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22/11/2017

Do Small Modular Acid Plants Fit in a World of Mega Plants?

Estudos Técnicos
Do Small Modular Acid Plants Fit in a World of Mega Plants?

Nelson P. Clark, Alexandre C. Bastida, Bruno B. Ferraro, Vitor A. Sturm

Clark Solutions, São Paulo, Brazil

Sulfuric acid plants get bigger and bigger. Thirty years ago a 2000 TPD acid plant could be called a large plant. Nowadays, plants as big as 4500 TPD and 5000 TPD are becoming a common size. In our current scenario, is there space for small 100-250 TPD acid plants?

While large smelting and fertilizer complexes are required to produce large amounts of acid either for pollution control purposes or as raw material, small consumers still struggle with logistics and supply reliability.

A small acid consumer in Brasil, Kalium Mineração, chose to build its own sulfur burning plant instead of buying acid. Due to the remote plant location, the company acquired a 150 MTPD modular, skid mounted plant.

The plant

Clark Solutions designed the 150 MTPD, sulfur burning, single absorption, hydrogen peroxide tail gas scrubber acid plant. Due to the logistics of the Kalium site, the company decided to build the plant on modular skids, which would be placed on civil basis and tied to each other at the site.

The plant consists of 9 modules (skids), each one with the dimensions of a 40-foot standard container, or smaller.

  • A sulfur melting and filtering skid: Consists of the sulfur melting and filtering areas and was designed to melt and filter up to 6 metric tons per hour of solid sulfur.
  • An air drying skid: Consist of a high silicon stainless steel pump tank, drying tower, and piping.
  • A compression and a sulfur burning skids: An air blower followed by a horizontal, fixed spray gun refractory lined burner, followed by the waste heat boiler. Saturated steam is produced at 10 bar for use at site operations.
  • A #1 converter skid: Gases from the waste heat boiler are admitted on a computerized fluid dynamics designed oblong converter with 2 passes separated by a superheater.
  • A #2 converter skid: Gases from the first converter are admitted on the #2 converter, which share the same dimensions as the #1 converter. Cooling of gases going to pass #4 is made by dilution air. From pass #4, the gases are cooled to 220 C on a boiler/economizer before being admitted into the SO3 absorption skid.
  • A SO3 absorption skid: Consists of a highly resistant austenitic stainless steel tower, pump tank, and piping system.
  • Two boiler skids: These boilers are placed after conversion passes to produce steam at 10 bar.
  • An H2O2 scrubbing skid: The single absorption arrangement does not allow conversions higher than 98.5 percent. So the remaining unconverted SO2 is driven to a plastic hydrogen peroxide scrubber where the acid gas reacts and turns into sulfuric acid. The acid produced is consumed as dilution in the plant.

The area required for Kalium’s acid plant is minimal. With very open space for maintenance and operation access, and large aisles between skids, the total area required by the plant is smaller than 500 square meters.

Figure 1: Conceptual arrangement from the plant.

The area required for Kalium’s acid plant is minimal. With very open space for maintenance and operation access, and large aisles between skids, the total area required by the plant is smaller than 500 square meters.

Production and consumption figures

The plant features a few interesting aspects; such as the production of low pressure steam, which Kalium’s process requires and would otherwise be produced by burning fuel.

Because the plant is a single absorption plant with a tail gas scrubber, the process consumes H2O2 as a scrubbing agent to produce weak acid. Emissions are below 100 ppm with this arrangement and the plant can turn-down up to 70 percent as there is no interpass circuit to affect the thermal balance.

Table 1:Procedural data from modular plant.

An interesting approach being evaluated, given the need for low pressure steam, is the usage of Clark Solutions’ patented indirect heat recovery system, Safehr®.

On Safehr, strong (99.0-99.5 percent) hot acid at 200 degrees C is used to absorb the SO3 at the tower. This acid, due to the heat of reaction, heats up to 220 degrees C and is cooled on a 310SS heat exchanger by heating Clark Solutions’ proprietary fluid, CS 270.

The fluid is totally inert to acid and water, so in the event of acid leak into the system, there is no risk of producing dilute acid and thus, no risk of catastrophic corrosion initiated by a leakage. In fact, as there is neither dilution nor heating of the acid, the leak will not increase as it normally does in heat exchangers, and the plant can even operate with an acid leak into the fluid, allowing programmed decisions on when and how to shut-down. No emergency shut-downs will happen.

Figure 2: Corrosion promoted in sulfuric contact with water (left) and with CS 270 fluid (right).

Figure 3: Conceptual arrange from SAFEHR®.

Safehr helps to increase the steam generation by almost 40 percent, and for each ton of acid Safehr can produce up to 0.50 additional tons of steam. Besides increasing steam production, Safehr actually reduces the requirement for cooling water in a conventional tower.

Table 2: Process data of the modular plant with SAFEHR®.

Overall benefits

The overall benefits of smaller modular acid plants include:

  • Steam and side products: Steam is a by-product of the process that can be a very valuable asset. The tail gas scrubber can also be designed to produce other by-products such as ammonium sulfate and sodium sulfate, among others.
  • Reduce transport costs: Transportation, especially in places with limited infrastructure, reduces both the amount of transport (3:1) as well as the inherent transport risk. Acid spills are more dangerous and harmful than solid sulfur spills.
  • Smaller storage requirement: Sulfur can be stored in open or enclosed piles and consumed as required, while acid requires large tanks.
  • Safehr construction: The modular design concept eliminates risks inherent to site construction. Modules are shop fabricated under much more stringent safety conditions.
  • Quality construction: Modules are shop fabricated and tested, guaranteeing a superior quality.

Conclusion

Even though acid plants are getting bigger and bigger, small, modular acid plants will continue to find applications for those customers who need 50-250 MTPD of acid and can benefit from the steam or power generated in such plants.

Corrosion promoted in sulfuric contact with water (left) and with CS 270 fluid (right).

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