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Sep 23, 2019

Improving energy efficiency with the Outotec Heat Recovery System – HEROS

With rising energy prices and the increasing need to make more efficient use of thermal energy, the economic importance of waste heat recovery in sulfuric acid plants is growing. When sulfuric acid is produced in a sulfur burning plant, approximately 98% of the energy input comes from the intrinsic chemical energy of the reactants.
HEROS heat recovery system at Dundee Precious Metals

Introduction

In a typical process cycle, ~60% of the total energy is recovered as high-pressure steam, ~3% is dissipated with the tail gas via the stack, ~0.5% is lost as sensible heat in the product acid, and nearly ~35–40% is available as low-level heat in the acid cooler system. (Figure 1)

Sankey Diagram of a typical sulfur burning sulfuric acid plant
Figure 1: Sankey Diagram of a typical sulfur burning sulfuric acid plant

The Outotec HEROSTM Heat Recovery System, first installed in 1989, generates steam using waste heat from the acid cooling process. It is designed as a peripheral system that can be taken in and out of operation without impacting the core plant’s production of sulfuric acid at normal plant load. The operating flexibility of the HEROS system was a fundamental demand that was fully incorporated into Outotec’s design. Acid-resistant bricklining and special stainless steel ensure the maximum possible reliability and widest possible operational window. An important side effect is that the HEROS system reduces the plant’s overall consumption of cooling water significantly. The steam can be used for heating purposes or for electricity production on site.

Process description

If a heat recovery system is to be used in an Outotec acid plant, the intermediate absorption consists of two stages, namely a Venturi co-current absorber and the downstream conventional absorption system based on a packed tower design with a dedicated acid circulation system that is installed upstream of the intermediate absorption tower.

Typical Outotec HEROS configuration
Figure 2: Typical Outotec HEROS configurationnfiguration

The HEROS system is an independently operable acid circulation system using sulfuric acid with a concentration of 98.5–99.7 wt.% H2SO4 on an elevated acid temperature level. On account of the absorption of SO3 and its reaction with water introduced in the Venturi, the acid temperature rises. A major part of the SO3 contained in the process gas is absorbed in the Venturi section, which means the sulfuric acid concentration has to be monitored and controlled in the Venturi circuit. The hot acid flows by gravity into a pump tank, from which it is pumped through the HEROS acid cooler back into the Venturi tower. Any residual SO3 not absorbed in the Venturi absorber is then removed in the downstream intermediate absorption tower.

The intermediate absorption system will be adjusted process-wise according to the operation mode (HEROS – on / HEROS – off). With this measure the sulfuric acid plant is able to operate at 100% plant load even if the HEROS system is out of operation.

HEROS design philosophy

Operational flexibility

If a customer requires the ability to vary the amount of LP steam production without altering the production of the acid plant, the Outotec HEROS system can include a controlled bypass of the Venturi absorber system. The intermediate absorption tower (IAT) is designed for 100% plant load, leading to the advantage that the LP steam can be easily adjusted in the HEROS Venturi while the absorption of SO3 in the IAT is ensured.

Safety

The safety aspect is clearly linked to minimizing corrosion in both normal operation and disturbed process conditions. The HEROS system features three different aspects: the new boiler concept, which leads to fast isolation of the boiler by gravity; the material selection of stainless steel together with bricklined equipment wherever possible, which leads to a wider operation window; and the enhanced safety with redundant instrumentation, which leads to early detection of leaks in the water/steam circuit and thus minimized corrosion.

Safety as a design consideration

Heat generated in the HEROS system is transformed into low-to-medium pressure steam in a specially designed heat exchanger. An improvement has been developed by Outotec that introduces a new design concept on the water side. The Outotec HEROS acid cooler design has been converted into a forced circulation water system. Instead of the conventional “reboiler” type, Outotec uses a much smaller shell and tube type vessel. This results in a greatly reduced amount of water in the system and hence it is easier and faster to separate/drain the water from the system in case of a leakage, which minimizes the exposure time and limits the dilution of the acid.

In contrast, a Kettle type boiler with U-tube bundle, which is not used by Outotec, is placed into an oversized shell filled with water. While this boiler type offers some advantages, such as a simple design, there are also some issues – particularly with the combination of acid and water. The large amount of water contained in the vessel leads to slow response times on load changes which in turn may cause strain on the components. The quantity of the hot water/acid mixture to be disposed of once leakages occur also represents a safety issue.

In forced circulation, the exchange of water between boiler and drum takes place using pumps. This not only reduces the amount of water in direct contact with the hot sulfuric acid, it also ensures a uniform temperature distribution in the heat exchanger. Due to the pump and the careful design of water distribution/steam separation piping, excellent water circulation is obtained immediately upon starting. Since all the water is brought quickly into circulation there are no differential temperatures within the boiler that restrict the startup rate. The higher temperature difference between the fluids allows for smaller heat exchange areas and smaller vessels. There are no constraints regarding the arrangement of the heat exchanger.

Safety based on material selection

It has always been recognized that the window of operation is crucial, and the system can fail despite all instrumentation and control precautions. Acknowledging this, the design must ensure that plant damage can be mitigated.

Any heat recovery process for LP steam production needs to operate the absorption plant with sulfuric acid at very high temperatures, typically 200–220°C. This is thermodynamically required when producing a saturated LP steam of, for example, 8 to 10 bar. Concentrated acid at 200°C or higher can be extremely corrosive unless a very strictly defined operation window is adhered to, with respect to acid concentration and temperature. A few suitable stainless-steel construction materials have been identified, but all are characterized by their window of operation, which is dependent on their material characteristics. (Figure 3)

Iso-corrosion of high alloy materials
Figure 3: Iso-corrosion of high alloy materials (0.1 mm/y)

The application of alloy 3033 is a major feature of our safety philosophy and represents an important minimization of risk. The substantially enlarged operating window of alloy 3033 ensures that deviations from the design acid concentration won’t jeopardize the boiler material. All other vessels in the hot acid circuit, such as the Venturi absorber and hot acid pump tank, are fully bricklined to withstand even the harshest operating conditions.

Safety through enhanced instrumentation

The rapid detection of any failure within the system is vital to minimize any repercussions caused by leakages, etc. The HEROS system offers a couple of monitoring and failure detection devices based on independent mechanisms. They ensure redundancy and fast response times to any system disturbances, helping to monitor:
• acid concentration,
• boiler water contamination,
• process and boiler feed water management, and
• plant load and energy balance.

Overall plant availability

It is essential to separate hot acid and boiler water as safely and quickly as possible should a leakage occur. Therefore, as mentioned above, a sophisticated control technology is also part of the process. This enables early detection of leakages or operational excursions and thus mitigates the damage to equipment and risk to personnel. Experience has taught us that reliance on mechanical devices such as emergency drain pumps etc. is somewhat critical, as they tend to operate all the time except when they are really needed. As a consequence, the HEROS system uses gravity to ensure drainage.

The HEROS acid cooler is arranged above the acid pump tank. Once the acid pump is switched off, the acid drains automatically back into the pump tank. The normal water circuit is isolated from the boiler by valves; steam and water are safely discharged via emergency drain valves. Both liquids, acid, and water/steam, are separated and completely discharged within a few minutes of leak detection. The emergency shutdown is, once confirmed by the operator, fully automatic and does not rely on manual intervention from field operators. The area can be completely isolated until the boiler has been safely drained to the bricklined pump tank. The fast drainage of acid back into the pump tank prevents further dilution of the acid. The bricklined vessel is designed to cope with any potential acid concentration. Acid pump shutdown almost immediately causes a collapse of pressure on the acid side. The large pressure gap between water and acid positively avoids contamination of the remaining water-steam mixture with acid.

In the unlikely event of a failure of the HEROS system, the plant is still capable of running at 100% capacity, which prevents a cold shutdown of the entire sulfuric acid plant with all the accompanying negative impacts on plant lifetime and overall availability. The acid and HP steam production at nominal capacity is guaranteed even if the HEROS system is not in operation. Once completely emptied it is isolated from the rest of the system by installation of blinds. Subsequently the plant is restarted. With the HEROS shut down, the Venturi acts simply as a gas duct and the SO3 is absorbed in the downstream absorption tower, meaning acid production continues at full capacity.

Retrofits

The HEROS system is particularly well suited for a retrofit into an existing plant, without losing its flexibility and reliability of operation. The only prerequisite is to allow for sufficient space to locate the Venturi section as close as possible to the absorber. Of course, allowance also has to be made for the space required for the pump tank and the boiler section. Retrofitting will have virtually no effect on plant operation when the HEROS system is out of service.

Conclusion

The Outotec HEROS heat recovery system can be taken out of operation while the acid plant continues normal production after the minimized downtime used to switch over the process mode.
The system can easily be retrofitted in existing sulfuric acid plants as it is designed to permit the shutdown of the HEROS system while the intermediate absorption tower remains in full operation. Additional production of low-to-medium pressure steam is achievable, depending on the plant configuration. Thus, with the Outotec HEROS system a large percentage of the “low-level” heat generated in the absorption section of the acid plant is transferred into valuable steam while the cooling water consumption is reduced by the same degree. In case of a greenfield HEROS system installation, the system can be further optimized to produce the maximum amount of steam possible.

Since the initial installation the concept has been further refined in order to increase efficiency and safety, culminating in recent installations in metallurgical plants in Turkey and Asia, as well as in sulfur burning plants in Egypt.

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