by: Steven M. Puricelli and Ernesto Vera-Castaneda MECS, Inc USA

Transcription

1 MECS SOLVR REGENERATIVE SULFUR DIOXIDE TECHNOLOGY by: Steven M. Puricelli and Ernesto Vera-Castaneda MECS, Inc USA Prepared for AMERICAN INSTITUTE OF CHEMICAL ENGINEERS 4798 S. Florida Ave. #253 Lakeland, Florida USA June 2013

2 MECS SolvR Regenerative Sulfur Dioxide Technology by: Steven M. Puricelli and Ernesto Vera-Castaneda MECS, Inc USA For many years, companies have searched for a way to remove low concentrations of sulfur dioxide from combustion gases and sulfur processing tail gases. One of the more promising technologies has been regenerative absorption. Many solvents have been proposed and utilized over the years, ranging from simple chemical solvents that take advantage of the sulfite/bi-sulfite system, to physical solvents that operate according to Henry s law, to the more sophisticated tertiary amines. Yet, none of these have achieved significant commercial success, notably due to high capital and operating expenses. As environmental regulations tighten up, the need for a regenerative sulfur dioxide technology is becoming increasingly important, because it addresses ever more stringent regulations while recovering a valuable commodity and not creating a secondary solid waste problem. Regenerative absorption and desorption systems have been around for many years and have been used extensively for CO 2 and H 2 S removal. Figure 1 shows a schematic of the MECS SolvR regenerative process. It is no coincidence that the MECS regenerative sulfur dioxide system looks very similar to classical systems on the outside. But, what is different is the solvent. HE To stack Recycled Buffer HE Water Column Feed SO2 Conditioned gas Water Column SO2 Loaded Gas Saturator Column Makeup NaOH Steam Absorber Column SO2 Loaded Buffer HE Stripper Column Steam SO2 Free Buffer Steam Recycle Water Crystallizer Figure 1: SolvR flow scheme

3 The SolvR Solvent In the search for the new sulfur dioxide solvent, MECS, Inc. (MECS) set the following criteria as the key performance metrics: Lower total installed cost than: o Current regenerative sulfur dioxide technologies o Double absorption (sulfuric) or SCOT (Claus plants) Lower operating costs than currently available regenerative technology o Steam consumptions in the range of 5-10 kg steam/kg SO 2 o Minimal losses of solvent in the treated gas o Solvent not degradable by sulfuric acid Significantly reduced emissions o Less than 20 ppm sulfur dioxide in the exhaust gas After an extensive computer aided search, MECS identified a family of non-toxic, noncorrosive solvents, with a high affinity for sulfur dioxide. Since this solvent has not been used in sulfur dioxide service before, the first step was to develop the vapor liquid equilibrium (VLE) for comparison to the theoretical values. As expected, the solvent exhibited a strong affinity for sulfur dioxide at moderate temperatures (up to 50 o C or 122 o F) and readily released the sulfur dioxide when the solution was heated to the boiling point, slightly above 100 o C (212 o F). Figure 2: Solvent absorption and desorption curves

4 The VLE data validated and calibrated the computer model, clearing the way for phase II of the development, bench scale operation. SolvR Bench Scale Operation The bench scale unit has been operating continuously, 24/7, since The goal is to establish and optimize the control parameters, check the stability of the solvent, develop the performance characteristics of the mass transfer equipment and to determine the suitability of various materials of construction for this service. The process flow scheme of the bench scale unit follows the same exact flow scheme of the commercial unit, which is briefly described below: Synthetic sulfuric acid exhaust gas is first quenched and hydrated via evaporative cooling to a temperature of about 30 o C. Excess water is added in this step to control the weak acid concentration. For gases contaminated with particulate, halogens, volatile metals, etc., a more extensive gas cleaning system must be incorporated for their removal. The saturated gas is then fed to the absorber for contacting with the lean solvent. The solvent has a high capacity for sulfur dioxide, thus reducing the amount of liquid needed to irrigate the packing. The absorber can economically operate at temperatures as high a 50 o C, which is easily and economically attained in most areas using cooling water. The solvent is capable of selectively and cost effectively operating over a wide range of concentrations, from as low as 200 ppmv to as high as 40 vol%. Figure 3: SolvR emissions vs. stripper performance

5 The treated gas with very low levels of sulfur dioxide is vented to the atmosphere. The goal was 20 ppm, but levels below 1 ppm were achievable. See Figure 3. The rich solvent leaving the absorber is then regenerated in the stripper by applying mild heat and/or reducing the pressure. Vacuum is not preferred because air in-leakage introduces oxygen into the stripper, which promotes sulfuric acid formation. The amount of steam used during regeneration is proportional to the desired level of sulfur dioxide emissions in the treated exhaust gas. Lower exhaust gas emissions require more thorough stripping of sulfur dioxide from the lean solvent and higher steam to sulfur dioxide ratios. The required steam pressure for the stripper reboiler is quite low, since the stripper operates at 100 o C to 105 o C. Steam usage is also very modest. Work is currently underway to validate optimized operating conditions that may result steam/so 2 ratios in the range of 5, when < 20 ppmv sulfur dioxide is required in the treated gas. The stripper overhead contains sulfur dioxide saturated with water vapor. The solvent is highly selective, so minimal amounts of other inerts, like CO 2, O 2 or N 2, are present in the sulfur dioxide product. The water saturated sulfur dioxide gas is partially condensed and sent to the rectifier column, where the condensed water is stripped of sulfur dioxide prior to recycling to the process or disposal in waste treatment. The recovered sulfur dioxide can be recycled and converted to sulfuric acid, or sulfur in the case of a Claus plant, thus increasing process efficiency. Alternatively, the sulfur dioxide can be used to produce other products, such as sodium bi-sulfite or can be sold directly as a refined product. Oxygen that is commonly present in feed gas streams will oxidize sulfur dioxide to sulfur trioxide. The solvent is indifferent to the presence of sulfur trioxide because it reacts with a sodium adjunct of the solvent, forming sodium sulfate. A solvent purification system is incorporated into the system to remove sulfates when it reaches high levels. Typically, the amount of sulfate produced represents less than one half of one percent of the sulfur dioxide treated. Caustic is added to the solvent on ph control to maintain solvent effectiveness. The sulfate byproduct is non-toxic and can be sold, disposed of as a solid or, depending on local regulations, sent directly to the sewer. Figure 4: Bench Scale Unit Corrosion Coupons

6 Extensive corrosion coupon testing was conducted during the initial two years of bench scale operation. Low cost 300 series stainless and FRP exhibited very low corrosion rates. Figure 4 shows 304L on the left and the remainder are carbon steel. The non corrosive properties of the solvent minimize the initial capital cost. Pressure drop through the regenerative process is lower than in sulfuric acid double absorption systems because the tail gas is treated in a single, low pressure drop absorbing tower with structured packing (design pressure drop is 5 kpa or 20 InWC), thus reducing the power and volume requirements on the gas compressor. This will greatly reduce the modifications needed to the compressor when treating the tail gas from an existing plant. Alternatively, existing units can be debottlenecked by increasing the gas strength and utilizing the SolvR system to treat the higher concentration tail gases. Commercialization of the Technology The next big step is to move the MECS SolvR technology from the bench to the field. A U.S. customer with a 200 MTPD single absorption acid plant will be the first commercial installation. The SolvR system will treat a tail gas of 23,800 NM 3 /hr (14,000 SCFM), containing 2,100 ppmv of sulfur dioxide. Figure 5: 3D Model of First Commercial SolvR Unit The MECS SolvR technology was chosen over double absorption because of its significantly lower capital costs and high sulfur dioxide recovery capabilities. No compressor modifications and minimal other changes were required in the sulfuric acid plant. The SolvR unit will be a modular design and will use low pressure 0.2 bar (3 psig) steam from the exhaust of the main compressor to operate the stripper reboiler. The sulfur dioxide recovered from the stripper will be returned to the drying tower for

7 recovery as sulfuric acid. Water recovered from the process will be used as dilution water in the sulfuric acid plant. The demonstration plant is expected to be operational in January Disruptive Technology The MECS SolvR technology is poised to become a disruptive technology. With the capability to handle a wide range of inlet sulfur dioxide concentrations and no catalytic equilibrium limitations, the sulfuric acid process can be greatly simplified with more attention paid to significantly increasing energy recovery. In a classic double absorption design, the following pieces of major equipment would be required: cold interpass heat exchanger, hot interpass heat exchanger, after interpass absorption converter pass(es), economizer, final absorber (including mist eliminator, distributors, packing, etc.) and acid cooler. In all, 6 pieces are required, of which 5 are sized for the gas volume. In contrast, the SolvR system consists of the following major equipment pieces: quench/absorber (with internals), stripper column (with internals), water rectifying column (with internals), solvent interchanger, overhead condenser and solvent purification system. In all, 6 pieces are required and only the quench/absorber is sized for the gas volume. Preliminary estimates indicate that the capital cost of SolvR will be significantly less than double absorption. Given that double absorption accounts for 25% of the total cost of a new plant, the overall capital savings will be notable. Optimization may further improve the savings. Hybrid Plants But, the most exciting potential SolvR brings is the prospect of a hybrid plant. That is, a sulfuric acid or Claus plant that is designed from a clean sheet of paper, taking advantage of the full benefit of SolvR in an integrated design. The first plant of this type has been named MAXENE for MAXimum ENErgy. This design integrates a single absorption acid plant with steam injection, HRS and SolvR. The result is a plant capable of ~1.5 T/T of 60 barg steam at 480 o C (900 psig at 900 o F). MAXENE can generate 27% more gross electrical power than a conventional plant at only a 10% capital premium over a conventional plant. It is worth noting that energy is recovered from the exothermic heat generated in all converter passes with steaming equipment, which greatly simplifies the flow scheme. Alternatively, flow schemes are being developed that will minimize the initial capital, albeit at the expense of energy recovery. However, in our current capital constrained

8 economic environment, a low cost hybrid plant may enable project to be executed that might not have been economically achievable in the past. Summary Over the years, there have been many attempts to commercialize a regenerative sulfur dioxide technology, but for the most part, the solvent failed to provide the economic incentive for widespread commercialization. The MECS SolvR technology, on the other hand, appears to have all of the right attributes needed for success: lower installed costs, lower operating costs and extremely low emissions. The validity of these claims will be confirmed when the first commercial unit is brought on line in early With a new emissions control tool in hand and a clean sheet of paper, MECS will be able to break free of classical design constraints and will be able to bring to the market lower cost or higher energy recovery hybrid designs that may very well redefine the industry, the way that double absorption did for sulfuric acid plants in the 1970 s. As this technology matures, it will break out of the classical applications in sulfuric acid and Claus and move into other industries with sulfur dioxide contaminated flue gases. This technology has the potential to have a major global impact across many industries. Copyright MECS, Inc. All rights reserved. The DuPont Oval Logo, DuPont TM, The Miracles of Science TM and all products denoted with a or TM are trademarks or registered trademarks of E. I. du Pont de Nemours and Company or its affiliates.