Achieving the holy grail of sulfuric acid production: profitability
Sulfuric acid plant operators have consistently sought to minimise capital costs and maximise profitability. Prior to the 1980s, many chose to do without advanced heat recovery systems. Improved technology and better plant design have since enabled more efficient facilities that typically include advanced waste heat recovery systems such as MECS® HRS™. The manufacture of sulfuric acid continues to be highly competitive. Additionally, the availability of large volumes of by-product sulfuric acid from metallurgical processes continues to create downward price pressure on sulfuric acid, which has further increased the focus on operating efficiency and capital cost. At the same time, operators need equipment that is reliable, limits the risk of corrosion and gas leaks, requires as little maintenance as possible, reduces emissions and, if possible, also allows them to recover waste energy. It’s a tall order.
In the past, attempts to cut capital and operating expenditure (CAPEX and OPEX) while reducing emissions failed due to technology limitations. Producers were forced to make choices, selecting one benefit but having to tolerate drawbacks in another area whether that was financial or environmental. The increasingly focused global drive to minimise both production costs and emissions at the same time means that plant design, equipment and operation must be viewed holistically. This market need has been the driving force for research and development at DuPont Clean Technologies for the past decade. The result of this work is MAX3™ technology, a highly efficient integration of a conventional single absorption sulfuric acid plant process with Heat Recovery System (HRS™) and MECS® SolvR® regenerative SO2 scrubbing technology. MECS® MAX3™ eliminates equipment, reduces operating costs, maximises energy recovery and delivers best in class SO2 emissions.
How does MAX3™ work?
Different, commonly available sulfuric acid plant designs all have their own relative merits and drawbacks. The focus in developing a plant design that would achieve maximum profitability and emissions control was on cutting operating costs, recovering more energy, and achieving best-in-class SO2 emissions. To do so, DuPont Clean Technologies designed a fully integrated single absorption MAX3™ flow scheme that combines a HRS™ plant with MECS® SolvR® technology (see Figure 1).
Operating and capital costs with MAX3™
The MAX3™ process offers substantial operating cost benefits when compared to a traditional double absorption sulfuric acid plant with HRS™. Overall energy recovery (sum of high- and intermediate-pressure steam rates) is slightly higher due to the choice of single absorption. In addition, the proportion of heat recovered as high-value, high-pressure steam is significantly higher (up to 20%) than in a conventional plant while intermediate-pressure steam is reduced by 40%.
At the same time, utility costs for a MAX3™ plant are low compared to those of a double absorption HRS™ plant while achieving nearly undetectable SO2 emission requirements. A MAX3™ plant will have lower electricity and caustic uses with equivalent or lower cooling water use.
The capital cost of a MAX3™ plant is roughly the same as for a conventional plant. Although elimination of double absorption equipment cuts initial capital costs this is offset by additional heat recovery equipment and the use of the SolvR® system.
Energy recovery with MAX3™
MAX3TM incorporates a number of MECS® technologies to increase production of high-value, high-pressure steam while also reducing cooling water consumption. The plant design achieves these results in a number of ways.
- Energy Recovery through MECS® HRSTM
MECS’ HRSTM technology reliably recovers intermediate-pressure steam from sulfuric acid plants to either generate electricity or replace steam produced by a fuel-fired boiler. The traditional MECS® HRS™ system recovers low temperature energy as intermediate pressure steam of up to 10 barg. By employing improved alloy materials, the high concentration (>99 wt%) strong acid circulating in the HRS™ operates at elevated temperatures allowing production of up to 0.5 tons steam / ton sulfuric acid. The inclusion of steam injection in the system further improves heat recovery by upgrading excess low-pressure steam to intermediate-pressure steam.
- Energy Recovery Facilitated by SolvR®
The integration of the high-capacity MECS® SolvR® solvent in the MAX3TM design offers several benefits. The efficient removal and recycling of SO2 by SolvR® makes it possible to switch sulfuric acid plants from double to single absorption, which not only eliminates an acid tower and cuts equipment costs, but also increases high-pressure steam production and keeps the plant’s capital costs roughly at the same level as a typical double absorption plant with HRS™. Further optimization and reconfiguration of the HRS™ and high-pressure steam systems provides a drastic improvement in high-pressure steam production while reducing intermediate-pressure steam and maintaining cooling water use at or below that of a conventional HRS™ plant.
- Single Absorption Energy Recovery Improvement
By switching to a single absorption plant, it is possible to operate without a low temperature economiser, eliminating the need to reheat gas from an interpass absorption step.
- Shifting Energy from Intermediate to High Pressure Steam
By reconfiguring the HRS™ to heat high-pressure boiler-feed water, energy shifts from intermediate pressure to high-pressure steam. At the same time, the increase in operating pressure of the deaerator improves both intermediate- and high-pressure steam production. Additional energy can be transferred to high-pressure steam by using an air heater on the combustion air for the sulphur furnace. The heat source may be intermediate- or high-pressure steam, or hot boiler-feed water, but in each case the net effect is increased high-pressure steam production.
- Low Temperature Energy Recovery with SolvR®
Integrating low-temperature energy recovery from the SolvR® overhead condenser reduces both cooling water and low-pressure steam use. As with any low temperature heat, the supply temperature of the treated water limits the amount of energy recovered.
- Additional Heat Recovery Options with MAX3™
Depending on specific site requirements, MAX3™ offers two additional energy recovery options. First is the option of installing a back-pressure turbine drive using high-pressure steam to meet the low-pressure steam requirements of the SolvR® system (as low as 0.2 barg). Secondly, any excess low-pressure steam generated by this turbine or by other on-site means can be upgraded to intermediate-pressure steam through the addition of SteamMax™ to the HRS™ to raise the steam injection rate from approximately 40% to as much as 90%.
- MAX3™ Steam Production
Incorporating each of these energy enhancements with MAX3™ results in high-pressure steam production of up to 1.55 tons steam / ton of sulfuric acid at approximately 45 barg and 400 °C. This increase in high-pressure steam production cuts intermediate-pressure steam production from the HRS™ to 0.29 tons steam / ton of sulfuric acid at approximately 10 barg saturated.
SO2 emissions with MAX3™
In a MAX3™ plant, the use of SolvR® for selective removal of SO2 from exhaust gases makes it possible to reduce SO2 emissions to almost zero. The removed SO2 is concentrated to 100% (saturated with water) and subsequently returned to the upstream process or condensed as a standalone high-value product. This proprietary, improved absorption solvent is non-toxic, non-corrosive, very stable and robust, and offers lower operating costs than competitive technologies.
The solvent forms a stable complex at relatively low temperatures (25-40°C). Thus, sulfur dioxide is efficiently removed from exhaust gases and then released when the solvent is heated to a temperature of about 100-110°C. The SolvR® solvent also has a high absorption capacity. In the SO2 Absorbing Tower a small flow of solvent efficiently removes the sulfur dioxide from the flue gas. Heat-stable salts are readily removed from the solvent and do not interfere with the solvent’s ability to absorb SO2 from the flue gas.
SolvR® is capable of removing sulfur dioxide from gas feed concentrations of 300 ppmv to 50 vol% in the inlet gas. Typically, the system will reduce emissions below 20 ppmv. If lower emissions are required, stripping steam can be increased incrementally and emissions can be lowered to below 10 ppmv. In the first commercial unit, operating data has shown that SolvR® is capable of reducing emissions to about 1 ppmv with higher solvent flows to the SO2 Absorbing Tower.
The key to improving profitability for sulphuric acid plants lies in addressing three critical aspects: energy recovery, emissions reduction, and cost. This the optimized MAX3™ plant design sets out to do.
About DuPont Clean Technologies
The DuPont Clean Technologies division applies real-world experience, history of innovation, problem-solving success and strong brands to help organizations operate safely and with the highest level of performance, reliability, energy efficiency and environmental integrity. The Clean Technologies portfolio includes STRATCO® alkylation technology for production of clean, high-octane gasoline; IsoTherming® hydroprocessing technology for desulfurization of motor fuels; MECS® sulfuric acid production and regeneration technologies; BELCO® air quality control systems for FCC flue gas scrubbing and other refinery scrubbing applications; MECS® DynaWave® technology for additional scrubbing solutions; and a comprehensive suite of aftermarket service and solutions offerings. Learn more about DuPont Clean Technologies at www.cleantechnologies.dupont.com.
About DowDuPont Specialty Products Division
DowDuPont Specialty Products, a division of DowDuPont (NYSE: DWDP), is a global innovation leader with technology-based materials, ingredients and solutions that help transform industries and everyday life. Our employees apply diverse science and expertise to help customers advance their best ideas and deliver essential innovations in key markets including electronics, transportation, building and construction, health and wellness, food and worker safety. DowDuPont intends to separate the Specialty Products Division into an independent, publicly traded company. More information can be found www.dow-dupont.com.
# # #
The DuPont Oval logo, DuPont™ and all products, unless otherwise noted, denoted with ™, ℠ or ® are trademarks or registered trademarks of E.I. du Pont de Nemours and Company or its affiliates.
Contact: DuPont Clean Technologies