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Desalination and Water Treatment li (2009) 73-80w w w . d e s w a t e r . c o m

A 1944-3994/1944-3986 © 2009 Desalination PubHcatíons. AU rights reserved

Used lubricating oil recycling using a membrane filtration: Analysis

of efficiency structural and composing

Yuhe Cao, Feng Yan, Jianxin Li* Xiaoping Liang, Benqiao He

Th e Key Lab of Hollow Fiber Membrane M aterials and Processes of Ministry cf Education,

School of Material Science and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300160, China

Tel. +86-22-2452 8 072; Fax. +86-22-2452 8001; email: [email protected] (j.X. Li) [email protected]

Received 29 March 2009; Accepted 31 August 2009

A B S T R A C T

Three kinds of polymer hollow fiber membranes—polyethersulphone (PES), polyvinylidenefluoride (PVDF), polyacrylonitrile (PAN)—were used for recycling of used lubricating oil. The

efficiency of membrane separat ion was characterized by m e a ns of the membrane reject ion and

the physical and chemical propert ies of the oils. The separation and analytical methods, for

exam ple, Fourier Transform Infrared (FTIR), U V-visible absorption spectra , column chrom atog-raphy, etc. were employed to illustrate the separat ion mechanism and the potential s t ructuresof used lubricant oil so as to assess the d i fferences betw een the used lubricating oil and the per-

meate. Results show that the membrane process can not only rem ove m etal part icles and dus t sfrom waste lubricant oi!, but also improve its liquidity and flash point. Further, the ultrafiltra-t ion membrane (PAN) has higher rejection than microfiltration membranes (PES and PVDF).The results of FT-IR and UV absorption spectra show that waste lubricant oil has almost 90%

long chain saturated hydrocarbons, which are originally from the base oil. The middle polari tycomposit ions may be lactones, esters, aldehydes, ketones, carboxylic acids, which may comefrom the oxidation of base oU and addit ives during the use of lubricating oil. Moreover, the

molecules of the reténtate contain arom atic rings as the basic unit in s t ruc ture . The conjugatedaromatic rings in the unit are mainly composed of two rings and three rings, whose connec-tions are "linear order", namely cata-condensed. Overall, it will provide much m ore referencedata for optimizing the regenerat ion processes of used lubricant oil.

Keywords: Hollow fiber membrane; Used lubricating oil; Column chromatography; UV-Visspec t rum; IR spec t rum

1. I n t r o d uc t i o n L ubr i c a ti ng o i ls are the m o s t va l ua b l e c o ns t i tue n t s in

l u » . - • ! _ , Jl. J £_:i; J crud e o i l . Was te or used lubr ica t ine oUs are bv- p r o d uc t sLubr ica t ing o i l s are us e d to reduce f rict ion and w e a r ^ ., , . , , , ,^ - ^ ,

bv inte rpos ing a film of m a t e r i a l be t w e e n r ubb i ng sur- "^ ?'̂ T "" , m a c hm e r y . T he y m us t be

f aces . Lubr ica t ing oUs mainly cons i s t of two materials ' fP '^ '^^d «" ^ ^^g""^^ bas i s m all o p e r a t i n g e q u i p m e n t

n a m e l y the b a s e oil and the chemica l add i t ives . Var ious ^"^ *" .*̂ ^ co r i t amina t ion f rom d i r t , meta l s c rapings ,

k i nd s of a d d i t i v e s are blended wi th the base o i ! acco rd - '''^^^'' ;"<^f"^P'^te products of c o m bus t i o n or o t he r

i.ig to its g r a d e and spec i f ic duty . These add i t ives can f^f ^ " ^ " ^ - 1 " prn^ciple, the c o m po s i t i o n of a typical used

be m e t a l l ic d e t e r ge n t s , z i nc d i a l ky! d i t h i o pho s pha t e s , l " bnc a t m g oil is a s tab!e d i spers ion of u n d e g r a d e d b a s e

ashles s d i spersants , ant i -oxidant , ant í -wate r , f r i c t ion °'^ ""'^ add i t ives wi th high concent ra t ion of m e t a l s ,

mod if ie r , ant i foam and po ur po i n t d e p r e s s a n t s [ 1] . l^""'^^' g " « ^ - \ f ^ "t^^^ ^f^^^^^<^ c o m p o u n d s c o m i n gf rom over lay of be a r m g s u r f a c e s and degradaf ion of the

f resh lubr icant components . The po lycycl ic a romat ic

^Correspond ing autho r . hyd roca rbon s (PAHs) wi!l be f o r m e d d u r i ng c o m bus t i o n

Presented at CESE-2009, Challenges in Environmental Science & Engineering, 14-17 July, 2009, Toionsville, Queenslan d, Australia.

d o i : IO.5OO4/dwt.2Oü9.845



y. Cao et al. / Desalination and Water Treatment 11 (2009) 73-80

in petrol engines and accumulate in lubricants over time.

PAHsareof particular concern due to their known carci-

nogenicity [2]. Therefore, recycling and refining of waste

into virgin lubricating oil may be a suitable option for

protecting the environment from hazardous waste [3].

Another benefit associated with used lubricating oilsrecycling could be the economic gain due to the high

price of mineral oils.

A number of processes such as coagulation, oxidiza-

tion of sulfuric acid, vacuum distillation [4], extraction

[5-71, hydro-treating [8] and adsorption [9-101 has been

used for purification and reprocessing of the lubricating

oils, ln addition to the low yield of oil, these traditional

treatment and regeneration technologies have impor-

tant disadvantages—second pollution and/or high cost

of energy. For this reason, improvement of processes for

regeneration of used lubricating oils is a pressing prob-

lem and the use of membrane technology is a promising

alternative for solving it.

Membrane technology for separation is a rapidly

emerging teclinology due to its continuous operation,

low energy consumption, easy adaptability, versatility

and the requirement for only relafiveJy mild operating

conditions in comparison with conventional separation

processes such as distillation, evaporation and crystalli-

zation. Membrane based separafions are well-established

technologies in water purification, protein separation

and gas separation. To date, commercial applications of

membrane technologies are limited to separations involv-

ing aqueous solufion and relative inert gases. The use of

membranes to treat non-aqueous fluids is an emergingarea in membrane technologies [11]. Membrane technol-

ogy is now being used for regeneration of used lubricat-

ing oils. Mynin et al. [12] reported the treatment of used

lubricant oils with inorganic membranes made in labo-

ratory condition. Results showed that the rejection was

98-100% for particulate contaminants, 25-40"''o for resins

and asphaltenes, 72-94% for ash content. The recovery of

regenerated oil was 85-90% with inorgarúc membranes.

The permeate was 6 L/(m^ h') at 0.6 MPa. In order to

remove the impurifies from a waste oil in high efficiency,

and to facilitate the separation of diluent and the cleaned

oil after permeation treatment, the waste oil was often

diluted with some diluents such as hexane, petroleumether, etc. before membrane separafion process [13-15].

Using membrane separation, the ash content of perme-

ate was found to decrease by 55-75% compared to the

convenfional method [16]. However, the diluents were

recycled from the permeate mixture by distillation which

was energy-consuming.

With thedevelopment of membrane technology, hol-

low fiber membrane has shown many advantages such

as high packing density, small footprint, etc. In the pres-

ent work, three kinds of hollow fiber membranes were

fabricated and used for purification and reprocessing of

used lubricafing oils. Tlie aim of this work is to inves

tigate the efficiency of membrane separation. Some

separation and analytical methods, for example, Fou-

rier Transform Infrared (FTIR), UV-visible absorption

spectra, column chromatography, etc. were employedto illustrate the separation mechanism and the compo

sitions and structures of waste lubricant oil purified by

hollow fiber membrane process so as to assess the differ

ences occurring between the used lubricating oil and the

permeate. It will provide much more reference data fo

optimizing waste lubricant oil regeneration process.

2. Experimental details

2.1. Materials

Waste lubricant oil used in this study was collected

from a vehicle garage. Tliree kinds of hollow fibermembranes—polyethersulphone (PES) with a pore size

of 0.1 [im, polyvinylidene fluoride (PVDF) with a pore

size of 0.1 |im and polyacrylonitrile (PAN) (MWCO

50 kDa) made by our laboratory—were used to separate

waste lubricant oil. Column chromatography filled with

silica gel (G60, Qingdao Haiyang Chemical Co., Ltd

China) were employed to separate different polarity

components from waste lubricant oil and permeate with

petroleum ether, ethyl acetate and ethanol as eluents

which were all analytical reagent grade.

2.2. Methods

2.2.1. Membrane separation processes and experimenta

design

A flow chart of regeneration of waste lubricant oi

with membrane technology is shown in Fig. 1. It can be

seen in Fig. 1 that the wasted lubricant oil was pretreate

by centrifugation for 30 min at the speed of 2000 r min

[12], and the pretreated waste lubricant oil was sepa

rated by hollow fiber membrane. The pretreatmen

could protect the membranes from the damage and foul

ing caused by large particles came from waste lubrican

oil. At the end of each separafion batch, the permeate

was sampled. The recovery was kept at 65'''o during al

the experiments. The PES, PVDF and PAN membrane

were carried out at 40̂ C and pressure 0.1 MPa.

The particle size distribution of the waste lubrican

oil and permeate was measured by Laser Particle Size

Analyzer (BT-9300H, Dandong Bettersize Instruments

Ltd, China). The transparency of oils wilJ present the

purity in a sense [17]. UV-visible absorpfion spectra

(UV-240, Shimadzu, Japan) were employed to measure

the absorbencies of the waste lubricant oil and perme-

ate so as to estimate (quantify) the membrane separation



Y. Cao et al / Desalination and Water Treatment 11 (2009) 73-80

E

O

10

Size (|im)

10 0

Fig. 3. Particle size distribution of the waste lubricant oil.

the particles which sizes were larger than 0.1 ^m wereremoved by the hollow fiber membranes.

In general, there are many impurities in the wastelubricant oil, such as sludge, carbonaceous particles,unbumed fuel, metal particles, tars, polymerized mate-rial and the like [12,18). These impurities are presenteither in suspension or in solution. Fig. 4 shows the pho-tos of the samples of waste lubricant oil and permeateobtain ed. It can be seen from Eig. 4 that the color of per-meate is red and transparent, while that of used lubricat-ing oil is black and non-transparent. This suggests thatthe hollow fiber membranes used in this study can effi-ciently rem ove most of the impurities.

In order to explore the efficiency of the membraneprocess, the chemical and physical properties of thewaste lubricant oil and permeate obtained by PAN weresum mariz ed In Table 1. As shown in Table 1, the viscosityof waste lubricant oil was 63.0 mm-s"' and lO.l m m's '^at40̂ C and lOO '̂C, w hile that of perm eate w as 29.8 m m - s '

and 5.3 mm^ s ' at 4lPC and lOO'̂ C. The viscosity ofpermeate became lower after membrane separation dueto elimination of sludge, carbonaceous particles, etcwhich were generated from all sorts of additives in lubri-cant oil product. Moreover, the flash point of permeateincreased from 214°C to 22O''C (Table I). Furthermore, theremoval rate of the cations in the waste lubricant oil wasover 70%. The ash content decreased from 0.82 wt % to0.17 wt % after membrane separation. As various addi-tives such as antioxidants, pour point depressants, viscosity index improvers, detergents, dispersants, and other

additives are added in the lubricant production in ordeto meet the deman ds of mod em engines, particularly fothose of internal combustion engines [19], the high ashcontent in the waste lubricant oil is typically associatedwith metals (mainly Zn and Ca) which as ingrediente ofadditives w ere added to the base oil [20].

Thus, the membrane process could not only removemetal particles and dusts from waste lubricant oil, bualso improve its liquidity an d flash point.

Fig. 4. Photos of the permeate and waste lubricant oil samples.

3.3. Com position analysis

Column chromatography was employed to separatewaste lubricant oil and permeate into different polar-ity fractions so as to illustrate the difference betweenthe used lubricating oil and permeate in their compo-sition. Three different elutions—Elution I (eluted bypetroleum ether), Elution II (eluted by ethyl acetate)and Elution HI (eluted by ethanol)—were obtained forpermeate by PAN separation. The elution results areillustrated in Eig. 5.

The waste lubricant oil was separated into foukinds of compositions which contents were 90.6 wt% inElution I, 3.8 wt % in EluHon II, 3.9 wt % in Elution lU



y. Cao et ai . j Desalination and Water Treatment 11 (2009) 73-80 77Table 1The chemical and physical properties of the used oil and permeate.

Index Waste oil feed

63.010.2

2140.82

54.911001860

60

Permeate

29.8

5.3220

0.17

14.1310286

10.8

Method of evc

GB/T 265-88GB/T 265-88GB/T 267-88GB/T 508-85

SH/T 0582-94SH /T 0309-92SH /T 0309-92SH /T 0061-91

Viscosity at 40°CViscosity at 100°C mm^ s"'Flash point (open cup)°CAsh content (%wt)

Metalcontent(ppm weight)

NaZnCaMg

120

TOO -

8 0 -

6 0 -

Ü4 0 -

2 0 *

///1 Waste lubricant oil

Petmeale

Left

Elution

Fig. 5. The elution results of waste lubricant oil and permeate.

1.7 wt % residual (Fig. 5). However, the permeate wasseparated into orüy two fractions. Their contents were95.6 w t "/. in Elution I and 4.4 wt % in Elution II. It imp liesthat the m emb rane process was able to eliminate someelutes in E lution 1Ü and the left (metal particles, carbondust, etc.).

3,4. Structure analysis

W aste/u sed engine oils differ in chemical and phy si-cal composition from virgin oil as a result of the changesoccurring during their use. Since membrane separa-tion in this study was merely a physical process, therewas no chemical change for the compositions of wastelubricant oil. We chose th e fractions ÍElution Ï, Elution II,Elution III) obtained from waste lubricant oil to inves-tigate their basic structures by FTIR and UV absorp-tion spectrum so as to analyze the differences occurringbetween the used lubricating oil and the permeate.

Fig. 6a show s the IR spectrum of Elution I. The bandat 1460 c m ' is the scissoring frequency of CH, gro ups

and that at 1379 c m ' is from the symm etrical bend ing

model of a CH.. group as show n in Fig. 6a. The po tentialfunction grou ps can be seen in Table 2. The absence of

bands between 1300 and 750 c m ' suggests a straightchain structure, while the band at 722 cm' indicatesthat there are four or more CH, groups in the chain.The C ^ H stretching bands confirm that no unsaturatedbond is presen t, since there are no ba nds ab ove 3000 cm '[21]. From the above observation, we can infer that theElution I only has saturated hydrocarbon chain, andthis part is mainly the base oil of lubricant oil.

Elution II was characterized by FT-IR. The band at1728 cm"' is assigned to the carbonyl stretching vibration(Fig. 6b). The band at 981 c m ' is inferred to be stretchingvibration of P-O-R, which m aybe come from the add itivesuch as zinc dialkyldithiophosphate. In addition, otherFT-lR bands of Elution II are sum marized in Table 3. It canbe found that there are possibly unsa turated hydrocarbonchains (at 1598 cm ') and azo groups (at 1259 and 1512 c m ' )in the fraction. Furthermore, it also can be seen that theband at 1728 cm ' is related to the carbonyl sti^tchingvibration of the oxidation produc ts (ASTM E-2412-0) [22].These have been identified as lactones, esters, aldehydes,ketones, carboxylic acids and salts, which could come fromthe oxidations of the base oil and additi\ es [23].

Tlie Elution III from waste lubricant oU was rejectedby th e PAN hollow fiber m em brane. The FTIR spectra ofElution III are shown in Fig. 6c and also summarized in

Table 4. It can be seen in Fig. 6c that the absorption band at1541 and 1631 c m ' responds to the stretching vibration ofC^=C. Tlieir potential function groups are aromatic rin^.These aromatic rings m aybe come from the oxidations ofthe aromatic compounds of lubricant oil. The absorptionbands at 1045, 1130 and 1209 cm ' respond to amidocy-anogens group of the com positions' molecular. The ami-docyanogens group m ay come from aromatic amine. Theabsorption band at 1735 cm ' corresponds to carbonylsstretching vibration, which maybe come from the oxida-tions of alkyl. The band between 3800 and 3000 cm' i srelated to hydroxyl groups or amidocyanogens associ-

ated, since the Elution in has strong polarity.



Y. Cao et al. / Desalimtion and Water Treatment 11 (2009) 75-80 79

Table 4

FTIR bands of the elution III.

rn Adscription Potential function groups

1045(w)1130 (w)1209 (m)1373 (w)1460 (s)1541 (s)1631 (w)1735 (w)2852 (s)2922 (s)2952 (s)3425 (s)

v C —ovC—OvC—O6CH,5CH,vC="C

v C = Ov C Hv C Hv C Hv O H V

v C—NvC—Nv C—N

v N H

NH

R—OH Ar—O—R Ar—H—COOH Ar—NH ArOH—COOH Ar—NH ArOH- C H ,—CH,—Ar—H Ar—NH NHRAr—H Ar—NH NHR—COOH —COOR—CH.—

— O H —COOH NH,

Note : w, weak intensity; m, middle intensity; s, strong intensity; N, stretching vibration; Ô, deformaHon vibration.

160 22 0 260 300

Wavelength (nm)

34 0 380 420

Fig. 7 UV spectra of elution III.

UV absorption spectrum was employed further toinvestigate the structures of fraction part from the usedlubricating o il. Fig. 7 shows the UV absorption spectraof elution in. There are maximum absorption peaksnear the wavelen gths of 222,280 and 330 nm . The ultra-violet absorption strength becomes weaker and weaker

with the increase in the wa\'elength. By using the sec-ond derivative spectroscopy analysis of the absorptiondata, the results display that Elution III has maximumabsorption peaks near the wavelengths of 222, 250, 258,269,294,308 and 315 nm. Further, the ultraviolet absorp-tion strength between 265 nm and 340 nm is very strong,which indicates that the compounds with mainly two orthree aromatic sheets exist in the reténtate fraction. Thisis consistent with the results obtained by Loeber et al.[23]. They employed model compounds. Comparedwith the model compounds' absorption wavelength, itis mainly naphthalene causing absorption at 220 nm.

The absorption near the wavelengths of 258, 269, 294,308 and 315 nm is cormected with three to four aromaticrings in mainly "linear order", namely cata-condensed.Since there is also absorptions near 340 nm, it indicatesthere are a few five aromatic rings and their aromaticrings are plane order, namely peri-condensed 123,24].

CH'erall, the resu lts of FT-IR and UV absorption spec-tra indicate that the molecules of Etution III contain aro-matic rings as the basic unit in structure, and have v erystrong hydrogen-bond action w ith each other.

4. Conclusion

Hollow fiber membrane filtration is a promisingmembrane technology for used lubricating oil recycling.The membrane process can not only remove m etal parti-cles and d usts from waste lubricant oil, but also improve



y. Cao et al /Desalination and Water Treatment 11 (2009) 73^0

its liquidity and flash point. Further, the ultrafiitrationmembrane (PAN) used has higher rejection than miero-filtration membranes (PES and PVDE). Furthermore,FT-LR and UV absorption spectra show that waste lubri-cant oil has almost 90% long diain saturated hydrocar-

bons, which are from the base oil. The middle polaritycompositions may be lactones, esters, aldehy des, ketones,carboxylic acids, which may come from the oxidation ofbase oil and additives during the use of lubricating oil.Moreover, the molecules of the reténtate contain aro-matic rings as the basic unit in structure. The conjugatedaromatic rings in the unit are mainly composed of tworings and three rings, whose connections are "linearorder", namely cata-condensed.

Acknowledgements

The authors gratefully acknowledge the NationalHigh Technology Research and Development Programof China ("863" Program, Grant No. 2009AA03Z223),National and Tianjin Natural Science Foundation ofChina (No. 20676100, 20876115, 08JCZDJC24000 andÜ8JCYBJC26400) for their financial support. ProfessorJianxin Li also thanks the financial sup port from the Pro-gram for New Century Excellent Talents in University(NCET-06-0250).

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