Liquid phase equilibrium of phenol extraction from bio-oil produced by biomass pyrolysis using thermodynamic models

Dewi Selvia Fardhyanti*,Bayu Triwibowo,Heri Istanto,Muhammad Khusni Anajib,Amalia Larasati,Windy Oktaviani

Department of Chemical Engineering,Faculty of Engineering,Universitas Negeri Semarang,Semarang 50236,Indonesia

Keywords:Biomass Pyrolysis Bio-oil UNIFAC NRTL

A B S T R A C T Utilization of biomass as a new and renewable energy source is being actively conducted by various parties.One of the technologies for utilizing or converting biomass as an energy source is pyrolysis,to convert biomass into a more valuable product which is bio-oil.Bio-oil is a condensed liquid from the vapor phase of biomass pyrolysis such as coconut shells and coffee shells.Biomass composition consisting of hemicellulose,cellulose,and lignin will oxidize to phenol which is the main content in bio-oil.The total phenolic compounds contained in bio-oil are 47.03%(coconut shell)and 45%(coffee shell).The content of phenol compounds in corrosive bio-oils still quite high,the use of this bio-oil directly will cause various difficulties in the combustion systemdue to high viscosity,low calorific value,corrosivity,and instability.Phenol compounds have some benefits as one of the compounds for floor cleaners and disinfectants which are contained in bio-oil.The correlation between experimental data and calculations shows that the UNIQUAC Functional-group Activity Coefficients(UNIFAC)equilibrium model can be used to predict the liquid-liquid equilibrium in the phenol extraction process of the coconut shell pyrolysis bio-oil.While the Non-Random Two Liquid(NRTL)equilibrium model can be used to predict liquid-liquid equilibrium in the extraction process of phenol from bio-oil pyrolysis of coffee shells.

1.Introduction

Theroleoffossilfuelsisstillcrucial,buttheireffectswillgraduallybe overtakenbynewand renewableenergysources.Alongwiththedepletion of world oil reserves and environmental issues,various researches on renewable energy are being actively conducted by various parties.Therefore,technology needs be able to overcome the problem.One technology that can be used is pyrolysis technology where the material used comes from organic waste(biomass).Pyrolysis technology can overcome organic waste problems and make the environment healthy by converting waste into value-added end product.

Biomass is an organic material derived from plants,algae,animal waste,industrial waste,forestry,and agricultural waste[1].Biomass can be used as pyrolysis raw material to produce liquid(bio-oil),gas and solid products.Various kinds of biomass waste can be used as raw materials for the manufacture of bio-oils,such as corncobs[2],sawdust and wheat bran[3],castor[4],oak[5],durian shell[6]and coconut shell[7].

IndonesiaisthethirdlargestcoffeeproducerintheworldafterBrazil and Vietnam by contributing about 6%of the world's total coffee production,also world's fourth largest coffee exporter with a market share of around 11%across the world.Coffee is one of the plantations with high economic value besides chocolate and tea.Based on data from the Indonesian Ministry of Forestry and Plantation,Indonesia's coffee production in 2013 reached 691160 tons with a plantation area of 47800 Ha[8].Waste from coffee processing that is abundant and not utilized optimally is the coffee shell.A large percentage of waste from coffee shells derived from plantations amounts to approximately 11.08%which potentially causes environment hazard.Coffee shells are a waste that has high lignocellulose content which has a potential to be used as bio-oil.Bio-oil is a blackish liquid fuel derived from biomass such as wood,coffee shells and other biomass.Bio-oil contains lignin components(23.4%),cellulose(34.2%)and hemicellulose(24.5%)[8].

In Indonesia,in addition to the coffee shell,one of the available biomassresources iscoconut shell.Availability of coconut shellcan be seen from the production of coconut fruit in Indonesia.Coconut is one of Indonesia's main commodities with an average production of 3000000 tons per·year[9].The utilization of coconut is very diverse,but the utilization and way of handling waste are still limited.Coconut fruit can be used as food in daily life for the purpose of direct consumption.Nowadays,lots of coconut shells become garbage that cannot be utilized.One of the waste handling is to utilize coconut shell waste as a base material in the manufacture of bio-oil.Characterization studies on coconut shells have been done through proximate,ultimate,and Thermo Gravimetric(TG)analyses.The results of this analysis indicate that coconut shell has a high potential to produce fuel fluid by pyrolysis conversion process[10].Chemical content of coconut shell is cellulose(34%),hemicellulose(21%),lignin(27%)and ash(18%)[11].

Before converted into bio-oil,coconut shell waste and coffee shell need to be treated first.It takes a process before converting the waste of coffee shell into bio-oil which is pyrolysis.Pyrolysis is the process of decomposition of a substance or material that is done at a relatively high temperature.Pyrolysis results in the form of gas,liquid and solid.The solid result of this process is char(char).Gas and liquid products can be used as a source of bio-oil.Bio-oil can be used as an alternative fuel that is easier to store and distribute than biomass in solid form.Bio-oil made from coconut shell material using slow pyrolysis with pyrolysistemperature250-300°Cproducesaliquidwhichisveryacidic and corrosive.The largest organic component in bio-oil is the lignin derivative of phenol,alcohol,organic acids and carbonyl compounds such as ketones,aldehydes and esters[12].The nature of the acid is caused by the content of the dominant phenol compounds.

Phenol compounds in addition to causing the smell of smoke in biooil,also showed antioxidant activity and corrosivity[13].Therefore,the use of this bio-oil will directly cause various difficulties in the combustion system due to high viscosity,low calorific value,corrosivity and instability[6].

Phenol needs to be extracted from bio-oil to reduce its corrosive properties.In addition,phenol is a compound that has a high economic value because it is expensive and widely used.Phenols have many important uses such as disinfectants,resins,pesticides,explosives,and drugs.The method that can be used to extract phenol compounds from bio-oil is by liquid-liquid extraction method using methanol solvent.This has been done by Jazbin?ek[14]on the isolation of phenol compounds from bio-oils from pyrolysis of forest waste and Mantilla[15]on the extraction of phenol compounds from bio-oils from pyrolysis of agricultural waste.Dahlia[16]conducted research on liquidliquid extraction of phenol compound from bio-oil of pyrolysis resulted of palm oil empty bunch using methanol solvent.The yield of phenol resulting from this extraction is 40%.In this study,it is expected that the liquid-liquid extraction method can be used to separate the phenol compoundsfromcoconutshellpyrolysisbio-oils.Thepurposeofphenol extractionistoimprovethequalityofbio-oilandreducethecorrosivity,so when used as fuel,it will cause no damage to the engine.

Biomass pyrolysis converted to bio-oil studies have been conducted.Bio-oil produced from the pyrolysis process can be used as fuel for engines,turbines and boilers.Further processing by the liquid-liquid extraction process will reduce the corrosive nature of bio-oil and obtained chemicals,in which one of them is phenol.Research on extraction of phenol compounds from bio-oil is still very limited.In addition,thermodynamic modelling study on liquid-liquid equilibrium of the phenol extraction process from bio-oil still needs to be investigated further.

To design extraction columns required much information such as liquid-liquid equilibrium data(liquid-liquid equilibrium).Thermodynamic models such as Equation of State or Activity Coefficient are used to correlate the experimental data to obtain optimal interaction parameters.Optimal parameters can be used to predict liquid-liquid equilibrium data to produce an equilibrium graph that is used to design extraction columns[17].The study on liquid-liquid equilibrium includes NRTL,e-NRTL,UNIQUAC,and UNIFAC.

Given the importance of thermodynamic modeling in this liquidliquid extraction process and the price and benefits of phenol in the industrial world,this research becomes an important thing to do.From the equilibrium data,the optimum condition will be obtained where the highest yield of extract is obtained in the extraction process of phenol compound from bio-oil from coconut pyrolysis.Liquid-liquid equilibrium data of the components of the system can be used in the design of the equipment in the separation process.

2.Experimental

2.1.Chemicals

AllreagentsusedhereareofA.R.gradeandusedasreceivedwithout further purification.Methanol,acetic acid,and chloroform were purchased from e-Merck(Germany).The distilled water is available at the laboratory.Coffee shell and coconut shell were obtained from public market in Semarang.

2.2.Equilibrium procedure

Instruments used in this study:

a)Shaker bath,product by Memmert WB14,SV1422,Scwabach,Germany.

b)GasChromatography(GC),productbyGC6820AgilentTechnologies,column:Rastek RXi-5MS002E.

c)Gas Chromatography-Mass Spectroscopy(GC-MS),product by GCMS-QP2010S Shimadzu,column:Rastek RXi-5MS.

The material systems and experimental conditions for the equilibrium extraction are summarized in Table 1.

Table 1 Material systems and conditions for equilibrium extraction

The bio-oil feed solution that contains phenol was extracted using two-stageextraction.The firststageusesdistilledwaterasa polarsolvent andchloroformasanon-polarsolvent.Extractionwasconductedwiththe ice-bath method at 4-5 °C,200 r·min?1and stirring time for 2 h.The extracts obtained at this first stage are used as feeds on the second stage extraction.At the second stage extraction,each extract was extracted using a methanol solvent under operating conditions shown in Table 1.

2.3.Analysis and characterization

After equilibrium was attained(60 min),the mixtures were poured into a separating funnel,settled for an hour and separated into two phases.Then the two phases were weighed.The extract phase and the raffinate phase of each extraction processes are analyzed by GC to obtain the concentration of each component.The principal conditions of this analysis are shown in Table 2.

Table 2 Conditions of analysis using GC

3.Results and Discussion

3.1.Bio-oil content

The chromatograms of the GC-MS analysis are shown in Figs.1 and 2.The results of the GC-MS analysis of the bio-oil showed that they contained more than six chemical compounds such as ethyl ester,phenol,furfural,acetic acid,and others.

Tables 3 and 4 represent that bio-oil contains more than 6 main components.Thepercentage(%)ofthecompositionsofthebio-oilanalysisbyGC-MSwasobtainedfromthepercentageareaofthepeakorthe height of the peak in the chromatograms.Using a method of GC-MS,this work presents a composition of the bio-oil generated in the experiment.The totalic phenolic compounds contained in bio-oil are 47.03%(coconut shell)and 45%(coffee shell).It was caused by the pyrolysis processing at 300-350°C(low temperature carbonization)that produced more substances containing ester and phenolic compounds.The composition of biomass such as hemicellulose,cellulose and lignin will be oxidized to phenol as the main component of the bio-oil.

3.2.Equilibriumdata onphenol extractionofbio-oil fromcoconut pyrolysis results

In this research the variables studied were temperature,speed of stirring and type of solvents(aqueous methanol and aqueous aceticacid).Generally,all extracts are rich in phenol but the mole fraction significantly depends on extraction conditions:temperature,speed of stirring and type of solvents.Methanol and acetic acid need to be mixedwithwaterbecausetheirpolaritiesarenotstrongenoughtoform an immiscible mixture with bio-oil[17].

Table 3 The main components of bio-oil produced by pyrolysis of coconut shells

Table 4 The main components of bio-oil produced by pyrolysis of coffee shells

Fig.1.Analysis result by GC-MS for the bio-oil produced by coconut shell pyrolysis.

Fig.2.Analysis result by GC-MS for the bio-oil produced by coffee shell pyrolysis.

The mole fractions of any components in the raffinate phase and the extract phase are constant after 60 min,so the system is assumed to be in equilibrium.The amount of water transferred from the solvent into the raffinate phase was negligible[17].

In this work,we report experimental data for the phenol extraction from bio-oil produced from coconut shell and the phenol extraction from bio-oil produced from coffee shell.

Tables 5 and 6 show the equilibrium of multi-component extraction data for aqueous acetone and aqueous methanol solvent in the extract phase and raffinate phase.

Table 5 Equilibriumof components extraction dataforphenol extraction using aqueousmethanol from bio-oil produced from coconut shell and coffee shell

Table 6 Equilibriumofcomponentsextractiondataforphenolextractionusingaqueousaceticacid from bio-oil produced from coconut shell and coffee shell

3.3.Liquid-liquid equilibrium for the phenol extraction process using UNIFAC and NRTL models

TheUNIFACandNRTLmodelswereusedtocorrelatetheexperimentaldataobtainedforthesystems understudy.TheUNIFACequationand NRTL equation[18]were used to correlate experimental phase equilibrium data.The activity coefficient models applied in this study are as follows:

a)UNIFAC:[18]

UNIFAC is a UNIQUAC Functional-group Activity Coefficients model based on the UNIQUAC equation by using functional groups of a molecule contained in the mixture to calculate the activity coefficients.The UNIFAC modeling is a continuation of the UNIQUAC modeling development.This model has the advantage of performing phase equilibrium estimation even in the absence of experimental data.This thermodynamic model is used to predict the estimation of non-electrolyte activityin non-idealmixtures.TheUNIFAC modelcalculates the activity coefficients by using functional groups present in the molecules that form the liquid mixture.If the value of the component activity coef ficient has a value close to one,this indicates that the component is in the ideal state in the system[19].The correlation in the UNIFAC model is used in order to reduce the number of binary interactions measured to predict the state of the system.The correlation for the UNIFAC model is shown as follows:

where:

ln γiCcombinatorial component

ln γiRresidual component

Jicomponent surface area component(i)

Licomponent volume fraction(i)

ripure component surface area parameter(i)

xipure component mol fraction(i)

qipure component volume parameter(i)

vk(i)number of sub-group(k)on component(i)

Rkparameter area for sub-group(k)(Rk=Awk/(2.5×109),where Awkis number of sub-group van der Walls surface area)

Qknumber of parameter volume subgroup(k)(Qk=Vwk/15.17,where Vwkis number of van der Waals volume subgroup)

amkgroup of interaction parameter from groups m and k[17]

xiliquid mol fraction of component(i)The UNIFAC equation can be obtained to:

b)NRTL:[18]

Non-RandomTwoLiquid(NRTL)isderivedfromtheScotttwoliquid modelanditis assumed thatrandomness isthesameasthemodelused on the Wilson model.The NRTL model has a non-random parameter(α12)making it possible to use in a wide variety of mixtures.The NRTL model uses only binary parameters to calculate the phase equilibrium properties.

The NRTL model is derived from the Scott two liquid model and it is assumed that the non-random parameter is the same as the model used in the Wilson Model.The NRTL equation is developed based on a two-fluid theory with the addition of a non-random parameter(α),which makes the equation applicable to different types of liquid-liquid mixture and equilibrium by choosing the appropriate α price.The parameter estimation for this equation is done by setting the α value at a certain value and subsequently optimizing the objective function to obtain two optimal parameter interactions.The recommended α value is 0.2-0.47.The value of α is often adjusted to certain conditions,but usually the value of α=0.3.Unlike the Wilson equation which contains two adjustable parameters,the NRTL equation contains three parameters,so the NRTL equation can be an alternative model if other models are inadequate.This equation can also estimate the equilibrium of multicomponent systems without additional parameters.

The NRTL equation can represent liquid-liquid equilibrium for both binary and multicomponent systems only with binary parameters.The equation is simpler than the UNIQUAC equation,but it has a disadvantage because it contains three parameters for each pair of components.The third parameter(α)is estimated based on the state of the component and sometimes for certain substances the extreme number is obtained.The NRTL equation is expressed:

where:

GEmolar Gibbs energy

R gas constant

T temperature

x liquid phase mol fraction

i,j,k component of 1,2,3,…,n

α non-random parameter for NRTL

g,G component parameter

γ activity coefficient

τ empirical constant for NRTL

Tables 7 and 8 show the value of activity coefficient on phenol extraction using methanol solvent obtained from calculation with UNIFAC and NRTL modeling,and the coefficient value of activity on each component shows that methanol and acetic acid component in theextract phase is in ideal condition because thevalue of activity coeffi cient obtained is close to one.The activity coefficient value of phenol and water in the extract phase is not close to one,it means that phenol and water components are far from ideal conditions for the system.The idealconditionsofthecomponents inthesystem show that thecomponentisdominant,sothepropertiesofthemixtureareclosertothedominant component properties either in the extract or raffinate phase.In theraffinate phase,it is known that the bio-oil has a coefficientof activity close to the one,indicating that the bio-oil is in ideal conditions forthe system,while the phenol and solvent(acetic acid or methanol)are far from ideal conditions.So non-ideality studies are very important for those systems[19].

Table 7 The activity coefficients were calculated by UNIFAC model

Table 8 The activity coefficients were calculated by NRTL model

The result of correlation of liquid-liquid equilibrium data by using UNIFACandNRTLmodelinggivesgoodagreementbetweenexperimental data and calculation.This is shown by the value of SSE(Sum of the Square of the Error),which is very small.The SSE values in UNIFAC and NRTL modeling can be seen in Table 9.The value of SSE is 0.015-0.554.

Table 9 Sum of the Square of the Error(SSE)

Based on Table 9,it can be seen that extraction with methanol solvent for the NRTL model has the smallest SSE value compared to the extraction with acetic acid solvent for the UNIFAC model.

Fig.3.Comparison of calculated and experimental mole fractions of the phenol extraction using aqueous methanol by UNIFAC model at 298 K.

where:

Xamole fraction of component in extract phase

Xbmole fraction of solvent in extract phase

Xcmole fraction of water in extract phase

Yamole fraction of component in raffinate phase

Ybmole fraction of solvent in raffinate phase

Fig.4.Comparison of calculated and experimental mole fractions of the phenol extraction using aqueous methanol by UNIFAC model at 313 K.

Fig.5.Comparison of calculated and experimental mole fractions of the phenol extraction using aqueous methanol by UNIFAC model at 323 K.

Fig.6.Comparison of calculated and experimental mole fractions of the phenol extraction using aqueous acetic acid by UNIFAC model at 298 K.

Fig.7.Comparison of calculated and experimental mole fractions of the phenol extraction using aqueous acetic acid by UNIFAC model at 313 K.

Fig.8.Comparison of calculated and experimental mole fractions of the phenol extraction using aqueous acetic acid by UNIFAC model at 323 K.

Fig.9.Comparison of calculated and experimental mole fractions of the phenol extraction using aqueous methanol by NRTL model at 298 K.

Fig.10.Comparison of calculated and experimental mole fractions of the phenol extraction using aqueous methanol by NRTL model at 313 K.

Fig.11.Comparison of calculated and experimental mole fractions of the phenol extraction using aqueous methanol by NRTL model at 323 K.

Fig.12.Comparison of calculated and experimental mole fractions of the phenol extraction using aqueous acetic acid by NRTL model at 298 K.

Fig.13.Comparison of calculated and experimental mole fractions of the phenol extraction using aqueous acetic acid by NRTL model at 313 K.

Ydmole fraction of bio-oil in raffinate phase.The smaller SSE values obtained show that the correlation between the experimental data and the calculation data indicates a better correlation.Good correlation results show that equilibrium modeling of UNIFAC and NRTL can be used to predict liquid-liquid equilibrium in the phenol extraction process from coconut shell pyrolysis bio-oil and coffee shell using methanol solvent and acetic acid.

Figs.3-14 show the visual comparison in which the calculated and experimental mole fractions in the extract phase and the raffinate phase for the methanol solvent system by the NRTL model are better than the other conditions(by UNIFAC model and acetic acid solvent).

Fig.14.Comparison of calculated and experimental mole fractions of the phenol extraction using acetic acid by NRTL model at 323 K.

4.Conclusions

NRTL modeling gives a good correlation to liquid-liquid equilibrium dataforaternarysystemonphenolextractionofbio-oilusingmethanol solvent compared to the UNIFAC modeling using acetic acid solvent.

The activity coefficients of water and phenol in their phase are far fromunity.Itmeansthattheyarefarfromtheidealcondition.Theactivity coefficients of methanol,acetic acid,and bio-oil in their phases are close to one.It means that they are nearly ideal in their phases.

Acknowledgments

The authors would like to acknowledge the Ministry of Research,Technology and Higher Education,Indonesia,for the financial support ofthisworkthroughtheresearchgrantof“ProdukTerapan”Universitas Negeri Semarang,Nomor:084/SP2H/LT/DRPM/IV/2017.