A CO2-controllable phase change absorbent solvent used to waste recycling of dining lampblack

2023-12-12 00:04WenjianYueXiaojiangLiJunhaoJingLiTongNaWangHongshengLuZhiyuHuang

Wenjian Yue ,Xiaojiang Li,4,,Junhao Jing ,Li Tong ,Na Wang ,Hongsheng Lu,2,3,,Zhiyu Huang,2,3

1 College of Chemistry and Chemical Engineering Southwest Petroleum University, Chengdu 610500, China

2 Engineering Research Center of Oilfield Chemistry, Ministry of Education, Chengdu 610500, China

3 Oil and Gas Field Applied Chemistry Key Laboratory of Sichuan Province, Chengdu 610500, China

4 Chongqing University of Science and Technology, Chongqing 401331, China

Keywords:Dining lampblack CO2-switchable Phase change Absorption Waste recycling Solvents

ABSTRACT Dining lampblack as a source of atmospheric pollution,urban residents had to spend a lot of economic costs all year round to solve its impact.However,traditional treatment methods often carry the risk of secondary pollution.The use of phase change absorption solvent(PCAS)controlled by CO2 can effectively absorb the oily components in dining lampblack,and smoothly avoid the generation of secondary pollutants and squandering of resources.The reversibility of PCASs under CO2 control was explained by pH changes and macroscopic visualizations.The absorption effects of favorable absorbents and PCASs on dining lampblack were compared and analyzed.The fatty acid (FA) in the oil absorption mixture was desorbed by interacting with D230.The results of GC/MS analysis on the oil components separated by desorption showed that the desorption of PCASs was effective for these refractory oil components.FAs can be enriched and applied to the subsequent dining lampblack treatment link to realize the waste recycling.In addition,the absorption and desorption of oily components by PCASs were combined with the CO2-controlled phase transformation of PCASs itself to achieve the absorption circulation of treating dining lampblack by using PCASs.

1.Introduction

In the past twenty years,the world economy has been growing steadily,and the catering industry has entered a prosperous period with the continuous improvement of people’s living standards in various countries.Of course,the booming development of the industry will lead to the drawbacks of the increase of its dining lampblack emission.Dining lampblack is a three-phase mixture of gas-liquid-solid,mainly including water vapor,oil mist and solid particulate matter[1].The two most obvious types of harmful substances are fine particulate matter(PM2.5 and PM10)[2,3]and volatile organic compounds (VOCs) [4,5].VOC treatment has always been the focus of environmental protection in the world.Alkanes,aldehydes,ketones and aromatic hydrocarbons in anthropogenic VOC in dining lampblack have the characteristic of strong hydrophobicity as oily components [6-8],which have become the key and difficult point of purification treatment [9-11].

Dining lampblack is formed in the cooking process of edible oil,and its components are affected[2,12]by the heating environment,cooking method,types of edible oils and food materials.This study will investigate various literature on dining lampblack,focusing on different edible oils and cooking methods to determine the most appropriate reagents for follow-up experiments.

The dining lampblack purification technologies adopted in China mainly include mechanical separation,electrostatic removal technique [13],washing method,filtration/adsorption [14],and composite processes [15].Most of the treatment methods have high requirements for equipment,and small and medium-sized restaurants do not have the economic strength to solve these problems.The most widely used are oxidation incineration and water washing,but oxidation incineration has secondary pollution emission,catalytic oxidation catalyst regeneration is difficult,and water washing has the drawbacks of high energy consumption,low purification efficiency,and secondary pollution of waste liquid[16].In particular,the hydrophobic VOCs belong to noncondensing gas,which can not be effectively treated [17],and is also difficult in the treatment of dining lampblack.

Water or aqueous solutions using bases,acids or oxidizing reagents are the most common absorbent,restricting the use of this process to water-soluble VOCs [18].There are already teams working on targeted uptake of hydrophobic VOC using green solvents,such as ionic liquids [19] and deep eutectic solvents [20].Avoiding or eliminating the disadvantages of solvents commonly used to capture VOCs in industrial efflux gases is the focus of research and development of new green solvents.

In the early study,it was found that fatty acids(FA)could combine with Polyether amine D230 to form a water-soluble complex,and this complex could be deconstructed by CO2stimulation [21],so that FA could be restored to the oil-soluble state.Such a chemical reaction cycle enables FA to realize the transformation of hydrophilicity and hydrophobicity,and the difficult-to-treat part of dining lampblack happens to have a fairly rich degree of FA.It is interesting to note that FA is strongly hydrophobic and its application as an oil-soluble extractant is quite mature [22].Through the dual action mechanism of physical absorption and chemical absorption,on the one hand,FA is absorbed and enriched.On the other hand,when FA accumulates to a certain extent,FA can be used as an oil-soluble absorbent to absorb other refractory oily components in dining lampblack,so as to achieve waste recycling.As shown in Fig.1.

Fig.1.CO2-controllable phase change absorbent solvent for dining lampblack treatment.

2.Materials and Methods

2.1.Materials

CO2and N2(purity >99%)were obtained from Chengdu Jinli Gas Co.,Ltd.Poly(propylene glycol) bis(2-aminopropyl ether) of average Mn~230 (D230),Toluene (purity ≥99%),oleic acid (OA,purity ≥98%) and heptane (purity >98%) were purchased from Shanghai Aladdin Biochemical Technology Co.,Ltd.Caprylic acid(CA,purity ≥99%),caproaldehyde (purity ≥99%),dioctyl adipate(DEHA,purity >99%) and tetraethylene glycol dimethyl ether(TEGDME,purity >99%) were purchased from Shanghai Macklin Biochemical Co.,Ltd.All materials were used directly without further purification.All the water used in the experiment is ionized ultrapure water by the ultrapure water machine (UPC-III-40L,Chengdu Chaochun Technology Co.,Ltd).

2.2.Physical property analysis

Thermogravimetric Analyzer (DSC823,METTLER TOLEDO,United states) was used to investigate the thermal stability of CA,OA and D230 (aluminum pan,N2flow,60 ml·min-1).The experiment of TGA was performed at 10 °C·min-1,respectively,in the ramp mode (from 40 °C ramp to 450 °C).

Measurements of the viscosity of these absorbents were carried out at the temperature 25 °C and at atmospheric pressure,using the HTHP rheometer (HAAKE MARS III,Thermo Fisher Scientific Co.,Ltd.).The density of the absorbent solvent at 25 °C was measured according to general rule for determination of density for chemical product(GB 4472-1984).All components were weighted by using the electronic analytical balance (FA2004B,Shanghai Youke Instrument Co.,Ltd.) with an uncertainty of ±10-4g.

The natural volatilization of the oily components was also tested and analyzed.50.00 g of the oily components (heptane,caproaldehyde and toluene) were placed in a surface dish with a radius of 30 mm and placed on an electronic analytical balance at room temperature (25 °C) for 1 h.The volatilization of the oily components at room temperature was obtained by measuring their mass changes before and after.

2.3.Phase change circulation

The combined system of D230 and FA has obvious CO2responsiveness,which facilitates FA in the system to switch between the two insoluble phases of oil and water.This phase change circulation can replace the desorption and regeneration link in the traditional absorption solvent.FA,originally a waste of lampblack,also becomes a recyclable absorbent solvent.The CO2switching behavior of CA and OA driven by 3%(mass)D230 solution was studied.At room temperature,CO2(gas velocity controlled at 100 ml·min-1)and N2(gas velocity controlled at 200 ml·min-1)were bubbled into FA-D solution formed by FA and D230 to realize hydrophilic and hydrophobic conversion of FA.The reversibility of the system was numerically verified by the reversible change of pH (measured by pH meter,PHS-3E,Shanghai Leizi Instrument Co.,Ltd.).

2.4.Absorption and desorption

Fig.2 shows the experimental set-up of the dynamic method implemented in order to obtain the absorptivityAfor 3 couples oily component/solvent (five solvents for each oily component).The purpose of this study is to compare the absorption differences between FA,excellent VOC absorbent and water for oily components in dining lampblack.As a consequence,the study no longer set up the air mixer and other devices to optimize the experimental process.

Fig.2.Experimental set-up for absorptivity.

Each experiment device is started in such a sequence to reduce the error between parallel experiments: constant flow N2purges and empties the gas in the system,the oil bath device sets the temperature for heat preservation,and the condensing device keeps the water circulation working.The above steps are maintained in a stable state for 15 min.Close the intake valve,inject the reagent into each bottle,wait for 2 min until the chemical reagent reaches the set temperature,open the intake valve and record the time for absorption test.

A polluted gas stream was generated by bubbling pure oily component in the volatile bottle with the carrier gas N2(gas velocity was controlled at (1±0.02) L·min-1viaa rotor flowmeter,MF5706-N-10,Guangzhou Dichuan Co.,Ltd.).The mass of the reagents in the volatilization bottle before and after the experiment were recorded asM1andM2,respectively.A tail gas receiver at the end of the system was used for quantitative control of the experiment,and its initial mass was 0 g.After completing the experiment without absorbent,the polluted air stream was switched and bubbled into the absorption bottle,and the massm1andm2of the reagents in the bottle before and after the experiment were recorded.The initial reagent mass of the volatilization bottle and the absorption bottle was set to 50.00 g,that is,the value ofM1andm1were 50.00 g.The data were taken from the average of the experimental results of two groups whose relative error is less than 0.5%.

The absorptivityAis calculated as Eq.(1):

The oily component (using the pipette to accurately transfer 1 ml) was solubilized with FA (1 ml) to form an oily mixture.The oily mixture was placed in a solution of D230 and reached equilibrium after sufficient contact for a period.FA will react with D230 and transfer to the water phase,so the oily component completes the desorption process (Fig.3).

Fig.3.Experimental set-up for desorption.

The oily components obtained by desorption were analyzed by GC/MS (7890A and 5975C,Agilent Technologies USA Co.Ltd.) to verify the loss of all reagents in the system.

2.5.Circulation of waste recycling

The CO2switching process of FA-D and the absorption and desorption process of oily components were combined to complete the construction of waste reuse cycle.The absorption mixture of oily component (heptane) and PCAS (CA) were mixed to carry out the simulation circulation experiment.

3.Results and Discussion

3.1.Establishment of system

Through literature investigation,the study integrated and referred to the dining lampblack produced by different types of cooking oils and cooking styles (Table 1),and listed its main components(Table 2),among which the main compounds were judged based on the components that accounted for the largest proportion in a certain class of compounds.The components of dining lampblack are mainly divided into four categories: fatty acids,alkanes,carbonyl compounds and aromatics.The components of dining lampblack are mainly divided into four categories: fatty acids,alkanes,alkenes,carbonyl compounds and aromatics.Generally,the fatty acid content of the products obtained by direct heating of edible oil is considerable,while the ranking of oil volatile componentcontentisalkanes>alkenes>carbonyl compounds >aromatics.Because the properties of alkenes are between alkanes and carbonyl compounds (the oil solubility of alkane chains and the oxidation susceptibility of unsaturated bonds),the study did not explore alkenes separately.

Table 1 Literature survey of dining lampblack

Table 2 Main ingredients of dining lampblack by literature survey

First of all,oleic acid,as a high content component[25]in most edible oils,need to be studied in depth,and it is quite reasonable to use it as a representative of unsaturated fatty acids.On the other hand,in particular edible oil,the distribution of fatty acids in different oils are different,for example,palm oil with palmitic acid as the main body is typical.The selected FA has good fluidity and thermal stability according to the principle of‘‘similar phase solubility”.Since the solvent in the circulating system is the best liquid at room temperature,octanoic acid is determined to represent saturated fatty acids,and oleic acid represents unsaturated fatty acids.Since the difference in physical and chemical characteristics of FA has no obvious influence on the system,such representation is reasonable.

According to the investigation results,heptane in dining lampblack has the highest content among all alkane compounds.The main aldehydes are acetaldehyde,caproaldehyde and nonanal among which the low molecular acetaldehyde is abundant at low cooking temperature,and the cooking method below the vapor temperature of water is unconventional.Therefore,this study only considered caproaldehyde,the aldehyde produced at higher cooking temperature.Toluene,as a recognized VOC,also is the highest content of benzene compounds in dining lampblack.Based on the above results,the representatives of oily compounds were determined,namely,heptane represents alkanes,caproaldehyde represents carbonyl compounds,and toluene represents aromatics.Itsrelated physical parameters are listed in Table 3.Obviously,they are all remarkably volatile at room temperature,especially heptane and toluene.

The results of volatilization are from experimental data Table S1(in Supplementary Material).

Dining lampblack is a gaseous gushing in the discharge outlet when the high temperature gas is liquefied for a long time to release latent heat.Obviously,the system can be treated in a high temperature environment is effective.The thermal decomposition temperature of PCASs was obtained by TG analysis to demonstrate its stability in high temperature environment(Fig.4).The thermal decomposition temperatures (Td) of the PCASs ranged from 97 to 174 °C,which were lower than the boiling temperature of each pure substance.The higherTdis obviously little influenced by the high temperature air flow in the discharge outlet,which implied the excellent temperature resistance characteristics of PCASs.Unsaturated fatty acids tend to oxidize during heating to form saturated fatty acids and the previous study has shown that the fatty acid with more double bonds is more prone to oxidation[28].This is the reason why saturated fatty acids are used for circulation,which also triggers the saturated fatty acid enrichment process.

Fig.4.TG and DTG curves of PCASs: (a) CA,(b)OA,(c)D230.

The physicochemical properties of the absorbents were listed in Table 4.Compared to the excellent absorbers DEHA and TEGDME,PCASs also have a lower viscosity and suitable density to become an ideal alternative to these organic solvents.

Table 4 Physicochemical properties of the absorbents

3.2.CO2-switchable behavior of PCASs

As we know,the ability of FA to realize CO2switching under the drive of D230 is the basis for the construction of the whole system.Thus,the CO2responsive behaviors of the FA-D were evaluated,as shown in Fig.5.Firstly,the homogeneous aqueous solution indicates that the complex FA-D (I) of FA and D230 has excellent hydrophilicity.Interestingly,the bi-phasic solvent (II) was obtained after bubbling CO2,and the homogeneous aqueous solution (FA-D,IV,it is also stage I of the second cycle) was presented again when D230(III)recovered by N2at 60°C addition combined with the separated FA again,which demonstrated the reversible phase change of this FA from hydrophobic state to hydrophilic state driven by CO2.With the first circulation of CA-D system as an example (Fig.5(a)),the pH value of the aqueous solution was decreased from 7.4 (I) to 6.67 (II),and was increased to 8.02 (III)upon N2/60 °C,finally restore to 7.25 (IV) after adding the separated CA.The reversible changes in pH value were highly consistent with the phase transition of CA.Similarly,OA-D system implements the above four steps in their entirety also (Fig.5(b)).

The system is regulated by CO2,and the chemical reactions that take place to follow Eqs.(2)-(4),of course,each phase corresponds to each equation.First,Eq.(2) illustrates the electrostatic binding of FA to D230 corresponding to its transition from a hydrophobic to a hydrophilic state (I).In Eq.(3),with the bubbling of CO2,the electrostatic force of FA-D was destroyed,FA could be reduced to a hydrophobic state and D230 was protonated to D(HCO3)2solu-tion(II)with a lower pH value.D(HCO3)2was reduced to D230(III)upon N2at 60 °C in the process of Eq.(4).More importantly,the phase transition of FA(Eq.(2))was presented upon a combination of protonated FA and deprotonated D230,obtaining the hydrophilic state (IV) again.The reversible phase transformation process of PCAS has established that this solvent has a simpler means of regeneration.

3.3.Absorption process of PCASs toward dining lampblack

The purpose of this study was to measure the absorption effect of PCASs for their ability to absorb oily compounds.Then the absorption experiment was carried out to test the absorption effect of oily component gas through FA solvent.Of course,in order to compare,we chose organic solvents DEHA and TEGDME with excellent absorption performance of VOC[29,30].The study is only to compare and evaluate the absorption effect,so the desorption experiment of the reference organic solvent will not be carried out.The experimental data of the absorbent absorption link are listed in Table S2 and Table S4,and the solvent absorptivity is shown in Fig.6.

Fig.6.The results of oily compound absorption by absorbents.

The results show that the blocking effect of oil-based solvent is better than that of water-based solvent.DEHA and TEGDME,as potent VOC absorbers,were indeed better at absorbing toluene and caproaldehyde,especially TEGDME.OA may be better at absorbing heptane,which may be due to the molecular structure of the long carbon chains.In conclusion,compared with the efficient absorbent,FA does not lag behind too much in performance,and it also has an excellent absorption effect for the oily component gas.FA has the regeneration means of CO2switch,so it has great development potential as a new absorbent to treat dining lampblack.

In this study,the experimental temperature of 90°C was chosen in the test of heptane absorption,because a large amount of heptane vaporized in the test of 100°C(which also shows that a large amount of heptane in dining lampblack is closely related to its own vaporization lead).The experimental data are shown in Table S3,such temperatures make the absorption process of the oily component like heptane liquefaction in a solvent,which is obviously not the best expression of ‘‘absorption”.Therefore,the absorption experiment at 90 °C slightly lower than the heptane boiling point(98 °C) was selected as the comparative analysis data.

The results of GC/MS analysis of the desorption experiments are shown in Fig.7.PACSs peak following their own boiling point order(Fig.7(a)).For the desorption process of the absorption mixture of heptane and toluene,no residual FA or D230 was detected in the oil substance obtained by the analysis (Fig.7(b),(d)),that is,it can be considered that the absorbent FA has achieved complete resolution.This shows that the selectivity of chemical reactions is better than that of physical separation alone.However,Residual FA and D230 were detected in the caproaldehyde precipitated from the absorption mixture for caproaldehyde(Fig.7(c)),which may be caused by the affinity of caproaldehyde to FA-D complex.Therefore,it can be concluded that PCASs will produce certain solvent loss with the increase of cycle times in the process of treating oil fume with more aldehydes.In addition,it is not ruled out that there are trace amounts of material outside the cycle with the accumulation of disadvantages.

Fig.7.GC/MS analysis spectra of desorption experiments: (a) PCASs,(b) heptane,(c) caproaldehyde and (d) toluene.

It should be added that in the actual application process,additional fatty acid as an absorption solvent is a quite good choice,because there may not be excessive fatty acid precipitation at the initial stage of absorption.In addition,in the experiments,the components with high boiling points (above 300 ° C,such as benzopyrene) were not dissolved in large quantities in the various reagents under study because of their solid form at room temperature.However,this does not affect the circulation operation of PCAS system.The content of high boiling point components in catering fume is very low,which generally provides the kernel for oily components to form tiny oil droplets,and the overall flow is still dynamic.Precipitation may be formed with the accumulation of the absorption process,and the separation operation of the solid-liquid two phases is easier.

3.4.Absorption circulation of PCASs toward dining lampblack

Here,the dining lampblack (heptane) was chosen to evaluate the practical application of PCASs(CA-D system)in the absorption process.As shown in Fig.8,the absorption circulation for dining lampblack was proposed based on the absorption and desorption approach for oily components by using PCASs.This absorption circulation is intuitively divided into three parts:(I) The saturated PCAS absorbed by the oily component reacts with the D230 solution,and the PCAS is transferred to the aqueous phase to complete the desorption step.The oily component remains in the oil phase and is removed by a simple phase separation operation;(II) The complex of PCASs and D in the aqueous phase realizes deprotonation under the regulation of CO2,and PCASs is reduced to the oil phase to precipitate out to complete the waste recycling;(III) D(HCO3)2aqueous solution obtained by protonation recovered to the original D230 solution upon N2/60 °C,thus the entire absorption circulation was closed.

Fig.8.Absorption circulation for heptane by using PCASs (CA-D system).

4.Conclusions

The absorption of three oily components by five absorbents(CA,OA,DEHA,TEGDME and water) was studied under high temperatures.Compared with DEHA and TEGDME,FAs have the same absorption effect on oily components,and both are more significant than water-based absorbent.Interestingly,with the assistance of D230,FAs can realize the transformation of hydrophilic and hydrophobic characteristics through the introduction and removal of CO2,thus completing the process of desorption from the absorbed oily mixture.According to the GC/MS analysis of the oily components separated by desorption,the absorption and desorption process of PCASs for heptane and toluene are almost without reagent loss,but the treatment of hexanal is not as excellent as the former two.The absorption circulation of PCASs toward dining lampblack was successfully completed,which indicates that the CO2regeneration and recycling method of PCASs provides a new idea for the treatment of dining lampblack.The combination of PCASs and purification equipment is expected to reduce the pressure of the end treatment method.

Data Availability

No data was used for the research described in the article.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the Chengdu Science and Technology Project of the Chengdu Bureau of Science and Technology(2021-YF05-00194-SN) and the Foundation of Department of Science and Technology of Sichuan Province (2022NSFSC0203).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2023.03.022.