One of the most common problems with air stripping towers is that over time they become fouled with solids, resulting in the loss of efficiency, capacity, and increased pressure drop. The added weight of the entrapped solids can also be a problem for the packing and other internals.

 The Variables

There are a number of variables that cause plastic packings to foul. The total surface area of the packing per unit volume is one important variable. However, the most important variables that cause fouling are the chemistry of the system conditions of the process. The shape of the packing elements, although important to the gas/liquid contacting, has been proven to have little effect on plugging or fouling problems. 

 The Truth

Recently, claims have been made that a particular shape of packing element is more resistant to plugging than others. These claims are based on "tests" in the field, where the variables are anything but controlled. The difference in fouling rates is surely due to different operating chemistry and the conditions of the process, and not the shape of the packing. Unfortunately, there is no single "truly non-plugging" packing shape.

 The Solution

Over the years, Jaeger Tri-Packs® have become the standard by which plastic random packings are measured. In the laboratory, as well as in the field, Jaeger Products, Inc. has accumulated a wealth of knowledge on how to deal with packing fouling problems while optimizing your stripping and absorption efficiencies. The next time you have a stripping or absorbing application, let Jaeger Products put their experience to work for you.
 
 

 How Can You Deal Effectively With Packing Fouling?

 Fouling and plugging of packings

 Random and structured packings are used in gas/liquid mass transfer operations such as distillation, absorption, and stripping in order to provide available surface for mass transfer. The mechanisms for the generation of active surface are varied, but can be summarized into two: formation of films and rivulets, and formation of drops and drips. In both cases, the geometry of the system is such that the ratio of liquid volume to surface area is very small.

 This small ratio maximizes mass transfer efficiency, but also promotes precipitation of insoluble compounds. A very common example is the precipitation of iron oxides onto plastic packing surfaces in air stripping units. These strippers are generally used to remove organic contaminants from source waters to acceptable limits. Oxygen from the air is simultaneously transferred into the water and this promotes the conversion of iron to oxidation states that are insoluble in water. These insoluble iron oxides precipitate out of the water and the crystals attach themselves to any available surface. As soon as a crystal attaches itself, it becomes a "seeding" site for other crystals to adhere and grow. A complicating factor is that the heavily aerated water is also an excellent medium for bacterial growth. Bacteria colonies in the water attach themselves to the packing and provide numerous sites for inorganic deposition and vice versa. Some forms of bacteria will use the iron oxides as a nutrient.

 Paradoxically, the high mass transfer efficiency provided by the packing promotes the deposition of the oxides and bacterial growth. Packings with high surfaces will be more efficient but would tend to promote fouling as well.

 
 
 
 

There is no magic cure for fouling. The composition of the water, the irrigation and aeration rates, and the operating temperature have much more to do with how rapidly a tower will foul than does the type of packing used. Figure 1 shows a conventional Pall ring that has fouled severely in an air stripping application. Pall rings are not considered among the "high efficiency" packings, but they plug nevertheless.

 Figure 2

 It is interesting to note in Figure 2 how the trough distributor at the top of the same stripper shows severe iron fouling as well. In other words, even the low-surface trough plugged significantly. Presumably this happened because entrained liquid droplets adhered to the surface of the trough, evaporated, and deposited the iron.
 
 
 
 

 

Figure 3 shows pieces of a supposedly "non-fouling" plastic random packing that is actually severely fouled. This piece came out of an air stripping unit in an area with high iron water.

All packings foul and one can be sure that the solution to a fouling problem can be found in good maintenance practices, good monitoring of process conditions, and good overall process design. Severely fouled packed beds are inefficient and cause high pressure drop. They can also be very dangerous since support plates are generally not designed to handle the weight of packing heavily laden with inorganic salts. In some extremes, the weight of the packed bed can increase by a factor of 10 or more as the packing fouls.

 What can be done to minimize the risk of fouling?

• Operate at higher water loads to eliminate dead spots in the packing
• Manipulate water chemistry by pH adjustment
• Optimize liquid distribution in the bed
• Pre-treat feed with sequestering agents and biocides
• Pre-treat with ozone or other strong oxidants
• Clean and maintain packing frequently

 There is no such thing as a "perfectly anti-plugging" packing. Jaeger Products, Inc. can provide assistance in analyzing an existing or potential fouling problem and provide a viable solution. Sequestering agents, ozone/detergent and acid cleaning technology in combination with sequestering agent chemicals are available from Jaeger that will work on any packing in water service, even the competition's, as long as the fouling is not excessive.

 Keeping Groundwater Air Stripping Units Clean and Unplugged

 As mentioned before, if the contaminated ground water contains free iron or other minerals, such as calcium and manganese, the action of the stripping air could cause some of these compounds to precipitate and foul the packing media. The degree of fouling is usually a function of several factors beyond the actual inlet conditions. Among them:

 1) If the packings or any of the other internals are exposed to ultra violet light, then algae growth will be accelerated. Algae formation creates an excellent base for mineral deposits, such as iron, manganese, and calcium.

 2) A packing that stays completely and continually wet, thereby constantly washing itself of the precipitate, seems to resist fouling and plugging. Thus, fouling and plugging can be accelerated by poor initial liquid distribution.

 The reality is that all air strippers will eventually lose some of their efficiency and capacity due to fouling, if the water is not pre-treated before entering the tower. The degree of fouling and the amount of time for the fouling to affect the performance of a stripper are functions of all of the above factors, plus other unique characteristics of a particular site.

 The best answer to the problem is a combination of good design and pre-treatment. Jaeger Products, Inc. can assist you in both activities to provide an effective solution to the problem of fouled packing. Our engineers will properly select the right internals for your tower to assure good liquid distribution and will also detail the recommended pre-treatment and maintenance options. Pre-treatment involves the continuous addition of chemicals to the water, to keep the minerals from precipitating and to prevent algae build-up during the stripping process. Jaeger Products has put together a complete treatment/ maintenance package to address plastic packing fouling in water service. Two processes are available: a pH adjustment process, using a mild acid solution, and a process that sequesters ions of insoluble salts and prevents them from precipitating.

 Furthermore, ozone or chlorine can be used to attack biological fouling in the contactors as well as in the packing itself. Without biological growth, the possibilities of inorganic fouling are greatly diminished. Ozone is an unstable compound in air and has a very short life reverting quickly back to O₂. Ozone emissions to the atmosphere or any post-treatment facilities should not represent a problem. Chlorine on the other hand can present complications of THM generation. Consult with Jaeger Products, Inc. when deciding which treatment technique to apply.

 An additional option for controlling inorganic salt and oxide deposition is to pre-treat the feed water with a sequestering agent that will maintain the solids in solution. The selection of the type and dosage of the sequestering agent can be done very precisely to ensure that the effluent water meets all drinking water standards and that it can be directed harmlessly to its desired destination. The inorganic polyphosphate agents that are used in this application have been approved by EPA, the US Department of Agriculture, and several state health agencies for use in potable water systems at concentrations well above those needed for treatment. This technique can also be used to clean fouled packings since the sequestering agent will tend to solubilize the deposited salts. The effectiveness of this wash can be substantially enhanced by combining it with the ozone treatment procedure outlined above.

 Keeping Wastewater Stripping Units Clean and Unplugged

 Organic stripping from waste waters presents additional fouling problems than the ones outlined above. Wastewaters typically have higher concentrations of organics, and in the case of steam strippers, operate at higher temperatures. These high temperatures often result in inorganic salt precipitation that can severely foul packed beds.

 The use of sequestering agents in wastewater strippers should be evaluated carefully since the consumption of chemicals can be very high due to the high concentrations and flows. Selection of a preventive process based on sequestering agents can still be relevant where wastewaters have moderate inorganic concentrations, such as the ones from chemical plants and oil refineries. Furthermore, correct design of a steam stripping system will direct the majority of the salt deposition to the feed preheater section of the process. Heat exchangers that can be easily cleaned are then a necessity.

 The most significant fouling problem found in wastewater strippers is caused by bacteria and algae growth promoted by high organic loads. This problem is more prevalent in air strippers, but it does present itself in steam strippers as well. Methods to control biological growth in wastewater strippers do not differ much from those outlined above for groundwater strippers. Good design of liquid distribution systems, combined with manipulation of the water chemistry, offer the best possibilities for control. Ozonation of the wastewater is certainly a viable alternative.

 How Can a Fouled Stripping Unit be Cleaned?

 In many cases, preventive maintenance, such as described above, is not performed. Severely fouled towers need to be cleaned, preferably without having to remove the packing and internals. Implementation of a cleaning procedure is not trivial since one has to consider many issues ranging from the chemistry of the system to the mechanical design of the stripper.

 There are two major issues to be addressed in terms of selecting a proper cleaning protocol: liquid distribution and proper chemistry.

 First and foremost, it is essential that whatever cleaning solution is used reaches the fouled areas of the packing. In towers where the fouling has been excessive, so that areas of the packing are completely plugged off, it would be impossible to reach the most critical portions of the packing by trickling the liquid down the media.

 If the mechanical design of the tower allows for liquid-full operation, then filling the tower with cleaning solution and recycling it is the best alternative. On the other hand, this procedure consumes large amounts of cleaning solutions. The next best choice is to trickle the liquid down the packing at the maximum possible rate and to feed gas (air, nitrogen, etc.) into the bottom of the tower at a rate that propitiates flooding. The volume of liquid required in this approach is significantly less but there is the requirement of gas flow.

 A stripper that is cleaned before severe fouling occurs, as described above, will be more readily irrigated properly by the use of the tower distributor. Nevertheless, it is recommendable that the tower be flooded with the cleaning solution as the first step in the cleaning process.

 There are some fairly effective "in-situ" cleaning techniques that can be very helpful, as long as the bed is not fouled too severely and it has remained wet. Washing the packed bed with a mild acid solution is an effective technique for removing some inorganic deposits. Ozone injection can clean plastic packings by breaking down the bacterial colony structure and allowing the salts to fall off the surface of the packing. Ozone injection, in combination with detergent rinses, provides a good maintenance solution. The user does need to perform the cleaning with certain regularity to prevent excessive buildup. The use of phosphates as metal sequestering agents can also be very effective in keeping metal salts and oxides in solution during heavy aeration.

 Most towers can be completely cleaned in 24 to 72 hours. Cleaning time is, of course, a function of the severity of the fouling problem and the size of the tower. In cases where a tower cannot be out of service for the entire cleaning period, the process can be alternated with normal tower operation. Additionally, the final filtration and neutralization process can be performed in a separate holding tank, thus allowing the tower to be put back in service at the earliest possible time.

Figure 4

 
 

Figure 4 illustrate the effectiveness of the clean-up process. The packings were severely fouled with iron deposits and biological growth. The picture show how different degrees of treatment can achieve remarkable results. These were packings that fouled while in air stripping service without water pre-treatment. It should be noted that structural damage of the polymeric packing pieces has never been a problem because of the relative short exposure times and the availability of oxidizable material.
 
 
 
 
 
 
 
 
 
 

 Figure 5 shows a complete cleaning and treatment process for a groundwater stripper with heavy biological and inorganic fouling tendencies. The inorganic polyphosphate is used to sequester iron, calcium and manganese ions to prevent their deposition.

  The correct selection of the chemicals to be used in the cleanup is also of extreme importance. The nature of the fouling needs to be identified as extensively as possible so that the proper combination of chemicals, and in the proper order, can be used. There are some general guidelines that can be established:

 1) Biological fouling (bacteria, algae). The chemical of choice for removal of biological fouling is an oxidant or a free radical generator. Ozone used as described above is very effective and less sensitive to liquid maldistribution effects. Furthermore, ozone is both an oxidant and a free radical generator. Other oxidants that are commonly used include potassium permanganate and hydrogen peroxide. Chlorine can also be used as a radical producer.

 2) Inorganic fouling by basic salts and oxides (for example calcium carbonate, iron oxide, calcium hydroxide, etc.). These can be removed by weak acid solutions. Mineral acids, such as phosphoric and nitric, are frequently used. Organic acids can also be used effectively. A sequestering agent can work in these applications but it would be significantly slower.

 3) Inorganic fouling by neutral or acidic salts and oxides (for example calcium sulfate, iron sulfate, calcium chloride, etc.). Acids will not be effective in removing fouling caused by these compounds. The best solution here is a combination of a sequestering agent with colloidal agents that can break crystal-crystal bonds and disperse the pieces. These fouling compounds are the most difficult to remove.

 It should be noted that acid cleaning can, in some cases, form new acidic salts that precipitate and aggravate the fouling problem significantly.

 Service from Jaeger Products

 Jaeger Products, Inc. offers its customers the necessary proprietary hardware and chemicals to perform preventive as well as corrective maintenance. The chemicals can be specially formulated for the particular application and can be supplied to the user on a pre-set schedule. Jaeger also offers the customer several different service options.

 1) Call Out - We will do a site assessment and provide a quotation for the complete cleanup. Our quotation will include complete mobilization, tower modification (if needed), and all equipment and treatment process.

 2) Contract Call Out Program - We will do a site assessment, and design and build all the necessary hardware to clean a tower by the end user or a qualified third party contractor. We provide all chemicals as well as supervisory resources on a time and rate basis.

 3) Contract Maintenance Program - We will do a site assessment, and design and build all the necessary hardware to clean a tower by the end user or a qualified third party contractor. The tower will be put on a regular maintenance program in which we will make regular follow-up visits to clean the tower or to supervise the cleaning of the tower by the owner or a contractor. We will also supply all the necessary chemicals.

 4) Continuous Maintenance Lease Program - We will do a site assessment, and design and build all the necessary hardware to clean a tower by the end user. Lease includes all chemicals, training of operating personnel, product information updates, and four (4) annual inspections of equipment.

 All equipment is packaged on a skid or trailer mounted, pre-wired, and factory tested and ready for operation. In many cases, one system can be designed to service more than one unit.

 Inorganic Polyphosphates

 Jaeger Products, Inc. offers water pre-treatment technology using inorganic polyphosphates. These chemicals are recognized as non-hazardous, therefore, permitted for human consumption and use in potable water distribution systems. The chemical is supplied in liquid form. Dosages are calculated for every case after a detailed analysis of the feed water is performed. They can be adjusted as the water composition varies with time.

 Polyphosphates maintain iron, calcium, and manganese in solution by complexing with the metal ions and forming large, soluble clusters that prevent crystallization and deposition of the metal salts. In some ways, these polyphosphates act as "molecular detergents" and can even be used to dissolve or disperse crystals by tuberculation.

 The temperature of the system, phosphate concentration, pH, and the reversion tendency or time stability of the polyphosphates all play a very important role when assessing pre-treatment possibilities. The conditions in a stripper can be very severe with respect to phosphate chemistry, since the dilutions are large, the pH can vary from 2 to 13, and temperatures can be high (i.e., in steam strippers where conditions can exceed 220 deg. F).

 These stringent requirements mandate the use of polyphosphate blends that can operate at high temperatures and low concentrations without significant reversion. The better polyphosphate blends offer synergistic effects that cannot be found in single polyphosphate. The technology exists today to produce high-performance polyphosphate blends that are uniquely suited for use in air and steam strippers. Some packing vendors offer this expertise to the users, as do some specialized water treatment chemicals suppliers.

 Summary

 If you are going to install a stripping unit, you should make provisions for keeping the packing clean. A few inexpensive modifications now can save time and money in the future. As a starting point, you may want to request our write-up on air stripping volatile organic compounds (VOCs) from groundwater.

 If you are operating a stripping unit in fouling service, you should begin now to pre-treat the feed water. Prevention is still the least expensive option. We have pre-treatment options to suit your specific needs.

 If you have a stripping unit that is fouled, we have effective technology for cleaning it without the expense and problem of removing the media from the tower.

 For us to recommend a pre-treatment procedure or a procedure for cleaning an existing stripping unit, please call or e-mail us.