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PHASE SEPARATION TECHNOLOGY
AIChE Annual International Phosphate Fertilizer & Sulfuric Acid Technology Conference
June 6 - 7, 2008 Sheraton Sand Key Resort - Clearwater Beach, FL
Cross-Flow Scrubber Design James Eldridge, Senior Applications Engineer, Kimre Inc; Miami FL, USA Background; Cross (Horizontal) Flow vs. Counter-Current-Flow: Cross-flow or, Horizontal-flow scrubber design is almost never mentioned in chemical engineering literature. Textbooks (1) used at universities for future engineers, undergraduate as well as graduate level students, invariably only discuss counter-current vertical scrubber tower design. Likewise, professional reference texts (2) generally provide little or no mention of horizontal cross-flow designs. And, when cross-flow design is mentioned, the discussion is usually very brief. The normal design discussion usually revolves around a packed tower and packing choices in consideration of improved gas to liquid surface contact, where the actual mass transfer of gaseous pollutant to liquid phase takes place. In nearly all cases, the packing discussion assumes that a random dump-packing product (saddles, rings, etc.) will be chosen to fill the tower. Thus a brief review of wet scrubber, counter-current packed tower design, is appropriate. Basic Scrubber Design: To design a typical scrubber tower, in this instance using an example of a wet scrubber for absorption of a gaseous pollutant into a liquid stream, the following criteria are required:
Total Air Flow: (Am3/hr; Nm3/hr or ACFM; SCFM) Air and Liquid Temperature: (C or F) Identity of Pollutant: (i.e., SO2; NH3; H2S; ClO2; etc.) Pollutant Concentration: (ppmv; % by mass; etc.) Desired Removal Efficiency: (% removal, ppm/unit volume, mg/unit mass, or volume)
Using the above data, the design engineer must then decide what type of scrubbing is required. An aqueous solution may need to be pH adjusted, have an oxidizing agent added, as well as other possible factors or adjustments, as they may apply. For example, as seen for odor problems at a wastewater treatment plant, multiple pollutants such as NH3 and H2S are often present. In order to target the individual pollutants, two stages or possibly more, of wet scrubbing may be required to compensate for conflicting chemistries. Ammonia is usually first scrubbed with an acid solution, then the hydrogen sulfide is scrubbed with an alkali solution, and an added oxidizing agent may also be required. After choosing the correct chemistry or chemistries, the design engineer must then proceed to crunch the numbers. The calculation exercise will determine:
G and L: Gas and Liquid Flow Ratios (lb/hr-ft2 or kg/hr-m2) These values, with some manipulation, will then be used to determine the required face area of the tower. Normally the sizing will be rounded up to the next available standard tower size, either round, square, or rectangular; in English or metric units.
The total height of packing in the tower is determined by: Ht = Nog x Hog where: Nog = Number of Gas Transfer Units and; Hog = Height of Gas Transfer Unit
P.O.Box571240 Miami FL332571240 USATel:(305)2334249 Fax:(305)2338687
Email:[email protected]:http://www.kimre.com
Koolmijnlaan201 B3582Beringen BelgiumTel:+32(0)11450758 +32(0)11450759
Email:[email protected]
(Note; for an air stripper column, where the mass transfer is from liquid to gas, not from gas to liquid as in the above, the terms would substitute Hol for liquid film controlled mass transfer.) Commercially available packing(s) have various values of Hog. The packing supplier provides these values which are often derived empirically. This value is an inverse relationship, the smaller the reported value (in feet or meters) indicates a higher claimed packing efficiency. Stated another way: if less depth of packing is required to achieve the same mass transfer efficiency, comparing one packing product to another, the product with the lower Hog value is seen as the more attractive packing. Selection of the lower value would facilitate a lower total packing depth, necessary to achieve the targeted performance and efficiency. The choice of packing is not always driven by which packing can be identified as being the most efficient for a given service. Generally, packing that is smaller in size, (i.e. a 1-inch size packing vs. a 3-inch sized product) will have the lower reported value of Hog. However, pricing for smaller sized packing generally makes for a higher total packing cost, as more pieces of the packing are required to fill a towers given unit volume (ft3 or m3). Smaller sized packing also tends to have greater pressure drop per unit depth (P/ft or P/m) vs. larger sized packing. Smaller packing choice is usually a cost adder, in terms of the initial purchase of the packing product as well as that for the fan size and horsepower requirements, as determined for that needed to motivate the gas through the scrubber systems predetermined minimum face flow area. As a result, the design engineer must arrive at an initial scrubber tower design that:
Achieves the required scrubbing efficiency. Can be manufactured economically through the selection of a standard tower size. Or, if a non-standard fabricated tower must be used, the engineer must consider whether the tower
is a practical size that can be locally fabricated, or can be shipped at a total reasonable cost (if it were fabricated some distance from the project site).
Operates at a predicted and reasonable (calculated and actual) pressure drop. The pressure drop correlates directly with the cost of the scrubber fan and the operating costs.
The system fan should not be excessively expensive, and should have a predictable long-term lower operating cost.
(For very large projects, a complete tower shipment is often not possible. This consideration is beyond the scope of this discussion, except to note that it is an additional cost adder, or it may be prohibitively expensive; as on-site fabrication or panelized construction may be required.)
The design engineer must perform iterative calculations, until all of the required criteria are satisfied. Gas Phase Absorption; Mass Transfer vs. Particulate Capture: Another very important consideration:
When an engineer derives a process design for a counter-current, packed-vertical-bed, wet scrubber; it is usually assumed that the species to be absorbed in the tower (e.g., H2S, ClO2, HCl, etc.) is gaseous.
For the above just cited examples; hydrogen sulfide and chlorine dioxide can be assumed to be totally gas phase. However, hydrochloric acid is notorious for forming extremely fine fogs or aerosols. Also called mist, the aerosols are typically composed of very small droplets, typically ~2-m droplet diameter. These small and lightweight particles, when carried along in the gas phase of a packed bed scrubber simply zigzag past the packing. Consequently, if the fine mists are not effectively captured in the towers top mist eliminator, they will exit the scrubber stack as a white fog. This occurs, despite the scrubber typically performing at 99% or greater efficiency for the gas phase HCl absorption, as the remaining presence of the fine aerosol HCl passes through the tower, ultimately forming the distinctive white plume. Pa
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P.O.Box571240 Miami FL332571240 USATel:(305)2334249 Fax:(305)2338687
Email:[email protected]:http://www.kimre.com
Koolmijnlaan201 B3582Beringen BelgiumTel:+32(0)11450758 +32(0)11450759
Email:[email protected]
In addition to acid gases, the problem of aerosol formation is a common problem in the manufacture of fertilizers. Typically in a scrubber processing phosphates, fluorine and silicon are common contaminants. HF and SiF4 are usually present as gaseous contaminants that must be removed before discharge to the atmosphere. However, HF like HCl, as well as H2SiF6 (formed when SiF4 reacts with water) form aerosols composed of very fine particles, typically 1 to 6-m in size. Examples of poor performance of air pollution control scrubbers at NPK plants are unfortunately all too common due to reliance upon gas phase mass transfer designs which ignore aerosols. Cross flow scrubbers, using the Kimre, Inc. KON-TANE structured mesh type packing and B-GON structured mesh mist eliminators, have been shown to be very reliable in this type of service (3). Cross Flow Design Advantages vs. Counter Current Design: The mathematics discussed above may be arcane; however the principles involved in counter-current flow, normally liquid down and gas up, are straightforward. Thus, over the years, the briefly discussed design model has become very well established within the chemical engineering profession. Cross-flow scrubbing, normally with the gas flowing horizontally with the liquid flowing perpendicularly downwards across the gas flow, does not present a simple and easily derived mathematical model. Indeed, the mathematics of a cross flow scrubber are so convoluted, mathematical solutions have essentially been ignored by academics and others for many years. It is very important to note that even though the counter current pattern is considered to be much more mathematically straightforward, than the cross-flow pattern as previously mentioned, values of Hog are almost always empirically determined. However, cross-flow scrubbers are quite common. A Google Internet search using the simple terms horizontal scrubbers returns a plethora of web sites.The University of Arizona chemical engineering department reports that horizontal, cross-flow scrubbers are the most common type of scrubber found in the semiconductor industry (4).This itself emphasizes the distinct advantages of the cross-flo
