Bing Zhou ,Bin Sun ,Wenjuan Qiu ,Ying Zhou ,*,Junqian He ,Xiao'ai Lu ,Hanfeng Lu ,*

1 Institute of Catalytic Reaction Engineering,College of Chemical Engineering,Zhejiang University of Technology,Hangzhou 310014,China

2 College of Environment,Zhejiang University of Technology,Hangzhou 310014,China

Keywords:Adsorption/desorption volatile organic compounds(VOCs)High humidity Macromolecule resin

ABSTRACT In many sources of volatile organic compounds(VOCs),large amountsof w ater vapor come from the air and the reactors.The relative humidity(RH)of exhaust gas is normally＞60%and is supersaturated.Maintaining the property of adsorbent on VOCs in a highly humid gas stream is a serious industrial problem.In this study,the adsorption/desorption behavior of toluene in a micro-mesoporous polymeric resin was investigated in a highly humid environment to explore the in fl uence of abound w ater vapor on resin adsorption and regeneration.This resin could selectively adsorb toluene at an RH of 80%,and its adsorption property was unaffected by the presence of water vapor.In the case of humidity saturation,the resin displayed a high adsorption capacity at a moisture content of＜30%.Therefore,the polymer resin is an excellent w ater-resistant adsorbent of VOCs.In the regenerative experiment,the resin maintained its original adsorption capability after four adsorption/desorption cycles of toluene purging w ith nitrogen gas at 120°C.The resin exhibited excellent regeneration performance at high humidity.

1.Introduction

Volatile organic compounds(VOCs)are among the most common atmospheric pollutants emitted by various industries,such as chemical,petroleum,medicine,paint,and printing industries[1-3].These compounds can be removed and recovered from gas streams through adsorption,w hich is one of the most attractive and effective removal and recovery techniques[4,5].The key component of adsorption isaporoussolid adsorbent.For example,activated carbon and molecular sieve are applied extensively during the removal of VOCs from gas streams because of their high porosity and large surface area[6-9].

How ever,in many sources of VOCs,a large amount of w ater vapor comes from air and reactors.The relative humidity(RH)of exhaust gas is normally more than 60%and even supersaturated because of a large amount of condensed w ater existing in adsorption containers.The competitive adsorption of organic molecules and w ater molecules also affectstheoverall performanceof adsorption[10].Activated carbon materialsare prepared through carbonization and activation.Activation is used for high-temperature treatment of w ater vapor in industries[11].A large number of oxygenated groups are inevitably found on the surface of activated carbon,and they enhance the selective adsorption capacity of w ater molecules.As a result,the capability of activated carbon to adsorb organic molecules w eakens at high humidity.Zeolite molecular sieves(ZSM-5 and USY)have a stable skeleton structure and a low Si/Al ratio and consequently exhibit strong moisture adsorption properties.Nevertheless,activated carbon and molecular sieve are unsuitable for application in high-humidity environments[12-14].Therefore,efforts have been devoted to developing highly hydrophobic adsorbents for the separation and recovery of VOCs from industrial gases.Porous polymeric resin show s potential for adsorption because of its controllable pore structure and on site hydrophobicity and regenerability.Resin is also frequently used as an adsorbent in puri fi cation and separation[15-17].

This study aimed to explore the in fl uence of abound water vapor on resin adsorption.Hydrophobic resin V-503(poly(styrenedivinylbenzene)polymer,The Dow Chemical Company,USA)w as used as an adsorbent to evaluate dynamic adsorption and desorption propertiesin humid environments.Toluene,w hich isa typical nonpolar and inert aromatic hydrocarbon,w as used as a model adsorbate,and the regeneration performance of the resin w as investigated through thermal nitrogen cyclic desorption.Our results could serve as a reference for future industrial applications of this resin.

2.Experimental

2.1.Materials and characterization

V-503 wasdried at a reduced pressure and at 60°Cfor 2 h to remove moisture and pore-forming agents.Toluene,w hich is a representative aromatic VOC,was applied during adsorption.

Theporetextureof thepolymeric adsorbent wasexamined through N2adsorption in an ASAP 2020 micropore analyzer isotherms at-196°C.Speci fi c surface area(SBET)was calculated following Brunauer-Emmett-Teller method by using adsorption data acquired at a relative pressure(P/P0)ranging from 0.05 to 0.25.The total pore volume w as estimated on the basis of the amount adsorbed at a relative pressure of approximately 0.9929.Pore size distribution curves w ere derived from the analysis of the desorption branch of the isotherm follow ing Barrett-Joyner-Halenda(BJH)algorithm.The thermal gravity(TG)and the derivative thermal gravity(DTG)of the samples were analyzed w ith a TG/DTG analyzer(NETZSCH,STA 409PC).The heating rate was set at 10 °C·min-1from 20 °C to 400 °C under an N2fl ow of 50 ml·min-1.

2.2.Adsorption performance test

2.2.1.Static adsorption of water vapor

In a static mode,the water vapor adsorption curve of the resin w as obtained by adsorbing deionized w ater relative to the saturation vapor pressure at 40°C,a saturated vapor pressure of steam of 7.38 kPa,and a mass concentration of 51.00 g·m-3.The cubic reaction vessel made of a thin iron w ire w as placed in 0.5 g of the sample and then positioned at 5 mm above liquid level.This reactor wasconnected to the electronic balance,and quality w as recorded every 2 min.The adsorption degree of w ater on the resin w as re fl ected by the change in quality.Similarly,the static adsorption curves of water vapor and toluene on the resin w ere obtained by adsorbing deionized w ater and toluene respectively relative to the saturation vapor pressure at 30°C and 50°C.

2.2.2.Dynamic adsorption of VOCs

A dynamic adsorption system was composed of three components,namely,a gasdistribution system,a detection system,and an adsorbent bed system[18].In this system,gaseous toluene used as a model VOC w as generated by bubbling liquid toluene at 0°Cw ith standard air fl ow,and the fl ow volume w as regulated w ith a mass fl ow controller.The generated gaseous toluene w as transferred to a buffer by air steam and diluted w ith air at the required concentration.The inlet toluene concentration w as controlled at 5000 mg·m-3,and the airspeed was maintained at 20000 ml·h-1·g-1.Hygrometric toluene gas(RH=80%)w as acquired by diluting air directly into deionized w ater at 30°C.Gaseous toluene in a dry or hygrometric state w as passed directly through a sample column at 30°C.The column(8 mm in diameter)w as packed w ith the adsorbent of approximately 0.5 g.The inlet and outlet toluene concentrations w ere monitored online by using a gaschromatograph(GC,6890 N,Agilent Technologies Co.,Ltd.)equipped with a fl ame ionization detector.Adsorption capacity w as calculated in accordance with Eq.(1)as follows:

where q is the dynamic adsorption capacity(mg·g-1);tsis the time to reach adsorption equilibrium(min);F is the fl ow volume of carrier gas(air)(167 ml·min-1);W is the mass of the adsorbent(g);C0is the inlet concentration of the adsorbed gas(mg·m-3),and Ciis the outlet concentration of the adsorbed gas(mg·m-3).

2.3.Desorption performance test

A desorption system comprised three components,namely,a temperature-controlled system,a gas detection system,and a desorption bed[19].After toluene underwent saturated adsorption,the samples w ere regenerated by a hot N2stream of 20 ml·min-1.At the beginning of desorption,N2was purged into the sample column,and the tube furnace w as turned on to heat up to 120°Cat a heating rate of 5 °C·min-1.The outlet toluene concentrations w ere monitored online through GC,and the entire desorption lasted 120 min.The samples w ere reused to measure the adsorption capability.The same procedure w as repeated four times during the subsequent recycling.

3.Results and Discussion

3.1.Surface area and pore volume distribution

The N2adsorption,desorption isotherms,and pore size distribution at-196°C of the polymer resin V-503 are demonstrated in Fig.1.According to the International Union of Pure and Applied Chemistry classi fi cation,the adsorption isotherm of V-503 w as close to type IV.In this case,the nitrogen adsorption isotherms show ed three typical steps as the relative pressure increased.First,the N2adsorption amount of thesample reached ahigh level(＞200 cm3·g-1)at arelativepressure of approximately 0.05,indicating that the adsorption curve exhibited a steep increase in the low-pressure region,w hich represented adsorption or condensation in small micropores.As the relative pressure increased(0.05-0.9),a type H4 hysteresisloop appeared,implying that V-503 was typically composed of microporous-mesoporous materials.Finally,the adsorbed amount increased quickly near thesaturation pressureof nitrogen becauseof activecapillary condensation,indicating the presence of some macropores in the resin.However,the adsorbed amount did not reach saturation until the relative pressure P/P0w as at 1.This condition show ed the multilayer adsorption phenomena on V-503.The pore size distribution curves based on the BJH method are show n in Fig.1.The average pore size w as 4.14 nm(Table 1).The surface area and pore volume based on N2adsorption and desorption curve at-196°C are also listed in Table 1.The resin,w hich exhibited a relatively high speci fi c surface area of 771 m2·g-1(w ith a microporous surface area accounting for 51%)and a large pore volume of 0.799 cm3·g-1,might present a suf fi cient adsorption potential[20,21].

Fig.1.N2 adsorption-desorption isotherms and pore size distribution for resin.

Table 1 Structural characteristics of the resin

3.2.Static adsorption performance

Fig.2 show s the static adsorption curve of dry resin under a hygrometric condition at 40°C.The curves can be divided into three stages.During the fi rst stage,the adsorption capacity increased rapidly because the adsorption position of the mesoporous pore was initially occupied by w ater vapor.When the adsorption time w as from 50 min to 250 min,w ater vapor entered the microporouspore,and the adsorption capacity increased slow ly.During the third stage,the adsorption reached equilibrium.V-503 obtained an adsorption amount of 126.13 mg·g-1.Consequently,the water content of V-503 wascalculated as 11.20%,indicating the certain hygroscopic property of the polymer resin in humid environments.In addition,the surface of dry resin produced static electricity w hen particles rubbed against each other.Therefore,this type of polymer resin should be preserved under moist conditions.

Fig.2.Static adsorption curves of water vapor(RH=100%)on the resin at 40°C.

Fig.3 presents the static adsorption curves of w ater vapor and toluene on the resin at 30 °C and 50 °C.The fi gure show s that the toluene adsorption capacity could reach 700 and 1200 mg·g-1at 30 °C and 50°C,respectively.V503 resin showed strong toluene adsorption.

3.3.Adsorption-desorption text under moist conditions

The in fl uence of RH on breakthrough adsorption curves is show n in Fig.4.The adsorption breakthrough time(126 min)and saturated adsorption capacity(210 mg·g-1)w ere calculated using the Y-Nequation.The per square nanometer of the pores could adsorb tw o toluene molecules.The result demonstrated that a RH of 80%elicited a nearly negligible effect on the adsorption of toluene on polymeric adsorbents probably because toluene molecules are strongly adsorbed on the surfaceby replacing water molecules[15].Thus,water w eakly in fl uenced toluene adsorption,and the effect of humidity on the adsorption of aromatic hydrocarbons by polymeric adsorbents could be disregarded in practical applications under these conditions.

Fig.3.Static adsorption curvesof water vapor and toluene on the resin at 30 °Cand 50 °C.

Fig.4.Adsorbent curves of toluene on the resin at dry and wet air.[C:5000 mg·m-3,GSHV:20000 ml·h-1·g-1,T:30 °C].

Fig.5.Desorption concentration and rate curves of toluene on the resin at 120°Cw ith N2 purging.(20 ml·min-1 N2,0～20 min:20-120 °C,20-120 min:120 °C).

Fig.6.Breakthrough curves and adsorption amounts of toluene on the resin at different w ater content.[C:5000 mg·m-3,GSHV:20000 ml·h-1·g-1,T:30 °C,RH=80%].

Fig.5 illustrates the obtained dynamic desorption curves of 20 ml·min-1N2gas at 120 °Cand the changes in the desorption rate.The outlet toluene concentration w as monitored by GCat an interval of 5 min.The highest toluene concentration of 140000 mg·m-3w as observed after 20 min.Thisconcentration exceeded thesaturated vapor concentration of toluene at 0°C.The result show ed that the toluene molecules on the surface of the resin w ere removed rapidly as temperature increased.The adsorbent immediately maintained a supernal concentration of desorption for 20 min at a constant temperature.The desorption rate w as approximately 80%.A high instantaneous desorption concentration corresponded to a high desorption rate.Although the concentration of toluene decreased quickly over time,the desorption rate reached 99%w ithin 120 min.Thermal nitrogen purging could be applied to remove toluene on polymer resin.

In industries,adsorbentscontain alarge amount of w ater vapor similar to VOCs because of oversaturated w ater gas stream.Therefore,resins w ith different w ater contents should be prepared by soaking them in deionized w ater and then by purging w ith N2at a normal temperature to simulate the actual condition.In the current study,a dynamic adsorption experiment involving toluene(RH=80%)on w ater-treated resin was conducted.The adsorption breakthrough curve w ascalculated(Fig.6).Thetoluene concentration rapidly reached thebreakthrough point,in which the outlet concentration was5%of the inlet concentration,w hen the moisture content of the resin w as more than 47.05%.The breakthrough curve became gentle because w ater molecules w ere replaced continually by toluene.The adsorption curve of the special resin w ith a moisture content of 36.63%w as close to that of dry resin.The adsorption breakthrough time and saturated adsorption capacity are listed in Table 2.The resin still had a good adsorption capacity in spite of a slight decline from 238.3 mg·g-1to 229.3 mg·g-1.The quality of the system w as also relatively balanced after adsorption.These results indicated that the initial w ater content slightly affected resin adsorption because it only retained the water content of the resin at less than 30%in the actual application.

3.4.Thermal stability text

Fig.7.TG,DTGcurves of toluene on the resin.

Table 2 Adsorbent properties of toluene on the resin at different water content.(RH=80%)

Fig.8.Breakthrough curves and saturated adsorptions of toluene on resin after maturing.(C:5000 mg·m-3,GSHV:20000 ml·h-1·g-1,T:30 °C,RH=80%).

The thermal stability of the resin was investigated by different aging times at speci fi c temperatures.The aging time and temperature range should bedetermined.Fig.7 presentsthemassloss(TG)and differential massanalysis(DTG)of V-503.The TGcurvesof the sample revealed one stageof weight lossfrom 70 °Cto 230 °C,with athermodesorption peak centered at approximately 150 °C.Three temperatures(120 °C,150 °C,and 180°C)w ere used to examine the stability of the resin.Fig.8 demonstrates the breakthrough curves of toluene on the resin aged at three temperatures.Toluene had no effect on adsorptive property after aging at 120°Cfor a long time.However,the breakthrough time lagged at 150°Cfor 30 h,and the saturation adsorption capacity decreased signi fi cantly from 238.3 mg·g-1to 203.1 mg·g-1.The collapsing of the porous structure of the macromolecule resin at a high temperature usually w eakens the adsorptive property.The results show ed that the observation w as true for hot desorption at 120°C.How ever,the thermal stability of polymer resin w as relatively poor,and the resin must be used at less than 150°C.

3.5.Regeneration capability

Thermal regeneration capability isoneof themost important criteria for excellent VOCadsorption.After four adsorption and desorption cycles,toluene adsorption w as slightly reduced after the desorption rate w as maintained up to 95%(Fig.9).The results clearly show ed that the adsorbed toluene could be effectively removed under hot N2at 120°C,indicating that the channel structure and surface properties of resin remain unchanged[22-24].Therefore,the resin exhibited excellent regeneration performance at high humidities.

Fig.9.Adsorbent/desorption properties of toluene on the resin after been cycle used for four times.[ads:C:5000 mg·m-3,GSHV:20000 ml·h-1·g-1,T:30 °C,RH=80%des:20 ml·min-1 N2,0-20 min:20-120 °C,20-120 min:120 °C].

4.Conclusions

The adsorption property of toluene on the resin with a moisture content of less than 30%is consistent in high-humidity environments(RH=80%)compared w ith that on dry resin.This result con fi rms that w ater vapor slightly in fl uences the entire adsorption system.The macromolecule polymer resin presents high-adsorption capacity after four adsorption and desorption cycles,and the desorption rate is up to 95%.Therefore,the polymer resin can be a remarkable adsorbent for VOCremoval in humid environments.