Recovery of organic solvents from dewaxed oil mixtures using organic solvent nanofiltration technology

Spratt, Alexander (2019-04)

Thesis (MEng)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: Solvents are valuable chemicals, which are conventionally recovered through energy intensive processes such as distillation. In lube oil dewaxing processes, four times more solvent relative to solute is required. Solvents, such as toluene and methyl ethyl ketone (MEK), are commonly used in the dewaxing process. Due to high-energy costs, potential alternatives to recover these solvents are investigated. An alternative to solvent recovery through distillation is the use of Organic Solvent Nanofiltration (OSN) technology, which incorporates nanofiltration membranes designed to separate solvent-oil mixtures. The Max DeWax operation using OSN technology demonstrated successful recovery of solvent, while providing lower energy usage in solvent recovery. The research documented in this thesis focused on solvent recovery, membrane performance, transport modelling and techno-economic evaluation. The main aim of the project was to investigate the viability of OSN separation as an alternative method to conventional process separation. OSN viability evaluation was accomplished through demonstrating the recovery of solvent-oil mixtures using novel membranes. Experimental investigations performed in this study focused on recovery of four commercially available solvents used in lube-oil dewaxing processes, namely toluene, methyl ethyl ketone (MEK), methyl-isobutyl ketone (MIBK), di-chloro-methane (DCM) and the solute species which represented long-chain paraffin solutions, n-hexadecane (C16H34). Duramem™150, Duramem™200 and Puramem™280 membranes were used for the recovery process. Operating parameters, which include pressure, solute feed concentration, solvent type and membrane type, were varied and the effects thereof on membrane performance were investigated. The investigation also focused on describing the mass transfer through membranes using transport models, such as solution-diffusion and pore-flow transport models, using Matlab R2013a. Modelling of the mass transfer of MEK and toluene, well-known commercial solvents used in lube-oil dewaxing and processing, was done according to two pore-flow models (PF-1, PF-2) and two solution-diffusion models (SD-1, SD-2) using Duramem™150, Duramem™200 and Puramem™280 membranes. Furthermore, the permeability of hexadecane was regressed to fit the experimental data. The viability of OSN operations in comparison to distillation operations was determined through a preliminary techno-economic evaluation using simulation software (Aspen Plus V8.8) to describe the mass and energy and provide supporting data for use in cost evaluation. Energy consumption, equipment performance as well as operating and capital costs were investigated. The main contributions made by this study are threefold: (i) to demonstrate the successful recovery of solvent-oil mixtures using novel membranes through experimentation, (ii) to describe the transport through membranes using transport models and (iii) to investigate the economic feasibility of OSN systems, using simulation software. i)Recovery of solvents from oil mixtures This study found that the recovery of MEK from solute was the most successful while providing high membrane fluxes and high rejections over the rest, followed by DCM. Overall, MEK and n-hexadecane, at feed concentrations above 20 wt/wt% separated using Duramem™150 membranes, provided >90% rejection, while permeating at fluxes of approximately 12 L.mˉ².hrˉ¹. Membrane performance and solvent behaviour of permeating species were found to be affected mainly by applied pressure, chemical properties that describe polarity such as di-electric constant and dipole moment as well as properties such as molar volume, viscosity and solubility parameters. ii)OSN modelling and simulation The transport of pure solvent and binary solvent-solute mixtures was described using transport models based on literature. Using MEK and toluene, the two-parameter pore-flow model (PF-2)and the classical solution-diffusion model (SD-1) provided relatively good predictions forthe transport through the polar stable membranes such as Duramem™150 and Duramem™200, but poor predictive models for non-polar stable membranes such as Puramem™280. The PF-2 model and SD-1 model provided Pearson coefficients of >0.988 and >0.996, respectively, for the Duramem™ series membranes. The SD-1 model was further improved after regressing the estimated permeability parameter of hexadecane which provided an optimized Pearson coefficient of 0.9995. iii)OSN solvent recovery For both OSN and distillation systems, while ignoring the cost of raw material, It was found that the energy required to recover a ton of MEK solvent by OSN (i.e. 2.5 kWh.tonsolvent_product-1)is approximately 50 times less than that of distillation (135 kWh.tonsolvent_product-1) with energy recovery incorporated, which also results in a lower carbon footprint. However, by using theNelson-Farrar cost index, it was found that the capital costs for OSN ($0.85 million) in 2015 were approximately ~25% of the capital cost for distillation (i.e. $3.36 million). The operating costs, while ignoring the cost of raw material and having a total operating feed capacity of 1 ton.hr-1, were approximately $0.075 million.yr-1 for OSN operation and $0.155 million.yr-1 for distillation operation with a recycling stream and heat integration, while providing solute rejections as high as 97%. Total operating costs of OSN are less than half the amount required for distillation with heat integration, where energy and maintenance costs differ significantly between the two processes.

AFRIKAANSE OPSOMMING: Oplosmiddels is waardevolle chemikalieë wat op gerieflike wyse hernu kan word deur energie intensiewe prosesse soos distillasie. In smeerolie ontwassingsprosesse word vier keer meer oplosmiddel relatief tot opgeloste stof benodig. Oplosmiddels, soos tolueen en metieletielketoon (MEK), word algemeen gebruik in die ontwassingsproses. Weens hoë energie kostes, word potensieel alternatiewe prosesse vir die herwinning van hierdie oplosmiddels ondersoek. ‘n Alternatief vir oplosmiddel herwinning deur distillasie is die gebruik van Organiese Oplosmiddel Nanofiltrasie (OSN) tegnologie, wat nanofiltrasie membrane inkorporeer wat ontwerp is vir die skeiding van oplosmiddel-olie mengsels. Die Max Dewax proses, wat OSN tegnologie gebruik, is in staat om oplosmiddel suksesvol te herwin terwyl die proses ook laer energieverbruik getoon het. Die navorsing wat beskryf word in hierdie tesis fokus op oplosmiddel herwinning, membraan werksverrigting, transmembraanvervoer modellering en tegno-ekonomiese evaluering van die OSN proses. Die hoofdoel van die projek was om die lewensvatbaarheid van OSN as ‘n alternatief tot konvensionele prosesse te ondersoek. Die evaluering van OSN lewensvatbaarheid is voltooi deur die herwinning van oplosmiddel-olie met nuwe tipes van moderne membrane te demonstreer. Die eksperimentele ondersoeke wat uitgevoer is het gefokus op die herwinning van vier kommersieel beskikbare oplosmiddels deur gebruik te maak van smeerolie ontwassingsprosesse. Die vier oplosmiddels waarvan die herwinning ondersoek is, was tolueen, metieletielketoon (MEK), metiel-isobutiel ketoon (MIBK) en dichloormetaan (DCM). Die opgeloste stof, wat langketting paraffien oplossings verteenwoordig het, was n-heksadekaan (C16H34). DuramemTM150, DuramemTM200 en PuramemTM280 membrane is gebruik in die herwinningsproses. Bedryfsparameters insluitende druk, opgeloste stof toevoerkonsentrasie, oplosmiddeltipe en membraantipe, is gevarieer en die effek daarvan op die membraan werksverrigting is ondersoek. Die ondersoek het ook daarop gefokus om die massa-oordrag deur membrane te beskryf deur gebruik te maak van vervoermodelle, soos oplossing-diffusie en porie-vloei modelle in Matlab R2013a. MEK en tolueen is bekende kommersiële oplosmiddels wat gebruik word in smeerolie ontwassing en prosessering. Modellering van die massa-oordrag van hierdie twee oplosmiddels is gedoen volgens twee porie-vloei modelle (PF-1, PF-2) en twee oplossing-diffusie modelle (SD-1, SD-2), deur gebruik te maak van DuramemTM150, DuramemTM200 en PuramemTM280 membrane. Verder is regressie gedoen van die membraan deurlaatbaarheid van heksadekaan om ‘n passing met die eksperimentele data te verkry. Die lewensvatbaarheid van die bedryf van OSN sisteme in vergelyking met distillasie sisteme is vasgestel deur ‘n voorlopige tegno-ekonomiese evaluasie te doen. Hiervoor is simulasie sagteware (Aspen Plus V8.8) gebruik om die massa en energie te beskryf en om ondersteunende data te genereer vir gebruik in koste evaluasie. Energieverbruik, toerusting werksverrigting asook bedryfs- en kapitale kostes is ondersoek. Die hoof bydraes wat gemaak is deur hierdie studie is drieledig: (i) demonstrasie deur eksperimentering van die suksesvolle herwinning van oplosmiddel-olie mengsels deur gebruik te maak van nuwe, moderne membrane, (ii) die beskrywing van die vervoer deur membrane deur gebruik te maak van vervoermodelle en (iii) die ondersoek van die ekonomiese lewensvatbaarheid van OSN sisteme deur gebruik te maak van simulasie sagteware. i) Herwinning van oplosmiddels uit olie mengsels. Hierdie studie het gevind dat die herwinning van MEK uit oplossing die suksesvolste was, gevolg deur dié van DCM. Hoë membraanvloei en verwerpings is hiervoor verkry. MEK en n-heksadekaan is geskei deur DuramemTM150 membrane by toevoerkonsentrasies bo 20 gew./gew.% en >90% verwerping is gelewer teen deursypelingsvloeie van ongeveer 12 L.M-2.h-1. Daar is gevind dat die membraan werksverrigting en oplosmiddelgedrag van die deurdringende spesie hoofsaaklik geaffekteer is deur toegepaste druk, chemiese eienskappe wat polareit beskryf soos diëlektriese konstante en dipoolmoment, asook eienskappe soos molêre volume, viskositeit en oplosbaarheidsparameters. ii) OSN modellering en simulasie. Die vervoer van suiwer oplosmiddel en binêre oplosmiddel-opgeloste stof mengsels is beskryf deur gebruik te maak van vervoermodelle wat gebasseer is op wetenskaplike literatuur. Met MEK en tolueen as oplosmiddels, het die twee-parameter porie-vloei model (PF-2) en die klassieke oplossing-diffusie model (SD-1) goeie voorspellings verkry vir die vervoer deur polêr stabiele membrane soos DuramemTM150. Die voorspellingsvermoë van die modelle vir die vervoer deur nie-polêr stabiele membrane, soos PuramemTM280, was egter swak. Die PF-2 model en SD-1 model het Pearson koëffisiënte van >0.988 en >0.996, respektiewelik, verkry vir die DuramemTM reeks membrane. Die SD-1 model is verder verbeter deur regressie van die geskatte deurdringingsparameter van heksadekaan wat ‘n geoptimiseerde Pearson koëffisiënt van 0.9995 gelewer het. iii) OSN oplosmiddel herwinning. Vir beide OSN sowel as distillasie stelsels, terwyl die koste van rou materiaal geïgnoreer word, daar is gevind dat die energie wat nodig is om n ton oplosmiddel te herwin van OSN (2.5 kWh.tonsolvent_product-1) ongeveer 50 keer minder is as dié van distillasie (135 kWh.tonsolvent_product-1) met energie herwinning ingesluit , wat ook ‘n laer koolstofvoetspoor tot gevolg het. Daar is egter gevind, deur gebruik te maak van die Nelson-Farrar koste-indeks, dat die kapitale koste

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