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Advances in sulphuric acid plant design

Vice President, Licensing, MECS Inc., USA Matthew Viergutz

Industry Background

Sulphuric acid plants often operate for several decades so industry investment decisions usually consider a combination of capital cost, operating cost, and long run emissions. MECS has long compiled feedback from the sulphuric acid industry to identify trends that can be used to drive long term innovation. And while capital cost has always been important, the global sulphuric acid industry has increasingly pushed for improved energy efficiency, reduced emissions, and new ways to upgrade or replace its aging assets. With difficult trade-offs guiding the choices available to sulphuric acid producers, MECS foresaw that a novel approach would be required to simultaneously improve emissions and reduce capital and operating expenses.

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Advances in sulphuric acid plant design

MECS has licensed its ClausMaster™ regenerative scrubbing technology for many years, with over10 references in a wide variety of applications. Figure 1 shows how this technology uses consecutive absorption/regeneration stages with a closed loop solvent to absorb and regenerate SO2 gas.

While the ClausMaster™ technology had successfully demonstrated its ability to remove SO2 from gaseous waste streams, it did so with a few relatively critical drawbacks. In the late 2000s, MECS reviewed ClausMaster™’s most glaring limitations and set out to develop an improved solvent that would be readily available worldwide, would use lower cost materials of construction, and would be designed to operate in the “sweet spot” of tailgas emissions from a single absorption sulphuric acid plant. This effort began with an extensive search of physical property databases to identify families of solvents meeting MECS’ rigid performance criteria. Next, a worldwide intellectual property review led to a determination that preliminary work would result in the development of a technology that was both free from infringement risk and also could be patented. Pilot plant tests verified the performance of the solvent, in both SO2 removal efficiency and resistance to corrosion. MECS’s upgraded regenerative technology was named SolvR® and was ready for commercialization in 2011.

SolvR®

MECS’s SolvR® technology, first used at a sulphur burning plant in the United States in 2014, follows the same principles of unit operations as the ClausMaster™ process. As shown in Figure 2, tailgas from a sulphuric acid plant is adiabatically hydrated in a DynaWave® scrubber and flows into a countercurrent absorbing column, where SO2 is absorbed into a circulating flow of solvent.

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Advances in sulphuric acid plant design

Clean gas exits the absorber at the top, and the rich solvent is pumped to a stripping tower which removes SO2 by steam stripping. SO2 is recycled to the front end of the sulphuric acid process, and lean solvent is pumped to the top of the absorbing tower. MECS also provides a solvent regeneration system to remove sulfates that will accumulate over time. Effluent from the SolvR® system is an aqueous sodium sulfate solution that can either be sent to battery limits or concentrated to produce higher grades of sodium salt. It should be noted that SolvR® is an upgrade over ClausMaster™ for several reasons. First, the SolvR® solvent does not require unique stainless steel materials, leading to a significant reduction in the cost of the system. Further, the SolvR® solvent is readily available and is much lower cost than other regenerative solvents used to remove SO2. Finally, steam consumption is less than in the ClausMaster™ process. And while the SolvR® system is a net consumer of energy, steam injection – another MECS technology – can be used to closely integrate heat recovery between the SolvR® and the acid plant in a breakthrough way.

Steam Injection

Steam injection, a technology first commercialized by MECS more than fifteen years ago, offers an economically advantageous method for maintaining concentration control in the Heat Recovery acid system. A portion of the water required for concentration control is provided through the steam injection vessel, and the remainder is provided in the Heat Recovery System (HRS) diluter. Low pressure steam is injected into the process gas in a steam injection chamber upstream of the heat recovery tower. Since the overall enthalpy of the water fed to the HRS is higher when steam is used, the latent heat from condensation boosts generation of HRS steam compared to <st1:stockticker w:st="on">HRS</st1:stockticker> designed without steam injection. Effectively, steam injection upgrades low pressure steam that would otherwise be vented to atmosphere.

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Advances in sulphuric acid plant design

About eight years ago MECS introduced a step change improvement to steam injection called SteaMax™, a sketch of which is shown in Figure 3 below.

In SteaMax™ HRS designs, MECS uses an even higher steam to liquid water ratio for dilution, approaching operation with little or no liquid water for dilution. This configuration multiplies the enthalpy effect of conventional steam injection, allowing most plants to realize gains in absorption heat recovery of 20-30% over a conventional <st1:stockticker w:st="on">HRS with steam injection</st1:stockticker>. But the most important benefit of SteaMax™ was not realizable until the development of SolvR®. Combining these two technologies results in a new process that promises to revolutionize the sulphuric acid industry: MAX3™.

MAX3™

Combining a single absorption sulphuric acid plant with SolvR® leads to both gains and losses with respect to heat integration. Additional heat is recovered because the interpass heat exchangers are not required in a single absorption sulphuric acid plant. But SolvR® consumes both low pressure steam and cooling water, so is there a way to integrate all of these technologies to offset utilities required in the SolvR®?

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Advances in sulphuric acid plant design

To solve this problem, MECS design engineers first started with the principle that essentially each kilogram of low pressure steam fed to the steam injection chamber is upgraded to ~10 barg steam in the HRS. While this is true in theory, it has been difficult to achieve in practice because few sulphuric acid plants have excess 2 barg steam available for steam injection. And the requirement for SolvR® to consume 2 barg steam would seem to leave no opportunity for integration. Nonetheless, the MAX3TM process overcomes these two fundamental challenges to simultaneously reduce capital cost, operating cost, and emissions. Figure 4 shows a flowchart of a turbine-driven main gas blower that produces 2 barg steam – inside the acid plant battery limits – which can feed the SolvR® and can be upgraded to either intermediate pressure steam or high pressure steam in the HRS.

This unique heat integration flow scheme overcomes common shortages of 2 barg steam while upgrading this low-grade energy source to higher value steam via SteaMax™. Figure 5 shows a case study comparing battery limit conditions of conventional, HRS, and MAX3™ processes.

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Advances in sulphuric acid plant design

By integrating the SolvR® and SteaMax™ systems, MECS shifts a groundbreaking portion of the steam produced in an HRS acid plant to high pressure steam, a benefit that has been unachievable with conventional acid plant technologies. And the MAX3™ process is able to achieve this upgrade at emission levels that are an order of magnitude lower than conventional designs. If a producer planned to build a sulphuric acid plant with an SO2 emission level higher than 30 ppmv, MAX3™ would recover even more energy than shown in Figure 5.

MECS sold its first MAX3™ plant in 2015 and it second in 2016. In both cases MECS successfully leveraged the flexibility of the SolvR® regenerative system to design unique configurations for each customer that minimized costly utilities and maximized profitable exports on a site-specific basis. These custom designs would not have been possible using conventional acid plant technology, which is typically restricted to a single configuration with only the ability to make small modifications. In moving the industry towards more customization, MAX3™ truly offers sulphuric acid producers the flexibility to evaluate numerous designs so that each new acid plant can be as profitable as possible.

Conclusion

Single absorption acid plants with regenerative SO2 removal systems have historically been CAPEX/OPEX prohibitive because of costly regenerative technologies, limited solvent availability, hazardous regenerative system effluent, and the high cost of the steam used to regenerate system solvent. But combining MECS’s revolutionary new SolvR® technology - that offers reduced cost and steam consumption - with MECS’s SteaMax™ technology and its ability to upgrade steam from 2 barg to as high as 60 barg, has finally led to a step change improvement that MECS expects to be the future standard configuration of new sulphuric acid plants worldwide.

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