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Superior performance by design

.1611 Peachleaf, Houston, Texas 77039

Phone:(281) 449-9500 Fax: (281) 449-9400(800) 678-0345

http://www.jaeger.com

JPI\1996ARST.DOC

Air Stripping of VOCs from Water

Jaeger Products, Inc.Houston, Texas

A very prevalent water pollution problem pertains to contamination by organic compounds that offerpotentially very important health risks. These organic contaminants are called volatile organic compounds(VOCs) when they have a certain propensity to evaporate away from the water into air. Many of these VOCsare only partially miscible with water but in general they all present a certain solubility. Table 1 lists someof the more common VOCs found in waters as well as some possible sources. The problem of reducingVOCs in water applies to ground waters, surface waters, and waste waters alike. The origin of the waterhas some important design implications that will be discussed later. This write-up will discuss theapplication of air stripping (also known as packed column aeration) for the removal of VOCs from water.

TABLE 1SOME COMMON VOCs FOUND IN WATER

VOC FOUND IN SOURCEbenzene ground waters gasoline leaks

waste waters process drains and effluentstoluene/xylene ground waters gasoline leakstrichloroethylene $ # solvent leaks into water tabletetrachloroethylene $ # $ $trichloroethane $ $ $ #dichloroethanes $ $ $ $trihalomethanes source waters chlorination/ozonation of

waste waters treated watersvinyl chloride waste waters plastics manufacturecarbon tetrachloride ground waters solvent spillsnaphthalene ground waters diesel spillsacetone waste waters solvent spills

ground waters $ #methyliso-butyl ketone ground waters gasoline leakschlorobenzenes waste waters process spills

source waters solvent spills

AIR STRIPPING . . .WHAT IS IT AND HOW DOES IT WORK?

The contaminants shown in Table 1 are considered VOCs because of their "volatility.# Volatile compoundshave a natural tendency to leave the water phase and evaporate into air or other gases that are free of thecontaminant. This high volatility makes these organic compounds easy to remove from water by puttingthe contaminated water in contact with air that is free of the pollutant. This process is called air strippingsince the VOC is "stripped" out of the water with air. In essence, the VOC is removed from the water andtransferred to the air.

JPI\1996ARST.DOC

Obviously air stripping by itself is not a solution since the pollution is only transferred from one phase toanother. Treatment of the VOC laden air that leaves an air stripper is necessary and several alternatives willbe discussed later.

VOC removals in excess of 99.99 % are possible with air stripping when a packed column is used. Packedcolumns offer very high removal efficiencies at very low pressure drops. The packing provides thenecessary surface and turbulence to allow the air to strip the organics effectively down to very low levels. Other high pressure drop devices have been used in air stripping service such as perforated trays, bubbleaerators and membranes, but undoubtedly packed columns are the devices of choice in air stripping sincethey can achieve extremely low effluent concentrations and they are the most economical to operate. Thisis especially true in applications exceeding feed flows of 50 gpm where the cost of operating the aircompression systems required for tray strippers become very large.

PACKED AIR STRIPPERS OR TRAY STRIPPERS?

The main advantages packed strippers have over tray strippers are:

a) Packed strippers are more economical and effective for larger flows (above 50 gpm).b) Packed strippers are more economical when large fractional removals are required (in excess

of 98%), such as in the case of waste-waters and highly contaminated source waters.c) Packed strippers operate at a fraction of the pressure drop of trayed ones. This reduces the

horsepower requirements for air movement in the stripper.d) Packed strippers are more economical when low volatility VOCs are to be removed. These VOCs

require higher air/water ratios that are better handled with packed columns.e) Packed strippers offer better turndown and operability range than tray strippers.

WHAT DOES A PACKED AIR STRIPPER LOOK LIKE?

In a conventional air stripper, the contaminated water is introduced to the top of a packed bed. The packingis usually a randomly dumped plastic media that is intended to provide a very large effective surface areaper unit volume for mass transfer. Water flows down the packed bed in opposite direction to ascending air.The air strips the VOC out and exits the top of the stripper. Clean water leaves the bottom of the packedsection into a sump.

The liquid is carefully distributed at the top of the tower by means of a specially designed liquid distributor.The packing is supported at the bottom of the bed by a support plate that is carefully designed not toconstrain the gas flow. In cases where the required packed bed depth exceeds 25 feet, separatelysupported beds are installed with a collection-redistribution stage between them.

Figure 1 depicts an air stripping system complete with off-gas treatment facilities. Materials of constructionfor the packing are generally polypropylene or polyethylene. The tower vessel is generally manufactured inFRP, aluminum, or stainless steel. Tower internals such as distributors and supports can be manufacturedout of polymeric materials, aluminum, or stainless steel. In drinking water applications, all materials shouldbe compatible with food service and meet FDA requirements.

Filtered air is introduced into the stripper by means of a blower. The blower can be installed in forced orinduced draft configurations depending on the site. Forced draft arrangements, such as the one shown inFigure 1, are more common and better suited for off-gas treatment.

JPI\1996ARST.DOC

HOW IS AN AIR STRIPPER DESIGNED?

The purpose of this section is to detail some of the general criteria used in designing air strippers for VOCremoval for ground and waste water applications. These guidelines are general and can be applied withoutmodification to the more conventional VOCs such as trichloroethylene (TCE), perchloroethylene (PCE),chloroform, benzene-toluene-xylene (BTX), dichloroethylene (DCE), trichloroethane, and carbon tetrachloride.These criteria apply only when air is the stripping medium and for dilute concentrations of VOCs in water(below 10,000 ppb).

JPI\1996ARST.DOC

HENRY'S LAW CONSTANT

The value of the Henry's law constant plays an important part in determining the required air to water ratioin the stripper. It can also be important in the determination of the number of transfer units and the heightof a transfer unit through its effect on the stripping factor.

Reliable data on Henry's constants (H) are not easy to find, especially since these constants are dramaticallyaffected by temperature and other solutes present in the water. In general H increases with temperatureand concentration of inorganic salts in the water. The effect of temperature should always be consideredwhereas the effect of inorganic salts is usually neglected since this represents a conservative assumption.

Henry's law is usually expressed as:

yP = H'x

where y is the mole fraction in the gas phase, x is the mole fraction in the liquid phase, P is the totalpressure and H' is Henry's constant. For the purpose of this document we will consider the Henry's constantto be in pressure (atmospheres) units and that the total pressure is always very close to one atmosphere:

y = H x

The numerical values of H and H' are the same since P=1.

The following table shows some estimates of values of H for some common VOCs:

Compound H Temperature deg. C

trichloroethylene 500 20perchloroethylene 800 201,1,1 trichloroethane 700 20chloroform 180 20methylene chloride 125 20o-dichlorobenzene 71 20BTX 240 20 (other data indicate a value of

400 for H at 20 deg. C)carbon tetrachloride 1183 20methyl-ethyl ketone 1.7 20methyl-isobutyl ketone 7.1 20ethylbenzene 389 201,1,2,2 tetrachloroethane 20 20

These numbers appear to be conservative and could be used for design. Always consult Jaeger Productsfor designs at different temperatures. Jaeger will assist you in the estimation of the correct value of Hbased on a very extensive, field-proven data base.

JPI\1996ARST.DOC

AIR TO WATER RATIO

The ratio of the volumetric air flow to the volumetric water flow (CFM air/CFM water) is commonly referredto as the air to water ratio. The recommended value of the air to water ratio varies for different VOCs inaccordance with variations of the Henry's constant and the hydraulic stability of the column. Considerationsof whether off-gas treatment will be required also affect the selection of the design air/water ratio sincelarger air volumes will be significantly more expensive to handle in an off-gas treatment unit.

The air to water ratio (A/W) is related to the stripping factor S by the following equation:

S = 0.00075 H (A/W)

Normally, stripping factors of between 10 and 15 work well in cases where no off-gas treatment is requiredso that as an initial guess one can use

A/W = 16, 000/H

to estimate the air to water ratio needed for the column. This guideline works well for components liketrichloroethylene and BTX. It should not be used strictly for components that have Henry's constants below100 since it would yield unreasonably high values of A/W. In reality, values of A/W above 500 are rare inVOC applications.

Lower stripping factors and air to water ratios are mandated when off-gas treatment is considered. Designsusing stripping factors between 3 and 7 are not uncommon. In these cases the design procedure for the airstripper becomes more sensitive and delicate. Please consult with Jaeger when your application requiresstripping factors below 10.

MASS TRANSFER EFFICIENCY

There are several ways to predict the mass transfer performance of a given packing under the requiredoperating conditions. In every case, the required height of packing will be the product of HTU times NTU.(Packed height = number of transfer units * height of a transfer unit). The values of HTU and NTU to beused will depend on the stripping factor and the inherent efficiency of the packing. NTU is a variable thatrelates exclusively to the stripping factor and the degree of removal. HTU relates to the stripping factor,liquid load, and the packing efficiency.

The equation to determine the value of NTUol (number of transfer units) for a VOC stripper is:

NTUol = (S/S-1) ln [ (1-1/S)xin/xout + 1/S]

S = stripping factor, xin = inlet concentration, xout = outlet concentration

At values of S = 12 or above, the previous equation can be approximated by

NTUol = ln (xin/xout)

To determine HTU, one can use a correlation or experimental data adapted to the conditions of the designand applicable to the packing being considered. Methods based on correlations can be fairly reliable ifapplied carefully to systems within the proven limits of the correlations. The best method currently availableto the public is a modification of the Onda method that was developed by the Separations Research Program

JPI\1996ARST.DOC

at The University of Texas at Austin. The use of the Onda method without modification is adequate in manycases and is also used extensively.

Experimental data is hard to come by and hard to validate but it represents the best basis for design. It isalways advisable to compare a design based on experimental data with a correlation method since this willprovide a good sense for the importance of the different variables on the design. Typical design values ofHTU derived from performance data on Jaeger Tri-Packs7 packings are shown in Table 2. The reader shouldunderstand that the selection of the proper value of HTU for design should be done by the packing supplier. Jaeger Products will provide guaranteed designs for VOC strippers so be sure and consult with us whenfinalizing a design.

TABLE 2HTU VALUES FOR JAEGER TRI-PACKS7 IN VOC STRIPPING

All data for trichloroethylene at 60 deg. F

PACKING A/W ratio Liquid load HTU

2" Jaeger Tri-Packs7 60 25 gpm/ft 2.9 ft2" Jaeger Tri-Packs7 90 35 3.2

3 " Jaeger Tri-Packs7 60 25 3.43 " Jaeger Tri-Packs7 90 35 3.7

EXAMPLE DESIGN

The accompanying chart gives quick estimation of the packed tower requirements for some of the easierVOC removal applications. In order to use the chart:

a) Determine the necessary air/water ratio for the selected contaminantA/W = 16 000/H (with H is the Henry's Law constant in atmospheres).

b) With the total water flow to be treated in a single stripper and the desired removal, read thenecessary packed height.

c) The packing size (Jaeger Tri-Packs7 1, 2, or 3.5 inches) is specified next to the column diameter.

NOTE: CONSULT JAEGER FOR DESIGNS REQUIRING HIGHER REMOVALS, LOWER AIR/WATER RATIOS,LOWER TEMPERATURE, OR DIFFERENT LIQUID LOADING RATES THAN THOSE SPECIFIED IN THE CHARTS

LIQUID AND GAS DISTRIBUTION

The performance of an air stripper is in many cases wrongly related only to the packing itself. In reality, thepacking performs only as well as the initial liquid and gas distribution allows it to. Badly designed liquiddistributors and inlet air nozzles are the most common problems found in non-performing air strippers. Careshould be taken to design and install proper distribution devices in the stripper. For this reason, JaegerProducts also offers a complete line of liquid and gas distributors to be used in air strippers.

The design and selection of a proper distributor is not simple and cannot be detailed here. For Jaeger tosupply a performance guarantee on a VOC stripper, we would have to supply the internals as well as thepacking or at least have access to review the drawings prior to installation. Contact Jaeger for complete

JAEGER PRODUCTS, iNC.HIGH PERFORMANCE TOWER PACKINS

AND COLUMN INTERNALS SELECTION CHART FOR AIR STRIPPERS FOR VOC REMOVAL(281) 449-9500 (for estimation purposes only)

Fax (281) 449-9400(800) 6778-0345

Number in table indicate required packed bed height in ft for a given removal, and air/water ratio. * Recommended packings

Air to water ratio ----------------------------------------------------------- 20 40 60 80 100 150 200Water flow (GPM) Tower ID (in.) Removal (%)

10 14 90 8' BED 8 8 8 7*(1" TTRIPACKS) 95 10 10' BED 10 10 9

99 16 16 16' BED 16 15

50 16 90 8 8 8 8 7*(1" TTRIPACKS) 95 10 10 10 10 9

99 16 16 16 16 15

100 *(2" TTRIPACKS) 24 90 9 9 9 9 9 9 995 11 11 11 11 11 11 1199 17 17 17 17 17 17 17

200 *(2" TTRIPACKS) 36 90 9 9 9 9 9 9 995 11 11 11 11 11 11 1199 17 17 17 17 17 17 17

300 *(3 1/2" TTRIPACKS) 48 90 10 10 10 10 9 9 995 12 12 12 12 11 11 1199 18 18 18 18 18 18 18

500 *(3 1/2" TTRIPACKS) 60 90 10 10 10 10 9 9 995 12 12 12 12 11 11 1199 18 18 18 18 18 18 18

750 *(3 1/2" TTRIPACKS) 72 90 10 10 10 10 9 9 995 12 12 12 12 11 11 1199 18 18 18 18 18 18 18

1000 *(3 1/2" TTRIPACKS) 94 90 10 10 10 10 9 9 995 12 12 12 12 11 11 1199 18 18 18 18 18 18 18

HOW TO USE THIS TABLE: 1) Calculate the required air/water ratio for the component of choice: A/W = 16,000/H where H is the Henry's constant in atmospheres. 2) At the design flow rate and rewuried percent removal, read the required packed bed height under the specified A/W RATIO.

size in parenthesis. Numbers to be used only when stripping factor is above 10 and temperature above 55 deg. F. Consult Jaeger Products for values outside the range of the table.

CONTACT JAEGER PRODUCTS FOR A GUARANTEED DESIGN

JPI\1996ARST.DOC

designs on the optimum internals for your VOC stripper.

AIR POST-TREATMENT

The air emitted from an air stripper frequently needs to be treated to remove the VOC before the air isexhausted to the atmosphere. This is commonly accomplished by gas phase carbon adsorption or by directcombustion. The size and cost of the post-treatment equipment is highly dependent on the air rate so thatthere is a great incentive to design air stripping systems with as little air flow as possible. This makes thedesign of the stripper very critical since all safety factors tend to be removed. Careful evaluation of theproposed air stripping system at low air flows is required and Jaeger can be of assistance here.

HOW CAN JAEGER HELP YOU IN AIR STRIPPING APPLICATIONS?

Jaeger Products has extensive experience in the successful design of air stripping systems for organicremoval and recovery. No other mass transfer equipment manufacturer has the number of successfulinstallations. Jaeger s mass transfer products have become the standard in air stripping installations.

Our engineering staff can provide you with a complete process design, with the necessary engineering tospecify the stripping column in detail, and supply you with all process specifications for the peripheralequipment as illustrated in Figure 1. Our data base is very extensive and chances are there are very feworganics we have not tackled. We can simulate and optimize a complete air stripping and off-gas treatmentunit using the most advanced and comprehensive models. Our calculations will account for unusualvapor/liquid equilibria and will incorporate the best mass transfer efficiency rating methods available.

We have a complete line of packings, trays, and tower internals that can satisfy any air stripping needs. Theperformance of the system depends heavily on the correct internals selection as well as on a good processdesign; Jaeger can assist you with both so that total responsibility is easily identified. Although we normallydo not provide turn-key systems, we can direct and/or assist you in such a project. We can also put youin contact with a systems manufacturer that would provide a turn-key project with Jaeger engineering andhardware.

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SOME PITFALLS IN THE DESIGN OF AIR STRIPPERS

Reliability of Henry's constant data

The design of an air stripper depends heavily on the value of the Henry's constant for the target VOC. Theliterature abounds with experimental values of Henry's constants but unfortunately they do not always agreewith other published values or with values apparent from field trials and installations.

The Henry's constant is a thermodynamic variable that depends only on temperature and composition. Manymisguided efforts have tried to link the value of Henry's constant to mass transfer performance by regressingvalues of H from actual stripping data. This is wrong and dangerous since a fundamental thermodynamicvalue is held dependent on totally unrelated things such as liquid distribution, packing shape and size,column levelness, gas distribution, instrument accuracy, etc.

Values of H derived in such manner should never be used for design since they will prove unreliable inscale-up and will undoubtedly supply wrongful answers.

The correct procedure is to determine values of H from good experimental data of volatility and solubilityand to determine column efficiency separately using the proper value of H. Consult with Jaeger for properand rigorous experimental data interpretation. No other mass transfer company has the level of experienceJaeger has in using field data to come up with reliable design. There is no magic involved, only the use ofgood engineering concepts and thermodynamic principles.

Designs at very low air/water ratios

The necessity of off-gas treatment in air stripping installations has created a very strong incentive fordesigning air strippers at very low air flows. Very often the cost of the treatment of the exhaust airdominates the total treatment cost and is roughly proportional to the air flow.

When the air/water ratio is very low (i.e., stripping factors below 3), the design of the air stripper becomesvery sensitive to the value of the Henry's constant and the packing efficiency. Inaccuracies in the estimationof these variables of less than 20% can result in calculated packed bed depths that are inadequate by 80to 120%. It is in these cases where Jaeger Products, Inc., can help you determine the exact towerdimensions and requirements for reliable results. Crude Henry's constant extrapolations and mistaken masstransfer data interpretation can lead to grossly under-designed strippers. Consult with Jaeger Products, Inc.when there is a need to minimize air flows. We have more and better experience than anyone in thebusiness when it comes to tough designs.

Designs at very low concentrations in the effluent

A pervasive problem in the application of air strippers is the fact that removal of VOCs to very low levels,say below 0.5 ppb, becomes very difficult if attention is not paid to the detailed design and layout of the airstripper. Very slight defects in the gas and liquid distribution can lead to strippers not performing properly. Many designers add extra safety factors to the packed bed depth to compensate for perceived problemsat very low concentrations. The fact is that the best way to ensure proper performance at such low levelsis to ensure that the gas is distributed properly at the bottom of the tower. Guidelines on the layout of thebottom section of the stripper that set the minimum distances between the gas inlet and the bottom of thepacked bed are very important as is the use of proper feed pipes and spargers. The air entering the towerfrom outside needs to be totally free of VOCs for these applications to be successful and measures mustbe taken to ensure that the air entering the tower is never contaminated with any of the exhaust air.

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Cooling of the effluent water

Contact between the air and the water in an air stripper leads to cooling of the water by evaporation. Theamount of cooling depends largely on the water temperature, the air temperature and humidity, and theair/water ratio. Very efficient packings for VOC removal will also be very efficient cooling devices leadingto potential problems, especially in cold environments. These problems can range from lack of the desiredremoval caused by low water effluent temperatures to actual freezing of the water in the stripper. It isalways advisable to check the heat balance of the system for the cold conditions to ensure that freezing hasbeen properly addressed.

Misuse of safety factors in design

Many performance specifications for air strippers include health safety factors in the inlet and outletconcentration requirements. Typically the effluent concentration is set at the detection limit of the VOC inquestion and the inlet concentration is an absolute maximum that will very rarely (if ever) present itself.

Unfortunately, some mass transfer device suppliers take advantage of this fact when presenting a designin a competitive situation. Their designs will be based on removals that are below the specified ones withthe "hope" that the specified levels will never present themselves and the performance of the stripperchallenged. Under-designs, such as these, give the false and dangerous impression that a packing is farbetter in performance than others. In reality, these suppliers are cutting corners at the expense ofperformance reliability of the stripper and are providing designs that do not meet the specified removal butonly meet the effluent characteristic.

The user must be aware of this practice and protect himself or herself against it by strongly requiring andverifying that all calculations and designs be based on meeting the specified outlet concentration, and thespecified inlet concentration. Jaeger Products, Inc. will always offer you a sound, safe, and economicaldesign without second-guessing the meaning of the specification. This is why we will always guaranteeour air stripper designs.

Fouling and plugging of packings

Paradoxically, the high mass transfer efficiency provided by the packing in a stripper promotes thedeposition of insoluble metal oxides and bacterial growth. Packings with high surfaces will be more efficientbut will promote fouling as well. There is no magic cure for fouling. The composition of the water, theirrigation and aeration rates, and the operating temperature have much more to do with how rapidly a towerwill foul than the type of packing used.

If the contaminated water contains free iron or other minerals, the action of the stripping air could causesome of these compounds to precipitate out and foul the packing media. Organic contaminants promotebiological growth that accentuates the fouling problem. The degree of fouling is usually a function of severalfactors 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 growthwill be accelerated. Algae formation creates an excellent base for mineral deposits, such asiron, manganese, and calcium.

2) A packing that stays completely and continually wet, thereby constantly washing itself of theparticipate, seems to resist fouling and plugging. Fouling and plugging can also be

JPI\1996ARST.DOC

accelerated by poor initial liquid distribution.

The reality is that all air strippers will eventually lose some of their efficiency and capacity due to foulingif the water is not pre-treated before entering the tower. The degree of fouling as well as the amount of timefor the fouling to affect the performance of a stripper is a function of all of the above factors plus otherunique characteristics of a particular site. It must also be noted that in many cases the fouling process isso slow that a contaminated site is essentially cleaned before fouling is a problem.

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. Ourengineers will properly select the right internals for your tower to assure good liquid distribution and will alsodetail the recommended pre-treatment and maintenance options. Pre-treatment involves the continuousaddition of a chemical or compound to the water to keep the minerals from precipitating and to preventalgae build-up during the stripping process.

All packings foul and, we assure you, the solution to your fouling problem can be found in good maintenancepractices, good monitoring of your process conditions, and good overall process design. A fouling problemwill not be resolved by trying a different packing unless important compromises are made in mass transferefficiency. Severely fouled packed beds are inefficient and cause high pressure drop. They can also be verydangerous since support plates are generally not designed to handle the weight of packing heavily ladenwith inorganic salts. In some extremes, the weight of the packed bed can increase by a factor of 10 or moreas the packing fouls.

What can you do to minimize your risk of fouling?

Operate at higher water loads to eliminate dead spots in the packingManipulate your water chemistry by pH adjustmentOptimize liquid distribution in the bedPre-treat your feedClean and maintain packing frequently (at least every year)

There are some fairly effective "in-situ" cleaning techniques that can be very helpful as long as the bed isnot fouled too severely and it has remained wet. One method is to clean while continuing to operate thetower by pre-treating the feed water with a slight overdosing of our chelating agent. This excess chemicalwill break the structure of the oxidized minerals into colloidal suspension and wash them away with thetower discharge. The rate of clean-up is a function of water chemistry, amount of fouling and percent of$clean-up# dosage. Rates of 110% typically clean moderately fouled towers in 90-120 days.Do not let anyone tell you that there is such a thing as a "perfectly anti-plugging" packing. Such a thing iscalled something else, maybe a tray or a spray column, or maybe just wishful thinking. Jaeger Products,Inc. can assist you in analyzing an existing or potential fouling problem and can provide you with a viablesolution. Jaeger is the only mass transfer equipment supplier that has the in-house capability and expertiseto tackle a tough fouling problem.

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GUIDELINES FOR EXPERIMENTAL PROCEDURES AND HARDWARE SET-UP FOR AIR STRIPPINGTESTS OF VOCs FROM WATER

The set-up of a test column to perform air stripping experiments to evaluate the removal of VOCs from waterunder different conditions has to be looked at carefully if repeatable and reliable results are to be obtained.When the objective is to measure commercial packing performance and to make comparisons in efficiency,the proper column set-up becomes critical.

Of equal importance are the experimental and analysis procedures that need to be followed in order toobtain reliable data. Care has to be taken in how the feed and effluent samples are handled betweencollection and actual analysis. Plastic packings are very susceptible to the effects of aging andcontamination by inorganic depositions; the researcher must be careful to assure that all the packingsevaluated have undergone the same conditioning.

This report will address some of the most important hardware and experimental technique considerationsthat are relevant when evaluating plastic packings in VOC stripping applications.

Hardware Considerations

Stripping Column - Material. Stainless steel, aluminum, or plastic (preferably fiberglass reinforced polyester)should be used in test columns to prevent the effects of water-induced- corrosion. If a plastic column isselected, then care should be taken so that the packed bed section of the column is not exposed to sunlightsince this will harm plastic packings and will promote biological activity. All other internals should be eitherstainless steel, aluminum, or plastic.

Stripping Column - Size. The column diameter should be at least nine times the nominal diameter of thepacking under study. A design with ratio of less than 9 to 1 could be subject to severe maldistribution,channeling and wall flow effects that would translate into poor performance. Furthermore, below this ratio,different packings would exhibit different degrees of performance deterioration so that comparisons madeunder these circumstances cannot be extrapolated to larger columns. Some packings with nominaldiameters of 3 or 3.5 inches exhibit performances similar to 2 inch packings. In these cases, it is probablyadequate (but not preferable) to test such packings in columns as small as 18" in diameter.

The packed height selected for the experiments should be the same for all the packings tested, sincedifferent packings have different abilities to maintain liquid distribution so that their local mass transferefficiency is depth dependent. For commercial size packings, a packed bed should be at least 5' deep andshould not exceed 25'. A 10' bed is considered standard. Care has to be taken to ensure that the columnis not so deep that the outlet concentrations go below the minimum reliable detection and quantificationpoint.

Liquid Distribution. The quality of liquid distribution in an experimental set-up is of critical importance to thereliability of the data for scale-up or for performance comparison. Different packings react differently to initialmaldistribution and these effects are almost impossible to quantify. This means that the use of an inferiordistributor will have different effects on different packings. Usually, the more modern, high efficiency andlow pressure drop random and structured packings, are affected more severely by initial maldistribution thanthe older random packings.

Drip pan or ladder type (perforated pipe) distributors are probably the best type for experimental work sincethey can offer substantial turn-down, if enough head is provided. A third choice would be the spray cone,as long as extreme care is taken to ensure that the nozzles deliver solid and homogeneous cones (which

JPI\1996ARST.DOC

by the way, is rarely the case).

The geometric coverage provided by the distributor should always be in excess of 80%. This means thatat least 80% of the cross section of the column receives the average liquid load. Manufacturers of liquiddistributors can quantify this and should be consulted when procuring a distributor. The pour point densityshould be at least of 5 points/square foot and ideally around 10 points/square foot. It seems that there islittle effect of the pour point density beyond 10 points/square foot.

The liquid distributor should be installed so that mass transfer above the packed bed is minimized so thatone sees the performance of the bed without the end effect caused by the distributor. This is a cleardisadvantage of the spray nozzle configuration since the angle of the spray cone has to be minimized andstill provide homogeneous coverage. The distributor should not be located more than three feet from the bedand ideally, this distance should be on the order of 12" or less.

Gas Distribution. This particular issue is often disregarded in the design of experimental strippers and canbe of extreme importance. Unfortunately, few guidelines exist for the design of effective gas distributors.A good gas distributor will have a pressure drop on the order of 1/2 to 2 inches of water but can detractfrom the capacity of the packing. The best way to ensure adequate gas distribution is to measure the gasvelocity profiles in the empty column, equipped only with the support plate, and if the profiles aresymmetrical, proper gas distribution can be assumed.

The location of the gas inlet nozzle should be perpendicular to the column axis and not tangential. It shouldbe located at least one column diameter or 20" below the support plate (whichever is less).

Bed Sampling. Intermediate bed samplers, when obtained properly, can be very valuable since they canprovide insight into liquid distribution issues and into how the performance of the packings change with beddepth. Unfortunately, the insertion of a bed sampler into the column has to, by necessity, disturb the liquiddistribution characteristics of the bed. The design of the intermediate bed samplers has to be done verycarefully, particularly in small (36" and less) diameter columns where high efficiency packing is to be used.Intermediate samplers should not be used at distances of less than 5' and the diameter of the samplerinserted into the bed should never exceed 1/2". The length of the sampler should be the same as thediameter of the column so that a representative sample can be obtained. Any other arrangement will onlyintroduce a maldistribution.

The sample to represent the bottom of the bed should always be taken at the bottom of the bed itself andnot in the sump below the air inlet if there is the opportunity for mass transfer to occur below the bed. Ifproper care has been taken in the placement and design of the distributor, the top sample can be taken fromthe distributor itself or from the feed line to it.

Wall Flow. The only important considerations to prevent the effects of wall flow are the proper selectionof the column to packing diameter ratio and the vertical alignment of the column. Trivial as it may sound,checking the vertical alignment of the column is usually disregarded. Misalignment can have an importanteffect on liquid distribution. The use of wall wipers is not necessary and can even be damaging to theperformance of high efficiency packings since they can introduce maldistributions the same way badlydesigned samplers can.

Experimental Procedure

Packing Installation. Always follow the recommendations of the packing manufacturer with respect toinstallation and storage. Plastic and ceramic packings are delicate and should be handled with care.

JPI\1996ARST.DOC

Packing Aging. It has been amply demonstrated by various researchers that the performance of plasticpackings changes with time due to changing surface characteristics and the slow loss of moldingcompounds used in the fabrication of the packing. The surface of a plastic packing suffers some changeswhen first put in service. It has been noted that the mass transfer performance of a plastic packing improvesas much as 20 to 30% over a period of about 150 hours of operation. Beyond that, there is littleimprovement in mass transfer efficiency and some loss of capacity and increased pressure drops can befound due to biological growth and inorganic salt deposition. At a minimum, plastic packings should betested after about one week of being "aged" in water since it appears that the changes in the surfacecharacteristics are due to the effects of water swelling and roughing the polymer.

When trying to make performance comparisons among different packings, it is very important that allpackings considered are "aged", or for that matter, "not aged" similarly.

Variables to be Measured and Peripheral Instrumentation. Air and water flowrates, inlet and outletcompositions, water and air temperatures, and pressure drops are the most important process variablesto be measured. The importance of properly measuring inlet and outlet water temperatures cannot beunderscored enough since the effects on the equilibrium constants can be very great.

The water chemistry can also be of great importance since it affects the equilibrium and can have an effecton packing performance through the deposition of salts or the existence of dissolved gases. The best wayto evaluate packing performance, particularly for comparison purposes, is by using synthetic, controlled feedstreams. As always, the calibration and accuracy of all instrumentation should be checked frequently sincemany instruments, especially flow, composition and pressure drop meters, can drift appreciably.

Sample Taking. All samples should be taken in such a way that the exposure to air is minimized. Sealedsample bombs should be used and filled to the limit to reduce the potential for exposure to air. The samplesshould be chilled immediately before transferring to the analysis vials. Once again, care has to be taken sothe contact with air is minimized in this step. The materials of construction of all sample handling equipmentshould be stainless steel or glass. The amount of polymeric materials used in things such as gaskets, lids,septa, syringes, etc. should be kept to an absolute minimum since VOCs absorb easily into them. Samplesshould always be treated to prevent bio-degradation and the formation of other products after they havebeen taken.

The useful life of VOC samples has been found to be 48 hours at best. After that time, variations areobserved that can be attributed to biological activity, absorption into the components of sample vials, andvolatilization. It is not recommendable to keep samples longer than 48 hours before analysis, and ideally,one would perform the analysis immediately after the sample is taken. Samples should never be frozen.

Analytical Techniques. Gas chromatography is the best method for VOC analysis in water. The sampleconcentration techniques that can be used vary and are usually the greatest source of error. EPA considerstrap and purge and extraction techniques as the ones of choice but they involve a great deal of samplehandling and offer numerous opportunities for introducing errors. We believe that a technique that reducessample handling by humans, such as head space analysis, can be more reliable and repeatable.

JPI\1996ARST.DOC

Calibration curves for the analytical instrument set-up should be run as frequently as possible, and actualexperimental sample analysis should always be mixed with analysis of known standard samples to ensureaccuracy. One should always be aware of the detection and reliable quantification limits of the technique.This limit is not the same as the detection limit in many cases.

The following pages are provided to help expediate any technical or sales information which you mayrequire. Please copy, fill out and fax any pertinent information and we will be glad to assist you with adesign.

JPI\1996ARST.DOC

To: JAEGER PRODUCTS, INC. Date: 1611 PEACHLEAFHOUSTON, TX 77039

(800) 678-0345Fax (713) 449-9400

GENERAL INQUIRY QUESTIONNAIRE FOR

1. My name isFirst Name M.I. Last Name

I am with:

Company Name:

Department:

Address:

Phone Number:

Fax Number:

2. We need:

Quotation

Design Information

For Packed Column Internal Products:

Column Packing - Materials of Construction

Mist Eliminator - Materials of Construction

Liquid Distributor - Materials of Construction

Packing Support Plate - Materials of Construction

Other - Materials of Construction

3. Our inquiry is for: Purchasing purposes

Planning

Study

Other

The Column Internals for: Retrofitting existing column(s)

New Packed Column(s)

Other

4. The dimensions of our existing Packed Columns are:

Column Diameter: Column Height:

Packing Height: Type and Size of

Existing Packing:

4a. This inquiry is for a new Packed Column.

Please determine packing height and diameter for us.

We have determined the packing height and diameter to be

and , respectively.

JPI\1996ARST.DOC

The Packed Column is for the following application(s):

Absorption, Desorption/Stripping, Odor Control, Fume Control

Biological Treatment

Other

6. The design column operating conditions are as follows:

Gas Liquid

(In) (Out) (In) (Out)

Vol. Flow rate, ACFM or SCFM GPM

Temperature, F

Pressure, Psia

Density, Lbs./Cu. Ft.

Flowrate, Lbs./Hr.

Viscosity, cp

Gas Composition (% by Vol.)

Liquid Composition (% by Wt.)

Allowable pressure drop (psi or $H2O#)7. We need demister(s) for the column. The mist droplet size distribution and required demisting efficiency are:

Size () Distribution (%) Demisting Eff. (%)

8. Remarks

John P Halbirt

John P Halbirt

striper design: and2: quote:

column3: column5: column6: column2: other: materials9: materials6: materials5: materials3: materials1: name/: name/address: date:

remarks: dem2: dem3: dem4: dis3: dis4: size2:

dis2: liquid23: 5:

other56: 25: