Research ArticleStudy on Reutilization of Pyrolytic Residues of Oily Sludge
Chao Tang,1,2 Jiaojiao Guan ,3 and Shuixiang Xie1
1State Key Laboratory of Petroleum Pollution Control, Beijing 102206, China2Chongqing Water Resources and Electric Engineering College, Chongqing 402160, China3School of Petroleum Engineering, Yangtze University, Wuhan 430000, China
Correspondence should be addressed to Jiaojiao Guan; [email protected]
Received 17 May 2020; Revised 26 August 2020; Accepted 30 September 2020; Published 23 October 2020
Pyrolytic residues of oily sludge are a kind of hazardous solid waste produced by high-temperature pyrolysis of oily sludge, whichstill contains a certain amount of mineral oil; improper disposal can cause secondary pollution. In order to reutilize the pyrolyticresidues of oily sludge, the pyrolytic carbon in pyrolytic residues is recovered by a combination of physical flotation and chemicalseparation, and they are used for the treatment of oilfield wastewater and adsorption of oil. +e results showed that the purity ofthe pyrolytic carbon is 95.93%; many pores of different sizes are distributed on the surface, with mainly mesoporous distribution.Specific surface area, pore volume, and average pore diameter reach 454.47m2/g, 0.61 cm3/g, and 6.91 nm, respectively. Ad-sorption effect of pyrolytic carbon on COD and oil in oilfield wastewater is better than that of activated carbon at the samecondition. With regard to adsorption on diesel and crude oil, the initial instantaneous adsorption rate of pyrolytic carbon is 3.8times and 1.86 times faster than that of activated carbon, respectively. When pyrolytic carbon reaches saturated adsorption,cumulative adsorption of activated carbon on diesel and crude oil is much lower than that of pyrolytic carbon.
1. Introduction
+e requirement of reduction, harmlessness, and reutili-zation for oily sludge treatment provokes oilfield enter-prises and researchers’ interest upon the pyrolysistechnology, which is regarded as the most promising oilysludge treatment technology owing to its advantages ofhigh reduction effect, high recovery rate of oil and gasresources, and capability to immobilize heavy metals [1–3].+e pyrolytic products of oily sludge include three phases:liquid products are water, low condensation point crude,etc.; gaseous products are usually CH4, CO2, CO, H2, etc.;and solid products are residues in the reactor generallycalled pyrolytic residues after pyrolysis reaction. At pres-ent, research of oily sludge pyrolysis is mainly concentratedon pyrolysis process, output, and properties of pyrolytic oiland gas, and little works have covered on pyrolytic residueswith a large portion of pyrolytic products containing un-recovered oil and some heavy metals, which will causesecondary pollution if not disposed properly [4–6]. +epyrolytic residues of oily sludge are listed in “National
Catalogue of Hazardous Wastes” in China, and the cor-responding disposal has become a bottleneck restricting thedevelopment of oily sludge pyrolysis technology in oilfieldenterprises [7].
+e paper studied on the pyrolytic carbon in pyrolyticresidues recovered by the combination of physical flotationand chemical separation, and pyrolytic carbon is used for thetreatment of typical pollutants from oilfield wastewater andoil adsorption for the characteristics of its high carboncontent in residues after pyrolysis of oily sludge. +erefore,the use of pyrolytic carbon was clarified and the reutilizationof pyrolytic residues of oily sludge was realized.
2. Experimental
2.1. Analytical Method
2.1.1. Composition of Pyrolytic Residues and -eir HeavyMetal Pollutants. Pyrolytic residues of oily sludge were fromthe oily sludge pyrolysis station Liaohe Oilfield. Watercontent and oil content of residues were measured according
HindawiInternational Journal of Analytical ChemistryVolume 2020, Article ID 8858022, 7 pageshttps://doi.org/10.1155/2020/8858022
to standards GB/T 8929-2006 and SY/T5118-2005, respec-tively. Residual content was calculated by the method ofdispersion. Heavy metal pollutants were analyzed by iCAPRQ ICP-MS.
2.1.2. Characterization of Products. Determination of ashwas based on the standard of GB/T7702.15-2008. Carbonpurity was calculated by the gravimetric method (the ratio ofthe product after removing the ash to the mass of theproduct). +e elemental analysis was adopted by Quantax200XFlash5000-10 EDS. Analysis of surface properties andSEM were performed by using NOVA-2000e N2 adsorptionASAP and Quanta250 tungsten filament scanning electronmicroscope, respectively. Determination of iodine value wasbased on the standard of GB/T7702.7-2008.
3. Results and Discussion
3.1. Composition Analysis of Residues and-eir Heavy MetalPollutants. +e composition of pyrolytic residues in Table 1indicates that the oil content of residues exceeded the controlstandard of no more than 0.3%mineral oil content regulatedin “Pollutant Control Standards for Agricultural Sludge” inChina, which cannot be disposed for agricultural use di-rectly. Table 2 displays the extraction toxicity of heavy metalsin pyrolytic residues, which was less than regulation valuesof “Identification Standards for Hazardous Wastes-Identi-fication for Extraction Toxicity” and the first-order of “In-tegrated Wastewater Discharge Standard” in China,indicating that the heavy metal pollution will not be causedin the process of reutilization.
3.2. Pyrolytic Carbon Recovery from Residues and ItsCharacterization
3.2.1. Pyrolytic Carbon Recovery Method in Residues. It wasidentified that the pyrolytic carbon in pyrolytic residues wasrecovered by a combination of physical flotation andchemical separation after lots of experimental exploration.First, residues were charged into flotation column. Carbonand ash were separated by bubbles under the action of thecollector because of the difference between hydrophobicityand hydrophilicity of the surface of carbon and ash. Afterthat, compound acid solution was added to residues at thesolid-liquid ratio of 1 : 8, heating reaction was carried out for2 h, acid-soluble ash was removed by washing, and thencompound alkali solution was added at the solid-liquid ratioof 1 :10, heating reaction was carried out for 2 h, and alkali-soluble ash was removed by washing. After washing andfiltering, the filtrate was neutral. And it was dried and thepyrolytic carbon was obtained.
Ash and carbon purity of pyrolytic residues, residues byflotation, acid-soluble product, and pyrolytic carbon weremeasured, and the results shown in Table 3 indicate that theash content of pyrolytic residues was as high as 47.43%. Afterflotation, acid dissolution, and alkali dissolution, the ashcontent was reduced gradually and the carbon purity wasincreased steadily. At last, the ash content of pyrolytic
carbon dropped to 4.07%, and the purity of pyrolytic carbonreached 95.93%. +e element composition of pyrolyticresidues, acid-soluble product, and pyrolytic carbon weredetermined by using the X-ray fluorescence spectrometer.Activated carbon was provided by Hongsheng ActivatedCarbon Factory and was used for comparative research atthe same time. +e results shown in Table 4 indicate that thecarbon content of the pyrolytic residues was close to 40%,followed by aluminum, silicon, and iron compounds; afterflotation and acid treatment, the compounds aluminum,calcium, and iron were removed a lot. After alkali treatment,the compound silicon was removed a lot. After physicalflotation and chemical separation of pyrolytic residues, theelemental compositions of the product (pyrolytic carbon)and activated carbon were very similar.
3.2.2. Characterization of Pyrolytic Carbon. Surface prop-erties and iodine value of pyrolytic carbon were analyzed,and the comparative study with activated carbon was per-formed. +e results shown in Table 5 indicate that specificsurface area of pyrolytic carbon was smaller than that ofactivated carbon, while pore volume and average pore sizewere larger; pore size distribution was mainly mesoporous.Pore size of activated carbon was small and was mainly madeup of micropores. Iodine value was related to developmentof micropores [8], which also showed that the developmentof micropores in activated carbon was better than that ofpyrolytic carbon. Surface appearance of pyrolytic carbon andactivated carbon was analyzed, respectively, under thescanning electron microscope (SEM). +e results shown inFigure 1 display that the surface of pyrolytic carbon wasrough, pore distribution was not uniform, and pore size wasrelatively large, which were consistent with the results of thesurface property test. While activated carbon was compact,the pore size was small and distribution was uniform.
3.3. Treatment of Oilfield Wastewater by Pyrolytic Carbon
3.3.1. Single-Factor Study. Pyrolytic carbon was used for thetreatment of typical pollutants (COD and oil) in oilfieldwastewater. Oilfield wastewater was from Liaohe Oilfield,and the wastewater was filtered by the filter paper for re-moval of oil slick and suspended solids before the experi-ment. +e content of COD in wastewater was measuredaccording to the standard of HJ 924-2017. Determination ofoil content was based on the standard of HJ637-2018. CODand oil content in oilfield wastewater was 578.73mg/L and52.59mg/L, respectively. Experimental method: a certainamount of pyrolytic carbon was added into a conical flaskfilled with 100mL of oilfield wastewater, placed in a water-bathing constant temperature vibrator for a certain period oftime, and then filtered. +en, COD and oil content of thefiltrate were determined.
Figure 2 shows the change of COD and oil removal ratein oilfield wastewater under different adsorption time pe-riods when the dosage of pyrolytic carbon was 2 g, whichindicated that with the prolongation of adsorption time, thetreatment effect of COD and oil in oilfield wastewater was
2 International Journal of Analytical Chemistry
also increased. When the adsorption time exceeded 60min,the removal efficiency of COD and oil in oilfield wastewatertended to be stable, while it can be determined that the mostsuitable adsorption time for pyrolytic carbon to removeCOD and oil in oilfield wastewater was 60min. Figure 3shows the changes of COD and oil removal rate in oilfieldwastewater under different dosages of pyrolytic carbon whenthe adsorption time was 60min, which display that with thedosage of pyrolytic carbon increasing, COD and oil removalrate of oilfield wastewater increased gradually. When thedosage of pyrolytic carbon was 2 g, COD and oil content ofoilfield wastewater reduced to 47.22mg/L and 5.81mg/L,respectively, which met the requirements for grade 2 of “+eNational Integrated Wastewater Discharge Standard” inChina.
3.3.2. Comparative Study between Pyrolytic Carbon andActivated Carbon. Treatment effect of COD and oil in
100ml oilfield wastewater was performed with a 2 g ad-sorbent dosage at 60min for comparison of pyrolytic carbonand activated carbon. Results shown in Table 6 indicate thatthe adsorption capacity of pyrolytic carbon was better thanthat of activated carbon for the treatment of COD and oil inoilfield wastewater. Due to the high proportion of mesoporesin pyrolytic carbon, which can provide more effectivestorage space and diffusion channels for macromolecularorganics in oilfield wastewater [9, 10], the microporousstructure of activated carbon was not conducive to liquiddiffusion and the treatment was not as effective as pyrolyticcarbon [11–13].
3.4. Adsorption of Oil by Pyrolytic Carbon. DN40mm× 1000mm plexiglass column was used for ad-sorption of oil by pyrolytic carbon, diesel, and crude oilwhich were experimental oils. +e device is shown inFigure 4. In the experiment, 200 g of pyrolytic carbon wasadded into the column, and then 400mL of oil was injected.Time was counted when the oil was injected, and thedownward scale of oil products at regular intervals wasrecorded. When the oil flowed out of the adsorption col-umn, the time of the first drop of oil flowing out of thecolumn was recorded. +is time was the saturated ad-sorption time of pyrolytic carbon. +rough mass-to-vol-ume conversion, oil cumulative adsorption of pyrolyticcarbon at different periods of the entire adsorption processwas obtained. +en, the instantaneous adsorption rate ofpyrolytic carbon at different periods was calculated. Ac-tivated carbon was used for a comparative studythroughout the experiment.
3.4.1. Study on Instantaneous Adsorption Rate.Instantaneous adsorption rate of pyrolytic carbon and ac-tivated carbon on diesel was studied, and the results areshown in Figure 5, indicating that, at the initial 0.5min of theexperiment, the instantaneous adsorption rate of pyrolyticcarbon on diesel was 146.63mg/(g·min) and activated car-bon was only 38.68mg/(g·min). +e initial instantaneousadsorption rate of pyrolytic carbon on diesel was 3.8 timesfaster than that of activated carbon. +e instantaneousadsorption rate of pyrolytic carbon and activated carbon oncrude oil was studied, and the results shown in Figure 6indicate that the instantaneous adsorption rate of pyrolyticcarbon on crude oil was 228.75mg/(g·min) and activatedcarbon was 123.03mg/(g·min) at the initial 0.5min; theinitial instantaneous adsorption rate of pyrolytic carbon oncrude oil was 1.86 times faster than that of activated carbon.When the adsorption time was extended, the instantaneousadsorption rate of pyrolytic carbon and activated carbon wasreduced both on diesel and crude oil. However, the in-stantaneous adsorption rate of pyrolytic carbon was always
Table 1: Contents of pyrolytic residues.
Sample Water content, w (%) Oil content, w (%) Residue content, w (%)Pyrolytic residues 7.81 1.83 90.36
Table 2: Extraction toxicity of heavy metals in pyrolytic residues(mg/L).
Sample Cr Hg Ni Cu Zn Cd Pb AsPyrolyticresidues 0.036 0.002 0.031 0.046 0.033 0.001 0.024 0.016
A 15 0.1 3 100 100 1 5 5B 1.5 0.05 1 0.5 2.0 0.1 1.0 0.5A: regulation values of “Identification Standards for Hazardous Wastes-Identification for Extraction Toxicity”; B: first-order of “IntegratedWastewater Discharge Standard.”
Table 3: Ash and carbon purity of pyrolytic residues at differentstages.
Figure 5: Comparison of instantaneous adsorption rate of pyrolytic carbon and activated carbon on diesel.
Insta
ntan
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Figure 6: Comparison of instantaneous adsorption rate of pyrolytic carbon and activated carbon on crude oil.
International Journal of Analytical Chemistry 5
faster than that of activated carbon. It can be seen that if anoil spill accident was encountered, pyrolytic carbon canprevent the spilled oil from spreading and polluting theenvironment quickly because of its rapid adsorption per-formance, and the advantage was more obvious than that ofactivated carbon.
3.4.2. Study on Cumulative Adsorption. Cumulative ad-sorption of pyrolytic carbon and activated carbon on dieseland crude oil was studied, and the results are shown inFigures 7 and 8, indicating that the adsorption of pyrolyticcarbon on both diesel and crude oil reached saturated ad-sorption in 60min, and saturated adsorption capacities were748.69mg/g and 787mg/g, respectively. Saturated adsorp-tion capacity of activated carbon was larger than that ofpyrolytic carbon, but saturated adsorption time was longer;adsorption time of crude oil to reach saturated adsorptiontime exceeded 700min, and adsorption on diesel was morethan 1600min. When pyrolytic carbon reached adsorptionsaturation, cumulative adsorption of activated carbon on
diesel and crude oil was much lower than that of pyrolyticcarbon.
4. Conclusions
+e pyrolytic carbon in pyrolytic residues of oily sludge wasrecovered by a combination of physical flotation andchemical separation. +e content of carbon in pyrolyticcarbon reached 91.03%, and the elemental composition wassimilar to that of activated carbon.
+e surface of pyrolytic carbon was rough and pores ofvarious sizes were distributed, and the pore size distributionwas mainly mesoporous. Specific surface area, pore volume,average pore size, and iodine value reached 454.47m2/g,0.61 cm3/g, 6.91 nm, and 327.71mg/g, respectively.
When the dosage was 2 g and adsorption time was60min, the treatment effect of pyrolytic carbon on COD andoil in oilfield wastewater was better than that of activatedcarbon. +e treated oilfield wastewater can reach the re-quirements for grade 2 of “+e National IntegratedWastewater Discharge Standard” in China. With regard toadsorption on diesel and crude oil, the initial instantaneousadsorption rate of pyrolytic carbon was 3.8 times and 1.86times faster than that of activated carbon, respectively.Saturated adsorption capacity of activated carbon was largerthan that of pyrolytic carbon, but it takes a long time to reachsaturated adsorption. When pyrolytic carbon reached ad-sorption saturation, cumulative adsorption of activatedcarbon on diesel and crude oil was much lower than that topyrolytic carbon.
Data Availability
+e table data, figure data, and other related data used tosupport the findings of this study are included within thearticle.
Conflicts of Interest
+e authors declare that they have no conflicts of interest.
Acknowledgments
+is work was supported by the Open Project Program ofState Key Laboratory of Petroleum Pollution Control underGrant no. PPC2018004, CNPC Research Institute of Safetyand Environmental Technology; Science and TechnologyResearch Foundation of Chongqing Municipal EducationCommission under Grant no. KJQN201803805; and ScienceResearch Foundation of Chongqing Water Resources andElectric Engineering College under Grant no. KRC201802.
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