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Designing for a sourer future

Achieving the right sulphur plant capacity and configuration is crucial to the proper operation of a refinery.

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Article Summary
Refiners around the world are processing a wider crude slate. They are also faced with the challenge of meeting a tighter sulphur specification for the products, hence the role of the sulphur plant and its design are gaining more importance.

A proper sulphur plant configuration needs to cater to the operational flexibility of the refinery. This needs to be decided based not only on the refinery turndown operation but also on the various crude assays that are being handled.

The best sulphur plant configuration avoids bottlenecks in refinery operation with minimum economic investment. This article will present several case studies on achieving the optimum sulphur plant configuration with varying crude compositions, refinery configurations and capacities, and provide generic guidelines for making decisions related to sulphur plant design during the refinery configuration phase.

The feasibility of a petroleum refinery depends on the inherent interaction between the choice of crude oil used and the desired type and quality of finished products to be generated. Using a more expensive light, sweet crude oil requires a simpler refinery configuration and hence a lower capital investment. But supplies of light, sweet crude oil are decreasing, and at the same time the gap between heavier and sourer crude prices is increasing. Refinery configurations are also inherently defined by the type of products expected and the quality to be achieved. Meeting the quality specifications of the final products is of utmost importance as environmental requirements are gradually becoming more stringent.

The average API gravity and sulphur content of refinery crude slate varies by region, and over time the average quality of global crude slate has been declining gradually. Figure 1 shows how crude slate is changing and is expected to change over the next few years. This confirms that total world crude oil reserves (crude oil of the future) are of lower API gravity and higher sulphur content than current world production (crude oil being processed presently). The harsh impact of the changing nature of crude oil has been moderated due to the effect of an increased usage of natural gas liquids and other unconventional crude oil sources, but the graph still shows that the change in sulphur content in crude is more prominent than crude complexity.

Simultaneously, product specifications with respect to sulphur content are becoming increasingly more stringent. The most recent is the decision adopted by the International Maritime Organisation (IMO) to limit the sulphur content in marine fuel to 0.5% by the start of 2020, whereas the same limit for the Sulphur Emission Control Areas (SECA) is already 0.1% from the start of 2015 (see Figure 2). This sulphur containment will have a huge impact on the global refining sector, requiring major changes in refinery configuration and operation. New process units will be required to upgrade and/or desulphurise the residue streams.

The impact on overall sulphur management in the refinery will also become vital, based on the modifications made on the overall refinery configuration and operation. The sulphur plant configuration needs to be optimised to limit any unnecessary impact on the total investment cost of the project without sacrificing flexibility in the overall refinery operation. The sulphur recovery unit (SRU) design should avoid bottlenecks with respect to the crude slate to be processed, the product specifications to be met and any other possible refinery operating scenarios. The following sections will illustrate and highlight with actual examples the defining parameters of SRU configurations. These examples are from four different refineries with different complexities and crude handling capabilities. The following discussions will also assist in achieving the most optimised solution based on the case studies reported, and will highlight not only the means and methods to arrive at the required sulphur plant capacity of the refinery, but also provide guidelines to select the most favourable train configuration of the sulphur plant.

Case study 1: single crude train 
– moderately sour crude
The first example presented here is a 9 million t/y refinery processing a mix of high sulphur crude and low sulphur crude and generating a mixture of fuels and hydrocarbons for downstream processes. Although the refinery is expected to operate with a variety of mixtures of low sulphur and high sulphur crudes, a ratio of 80-20 is the worst feed with respect to the sulphur content and is considered for the sulphur balance shown in Table 1. Based on the actual mathematical sulphur recovery capacity, the real capacity of the SRU is generally selected by adding a margin. The margin is typically 10% or 15% over the calculated plant capacity. So for this example, the real capacity will typically be 660 t/d rather than 
600 t/d.

A single train SRU with the required capacity will have no operational flexibility; designing it for a 2 x 660 t/d configuration will attract a lot of unnecessary installation cost. Thus the prudent option for this scenario would to select a 2 x 400 t/d sulphur plant configuration which will allow for refinery operation at 70% capacity even with one SRU train operating with optimum additional installation cost.

Case study 2: multiple crude trains
– high sulphur crude
The second case study is for a refinery predominantly handling high sulphur crudes and consists of three crude trains, each capable of handling 9 million t/y of crude oil. The refinery is designed for specific cases: present case for high sulphur crude oil; and future case for a very high sulphur crude oil. The sulphur balances for these two cases are shown in Tables 2a and 2b, respectively. For Case 2a, the required sulphur plant capacity is around 2700 t/d, while for Case 2b the capacity is calculated as almost 3200 t/d after considering the required design margin.

Selection of the sulphur plant configuration also needs to take into account a refinery configuration of three crude trains, hence the sulphur plant needs to have at least three trains so that the shutdown and maintenance plan of any of the SRU trains may be grouped with any one of the crude trains.

The selected SRU configuration is 3 x 1000 t/d for the present case, keeping the provision for a fourth train to cater for future needs. This configuration provides the optimised installation cost for the present scenario, yet it has the capability to give the refinery flexibility with respect to its shutdown and maintenance plans. Each of the three SRU trains can easily be linked to the shutdown and maintenance of any of the three crude trains. This operational flexibility allows the refinery to operate at about 85% even when one of the SRU trains becomes unavailable.

When the future train is added, operational flexibility will be further increased, and the refinery will be able to operate at full capacity even with one SRU train out of operation as the three operating SRU trains 
(3 x 1000 t/d = 3000 t/d) will meet sulphur demand for the worst crude case (Case 2b) of the refinery, even with some design margin. Keeping the capacity of the future train the same as the installed SRU train capacity also offers the benefit of drastically reducing the engineering and procurement cycle when the fourth train is installed.
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