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Removal of benzene fromindustrial wastewater byvapor strippingJ. B. Phillips a ba Department of Chemical Engineering ,Tulane University , New Orleans, LA, 70118b Engineering Development Institute , 8627North 106th Street, Milwaukee, WI, 53224Published online: 15 Dec 2008.

To cite this article: J. B. Phillips (1995) Removal of benzene from industrialwastewater by vapor stripping, Journal of Environmental Science andHealth . Part A: Environmental Science and Engineering and Toxicology, 30:5,1075-1090, DOI: 10.1080/10934529509376250

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J. ENVIRON. SCI. HEALTH, A30(5), 1075-1090 (1995)

Removal of Benzene from IndustrialWastewater by Vapor Stripping

J. B. Phillips*Department of Chemical Engineering

Tulane University, New Orleans, LA 70118

Abstract

Four types of vapor stripping processes are designed in an effort toinvestigate methods to treat industrial wastewater in compliance withthe Environmental Protection Agency's recently promulgated BenzeneNational Emission Standards for Hazardous Air Pollutants (NESHAP)regulations. Each vapor stripping technology involves a unit operationproducing a benzene-enriched vapor stream and a wastewater streamcontaining less than 10 ppmw benzene, per the applicable regulation.Steam stripping, vacuum stripping, air stripping, and natural gas strip-ping technologies are designed and evaluated. It is concluded that: (1)steam stripping involves high operating costs due to the need to preheatthe feed; (2) vacuum stripping requires high capital investment becauseof the number of equipment items needed; (3) air stripping provides onlya partial solution to the problem, as it converts a water pollution probleminto an air pollution problem which must be addressed; and (4) naturalgas stripping is the solution with fewest disadvantages of those examined,and therefore is the recommended method.

* Until September 1995, address correspondence to: J.B. Phillips, Engineering DevelopmentInstitute, 8627 North 106th Street, Milwaukee, WI 53224.

1075

Copyright © 1995 by Marcel Dekker, Inc.

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Introduction

The Environmental Protection Agency's Benzene National Emission Stan-dards for Hazardous Air Pollutants (NESHAP) Regulations (40 CFR Part61) are spurring interest by manufacturers in the chemical, petrochemical,and petroleum industries in novel technologies to treat produced water andwastewater containing benzene [1]. Typically, benzene/water mixtures in suchfacilities require two-stage treatment consisting of: (1) physical or chemicalmeans to reduce the benzene level to less than 10 ppmw in a confined envi-ronment, and (2) biological treatment prior to discharge to a river or otherwaterway. Because biological treatment usually occurs in open ponds, theinitial physical or chemical treatment is necessary to prevent excessive evap-oration of benzene into the air. Although the secondary biotreatment is animportant part of the pollution control process, this work will concentrateon the initial treatment options required to reduce benzene concentration inwastewater to below 10 ppmw.

All four methods examined herein represent technologies which haverecently begun to emerge and gain attention as promising candidates forindustrial-scale treatment. Initial studies have been conducted on a numberof treatment options which may be adequate to ensure compliance with theapplicable regulations. The experimental work which has been conducted todate on the subject technologies allows the design and evaluation of equip-ment systems which can be put into industrial practice.

Steam stripping of benzene from an oil film is the subject of a study byBrodskii, et al. [2]. The study concluded that the stripping rate from an oil filmwas accelerated by 100-200% over that in a traditional bubble-cap tower, in-dicating that the stripping rate is strongly related to the surface area availablefor mass transfer. The study also developed a mathematical model to describethe performance of a thin oil film desorption unit, as it typically would oper-ate in a coking plant. In addition to surface area, the benzene stripping ratewas found to be dependent on the oil temperature, pressure, and steam feedrate. The dependence on surface area is the subject of a review by Huang [3].

Steam stripping also has been used to remove benzene and other pollu-tants from groundwater [4]. Solomon and Peterson demonstrated that ben-zene, toluene, ethylbenzene, xylene, trichloroethylene, pentachlorophenol,and 1,2,4-trichlorobenzene can be removed from groundwater through theuse of steam stripping, although the steam-to-wastewater ratio is high. Whilethe issue of stability was not addressed by Solomon, it should be noted that

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VAPOR STRIPPING 1077

failure to preheat the groundwater can cause unstable operation of a steamstripper. Other methods (i.e., alternatives to vapor stripping) for remedia-tion of groundwater have been evaluated in significant detail, but each of thebetter-known treatments has pronounced disadvantages [5].

Vacuum stripping of wastewaters also has been the result of some initialexperimental work. Khlevnoi, et al. [6] examined the flash vacuum strippingof hot coking wastewaters. It was found that atmospheric emissions of aro-matics were reduced, as were fixed nitrogen emissions. Moreover, the cool-ing which took place due to the evaporation process resulted in more efficientoperation of the downstream biotreatment process contained in the subjectsystem.

It has been demonstrated that air stripping can be used to remove ben-zene, toluene, w-xylene, /j-xylene, ethylbenzene, styrene, chlorobenzene, p-dichlorobenzene, and 3-pentanone from wastewaters [7]. The specific tech-nique used in the work is referred to as bubble column aeration, which al-lowed a unit operation approaching counter-current flow in that the air wassparged into the bottom of the column, and the water samples were removedfrom the water at the air injection point. The experimental setup allowedcalculation of Henry's Law constants:

y = K'x (1)

where:

y solute concentration in the vapor phase in equilibrium with the solution,g/L

x solute concentration in the solution, g/LK Henry's Law constant (dimensionless)

For benzene, a Henry's Law constant of 0.222 was reported. In the samework, increasing the ionic strength of the aqueous source phase was found toincrease the Henry's Law constant monotonically. The effect was likened toa "salting out" phenomenon. Conversely, addition of alcohols to the aqueoussource phase was found to decrease the Henry's Law constants. The Henry'sLaw constants for all of the nonpolar substances examined were of the sameorder of magnitude, but the Henry's Law constant for 3-pentanone was abouttwo orders of magnitude lower due to the polar oxo group.

Related work examined air stripping for the removal of benzene andother organics from water produced at offshore petroleum production plat-forms [8]. Benzene, toluene, butane, pentane, hexane, and cyclohexane were

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stripped from produced water by bubbling air through the water in a batchprocess. The Henry's Law constants for several substances were reportedin this work, among these being a Henry's Law constant for benzene of0.00549 atm • m3/mol. When this is converted to a dimensionless Henry'sLaw constant, as in the work by Harkins, et al. [7], a Henry's Law constant of0.231 is obtained. This is in good agreement with the value of 0.222 reportedby Harkins. The study by Fang and Lin [8] considered two different sourcesof organic pollutants in produced water: those found naturally as the wa-ter enters an offshore production platform, and those added at the platformto facilitate operability. Substances in the latter category include methanol,which is added as an antifreeze, and formic acid, which is used for its biocidicproperties. While air stripping was found to be effective in removing thosesubstances naturally present in produced water, it was not able to removemethanol, formic acid, acetic acid, ethanol, or isopropanol which are addedto the produced water at the platform. It is clear that those substances whichare removed most easily are the nonpolar ones, whereas substances contain-ing polar groups (hydroxyl or carboxyl) remain dissolved in the water. In thisrespect air stripping can be considered to have similar operability prospectsas steam distillation [9].

Similar work on the use of air stripping to remove trace concentrationsof volatile organics, and aromatics in particular, from groundwater was con-ducted by McFarland [10]. It was found that air stripping can be an effectivemeans to reduce the level of aromatics such as benzene, toluene, and xylene(BTX) in groundwater to below 10 ppb. Multiple air strippers in series wereable to reduce aromatics levels to below 1.0 ppb. The flow configuration inthis work was a packed tower, with groundwater being introduced at the topof the column and the air inlet at the bottom of the column. Such a coun-tercurrent flow operation allows the air-to-water ratio to be minimized. (Itshould be noted that any vapor stripping technology can be operated withcountercurrent flow in order to minimize the vapor-to-liquid ratio.) Typicaldesign parameters given by McFarland are shown in Table 1.

As mentioned above, other technologies have been tested for their ap-plicability to the removal of aromatics from wastewater. Schuckrow andPajak compared the efficacy of activated carbon adsorption, resin adsorp-tion, biological treatment (both aerobic and anaerobic), and stripping [11].Bench-scale testing of the technologies provided some valuable informationon which methods were the most promising. The results of their experi-ments indicated that granular activated carbon adsorption followed by ac-

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VAPOR STRIPPING 1079

Table 1: Typical air stripper design parameters [10].

Parameter Recom. min. Recom. max.

Col. diameter, cm (in)Col. height, m (ft)Packing height, m (ft)Air-to-water ratio, by volumeHydraulic loading rate,L/min/m2 (GPMa/ft2)Air loading rate,Std. m3/min/m2 (SCFM6/ft2)

10.0 (4.0)3.5(12)2.5 (8.0)50:1

200 (5.0)

30 (100)

48 (18)6.0 (20)4.5 (15)250:1

1200 (30)

150 (500)

aGPM: gallons per minute.6SCFM: standard cubic feet per minute.

tivated sludge treatment can achieve nearly complete removal of aromatics.Unfortunately, sustained removal of aromatics could not be effected undercontinuous operation. Similar operability problems were experienced by Sel-vakumar and Hsieh [12] in experiments conducted on adsorption of aromat-ics by biomass. These researchers found that although an inert microbialbiomass secondary sludge can adsorb aromatics initially, and that the ad-sorption follows the Freundlich isotherm, in time the sludge will release aro-matics into soil and groundwater, thereby preventing landfarm application.Activated carbon treatment of wastewater containing aromatics can be ef-fective [13], but high loadings or high volumes of processed wastewater thatwould be typically found in an industrial operation make such an alternativecost-prohibitive. Thus, it can be argued by default that vapor stripping is themost promising alternative for the treatment of industrial wastewater con-taining aromatics such as benzene.

The remainder of this work concentrates on the design of equipment sys-tems to implement the aforementioned four vapor-stripping technologies-steam stripping, vacuum stripping, air stripping, and natural gas stripping.The advantages and disadvantages of each variety of stripping process areevaluated, and a preliminary economic analysis of each process is conducted.

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Design Methodology

A. Steam Stripping

For the design of steam stripping systems, a process simulator was employed(ChemCAD II, Chemstations Software). The flash drum unit operation wasused to model a single-stage stripping process. For steam stripping, feeds tothe flash drum consisted of a water stream containing up to 1700 ppm ben-zene, and steam. For all stripping operations examined, an upper limit of1700 ppm for the benzene content of the wastewater stream was chosen be-cause this value represents the solubility limit of benzene in water at ambientconditions. If, in the field, the wastewater contained more than 1700 ppmbenzene, the excess could be removed by phase separation (i.e., skimming).The flash drum and any feed to it were specified to be at ambient pressure(1.0 atm). In the case of steam stripping, the feed temperature was 310 K(100°F) for the wastewater stream and 373 K (212°F) for the steam. Steamstripping could be conducted either in a single-stage operation, such as aflash drum, or in a multistage (i.e., packed) tower, with steam being addedat the bottom of the tower and wastewater being introduced at the top. Mul-tistage contacting typically allows the requisite separation to be performedwhile minimizing the vaponliquid ratio. The single-stage and multistage con-tacting configurations for steam stripping are shown in Figure 1.

B. Vacuum Stripping

The ChemCAD II process simulator also was employed to design vacuumstripping systems. The key parameter which determines the separation effi-cacy of a vacuum stripping system is the pressure. Ambient temperature andadiabatic operating conditions are specified, with changes in pressure causingchanges in the amounts of liquid and vapor product, and differing composi-tions of the products. As in the design of steam stripping systems, a flash drumunit operation can be used to predict the performance of a single-stage con-tacting operation, and a packed tower can be used to evaluate any improve-ment in performance that multistage contacting would produce. In contrastto steam stripping, vacuum stripping only requires one feed (the wastewaterstream) to the flash drum (or tower). If a packed tower is used, the wastewaterfeed is at the top stage. The single-stage and multistage contacting configu-rations for vacuum stripping are shown in Figure 2.

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VAPOR STRIPPING 1081

CondenserO/W Separator

Benzene

Water/Benzene

Steam

Water(Benzene-saturated)

Steam Stripper

Clean Water(<10 ppm Benzene)

Figure 1: Schematic of steam strippingprocess.

C. Air Stripping and Natural Gas Stripping

Laboratory data provided by Harkins, et al. [7] and Fang and Lin [8] allowedair stripping processes to be designed with a higher confidence level than de-signs based solely on simulated data. The Henry's Law constant for benzenewas employed to determine the liquid:vapor ratio required to perform the

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Condenser

Vacuum Pump

O/W Separator

-10kPaBenzene

Water/Benzene

Water(Benzene-saturated)

Vacuum Stripper

Clean Water(<10 ppm Benzene)

Figure 2: Schematic of vacuum stripping process.

separation. For air stripping or natural gas stripping, it is possible to removebenzene in a single stage by bubbling the vapor into a tank containing wastew-ater, or in a multistage operation by introducing the wastewater into the topof a packed tower and connecting the vapor inlet to the bottom of the tower.The single-stage and multistage contacting operations for air stripping andnatural gas stripping are shown in Figure 3.

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VAPOR STRIPPING 1083

Benzene Vapor in• Air or NG

Water/Benzene

Air or NG

ooOo oI T T !

CleanWater

Clean Water(<10 ppm Benzene)

Figure 3: Schematic of air stripping and natural gas stripping processes(single-stage contacting, right; multistage contacting, left).

Results

In general, it is recognized that multistage contacting operations are moreefficient than single-stage operations and are cheaper to operate. However,near-term compliance with applicable regulations may dictate the level of so-phistication which operating companies can afford to consider. Accordingly,the remainder of this work will concentrate on the design of single-stage sys-tems.

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A. Steam Stripping

The wastewater feed to the steam stripper contained between 50 ppm and1700 ppm benzene. (The 50 ppm figure was based on a case study.) Vary-ing amounts of steam were added, and the concentration of benzene in theliquid product stream was examined. It was anticipated that progressivelyincreasing the steam-to-wastewater ratio on the input to the stripper wouldresult in a monotonic decrease in the benzene content of the liquid effluent.However, the process simulator predicted unstable operation of the strippingunit unless the wastewater was preheated to near its boiling point. Preheat-ing was judged to be uneconomical, because of the capital costs associatedwith installing a preheater, as well as the operating costs involving the inputof considerable energy into a wastewater stream. Thus, further investigationinto steam stripping was not pursued.

B. Vacuum Stripping

Initial simulations of vacuum stripping systems concentrated on the removalof benzene from a wastewater stream containing 50 ppm benzene. Using asingle-stage separation system, the pressure in the flash drum was varied untilthe effluent liquid stream contained less than 10 ppm benzene. This occurredat a pressure of about 9 kPa (slightly less than 1.5 psia). The relationship be-tween flash drum pressure and liquid effluent benzene concentration is shownin Figure 4. It is observed that the level of benzene in the liquid effluent is astrong function of flash pressure, decreasing monotonically with decreasingpressure.

The fraction of benzene removed from the subject wastewater stream isalso strongly a function of pressure. Figure 5 shows that the percent benzeneremoved initially increases rapidly with pressure reductions from 9.65 kPadown to approximately 9.0 kPa, whereas additional benzene removal requiresmore drastic pressure reductions. As benzene removal approaches the 80-90% range, the water-to-benzene ratio in the overhead stream is seen to in-crease dramatically (Figure 5). Reducing the benzene content in the sub-ject wastewater stream to 10 ppm will result in an overhead stream water-to-benzene ratio of approximately 20, and additional reductions in the benzenecontent of the wastewater stream result in even higher overhead water loads.This is significant because the overhead processing equipment must be sizedto handle the vapor loads for both water and benzene. (See Figure 2.)

Capital costs for vacuum stripping are anticipated to be high. In addi-tion to a stripping tower or flash drum, an overhead condenser and vacuum

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8.250 8.500 8.750 9.000 9.250

Pressure (kPa)

9.500 9.750

Figure 4: Effect of pressure on liquid effluent benzene level in a vacuum strip-ping process (dashed line indicates 10 ppm regulatory threshold).

Kg4) c

o

80-

60-

40-

20-

0-

~ — ^

Water/Benzene Ratio -* .

1 1

Percent BenzeneRemoved from

^ \ ^ Wastewater

\

\\

1 1

8.250 8.500 8.750 9.000 9.250

Pressure (kPa)

9.500 9.750

Figure 5: Effect of pressure on benzene removal, and on overhead water-to-benzene ratio in a vacuum stripping process.

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pump are needed. Both of these equipment items must be sized to processthe total vapor flow. In particular, sizing a vacuum pump to handle a benzenevapor and water vapor stream in which the benzene vapor is only-5% of thetotal (Figure 5) will cost about six times as much as a unit which only wouldneed to process the benzene vapor. (Based on the power law cost estima-tion method discussed and analyzed by Garnett and Patience [14].) Due tothe high associated capital costs which are expected to be incurred up front,further investigation into the design of vacuum stripping systems was not pur-sued.

C. Air Stripping and Natural Gas Stripping

The experimental data reported by Harkins, et al. [7], and Fang and Lin [8]allowed calculation of vapor-to-liquid ratios for air stripping and natural gasstripping to specified target values. Using a wastewater feed stream contain-ing 1700 ppm benzene (the saturation limit), effluent benzene levels werecalculated as a function of the inlet vapor-to-liquid ratio. The results, shownin Figure 6, indicate that a molar vapor-to-liquid ratio of approximately 0.57:1will result in an effluent benzene level of 10 ppm. (Conversion to units com-monly used in industrial operations yields a volumetric ratio of just over100 SCFM gas per GPM liquid.) While it is expected that there would bedifferent mass ratios for air stripping vs. natural gas stripping, the volumetricratios and the molar ratios should be the same for the two methods.

This leaves the question of whether to specify air stripping or natural gasstripping as the technology of choice for removing benzene from industrialwastewater. A number of factors need to be considered in making this deci-sion. In air stripping operations, the expected products would be (relatively)clean wastewater and an air-in-benzene mixture. Treatment of the air-in-benzene mixture can be problematic. In essence, air stripping has converted awater pollution problem into an air pollution problem. One possibility wouldbe to pass the vapor through carbon canisters [13], but there are a numberof disadvantages to this approach. First, the cost can be high, because thebenzene-to-carboh ratio must be kept low in order to prevent benzene break-through. Beyond that, the carbon canisters must be regenerated periodically.If this is done with steam, then the steam must be condensed and treated. Airemissions of benzene also can result from the regeneration process. At best,it can be concluded that air stripping provides only a partial solution to theproblem at hand.

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VAPOR STRIPPING 1087

1.000-•

200.0

-•150.0

• • 1 0 0 . 0

•50.0

20 40 60 80

Benzene Level in the Aqueous Phase (ppm)

100

o

aDC

Figure 6: Relationship between inlet vapor-to-liquid ratio and liquid effluentbenzene level (dashed line indicates 10 ppm regulatory threshold).

Moreover, there is a safety concern associated with air stripping. If thecontaminatedwastewater contains significant concentrations of other volatileorganics, the vapor effluent may be explosive. Because air stripping is notinherently safe, alternatives should be given due consideration.

Natural gas stripping does not possess any of the aforementioned disad-vantages associated with air stripping. Effluents from a natural gas strippingprocess are the clean wastewater and natural gas which contains benzene.If the stripping is conducted with natural gas which was designated for fuelpurposes, then the benzene would be combusted along with the natural gas.Thus, natural gas stripping provides a complete solution to the benzene prob-lem, because the benzene is converted to carbon dioxide along with the nat-ural gas.

In addition, natural gas stripping does not produce potentially explosivemixtures of air and hydrocarbons, as might arise during the operation of an airstripping process. If the decision to operate an air stripping process was made,one use of the vapor effluent might be as the oxidant for a boiler or equivalent

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fuel-burning system. While this would result in conversion of stripped ben-zene to carbon dioxide, it does not address the explosion issue. In contrast,natural gas stripping produces a vapor effluent which is above the upper ex-plosion limit, and which remains so until it is introduced into a combustionchamber with oxidant (i.e., air).

Conclusions

Steam stripping, vacuum stripping, air stripping, and natural gas stripping sys-tems have been designed for the removal of benzene from industrial wastew-ater.

Steam stripping is likely to be of greatest use when preheating the wastew-ater is not required (i.e., stripping benzene from hot wastewaters). However,the unfavorable economics of preheating the wastewater suggest that steamstripping usually will not be the method of choice.

Vacuum stripping may be most effective when stripping only small quanti-ties of benzene is needed. Unfortunately, as the amount of benzene and wa-ter vapor produced in a vacuum stripping unit increase, so do the costs of theoverhead equipment. This factor becomes pronounced as the benzene levelin the wastewater is reduced and the water-to-benzene ratio in the overheadsis increased. Thus, capital cost considerations suggest that vacuum strippingalso will not be the technology of choice.

Air stripping and natural gas stripping can be accomplished simply andeffectively; both methods can reduce the benzene level in wastewater to under10 ppm. A molar vapor-to-liquid ratio of 0.57 is required in either case. Airstripping is disfavored because it merely converts a water pollution probleminto an air pollution problem, and because it produces a potentially explosivemixture inside the stripping equipment. Neither of these disadvantages isassociated with natural gas stripping. The results of this work suggest thatnatural gas stripping is the technology of choice for removing benzene fromindustrial wastewater.

Notation

BTX benzene, toluene, and xyleneGPM gallons per minuteNESHAP National Emission Standards for Hazardous Air PollutantsSCFM standard cubic feet per minute

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VAPOR STRIPPING 1089

K Henry's Law constantx benzene level in the liquid phasey benzene level in the vapor phase

References

[1] United States Environmental Protection Agency. National emissionstandards for hazardous air pollutants; benzene waste operations; finalrule; 40 CFR 61. Federal Register, 58(4):3072-3105,1993.

[2] E. V Brodskii, S. V. Gudimenko, and N. I. Potemkina. Height of transferunit in the desorption of benzene hydrocarbons from absorption oil infilm apparatus. Metall. Koksokhim., 73:94-97,1981.

[3] H. T. Huang. Environmental-pollution surface chemistry. Chieh MienHua Hsueh Hui Hsin, 1(3):2-6,1981.

[4] R. L. Solomon and D. J. Peterson. An innovative solution for steamstripping of volatile organic components and high boiling pollutantsfrom groundwaters with the aquadetox stripping system. In Proc. of the48th International Water Conf., pages 310-315. 1987.

[5] P. A. Hajali and L. W. Canter. Rehabilitation of Polluted Aquifers.NCGWR 80-12, National Center for Ground Water Research, 1980.

[6] I. S. Khlevnoi, V. A. Chimarov, A. L. Shtein, A. V. Popov, V G. Gor-shkov, and A. I. Bogdanov. Cooling of wastewaters. KoksKhim., 1:45-46,1988.

[7] B. Harkins, T. L. Boehm, and D. J. Wilson. Removal of refractory or-ganics by aeration. VIII. Air stripping of benzene derivatives. Sep. Sci.Technol., 23(l-3):91-104, 1988.

[8] C. S. Fang and J. H. Liri. Air stripping for the treatment of producedwater. J. Pet. Technol, May:619-624, 1988.

[9] R. Adams, J. R. Johnson, and C. F. Wilcox. Laboratory Experiments inOrganic Chemistry, pages 60-66. Macmillan, New York, 1970.

[10] W. McFarland. Air stripping removes petroleum from groundwater. Wa-ter/Engrg. Mgmt., May:48-52, 1989.

[11] A. J. Schuckrow and A. P. Pajak. Bench scale assessment of concentra-tion technologies for hazardous aqueous waste treatment. In Land Dis-posal: Hazardous Waste, Proc. Annu. Res., PB81-173882, pages 341-351.1981.

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[12] A. Selvakumar and H. N. Hsieh. Adsorption of organic compounds bymicrobial biomass. Int. J. Environ. Stud., 30(4):313-319, 1987.

[13] A. Yoshihara, T. Yoshihara, K. Okata, and S. Sakaguchi. Activated car-bon for water treatment. Japan, 71 25,490 (cl. C 01b), 23 Jul 1971, Appl.62 34,044, 1972.

[14] D. I. Garnett and G. S. Patience. Why do scale-up power laws work?Chem. Eng. Prog., 89(8) :76-78, 1993.

Received: November 1, 1993Accepted: April 16, 1994

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