INNOVATIVE FILAMENT TECHNOLOGY IMPROVES PERFORMANCE AND LONGEVITY OF SULPHURIC ACID MIST ELIMINATORS

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1 INNOVATIVE FILAMENT TECHNOLOGY IMPROVES PERFORMANCE AND LONGEVITY OF SULPHURIC ACID MIST ELIMINATORS Authors: Daniel Egger, Sulzer Chemtech Ltd, Winterthur, Switzerland Loris Tonon, Sulzer Chemtech Ltd, Winterthur, Switzerland ABSTRACT Proper selection of internals for mist elimination in sulfuric acid drying and absorption towers is still a challenge from the performance and material points of view. In sulfuric acid drying towers, knitted wire mesh pads are the principal type of equipment used. Poorly designed, corroded or fouled drying tower mist eliminators are common sources of excessive entrainment. Therefore, mist loading in the downstream absorbers is significantly influenced by the efficiency of the drying tower mist eliminator. In drying towers the operating life of mist eliminators is affected by two primary factors; corrosion and fouling. A new PTFE filament used in the Sulzer KnitMesh XCOAT mist eliminator provides an innovative approach to these problems: Corrosion: The Sulzer KnitMesh XCOAT mist eliminator is recognized for its high corrosion resistivity and high resistance to deformation in a wide range of operating conditions including high temperatures. Fouling: Due to the unique behavior of the PTFE surface, any deposition and fouling can be more easily removed from the surface than from any other material. The PTFE coating of the Sulzer KnitMesh XCOAT promotes better flushing of solids out of the mist eliminator. This characteristic results in extended service life of the mist eliminator compared to other corrosion resistant materials. The innovative Sulzer solution combines the chemical resistivity of PTFE and the mechanical strength of metal alloys. These characteristics are ideally suited for applications in highly corrosive environments like sulfuric and acetic acid, with extended service life of the mesh mist eliminator. Enhancement of corrosion resistance and prolonging operational lifetime resulting in reduction of costs for replacement are only a few of the advantages provided by the Sulzer KnitMesh XCOAT mist eliminator. This paper introduces the Sulzer KnitMesh XCOAT mist eliminator, compares the benefits and limitations as well as installation, maintenance, and operating costs that should be anticipated for the various options in sulfuric acid drying towers.

2 1 INTRODUCTION Typical applications suffering from mist contamination include sulfuric, phosphoric, acetic and nitric acid plants. Liquid entrainment in a process gas stream can be formed by either dynamic processes or thermodynamic changes in a system: Dynamic processes such as contact between gas and liquid phases in a mass transfer operation or thermal processes such as condensation. Thermodynamic changes in a system such as chemical reactions or vapor condensation. Condensation of mist can occur during heat exchange processes of saturated gases, reaction of gases which produces a liquid, sudden release of pressure or by mixing hot and cold gas streams. 1.1 Challenge from Strong Sulfuric Acid Applications Proper selection of equipment for mist elimination in sulfuric acid drying and absorption towers is still a challenge from the performance and material points of view. Operating evidence shows that the gas-liquid interface represents one of the highest risks of corrosion and since mist eliminators operate in this region, they are one of the most challenging corrosion environments in the plant. In sulfuric acid drying towers, knitted wire mesh pads are the principal type of equipment used. One of the common sources of excessive entrainment are corroded and fouled mist eliminators used in drying towers which causes acid condensation during shut-downs and thus degradation of the catalyst. Drying the gas will also help to maintain a clear stack by avoiding the formation of excessive amounts of sub-micron mist droplets which may overload the mist eliminator in the final tower. Several factors have therefore to be considered in the selection of the mist eliminator type and the material of construction where the longevity depends on the operating conditions, acid concentration and used material. 1.2 Droplet Size In Sulfuric Acid plants acid droplets are formed by condensation, chemical reaction and shear forces. Fume (<1 micron size) and mist (<10 micron size) are generated from the sudden or shock cooling of hot gas containing SO 3. Generally at the inlet of absorber towers, chemical reaction between SO 3 and water will also produce a mist or fume of sulfuric acid. Mechanically formed droplets usually range in sizes from 3 to greater than 800 microns. Droplets can be sheared from wet surfaces such as liquid distributors or packed beds where there is high relative velocity between the gas and liquids. If the gas is travelling too fast to allow the liquid droplets to settle out under gravity they become suspended in the gas stream.

3 In order to understand the importance of mist elimination, it is useful to review the mechanism of separation and the range of droplets that different types of mist eliminators can operate at. Figure 1 below summarizes the range of droplets that different types of separators are capable of removing: Figure 1 - Particle classification and useful separator equipment versus particle size (Fair et al., 1997) 1.3 Mechanisms of Mist Removal The four basic mechanisms of droplet removal are as follows (Figure 2). Diffusional Deposition This mechanism is only effective in separation of very finely dispersed liquid aerosols with droplets typically smaller than 1 micron which are small enough to be affected by Brownian Motion ( Low gas velocities are required to allow Brownian Motion. Inertial Interception Inertial forces act on the droplets producing relative motion between the gas flow and the path of motion of the droplets. If the inertial forces are large enough, the droplets reach the impingement

4 surface, rest on the impingement surface or coalesce with other droplets that already existed on the surface into a big drop or film, and finally run out of the mist eliminator. Direct Interception Direct Interception assumes that a droplet of a given diameter and negligible mass follows the stream line around the target wire or fiber and is separated as it passes close enough to touch the target. Gravitational Deposition Gravitational Deposition works on the principle that large, slow moving droplets may separate from a gas stream under gravity. This is restricted to large droplet sizes and low superficial gas velocities making separator dimensions very large and uneconomical. Figure 2 - Mechanisms of droplet removal In drying towers the median droplet sizes are typically greater than 3 microns. High separation efficiency is required. Under these circumstances direct and inertial interception are the most appropriate mechanisms and separation is best achieved by impingement of droplets on the wire and fibers of high performance mesh mist eliminators. 2 SULPHURIC ACID MIST ELIMINATORS Several types of mist eliminators are available for separation of entrained liquid. Sulzer mist eliminators are used in many different applications including drying, distillation, absorption, stripping, etc and provide an effective solution to liquid entrainment problems. Knitted wire mesh mist eliminators are used as a low cost, highly versatile and efficient method of removing liquid entrainment from gas streams. They are produced as a bed of knitted mesh which presents a tortuous path and large surface area to the droplets entrained in the gas stream. Separation is

5 achieved by impingement and capturing by the filaments of the mesh where droplets coalesce and drain. High performance mesh mist eliminators provide excellent separation efficiency down to droplet sizes as small as 2 microns and with a pressure drop typically less than 7 mbar. 2.1 Impact on Lifetime due to Corrosion Most common drying tower mist eliminators comprise of wires which have a standard diameter of 0.28 mm. Even with low corrosion rates, it will not take long for the wires to corrode through. Assuming a corrosion rate of 0.1 mm/year the wire will be destroyed in 2 years or less. In common cases Nickel based alloys such as Alloy 20 can be used to reduce the rate of corrosion and to improve the service life of the mist eliminators. However, there are cases where even Alloy 20 does not resist because the temperature and/or sulfuric acid concentration ends into a range where increased corrosion occurs. Drying tower mist eliminators face dangers from such swings in the acid concentration and temperature. Both of these factors can result in a significant decrease of service life. Silicon containing stainless steels (such as UNS S30600, S30601 and S32615) provide an improved corrosion resistance towards sulfuric acid. But, depending on the alloy type, operating temperature and acid concentration, the corrosion rate can rapidly achieve a critical level for mist eliminator wires where only 0.05 mm of corrosion allowance exists. In addition, the presence of fluorides should be avoided since it will attack the silicon in the alloy. Non-metallic materials such as fluoroplastics like PTFE, PFA, ETFE or FEP also are known to be very resistant to corrosion. However, they lack mechanical strength and are prone to shrinkage at higher temperatures causing gaps between the sections and gas bypassing. Amongst these materials, PTFE shows the best results in resistivity and durability at high temperatures and it s totally resistant to sulfuric acid. 2.2 Extension of Service Life with the Sulzer KnitMesh XCOAT Mist Eliminator To address above concerns, Sulzer introduced an ideal mist elimination solution for corrosive media. The concerns related to corrosion, shrinkage, collapsing is now solved by the new Sulzer KnitMesh XCOAT mist eliminator (Figure 3) which combines the excellent chemical and temperature resistance of PTFE with the mechanical strength of a stainless steel wire. The Sulzer XCOAT material consists of a 100% pure PTFE monofilament which contains a stainless steel wire as a core. This leads to an extremely high resistance to deformation, corrosion and temperature, and makes it highly ideal for corrosive environments such as sulfuric and acetic acid applications. Characterized by these features, the Sulzer KnitMesh XCOAT is the world s first, unique and patented wire mesh mist eliminator being a PTFE monofilament and integrating all the valued advantages of metal wire mesh pads.

6 Figure 3 Sulzer KnitMesh XCOAT Mist Eliminator The Sulzer XCOAT material is specifically developed for corrosive environments where other materials are considered unsuitable or have limited operational life. An investment in this product will be rewarded by extended service life and less maintenance work, labor and equipment costs Corrosion Resistance Field and lab tests have been conducted to demonstrate the corrosion resistance of the Sulzer KnitMesh XCOAT. Figure 4 below compares the corrosion between the Sulzer XCOAT and various stainless steel and alloy wires in the presence of 93-98% sulfuric acid, at C over the period of 1 year. Figure 4 Comparison of corrosion between the KnitMesh XCOAT and various metal wires

7 Figure 4 shows that the Sulzer KnitMesh XCOAT is not affected at all whereas metal wires suffer from corrosion, which can get even worse if the sulfuric acid concentration and temperature changes. Figure 5 shows a corrosion example of an Alloy 20 wire mesh mist eliminator caused by swings in acid concentration and temperature. Figure 5 Destroyed wire mesh mist eliminator Fouling Resistance In addition to the high corrosion resistivity, the Sulzer KnitMesh XCOAT mist eliminators are also resistant to fouling and withstand the build up of solids better than conventional materials. The PTFE promotes better flushing of solids out of the mist eliminator. This characteristic also results in significantly extended service life of the mist eliminator Pressure Drop and Efficiency The pressure drop and the grade-efficiency of mist eliminators are primarily considered for the operating cost and performance of a unit. Table 1 lists typical clean pressure drops for commonly used Sulzer mist eliminators at the equipment s design capacity. Figure 6 provides the pressure drop for the Sulzer KnitMesh XCOAT 9033.

8 Product Sulzer KnitMesh Type Clean Pressure Drop Traditional All metal mesh pad mbar High Efficiency Plastic mesh High Efficiency Metal mesh Highest Efficiency Sulzer KnitMesh XCOAT Highest Efficiency Sulzer KnitMesh XCOAT Highest Efficiency and Capacity Sulzer KnitMesh V-MISTER XCOAT 9048-FEP 9798-H2SO XCOAT 9798-XCOAT V-MISTER 9798-XCOAT 5.2 mbar 1.9 mbar 6 mbar 5 mbar 7 mbar Table 1 Sulzer mist eliminator types and clean pressure drop Figure 6 Pressure drop of the Sulzer KnitMesh XCOAT 9033 (100 mm pad) The separation characteristic of a mist eliminator is best described by the so called grade-efficiency curve, which is the separation efficiency for a given feed droplet size or range of droplet sizes. Figure 7 provides the grade-efficiency for the Sulzer KnitMesh XCOAT 9033 and 9798 in drying towers. 100% of the entrainment larger than 2 microns and a significant percentage of droplets below 1 micron diameter are removed.

9 Figure 7 Grade-Efficiency curve Sulzer KnitMesh XCOAT 9033 and CASE STUDY 1: REVAMP OF DRYING TOWER IN SULFURIC ACID PLANT If a sulfuric acid plant is linked to a Zinc factory then the acid shall not be contaminated with traces of metal corroded and eroded from the plant equipment. Accordingly, a knitted fluoroplastic filament mist eliminator with fluoroplastic coated metal grids has been used in the drying tower. The tower is lined with heat-resistant bricks. The mist eliminator couldn t resist the high temperature and showed a severe shrinking of the pad segments after 6 months of operation (Figure 8). Figure 8 Case of a shrunk fluoroplastic filament mist eliminator

10 After a Sulzer KnitMesh XCOAT sample has been successfully tested in the column during almost one year the customer decided to replace the previously fluoroplastic mesh pad design with the new Sulzer KnitMesh XCOAT mist eliminator having a metal grid system with a fluoroplastic coating in order to meet the specific process requirement (Figure 9). Figure 9 Sulzer KnitMesh XCOAT with metal grid system with a fluoroplastics coating The Sulzer KnitMesh XCOAT is successfully in service since the beginning of 2010 and is still in perfect condition. This nice example manifests that mist eliminators which last for several years are saving labor and equipment costs needed to remove and replace shorter service life alternatives. Further, it illustrates that mist eliminators with longer service life also can be counted on to provide assurance: that expensive downstream equipment will be protected and that the quality of intermediates and end products will be met through the full operating cycle. 4 CASE STUDY 2: REVAMP OF KO-DRUM FOR RAW GAS WITH DUST A Raw Synthesis Gas KO Drum, equipped with three bubble cap trays and one radial vane separator, shall be improved for the removal of entrained droplets with the size of >2 microns. In order to decrease the dust loaded liquid entrainment from the raw gas KO drum, the radial vane separator shall be replaced by a wire mesh mist eliminator having a spray nozzle wash system. Due to the dust loading the customer was looking for a solution with excellent antifouling characteristics. Additionally, the new device has to withstand high operating temperatures of up to 207 C. The Sulzer KnitMesh XCOAT mist eliminator (Figure 10) has been selected where easy washout is achieved due to the low adhesiveness of debris on the PTFE surface (Teflon pans effect). Due to this unique behavior of the PTFE surface, any deposition and fouling can be easier removed from the surface than on any other conventional materials. Beside the self-drainage of liquids and solids from the packing the flushing will be supported with a suitable spray wash system.

11 Further, the Sulzer KnitMesh XCOAT is capable of operating at temperatures up to 260 C and for a short time at 280 C. Figure 10 Sulzer KnitMesh XCOAT for a Raw Gas KO Drum First results are expected soon from this dust loaded raw gas revamp case story. 5 SUMMARY In sulfuric acid applications entrainment needs to be efficiently separated from the gas stream in order to avoid problems or damages in the downstream equipment. The most useful mist eliminator can be selected by understanding the causes of the entrainment and the downstream problems caused by this entrainment. Thereby, efficiency, pressure drop and service life of the mist eliminator are affected by the operating conditions of each plant. Benefits of longer turnaround cycles, payback and OPEX calculations as well as the maintenance culture have an impact on the selection process by the plant management. The provided information here should help to understand the benefits of the Sulzer KnitMesh XCOAT mist eliminator by comparative information on service life, efficiency and capacity. The advantages of using the Sulzer KnitMesh XCOAT are summarized below: Recognized for its high corrosion resistivity and high resistance to deformation including high temperatures up to 260 C and for a short time at 280 C. Offers significant improvements in operating service life with less maintenance, shutdown and safety related work. Meets the highest requirements on efficiency and pressure drop

12 Can be combined with Sulzer s proprietary V-MISTER technology in order to increase the capacity Allows retrofits without modifications Offers a significant decrease of fouling which results in longer service life Investment will be paid back in a short time. 6 REFERENCES 1. Egger, R.D., "The ideal solution for corrosive media" Sulzer Technical Review 1/ Egger, R.D., and Robinson, M.R., "Removing liquid from gas" Sulzer Technical Review 3/ Fair, J.R., Steinmeyer, D.E., Penney, W.R., and Crocker, B.B., (1997) Gas absorption and gasliquid system design, in: Perry s Chemical Engineers handbook (R.H. Perry, D.W. Green, and J.O. Maloney, eds), 8th edition, Section Azwell, D.E., and Ziebold, S.A., MECS Inc., Debottlenecking Mist Eliminator Installations in Sulfuric Acid Plants, AIChE Clearwater Craig, B.D., Handbook of Corrosion Data, 2 nd Edition 6. Louie, D.K, Handbook of Sulfuric Acid Manufacturing 7. Francis, R., RA Materials, UK, The performance of stainless steels in concentrated sulphuric acid 8. Sulzer KnitMesh V-MISTER, Technology/Mist-Eliminators/Sulzer-KM-V-MISTER