented in addition to the air-only mode. It is common design practice to include provision for oxygen enrichment as a means of increasing SRU throughput. The level of oxygen enrichment will depend on the level of increased throughput to the SRU. Oxygen enrichment is also commonly used in debottlenecking SRU facilities to provide expanded processing capacity. There are three levels of oxygen enrichment: low, medium and high. Figure 3 shows capacity increase for a SRU as a function of oxygen concentration in the process air.

NH3 destruction
NH3 is present in the acid gas from the SWTU. It is converted to nitrogen in the SRU reaction furnace. Residual NH3 leaving the reaction furnace and at the inlet to the first Claus reactor usually should not exceed 120 ppmv. For multiple trains, the design usually includes provision for proper distribution of amine acid gas and SWTU acid gas to each plant to ensure that NH3 is destructed in the Claus thermal stage.