Scalable Energy Recovery Systems for Sulfuric Acid Plants
Vitor A. Sturm¹, Bruno B. Ferraro1, Michael D. Montani1, Nelson P. Clark¹
¹Clark Solutions, São Paulo, SP, Brazil
This article illustrates a new approach on energy recovery for sulfuric acid plants. A scalable unit was designed to recover heat loss, reducing cooling water and increasing steam output without compromising operation in a completely isolated and reliable system.
Keywords: Sulfuric Acid; Energy Recovery
1.SULFURIC ACID PLANTS AS THERMAL PLANTS
Contact sulfuric acid plants operate with exothermal unit operations – sulphur burning at the furnace, contact catalytic oxidation of SO2 to SO3 at the converter and SO3 absorption with acid at the absorption towers.
Typically a sulfuric acid plant will recover heat from the furnace and the converter and transform into high pressure steam – roughly 2/3 of all generated heat is recovered.
Unfortunately, the remaining 1/3 of generated heat, the portion that originates at the absorption towers, is transformed into heat waste and disposed through the cooling water. This loss is due to the low temperature of the returning concentrated acid, typically between 100 and 120°C – which cannot be efficiently recovered through steam.
Some technologies changed operating temperatures to increase the available energy recovery from the acid, ranging from 180 to 240°C, but affected materials, operational conditions and incurring disadvantages such as increased corrosion and H2 production.
Clark Solutions designed an isolated system to retrieve lost energy without compromising operation. This safe heat recovery system (SAFEHR™ BFW) can increase plant revenue and reduce operational costs.
2.ENERGY RECOVERY SYSTEM
A sulfuric acid plant of 3000 MTPD can generate up to 200 MW of heat, with 135 MW recovered through steam and 65 MW lost at the cooling water.
A scalable and isolated recovery unit was designed by Clark Solutions to partially recover lost heat. The system consists of plate heat exchangers to minimize temperature approach at the hot sides and a water closed circuit to transfer heat from the acid heat exchanger to the boiler feed water heat exchanger.
Operating this way, there is control over any eventual leaks with instrumentation at the closed circuit. The closed circuit also prevents process contamination with acid and could be applied to heat not only the boiler feed water but also other plant utilities. Heating boiler feed water debottlenecks steam capacity.
A schematic of all lost heat at the shell and tube heat exchanger can be observed in Figure 1:
Figure 1. Typical heat loss for 3000 MTPD sulfuric acid plant
Figure 1 also shows the average use of cooling water.
A schematic of Clark Solutions SAFEHR™ BFW is shown in Figure 2.
Figura 2. Esquemático do SAFEHR™ BFW
The schematic from Figure 2 shows an increase in recovered energy; roughly 10% of lost heat is saved in this particular case. This recovery can be increased depending on plant steam capacity and auxiliary utilities, as well as heat necessities from other surrounding plants. Figure 2 also shows the decrease in cooling water when the SAFEHR™ BFW is operational.
Figure 3. 3D schematic representation of the SAFEHR™ BFW
Figure 3 depicts a 3D schematic of the SAFEHR™ BFW. The system can be adapted to fit any facilities need, providing heat to water or special fluids, such as electrolyte solutions as a case example.
The SAFEHR™ BFW can provide further savings and revenue. Table 1 shows approximated savings for a recently designed system.
Table 1. Revenue and savings overview for the SAFEHR™ BFW
Payback will depend upon plant capacity and energy prices, but estimates between 2 and 3 years on recent cases assessments for energy income – without considering decrease of cooling water, make-up and water treatment costs.
Process improvements are constantly sought after to increase revenue and decrease costs. Clark Solutions low profile safe heat recovery (SAFEHR™ BFW) system is an easy to install-easy to operate unit which can increase energy output and reduce cooling water related operational costs without affecting operation.
Clark Solutions is always available to support enquires.
SAFEHR™ is a registered trademark of Clark Solutions in Brazil.