Improved Sulfuric Acid Plant Absorbing Tower Efficiencies
Sulfuric acid is an important large-scale world chemical commodity. In the last step of the contact process, liquid sulfuric acid is made in strong acid absorbing towers. Therefore, highly efficient absorbing towers are desired for optimal performance, service life and cost. Unfortunately, part of the sulfuric acid in the absorbing tower is converted to mist so that absorbing tower installations most often use high efficiency Brownian diffusion fiberbed mist eliminators for product recovery, protection of downstream equipment and stack acid mist control.
Hanging style diffusion fiber beds have been the preferred arrangement with top element flange attachments on the clean side of the process making installation and troubleshooting easier compared to standing elements. These types of mist eliminators provide better performance at reduced rates, when process upsets occur, and long term when tower components wear results in higher mist levels. Also, advances with diffusion fiber beds over the years have led to significant cost and energy savings.
There are however common problems frequently associated with liquid seals for hanging style mist eliminators. This article describes a new proven alternative that eliminates drain legs routed to trough distributors or seal leg/seal cup assemblies thereby saving cost, plant downtime and maintenance.
Mist Formation in Absorbing Towers
The amount of small particle sulfuric acid mist (< 5 microns) and large particle sulfuric acid spray (> 5 microns) formed in absorbing towers depends on several factors such as tower design, operation, condition of equipment, gas inlet composition and gas inlet conditions. When one or more of these factors are “out of whack” from normal design practice, a large amount of acid mist and/or acid spray can be formed in the tower.
In general, larger acid spray particles are formed by interaction of gas and liquid acid around the distributor. A portion of these large acid particles is shattered and carried upwards by the gas to mist eliminators. A significant amount of submicron mist can also be formed when a high level of sulfuric acid vapor is present and condenses from the gas phase into submicron particles. Acid spray and mist if not controlled will corrode downstream ductwork and heat exchangers, damage catalyst and cause atmospheric pollution.
Brownian diffusion fiber bed mist eliminators have been used for effective removal of sulfuric acid mist in absorbing towers since they eliminate both large acid spray particles and submicron acid mist. A tower design and operation that minimizes mist and spray formation also helps mist eliminators to perform better, ensuring only clean gas goes to downstream equipment.
As a simple example of how tower design affects mist formation and ultimately downstream mist eliminator performance, here is a comparison of two types of distributors (Fig. 1). The first distributor shown on the left side is a MECS® UniFlo® using “downcomers” buried into the tower packing to prevent formation of acid spray while the second type, shown on the right, is an “open” distributor spraying acid upwards to impaction plates.
The open up-spray distributor with top impaction plates generates a high level of acid spray that overloads downstream mist eliminators resulting in re-entrainment, drip acid carry-over and high maintenance over time. A properly functioning distributor is crucial for optimal mist elimination, and DuPont Clean Technologies always recommends combining the MECS® UniFlo® distributor with MECS® Brink® Mist Eliminators.
Mist Collection Mechanisms
Brownian diffusion fiber bed mist eliminators provide high efficiency because they are designed to use all three primary mist collection mechanisms: inertial impaction, direct interception and Brownian diffusion. Inertial impaction and interception are the main collection methods for removing larger acid particles from process gas. However, the third mechanism, Brownian diffusion, removes submicron acid mist and is exclusively utilized by Brownian diffusion fiber beds.
All molecules in a gas stream are in constant random motion. The smaller particles pick up random motion by colliding with surrounding gas molecules. The smaller the particle, the greater the random motion and the more likely it will contact a target and be captured as it passes by collecting fibers in the gas stream. Since visible stack emissions are primarily sub-micron in size, high efficiency fiber beds utilizing the diffusion collection mechanism are required to eliminate visible opacity.
With impaction devices, efficiency decreases as the gas and particle velocity decrease because particles have less momentum and do not deviate from the gas stream. With Brownian diffusion mist eliminators, collection of submicron particles increases as gas velocity decreases because small particles have more residence time in the collecting media to strike a target along their random path travel. Therefore, diffusion fiberbeds are more effective in controlling mist removal during reduced plant rates.
Brownian Diffusion Fiber Bed Mist Eliminators
Dr. Joe Brink developed the “first-generation” Brownian diffusion HE (High Efficiency) mist eliminator in the 1950s (Fig 2). This element utilizes a hand packed thick fiberbed between concentric wire mesh screens. The element can achieve high efficiencies ranging from 90 to 99.9+% depending upon the application. For strong sulfuric acid installations, chemically resistant glass fibers are used as collecting media. One concern with hand packed fiberbeds is uniformity. As a result, more fiber is used to achieve higher efficiencies compared to newer fiber bed technologies. There is also a limit with operating bed velocity and inlet mist loading which affects re-entrainment formation. Furthermore, when elements need to be repacked, bulk fiber packing is more difficult to remove compared to newer fiber bed designs
The "second generation" Brownian diffusion fiber bed mist eliminator called the ES (Energy Saver) was developed in the late '70s and is commonly used in absorbing and heat recovery towers today (Fig. 3). A roving fiber is wound with computer control (not hand wrapped), resulting in exact placement of collecting fibers and a highly uniform packing distribution. Pressure monitoring during manufacture assures exactly matched elements to optimize mist capture in multi-element installations with high efficiency. For many applications, ES diffusion fiberbeds typically provide up to ~20 percent more gas throughput compared to bulk packed elements for the same pressure drop and mist collection (or equivalent reduction in pressure drop for the same number as bulk packed elements). This is due to a more uniform wound roving packing structure along with unique wetting properties of roving fiber.
A standard feature of the ES is a bi-component fiber bed design for eliminating re-entrainment. This design utilizes a proprietary drain layer of coarse fibers oriented downstream of the fine fiber collecting layer. Liquid films that would normally form and burst at the gas discharge surface of the fine fiber collecting layer are drawn into the drainage layer and drain by gravity. This allows the element to operate at higher bed velocities and inlet mist loadings compared to the original bulk packed fiber beds that only use one fiber layer.
The “third generation” Brownian diffusion mist eliminator is a multi-layer fiberbed called the XP (for eXtra Performance). The XP was developed in the mid-2000s and has been installed in many absorbing towers demonstrating up to ~50 percent more gas throughput compared to the original hand packed elements introduced by Dr. Joe Brink in the 1950s (or equivalent pressure drop reduction using the same number of original hand packed elements). Part of the increased XP performance compared to earlier fiberbed designs has been due to development of a proprietary mat that is angle wrapped, resulting in a highly uniform pack combined with the unique properties of fine collecting fibers.
The Traditional Hanging Element Liquid Drain
The traditional hanging element “drain” is a seal leg/seal cup design (shown in Figure 3), which has been used for over a half century in the sulfuric acid industry to remove collected sulfuric acid mist from hanging style Brownian diffusion mist eliminators. The term “seal” is often used because this arrangement has a liquid seal allowing liquid to discharge from the inside of the element to the outside against pressure differential across the element. Collected liquid in the bottom of the element drains into the element “seal or drain” leg. The draining liquid flows into the seal cup and then overflows resulting in large acid droplets dripping back down onto the tower packing below. Actual liquid level in the seal leg during operation is determined by operating pressure drop across the mist eliminator and the density of the liquid. Even with proper design of the element drain, this arrangement requires attention and maintenance during plant operation to assure proper mist eliminator performance.
Consequences of Failed Hanging Element Drain
When a hanging element drain is blocked, collected liquid in the bottom of the element rises and tries to drain back through the fiberbed against the gas flow. The collecting fibers become saturated and gas flowing through the fiberbed creates bubbles at the liquid level/fiberbed interface which in turn generates small acid particles by film shatter. Acid particles several microns in diameter are then carried downstream by the gas exiting the mist eliminators, exposing downstream equipment to drip acid resulting in corrosion and maintenance.
Another serious operating condition occurs when a drain seal fails, resulting in gas bypassing an element flowing upwards through a “blown” seal leg. Gas bypassing through an element seal leg prevents liquid collecting in the bottom of the element from draining down through the seal leg. A large amount of re-entrainment is formed by gas vertically discharging though the bottom element drain coupling and consequently shearing collected liquid in the bottom of the element into small particles that are carried downstream (Fig. 4).
“Blown” Seal Leg Example
A “blown” seal leg, two inches in diameter and three feet long, exposed to a 250 mm w.c. element pressure differential will bypass ~460 cfm of dirty gas with an upward gas velocity, leaving the seal leg at ~350 fps. At this velocity, there is plenty of gas shear to atomize acid accumulated in the bottom of the element into small particles that “re-entrain” or discharge from the element with the exiting gas, resulting in corrosion of downstream equipment.
Alternative to Traditional Hanging Element Drains
The AutoDrain™ (AD) option for hanging style MECS® Brink® Mist Eliminators is currently in service in many absorbing tower installations (Fig 5). AD effectively drains acid from mist eliminators without the use of expensive drain legs/seal cups or complex drain leg piping systems routed to MECS® UniFlo® distributors. AD eliminates the hazard of working in acid resistant apparel required when attaching drain legs to hanging elements below the tube sheet, which is a very dangerous work area. When elements are removed from a tower, this same hazardous work must be performed to de-attach drain legs before they can be lifted out. AD prevents this hazard as well.
AD eliminates element seal legs directly routed to trough distributors or seal leg/seal cup assemblies which are the more frequent maintenance trouble spots in the tower and one of the more difficult areas to troubleshoot. Also, when maintenance is required on tower acid distributors, hanging drain legs (pipes) from the mist eliminators are a significant obstruction to anyone working in acid resistant apparel which can result in falls and injury. AD averts this safety hazard as a set of mist eliminators built with AD does away with all the hanging drain legs or seal legs/seal cups and so presents significantly fewer obstructions under the mist eliminators which leads to easier and safer maintenance on tower components (Fig. 6).
During operation using AutoDrain™, the inside bottom element plates will typically be dry, while bottom plates of standard elements with traditional drains will typically have accumulated liquid agitated by gas near drain couplings, resulting in re-entrainment formation. Thus, elements using AutoDrain™ reduce re-entrainment formation on the clean gas discharge side leading to increased protection of downstream equipment.
Increased sulfuric acid plant efficiencies can be realized by operating state-of-art UniFlo® absorbing tower distributors combined with new high efficiency Brink® Brownian diffusion fiberbed mist eliminators using AutoDrain™ to achieve significant cost and energy savings over the life of the plant