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Superior performance by design

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

http://www.jaeger.com

JPI\1996STMT.DOC

Removal of Organics From Water Using Steam Stripping

Jaeger Products, IncHouston, Texas

Dilute mixtures of organic materials in water can be concentrated by a process known as steam stripping. The end products of such operation are a clean water stream almost devoid of organic materials, and ahighly concentrated organic stream suitable for recycle to a process or for disposal. The use of heat in theform of steam as a separating agent offers significant advantages over other methods, such as inert gas (air)stripping.

WHY USE STEAM STRIPPING?

Steam stripping for water clean-up is essentially a distillation process where the heavy product is water andthe light product is a mixture of volatile organics. These organics are present in the feed water in relativelysmall concentrations. The process of steam stripping takes place at high temperatures compared to airstripping, usually very close to the boiling point of water. Since the volatility of the organics is a very strongfunction of temperature, the high stripping temperatures inherent in steam stripping allow for the removalof heavier, more soluble organics that are not strippable with air.

Another very important feature of steam stripping is the fact that no off-gas treatment is needed, and thatthe only waste stream generated is a small amount of very concentrated organics. These are easily dealtwith by incineration, biological treatment, or recycled to process.

In summary, steam stripping is a good solution for wastewater streams that contain fairly soluble,non-volatile organics and where no off-gas stream is desired. On the other hand, steam striping doesnecessitate the presence of steam (or process heat) and would tend to be more capital intensive than airstripping. Ideal settings for steam stripping are oil refineries, petrochemical, and chemical plants.

WHAT IS STEAM STRIPPING?

A wastewater stream is heated and put in intimate contact with steam in a packed or trayed tower. Thecombined effects of the steam and heat, or temperature cause organic material to transfer from the liquidto the vapor phase. This material is then carried out with the vapor. As contacting proceeds down thetower, the wastewater becomes leaner in the organic material while the vapor phase becomes moreenriched as it travels up the tower.

Steam is injected at the bottom of the tower to provide heat and vapor flow. Clean water leaves the bottomof the tower. The wastewater is fed at the top of the tower and the steam leaves the top heavily laden withorganic material. This steam/organic combination is condensed and processed further as detailed in thenext few pages. The net effect achieved in the steam stripper and condenser is that a contaminatedwastewater and steam are injected into the tower and a clean water stream is obtained. A low-volume,

JPI\1996STMT.DOC

but concentrated water/organic mixture, is also obtained as a by-product.

WHAT DOES A TYPICAL STEAM STRIPPING UNIT LOOK LIKE?

The configuration of a steam stripping unit can vary depending on the characteristics of the organic materialto be removed, and on what is to be done with it in terms of disposal and recycle. As a minimum, a steamstripping unit will look like the unit depicted in Figure 1. It is important to note that heat recovery from thebottom product is necessary for economical operation. Operations at reduced pressure do not needrecovery exchangers, but operate at lower temperatures and larger steam rates. The towers also tend tobe a bit larger in vacuum operations.

Steam requirements for stripping vary with the operating pressure, the type of organic, and the degree oforganic removal/recovery. Further, steam requirements for heat balance purposes need to be accountedfor. A very important consideration in the design of a steam stripper is the fact that the column needs tobe capable of handling enough steam flow to operate without the benefit of the recovery exchanger. Thisfeature will be needed during start-up and when the exchanger is out of service for cleaning.

Some organic materials are not totally miscible in water and separate into a distinct organic phase whenthe concentration exceeds the solubility limit. Most aromatics and halogenated organics fall in thiscategory. Steam stripping applications for these types of compounds can be very effective, since a goodpart of the concentration of the organic can be accomplished in a decanter as indicated in Figure 2. In thiscase, the water layer is recycled to the stripping column for reprocessing. The design of the decanter posessome interesting questions since the water flow is generally significantly larger than the organic flow. Furthermore, in some cases (benzene, toluene, etc), the organic layer is the lighter of the two liquid phases. In applications involving halogenated organics, the organic liquid is heavier than water. Needless to say,good models to predict the phase behavior of the system in question are essential.

Figures 3A and 3B are refined versions of the flowsheet in Figure 2. These arrangements are needed whenbetter organic recoveries are needed from more dilute streams. The selection between Figure 3A and 3Bdepends solely on the equipment sizing. Figure 3A is used when required steam flows are larger (lessvolatile compounds).

Figure 4 is applicable when the organic material to be removed exhibits very high solubility in water. In thiscase, a refluxed distillation column is needed to achieve high organic concentrations.

Other variations on the same flowsheets shown above include the use of reboilers instead of direct steaminjection and operation at reduced pressure to reduce operating temperature.

JPI\1996STMT.DOC

CHEMISTRY, CHEMISTRY, CHEMISTRY!!

It is of crucial importance that the designers and operators of steam strippers understand the chemistry ofthe system, since lack of operability and maintenance problems occur frequently because of faultychemistry.

This is of particular importance in systems that include a multitude of pollutants, since interaction amongthem can be large. An excellent example is the typical mixed wastewater from a chemicals manufacturingfacility that includes inorganic acids, organic pollutants, and dissolved gases. As the gases, such as CO2and or NH3, are stripped, the pH of the water changes causing potential solids precipitation. This isaggravated by the fact that steam stripping temperatures often exceed the precipitation temperature forsalts, such as calcium carbonate.

The volatility of the compounds to be stripped is often affected by the water chemistry present. Accuratepredictions of the volatility are of extreme importance for proper stripper design; the operators of strippingsystems should always be aware that changes in the chemistry of the incoming water can affect theremoval efficiencies observed in the stripper.

Jaeger Products, Inc. has more experience than any other mass transfer supplier in tackling tough strippingproblems from the chemistry to the equipment.

SOME PITFALLS IN STEAM STRIPPING SYSTEM DESIGN.

Several aspects of the design of steam stripping systems are very crucial and not immediately obvious. First is the accuracy and reliability of equilibrium data. Steam stripping is a situation where the old reliableHenry's law just isn't applicable due to the broad concentration ranges, high temperatures, extensiveinteractions between components, and the existence of two liquid phases. The thermodynamic model ofchoice for steam stripping systems is one based on activity coefficients that can predict immiscibility. Nomodel fits this function better than the NRTL activity coefficient model (non-random two liquid modeldeveloped by Prausnitz and co-workers). Pilot and laboratory tests to establish the adjustable parametersin the NRTL model for the mixture in question are advisable, but solubility and vapor pressure data cansuffice as a good approximation.

Wastewaters can be very fouling, especially when the temperature is raised and inorganic salts precipitate. In typical steam stripping configurations, most of the fouling will occur in the recovery exchanger anddesign provisions are needed to allow for frequent cleaning. In the absence of a recovery exchanger, thebrunt of the fouling will be taken by the stripper itself. In such cases, the use of trays can be a way to avoidplugging even though packings would yield better performance characteristics. The use of sequesteringagents is also a good solution for reliable and lengthy operation.

Materials of construction need be some grade of stainless steel or a high performance plastic due to thevaried and changing nature of the water chemistry. Capital savings by use of lesser materials ofconstruction generally translate into severe problems and added expense later.

Start-up of any steam stripper requires heating of the feed water to the operating temperature in thestripper. This added heat has to be supplied in the form of steam at the bottom of the stripper. Designprovisions need to be made to accommodate this larger, but temporary, steam flow in the stripper. Thiscapability is also desirable to allow for continued operation while cleaning of a fouled recovery exchangertakes place.

JPI\1996STMT.DOC

Design at low stripping steam rates is desirable since it reduces the downstream processing requirements. Figure 5 illustrates how sensitive the process is to steam flow. Optimum designs require stripping factorsbetween 1.5 and 4. These stripping factors mandate more stages for separation and taller packed heights. Design under these conditions becomes very sensitive to the reliability of the equilibrium data and the masstransfer models. This is also the case where excellent packings and internals are necessary and wherevendor experience in design of steam stripping systems is invaluable.

THE STEAM STRIPPER AND OTHER COLUMNS IN THE SYSTEM.

The contacting devices in the steam stripping system are where the mass transfer takes place. They arevertical countercurrent vessels filled with a mass transfer device. In general, these devices are either sievetrays, random packings, or structured packings (the level of efficiency and capacity follows the same orderand so does their sensitivity to fouling).

The columns are also equipped with liquid distributors and support plates for the packing. In the case ofdeep bed requirements, intermediate liquid collectors and redistributors are also installed to ensure goodperformance. Figure 6 shows different combinations of internals that can be installed in a steam stripper. In most cases though, only combinations of trays and packings (with the associated internals) are used. Jaeger Products, Inc. offers all internal devices necessary for steam strippers and distillation columns ina variety of designs and materials to suit the application.

HOW CAN JAEGER HELP YOU IN STEAM STRIPPING APPLICATIONS?

Jaeger Products, Inc. has extensive experience in the successful design of steam stripping systems fororganic removal and recovery. Our engineering staff can provide you with a complete process design, andwith the necessary engineering, specify the contacting column in detail, and supply you with all processspecification for the peripheral equipment as illustrated in Figure 7. Our database is very extensive andchances are there are very few organics we have not tackled. We can simulate and optimize a completesteam stripping and solvent recovery unit using the most advanced and comprehensive models. Ourcalculations will account for unusual vapor/liquid equilibria and will incorporate the best mass transferefficiency rating methods available.

We have a complete line of packings, trays, and tower internals that can satisfy any steam stripping needs. The performance of the system depends heavily on the correct internals selection as well as on a goodprocess design; Jaeger can assist you with both so that total responsibility is easily identified. Althoughwe normally do not provide turn-key systems, we can direct and/or assist you in such a project. We canalso put you in contact with a systems manufacturer that would provide a turn-key project with Jaegerengineering and hardware.

JPI\1996STMT.DOC

THE JAEGER ADVANTAGE

Typical Steam Stripping Applications

Benzene removal from waste watersSour water (H2S and NH3) stripping

Phenol recoveryAcetone removal/recovery from waste waters

Oxygenate (MTBE, MEK) removal/recoveryRemoval of chloroform, bromoform and other halogenated organics from water

Removal of various organics from quench watersConcentration and organics recovery from leachates

Alcohol (ethanol, propanol, IPA, butanol) removal from waterSolvent recovery or removal (tetrahydrofuran, hexane, heptane)

Steam stripping facts

Capable of achieving very high removals and low effluent concentrationsMost economical removal technique at feed concentrations above 0.1% weight organics

Cost effective at feed concentrations as low as 200 ppmCan produce a re-usable concentrated product

Minimizes air emissionsReduces loads to incineration

Can be operated at vacuum or pressure depending on needs with little penaltyCan be made very energy efficient with heat recovery

Fouling is a continuous concern

Typical hardware for steam strippers

Sieve trays for fouling service (SS, Monel)Metal random packings for most applications (SS, Monel)

Plastic random packings for acid service (GFPP, Noryl, PVDF, Teflon)Metal structured packings for high efficiency/capacity (SS, Monel, Aluminum)

Column internals to include: distributors, redistributors, supports, and mist eliminators

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STEAM STRIPPING

Application information for design(Copy, fill out, and fax pertinent information and we will be glad to assist you with a design.)

Company

Person Responsible

Address

Telephone Fax

Your Reference Date

Description of problem, diagram:

JPI\1996STMT.DOC

Utilities Available:

Heating medium: Saturated steam Heat transfer oil Hot water

for steam: pressure psi, temperature °F

Coolant: Water BrineTemperature Inlet--summer °F, winter °F

Outlet--maximum °F, minimum °F

Mass balance for continuous rectifying column

Streams Feed F = lb/hDistillate D = lb/hBottom product B = lb/hSteam S = lb/h

Composition of streams or desired purities

Please place a check against the units in which the specification is made:

lb/h Weight % Mole fraction PPM PPB

Table 1A

Component (I)NameMole Mass

1 2 3 4 5 Total

Feed F

Distillate D

Bottom

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Data for separation problem

Column operated: continuously intermittently

Maximum bottom temperature tolerated °F, bottom pressure psia

or head pressure psia, and maximum pressure drop tolerated psi

Calculated pressure performance data (if separation problem has been calculated by the customer)

Number of theoretical stages in rectifying section (section D) =

in stripping section (section B) =

Total

Loading Nominal load = 100% (Load range) - %

Column head: Gas GD = lb/h M = lb/lbmol pD = psia

Liquid LD = lb/h ρL = lb/lbmol tD = °F

Bottom: Gas GB = lb/h M = lb/lbmol pB = psia

Liquid LB = lb/h ρL = lb/lbmol tB = °F

Feed liquid at boiling point vapor partly vapor flash %

Is there danger of precipitation? yes/nofoaming? yes/no

Column sizing by Jaeger by customer

Column internal diameter in.

Packing type total h eight ft.

Number of sections , rectifying section , stripping section

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PHYSICAL DATA OF THE PURE COMPONENTS

Table 2

Designation of components Units 1 2 3 4 5

Name of components -

Molecular Weight lb/lbmol

Density °F (liquid) lb/ft;

G L G L G L G L G LDynamic viscosity G: vapor _____ °F

L: liquid _____ °F

cp

Heat of evaporation Btu/lb

p/H t(°F)p or Hin

atmosphere

A= A= A= A= A=B= B= B= B= B=C= C= C= C= C=

Boiling point(Vapor pressure curves)of the pure components)orAntoine constantslog p = A - B/(C+t)orHenry=s constants (H)