Design of sulphur recovery units

Cost-effective methodologies for SRU developments should incorporate advanced structural design aspects with process design improvements.

Osama Bedair

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Article Summary

Much of the efforts directed toward SRU process design improvements neglect structural design aspects. To bridge the gap between process improvements and structural design, advanced structural methodologies have been proposed that can be utilised to optimise the cost of SRU design. The sulphur vessel supports are categorised into three systems based on loading mechanisms during operation and testing.

Sulphur recovery units (SRUs) are a vital facility used for the treatment of petroleum and natural gas to eliminate sulphur compounds from hydrocarbon products. Sulphur compounds in crude oil and natural gases are absorbed as acid gas (Hâ‚‚S & CO2) and converted to sulphur in the SRU. Sulphur material is commonly used to manufacture fertilisers, pharmaceuticals, cosmetics, and in the rubber industry.

Existing sulphur recovery technologies require high operating costs to meet emission requirements. The increasing demands on SRU efficiency to minimise sulphur emissions have triggered significant efforts to develop optimisation studies. Efforts were made1-5 to advance the process design of SRUs. Unfortunately, these techniques overlooked critical structural issues. Against this backdrop, it is important to address SRU structural requirements. The methodologies presented herein are based on actual cost evaluations using industrial data.

SRU components
Primary SRU components include piperacks, horizontal vessels, buildings, and sulphur storage facilities (see Figure 1). Structural components include:
Å’    Sulphur vessels SV01-SV-09
    Piperacks
Ž    Sulphur storage pit
    Substation building (SB) and control building (CB)
    Drums, condensers and heat exchangers.
Piperacks are used to support pipelines transporting process material from or to the SRU. Horizontal vessels can be categorised as:
a) Steel mounted saddle support (SMSS) system
b) Integrated saddle support (INTS) system
c) Isolated saddle support (ISS) system.

The SB houses electrical equipment and is sized based on the interior spacing. The CB is a single-story building housing critical instrumentation controls and is occupied continuously.

SMSS system
The SMSS model is recommended for vessels required to be placed at high elevations. Examples of this system are vessels SV07-SV09 in Figure 1. The elevated sulphur vessels using SMSS are mounted on steel skids attached to the primary steel framing. Figure 2 shows various schemes that can be used for the SMSS system. For illustration, Figure 2a shows the support system of stacked vessels (VA) and (VB) located at elevation (z)=14m above grade. A steel framing is provided to support the vessels, as shown in the front view. The two vessels are connected using an intermediate saddle, as shown in Figure 2a. The saddles of the lower vessel (VA) are connected to the steel skid support connected to the primary framing, as shown in the enlarged front view.

Figure 2b shows other examples of vessels supported using the SMSS system. Inlet and outlet pipes and junction boxes are supported on the same platform. Figure 2c shows control valves used to regulate gas flow in pipelines and vessels. Mechanical equipment is shown in dark grey. Steel supports for vertical and horizontal pipe bends are also shown. Pipe shoes/guides are not shown to maintain image clarity.

INTS model
The INTS model is used when the vessel support is combined with external steel framing supports. In this case, load interaction must be considered in the design of the pile cap. An example of this support system was previously shown in Figure 1 for vessels SV01-SV03. Depending upon the size of the vessel, two or more saddle wall supports can be used to reduce the loads on each wall. As illustrated, vessel SV01 is supported by four saddle walls, while SV02 and SV03 are supported by two saddle walls.

To provide further insight, Figure 3 illustrates details of the INTS system from various angles. The x-axis and z-axis directions are also shown to identify the snapshot orientation. Figure 3a shows an example of the INTS model supporting a vessel using four concrete walls. This scheme will be referred to hereafter as ‘Case A’. The vessel support of Case A is integrated with the service platform framing. The concrete walls are shown as red bricks, and the vessel is coloured silver. Enlarged detail of the front part is zoomed in Figure 3b. An alternative INTS system for two horizontal vessels is shown in Figure 3c. This support scheme is referred to as ‘Case B’. The saddles of each vessel using this model are supported using two concrete walls.

Figure 4a illustrates the loading mechanism during operation to support the INTS Case A system. In this scheme, four saddle walls are required to support the vessel. Note that the vessel diameter changes between the intermediate saddle walls. For smaller vessel sizes, two wall support systems can be used, as shown in Figure 4b. The wall size should be carefully determined to accommodate the vessel kinematics. Walls supporting fixed saddles in both schemes are denoted by W1, while walls W2 to W4 support the sliding saddles.

Concrete pile caps are used to transfer the loads from saddle walls to the piles. The wall height is denoted by Dw and is determined by piping design. Base plates are used to connect the vessel saddles to the concrete walls. Figures 4c and 4d show sections through the saddle base plates. The fixed saddle is anchored to the concrete wall support using four anchor rods. It is reasonable to assume in the analysis model that it simply supported boundary conditions for this support type. Teflon coating is bonded to the sliding base plates to reduce friction forces during operation. A roller support can be assumed to simulate the saddle sliding condition.

During operation, the vessel expands in the direction shown by the top red arrow. As a result, the rear saddle supported by W1 restrains the motion, while the front saddles supported by walls W2 to W4 allow the vessel to slide. The thrust forces on the rear support ‘A’ could be significant to result in using  a massive concrete wall size. Note that the vessel load distribution depends upon the number of saddle walls used in the support system.

Figure 5 shows the concrete support system used for INTS. Concrete walls W1-W4 support the horizontal vessel, as shown by hatched rectangles. Column supports frames F2 and F3 are shown by hatched squares. Pile-caps F1, F2 and F3 also support saddle walls. Pile-cap F1 has trapezoidal geometry and is supported by three piles. During  the design phase, it was found that this geometry is very effective to use at this location. Concrete wall W1 is projected from F1 and contains the fixed vessel saddle. Note that the F1 model is not supporting any platform column. Pile-cap F2 supports walls W2 and W3 that contains a sliding saddles and two platform column pedestals along gridlines A-5 and B-5. Note that the pedestals shown by the narrow, hatched rectangles support the platform columns. Pile-cap F2 supports concrete walls W2 and W3. Pile-cap F3 supports concrete wall W4 that contain sliding saddle and two column pedestals along gridlines A-6 and B-6. Twelve isolated pile supports are denoted by F4. Circular pedestals are projected from the piles directly to support isolated platform columns. Note that an identical concrete pedestal size is used for all platform columns.

Base plates of the columns are shown in blue and anchored to the pile head using four anchor rods. Teflon coatings are used on the sliding side with a low coefficient of friction to reduce the induced forces during operation. It is also recommended to reinforce the saddle plate using equally spaced vertical stiffeners on both sides.

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