Intensified Distillation-Based Separation Processes: Recent Developments and Perspectives Greater sustainability can be achieved by decreasing the production costs, energy consumption, equipment size, and environmental impact as well as improvement of the raw material yields, remote control, and process flexibility. Process intensifi- cation (PI) as the main route for improving the process performance is used widely in heat transfer, reactions, separation, and mixing, which results in plant compactness, cleanliness, and energy efficiency. Some of the main intensified sep- aration processes and improvement mechanisms are reviewed briefly with the main focus on the PI of distillation processes, which are the most important sepa- ration methods. In addition to these technologies, the potential and reliability of reactive separation processes are addressed briefly, which will enable higher effi- ciency and capacity. Keywords: Energy efficiency, Innovative systems, Process intensification, Reactive separation processes Received: October 26, 2015; revised: March 13, 2016; accepted: September 09, 2016 DOI: 10.1002/ceat.201500635 1 Introduction The development of industrial systems for the year 2050 has been well defined in the recent Research Agenda with many strategic sectors, such as water, energy, food, and health, expected to take place consistently based on process intensifica- tion (PI) principles [1]. They require innovative methods for the design of equipment and processes that are expected to achieve significant improvements, such as higher energy effi- ciency, capital reduction, safety, environmental impact, and improved raw-material yields [2, 3]. However, there are several hurdles preventing the rapid implementation of PI technolo- gies in the process industry, such as the higher failure risk to the process industry, higher scale-up knowledge uncertainty, higher equipment unreliability, and increased safety, health, and environmental risks [4]. Separations, which currently account for 60 to 80 % of the process cost in most mature chemical processes, can be improved by using PI technologies [5]. Among these separa- tion processes, distillation, which comprises the largest share of the industrial energy use for separations, has attracted consid- erable attention to improve efficiency. A membrane that con- sumes less energy has the potential to replace conventional energy-intensive technologies. However, many challenges need to be overcome before the membranes can be scaled up using PI technologies. Considerable efforts have been made in the past to make new developments that go beyond traditional separation processes [6]. Materials and process-development strategies for improving the separation energy efficiency include replacing high-energy technologies, such as distillation, drying, and evaporation, with low-energy technologies, such as extraction, absorption, ad- sorption, membrane separations, crystallization, and physical- property-based operations or adopting PI strategies [7]. In par- ticular, to intensify the separation process, there are two areas [6]: process-intensifying equipment, such as Higee (high grav- ity) referring to rotating packed bed (RPB) [8], and process-in- tensifying methods, such as hybrid processes [9], dividing-wall column (DWC) [10], integration of reaction and separation [11], and techniques using alternative energy sources, including microwave, centrifugal field, and electric fields [3] (Fig. 1). Reactive separations, which are formed through the combi- nation of a selected separation process with a chemical reaction within a single unit, have attracted considerable attention in industry and academia. This technology can improve the reac- tion conversion and selectivity by removing the products from the reactive section and circumventing/overcoming the separa- tion boundaries, such as azeotropes in the distillation process [12]. However, reactive separation processes have some limita- Chem. Eng. Technol. 2016, 39, No. 12, 2183–2195 ª 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cet-journal.com Nguyen Van Duc Long 1,2 Le Quang Minh 1 Faizan Ahmad 3 Patricia Luis 4 Moonyong Lee 1 1 School of Chemical Engineering, Yeungnam University, Gyeongsan, South Korea. 2 Department for Management of Science and Technology Development & Faculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, Vietnam. 3 School of Science and Engineering, Teesside University, Middlesbrough, United Kingdom. 4 Materials & Process Engineering (iMMC-IMAP), Universite ´ Catholique de Louvain, Louvain-la-Neuve, Belgium. – Correspondence: Prof. Moonyong Lee ([email protected]), School of Chemical Engineering, Yeungnam University, Gyeongsan 712-749, South Korea. Review 2183
Greater sustainability can be achieved by decreasing the production costs, energyconsumption, equipment size, and environmental impact as well as improvementof the raw material yields, remote control, and process flexibility. Process intensifi-cation (PI) as the main route for improving the process performance is usedwidely in heat transfer, reactions, separation, and mixing, which results in plantcompactness, cleanliness, and energy efficiency. Some of the main intensified sep-aration processes and improvement mechanisms are reviewed briefly with themain focus on the PI of distillation processes, which are the most important sepa-ration methods. In addition to these technologies, the potential and reliability ofreactive separation processes are addressed briefly, which will enable higher effi-ciency and capacity.
Keywords: Energy efficiency, Innovative systems, Process intensification,Reactive separation processes
Received: October 26, 2015; revised: March 13, 2016; accepted: September 09, 2016
DOI: 10.1002/ceat.201500635
1 Introduction
The development of industrial systems for the year 2050 hasbeen well defined in the recent Research Agenda with manystrategic sectors, such as water, energy, food, and health,expected to take place consistently based on process intensifica-tion (PI) principles [1]. They require innovative methods forthe design of equipment and processes that are expected toachieve significant improvements, such as higher energy effi-ciency, capital reduction, safety, environmental impact, andimproved raw-material yields [2, 3]. However, there are severalhurdles preventing the rapid implementation of PI technolo-gies in the process industry, such as the higher failure risk tothe process industry, higher scale-up knowledge uncertainty,higher equipment unreliability, and increased safety, health,and environmental risks [4].
Separations, which currently account for 60 to 80 % of theprocess cost in most mature chemical processes, can beimproved by using PI technologies [5]. Among these separa-tion processes, distillation, which comprises the largest share ofthe industrial energy use for separations, has attracted consid-erable attention to improve efficiency. A membrane that con-
sumes less energy has the potential to replace conventionalenergy-intensive technologies. However, many challenges needto be overcome before the membranes can be scaled up usingPI technologies.
Considerable efforts have been made in the past to make newdevelopments that go beyond traditional separation processes[6]. Materials and process-development strategies for improvingthe separation energy efficiency include replacing high-energytechnologies, such as distillation, drying, and evaporation, withlow-energy technologies, such as extraction, absorption, ad-sorption, membrane separations, crystallization, and physical-property-based operations or adopting PI strategies [7]. In par-ticular, to intensify the separation process, there are two areas[6]: process-intensifying equipment, such as Higee (high grav-ity) referring to rotating packed bed (RPB) [8], and process-in-tensifying methods, such as hybrid processes [9], dividing-wallcolumn (DWC) [10], integration of reaction and separation[11], and techniques using alternative energy sources, includingmicrowave, centrifugal field, and electric fields [3] (Fig. 1).
Reactive separations, which are formed through the combi-nation of a selected separation process with a chemical reactionwithin a single unit, have attracted considerable attention inindustry and academia. This technology can improve the reac-tion conversion and selectivity by removing the products fromthe reactive section and circumventing/overcoming the separa-tion boundaries, such as azeotropes in the distillation process[12]. However, reactive separation processes have some limita-
–Correspondence: Prof. Moonyong Lee ([email protected]), School ofChemical Engineering, Yeungnam University, Gyeongsan 712-749,South Korea.
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tions, such as complex modeling needs, increased operationalcomplexity, limited applications, significant development costs,and extensive equipment design efforts [3].
This paper reviews some of the intensified separation pro-cesses and improvement mechanisms. In particular, this studyfocuses mainly on the PI of distillation, which has attractedsignificant interest from industry and academia. In additionto these technologies, the potential and reliability of reactiveseparation processes are addressed briefly, which would ena-ble the intensification of various production processes. Thispaper proposes and evaluates new efforts for the membrane-assisted reactive dividing-wall column (MRDWC), which isan innovative configuration combining reactive distillationused to overcome the chemical equilibrium and the dividing-wall column employed to reduce the energy requirement, witha membrane to overcome the azeotrope in azeotropic distilla-tion systems. The most important and interesting recent devel-opments in the current main research areas are summarized tohighlight the importance as well as the effects, challenges, andfuture prospects of PI for distillation-based processes.
2 Intensification of Separation Processes
2.1 Distillation
Distillation is a highly energy-demanding unit operation andnumerous modes of energy, or process-intensification alterna-tives, which are based either on the application of additional andalternative energy forms or on the manipulation of variousstructural parameters [13, 14], have been proposed and imple-mented over the years. Several reports considered intensifieddistillation, such as DWC [15], internal heat-integrated distilla-tion columns (HIDiCs) [16], Higee distillation [17], and even cy-clic distillation [18], as well as innovative reactive distillation[19] and reactive DWC [20], which will be discussed in Sect. 3.High-performance trays/packings are not reported in this paper.
2.1.1 Dividing-Wall Column
The thermal coupling technique is a good mechanism forimproving the conventional distillation sequence [21]. Amongthe configurations that employ the thermal coupling technique,a DWC is one of the best PI technologies and the most widelyapplied in industry, as shown in Fig. 2. This industrially proventechnology results in a reduction of the quantity of heatexchangers, enhanced separation efficiency, and energy andcapital-cost savings. Recently, the application of DWCs wasextended to azeotropic [22], extractive [23], and reactive distil-lations [24]. Several excellent review papers about DWCs havebeen published. In particular, a recent review by Dejanovic etal. [25] gave a comprehensive overview of DWCs, which cov-ered both theoretical descriptions and the patent area, whileanother review by Yildirim et al. [26] focused on current indus-trial applications of DWCs and related research activities,including the column configuration, design, modeling, andcontrol issues. Long and Lee [27] reviewed the retrofitting ofdistillation columns using a thermally coupled distillationsequence (TCDS) and DWC to improve energy efficiency.
The design, control, and application of DWCs of the three-component mixtures are well established [25]. Recently, theextension of DWCs to the separation of more than three com-ponents including the Kaibel, Sargent, and Agrawal arrange-ments have attracted attention (Fig. 3) [28]. The Agrawalarrangement, which showed better results in this study, wasapplied to improve the natural-gas recovery process [29]. TheKaibel arrangement is an interesting alternative considering itssimplicity by withdrawing a second side stream of a DWC. Thedesign of this column was almost solved, but the operation andcontrol still remain open issues [30]. The Kaibel arrangementis difficult to handle when minimizing the vapor flow rate at agiven product purity, whereas it appears to be easier to operatewhen the product purities are free. This arrangement can beused in refinery plants where the product purities do not play avital role or when a bottleneck problem occurs in the process
[30]. Recently, the Kaibel arrange-ment was investigated further bymeans of an examination of theeffects of vapor split manipulation[31].
Because DWCs can enhance theenergy efficiency in the distillationsequences, considerable effortshave been made to retrofit existingdistillation columns to reduce theenergy requirements and/or in-crease the capacity while maintain-ing the main product purity andrecovery (Fig. 4) [27, 32–34]. In anycase, a successful retrofit project isnormally based on the maximumemployment of existing equipmentto reduce the investment cost [35].Therefore, a dividing wall can beadded to the existing distillation toform a DWC for improving energyefficiency. In addition, one of thekey ideas in de-bottlenecking is tohave the DWC manage the in-creased load [36].
Most reports on the columninternals of DWC use the packingtype [26]. Recent efforts on thedesign of DWCs with trays, whichis important for systems operatingat high vapor loads, using compu-tational fluid dynamics were re-ported [37]. This study pioneersthe studies on the design of trays orpacking type, hydraulic and operat-ing conditions analysis for a rangeof DWC systems, particularly for
Figure 2. Schematic diagram of the (a) DWC, (b) TDWC, and (c) BDWC.
Figure 3. Schematic diagram of the (a) Kaibel, (b) Sargent, and (c) Agrawal arrangement.
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applications to floating, production, storage, and off-loadingfacilities (FPSO) and floating liquefied natural gas (FLNG),which have significant motion. DWC applications are expectedto increase and become a standard distillation piece of equip-ment over the next 50 years [38].
The heat-integrated distillation column (HIDiC) is the mostrevolutionary approach to heat pump design [39]. Remarkably,up to 70 % energy savings can be achieved using a HIDiC sys-tem compared to the conventional distillation columns; this isachieved by utilizing the heat from the rectifying section to boilthe stripping part of the column. However, many challenges,such as high investment costs, complex design, and controlproblems, need to be overcome before HIDiC can be con-ducted on a commercial scale [14].
Recently, an excellent review [39] provided a comprehensiveoverview of the latest developments of HIDiC, covering all themajor aspects related to the working principle, thermodynamicanalysis, potential energy savings, various design configura-tions, and construction options, design optimization, processcontrol, and operation issues, as well as pilot scale and poten-tial industrial applications. Remarkably, a development in thisarea was reported with a trade name, SuperHIDiC (Fig. 5),which was developed by Toyo Engineering Corp. [40]. How-ever, the separation of multicomponent mixtures using thistechnology is still an extremely challenging research issue [39].
2.1.3 Higee Distillation
Higee, which is a synonym for high-gravity technology, andutilizes centrifugal fields to form a rotating packed bed (RPB),provides another mechanism to improve the separation effi-ciency [8, 41, 42]. The fundamentals of mass transfer in RPBsneed to be explored and understood fully. The flow patternsinside a rotating packed bed are difficult to observe inside arotating packing [13]. Recently, a novel high-gravity device, therotating zigzag bed (RZB) shown in Fig. 6, which has a uniquerotor combining the rotational part with a stationary one, wasdeveloped to overcome the disadvantages of the rotating bed[43].
The application of Higee distillation is rather new [44], andit has not yet been established in industry. Nevertheless, inChina, approximately 200 units of a RZB have been commer-cialized [42]. Offshore applications of distillation might alsobecome feasible because the Higee unit is easier to operatewhen there is movement [13, 45].
2.1.4 Cyclic Distillation
Cyclic distillation is another intensified unit that has attractedincreasing attention. This enhances the separation efficiencybased on separate phase movement, which can save energyrequirements substantially [46, 47]. The cyclic operating mode
Figure 4. Schematic diagram of promising configurations forretrofit using DWC.
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consisting of a vapor flow period and a liquid flow period canachieve higher separation efficiency and higher product quality,as well as reduce the energy requirement and number of traysas compared to conventional distillation [18, 47].
However, cyclic distillation has several limitations [47–49].The application to vacuum distillation is difficult. The perfor-mance improvement is difficult to achieve when the columnhas more than ten simple trays [48]. Furthermore, the model-
ing and design of cyclic distillation col-umns is rather limited. Recently, a cyclicdistillation column on the industrial scalewas modeled with significantly improvedseparation efficiency compared to the con-ventional bubble-cap trays column, exceed-ing 200–300 % [47]. Sluice-champer trayswere proposed to prevent the limitationsassociated with the number of trays (Fig. 7)[47, 49]. The performance of a pilot-scalecyclic distillation column was first reportedfor ethanol/water separation [49].
2.2 Other Separation Processes
Considerable attention has been paid to thedevelopment and application of PI on sepa-ration, which requires substantial energy inmany chemical industries. Tab. 1 lists theother selected separation-process-intensifi-cation technologies. In recent years, amongthe various separation processes available,membranes have attracted substantial in-terest from industrial and academic per-spectives. Membrane engineering has thepotential to contribute towards process in-tensification by replacing the conventionalenergy-intensive separation techniques,such as distillation and evaporation, toachieve the selective and efficient transportof specific components, to improve the per-formance of the reactive processes, and toprovide reliable choices for environmen-
tally friendly industrial growth [70, 71]. Over the last fifteenyears, some membrane-based processes have been commercial-ized, such as membrane distillation, membrane absorbers,
Table 1. Summary of other selected separation-process-intensification technologies.
Process type Equipment type(or name)
Intensification technique(or mechanism)
Main remarks Ref.
Absorption Higee Rotating packed bed Significant volume reduction can be achieved using Higee overconventional packed beds.
[50]
Reactive absorption (RA) Integrating of reaction andabsorption in one single pieceof equipment
The most common use of RA is for the separation of gas mixtures(e.g., CO2, H2S, NOx, and SOx) in a solvent. The extensive effortsin CO2 capture and storage mainly stimulate the current growthof RA processes. Owing to the large chance for enhancing theregeneration of the solvent, which is responsible for up to 70–80 %of the operating costs, most current research had focused on thedevelopment of new solvents.
[51]
Adsorption Temperature swingadsorption (TSA)
The molecules are stronglyadsorbed at low temperatures,and the molecular sieves areregenerated at hightemperatures.
The CO2 capture from flue gases can consider TSA as aninteresting option. A gap between the emerging works aimedat determining quantitative structure-property relationshipsof adsorbents and the actual process performance still remains.
[52]
Pressure swingadsorption (PSA)
The molecules are stronglyadsorbed at high pressures,and the molecular sieves areregenerated at low pressures.
PSA is normally preferred over TSA because of its loweroperating cost.
[53]
Electrothermal swingadsorption (ESA)
Passing electricity through thesaturated adsorbent and theheat generated by the Jouleeffect facilitates the release ofgas.
This technology proposed for CO2 capture can potentially bemore energy effective than conventional TSA and PSA, thusreducing the CO2-capture cost.
[54]
Centrifugal adsorptiontechnology (CAT)
Centrifugal force CAT can lead to very good separation efficiencies but it requiresvery compact adsorption equipment with high capacity.
[55]
Reactive adsorption Integrating the mechanismof reaction and adsorptionin one single unit
This technology still needs to be applied commercially eventhough it is well proven on the laboratory scale.
[56]
Extraction Podbielniak extractor Centrifugal force Intense contact between the two liquids and generation of aninterfacial surface area for mass transfer.
[57, 58]
Electric field A 2- to 6–10-fold increase in the transfer rate can be achievedrelative to the no-field case.
[59]
Ultrasound The yield and mass transfer in many solid-liquid extractionprocesses can be enhanced.
[60]
Microwave The extraction time was reduced, less solvent was used, and theamount of compound extracted was increased.
[61]
Reactive extraction (RE) Integration of reaction andextraction in one single pieceof equipment
Most studies in the literature are focused on equilibrium, kinetics,and application of RE. The supercritical reactive extraction (SRE)process was also suggested for biodiesel production to achieve afaster rate. However, more fundamental research and developmentare needed before this relatively new technology can be used on acommercial scale.
A ‘‘research boom’’ and commercialization efforts for MD havebeen observed and realized. However, new and better membranes,improved design of the membranes and of modules, and betterengineering overall are necessary for further industrial exploitationof this technology.
[64]
Membrane absorption/stripping
Selective transportation of thegaseous component througha membrane while thecomponent is dissolved in theabsorbing liquid. In membranestripping, selected componentsare separated from the liquidphase using a stripping gas.
Most of the literature deals with a variety of membranes andmembrane materials at different pressures and temperaturesof operation using different classes of absorbents. The mostimportant industrial application area is the capture of CO2
from flue gas with significant reductions of up to 70–75 and 65 %in terms of the weight and size of the equipment, respectively.This can be advanced for offshore technology.
[65, 66]
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membrane extractors, membrane strippers, and membrane bi-oreactors [69].
3 Intensification Using ReactiveSeparations
The development and application of reactive separation pro-cesses integrating reaction and separation in a single set ofequipment has attracted significant interest that has resulted inequipment-size reduction, improved separation, a cheaper pro-cess, and improved reaction efficiency [9]. The integration of areaction with separation has been investigated extensively forreactive distillation but it has received less attention for reactiveabsorption, reactive adsorption, reactive extraction, and reac-tive membranes or membrane reactors.
Reactive distillation (RD) has been the most extensivelyresearched PI method over the last two decades [72]. Com-pared to conventional reaction-distillation sequences, theseso-called RD processes have enabled higher reactant conver-sion, product selectivity, as well as lower energy, water, and sol-vent consumption, thereby leading to reduced investment andoperating costs [73]. Normally, RD is applied to etherification,esterification, and alkylation on an industrial scale [74]. How-ever, compared to the design of a conventional distillation col-umn, it is much more sensitive to pressure due to the need for
a match between the temperature favorable for the reactionand the temperature favorable for separation [75].
The thermal coupling concept, which improves the thermo-dynamic efficiency, can be used to improve RD. In particular,TCDS and DWC, which are the best examples of proven PItechnology, are proposed to form a thermally coupled reactivedistillation sequence (TCRDS) (Fig. 8) [76] and reactive divid-ing-wall column (RDWC) (Fig. 9), respectively, as an attemptto improve the energy efficiency and/or increase the capacity.Up to now, only a few industrial applications of RDWC havebeen reported [26].
In addition, because membrane separation, such as pervapo-ration and vapor permeation, is not limited by vapor-liquidequilibrium, they can be considered to separate several non-ideal aqueous-organic mixtures, which form azeotropes [9].Therefore, a hybrid configuration comprised of membrane-as-sisted RD is attractive for improving many chemical processessustainably. The separation process can be intensified furtherby using a novel configuration of a MRDWC (Fig. 10), whichintegrates the RD used to overcome chemical equilibrium andthe DWC employed to save energy with a membrane to over-come the azeotrope. Note that the unreacted reactant separatedby the membrane can be withdrawn or recycled in theMRDWC. This intensified configuration can also be consideredin a retrofit design. More detailed research will be needed tobetter understand this process as well as to quantify the syner-gistic effects between the three unit operations. Insight analysis
This is characterized by theabsence of pore diffusion,which is the main transportresistance in conventionalcolumn chromatographyusing porous particles.
Membrane chromatography offers a process-intensive choice fordifferent chemical processes, such as ion exchange, metal affinity,and protein and reverse-phase chromatography. Until now, theliterature mostly discusses the approximate solutions neglecting thedispersion and kinetic aspects. In the future, the complex modeling,membrane support materials, surface chemistry, and device designneeds to be assessed.
[67, 68]
Membrane extraction The treated solution and thesolvent are separated fromeach other using a solid orliquid membrane.
On the commercial scale, hollow-fiber membrane solvent-extraction units are mostly used. Hydrophobic hollow fibers withhigher solvent resistance are required for most industrialapplications. Hydrophilic hollow fibers with an aqueous phasein the pores are needed for processes that involve organic solventextraction.
[69]
Membranecrystallization
A modified form of membranedistillation that combines theprinciples of membrane andcrystallization to achieveseparation
The study of a membrane crystallizer is a response to thehigh-market demand of high-value-added products. Moreover, italso allows control of the crystal shape, enhances crystal growth,and achieves selective crystallization. These advantages overconventional separation techniques make it an interesting processin the field of process intensification.
[66]
Membrane reactors(MRs)
Integration of the reactionand membrane in a single unit
The recent development of catalytic membranes and membranereactors, together with the membrane contactors units, mightpresent crucial opportunities for the redesign of industrialprocesses based on integrated membrane systems. However, theimplementation of commercial-scale membrane reactors is quitelimited. Therefore, the development of very thin, flawlessmembranes that can withstand high-pressure and high-temperature conditions is needed.
[56]
Table 1. Continued.
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should be studied and analyzed, which can be used to definethe operating range of this novel process.
Microwave-assisted RD has attracted considerable researchattention with its theoretical potential for enhanced separationefficiency and reaction rates [12]. However, there is a conflictin the obtained results. Werth et al. [77] examined the impactof microwave irradiation on the separation and reaction in aRD column, but observed no apparent enhancement either inthe separation efficiency on the macroscopic scale or in thereaction rate. In contrast, the recent result from the applicationfor acetic acid and ethanol esterification [78] showed theinstantaneous improvement of ethyl acetate content up to6.9 % under the same reflux ratio, and 6.7 % under the sameacetic acid feed-flow rate compared with the RD process. Fur-ther study will be needed to prove the potential and feasibilityof the concept of microwave-assisted RD.
A novel PI approach integrating a cyclic operation and RDin a single unit that outperforms classic RD was proposedrecently [79]. This study developed a rigorous mathematical
model and tested it with the synthesisof dimethyl ether to reveal the key ben-efits of a RD with cyclic operationmode.
Tab. 1 summarizes the brief mecha-nisms, the main applications, and re-cent status of other selected reactiveseparation processes. Up to now, RDand RA have been commercialized,whereas some processes, such as reac-
Figure 8. Schematic diagram of the TCRDC with (a) side rectifier and (b) side stripper.
a) b) c)
Figure 9. Schematic diagram of the (a) conventional reactive dividing-wall column, (b) top reactive dividing-wallcolumn, and (c) bottom reactive dividing-wall column.
Figure 10. Schematic diagram of the hybrid membrane-RDWCsystems.
Figure 11. Membrane reactor scheme [2].
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tive adsorption, reactive extraction, and membrane reactors(Fig. 11), still require significant fundamental research anddevelopment before becoming viable on a commercial scale.
4 Industrial Separation ProcessApplications
Because PI can reduce the investment cost and inventory, andimprove the heat management/energy utilization, it is expectedto have wide-ranging applicability, ranging from petrochemi-cals and bulk chemicals [4], fine chemical and pharmaceuticalsindustries [3] to biofuels [80], carbon capture [81], and off-shore processing [82]. Owing to the large volume productionof petrochemicals and bulk chemicals, reducing the energyconsumption and environmental impact are significant moti-vations of technology innovation. However, achieving improve-ments in the selectivity, yield, and processing time are moreimportant for fine chemicals and pharmaceuticals because theenergy cost comprises a smaller fraction of the production cost[7].
4.1 Intensified Separation Systemsin Petrochemicals and Bulk Chemicals
Numerous applications of PI technologies have been applied inpetrochemicals and bulk chemicals. Among them, reactive dis-tillations and DWCs are the most popular technologies with arange of applications. In particular, reactive distillations andDWCs have been implemented on a commercial scale in thepetrochemical industry more than 150 times [83] and morethan 100 times [4, 84], respectively.
Eleven chemicals have been identified to have significantopportunity for energy savings through the implementation ofPI technologies: ethylene, ethanol, chlorine/sodium hydroxide,ammonia, nitrogen/oxygen, ethylene chloride, propylene, ben-zene, ethylene oxide, methanol, and acetone [85]. Future appli-cations in these industries can consider many types of innova-tive multifunctional processes, such as RDWC, extractiveDWC, and hybrid configurations.
4.2 Intensified Separation Systems in FineChemical and Pharmaceutical Industries
Fine chemicals, which are normally used as starting materialsfor specialty chemicals, such as pharmaceuticals or agrochemi-cals, are produced in limited volumes with most operationsbeing batch based. Besides the trend of changing to continuousprocesses, many applications of PI technologies in these areashave been realized. The potential benefits of PI that have beenidentified are significant with overall cost reduction expectedto be up to 20 % (5–10 years) and 50 % (10–15 years) [86].
In particular, membrane chromatography has been appliedto a wide range of compounds, particularly for purification ofproteins [68]. Electrostatic fields have been employed toimprove the penicillin extraction process [3]. Drying, which is
a commonly used technique for pharmaceutical applications,has been intensified by using ultrasound, microwave, and elec-tromagnetic techniques, etc. [87, 88].
4.3 Intensified Separation Systems in RenewableEnergies (Biofuel)
The production of biofuels already takes advantage of advancedPI technologies, such as reactive separation processes [89].Most studies on reactive separation processes for biodiesel pro-duction were based solely on conventional reactive distillation[90, 91], or alternatives, such as entrainer-based [92] or dualreactive distillation [93] or reactive absorption [94, 95]. In par-ticular, a novel reactive-absorption-based biodiesel process wasproposed, which is a simple and robust process, achieving highconversion and selectivity, with no thermal degradation of theproducts, and no waste streams, as well as reducing the capitalinvestment and operating costs [94]. Pervaporation, which isalso described as a PI technology, can be applied to biodieselproduction [89].
The application of advanced or intensified distillation sys-tems is expected to be promising when continuing to developthe concept of biorefineries away from single-product systemsto multiple-product systems [96, 97]. In addition, the integra-tion of bio-based raw materials into existing conventionalplants will further push those intensified systems or hybridprocessing [96]. However, the challenges to the implementationof distillation systems to biotechnological processes remainwith respect to the operating conditions, such as handling solidsystems (e.g., enzymes, cells) and highly viscous systems, whichwill require the development of new distillation systems, suchas Higee distillation, which could fill this gap in specific cases[12].
An area of chemical engineering technology that has possiblygrown most rapidly within the past few years is carbon capture,the main aim of which is to reduce CO2 emissions from allplants that burn fossil fuels, such as steel works, offshore facili-ties, chemical plants, and possibly the largest challenge, powergeneration plants on shore, in particular, but not limited to,coal-fired power stations [3]. Normally, a packed column isdesigned to remove CO2 by chemical absorption [98, 99]. Toimprove the mass transfer rate in a conventional packed bed, arotating doughnut-shaped packing device, which refers toHigee or RPB, was suggested [100]. This technology promotessize and weight reduction, enhances the inherent safety withlower inventories, improves energy consumption, lowers thecapital cost, and addresses environmental concerns [81, 101].The associated centrifugal acceleration leads to droplet flowand film flow of liquids in the unit. This will increase the inter-facial area and consequently mass transfer. Based on this, thevessel size will be reduced significantly compared to conven-tional absorbers [101, 102].
Furthermore, membrane technology can be considered apromising separation method that overcomes the disadvan-tages of a gas absorption tower [103]. Considerable efforts havebeen made to increase the membrane performance with twomain approaches: gas permeation and membrane contactor.However, there is still a significant gap between the laboratoryand commercial scales in the industry [104]. This is particular-ly important for intensifying the desorption process in thestripper. In particular, agitation, ultrasound, and microwavescan be used to improve the release of CO2 [3, 105].
4.5 Intensified Separation Systems in OffshoreProcessing
Normally, trays are preferred in the distillation of hydrocar-bons. However, trays should not be applied to the distillationand separation columns of an offshore plant, such as FPSO andFLNG, which must have the ability to withstand waves [106].Therefore, packed-type column designs are suitable becausethe columns can withstand motion [82]. The walls formed bythe corrugated sheets reduce the impact of motion and makestructured packing a better choice than random packing [107].Furthermore, the use of a DWC for offshore FLNG plants,forming an energy-efficient and compact NGL recovery pro-cess, has been suggested [82].
In seawater deaeration or deoxygenation, contact plays animportant role in the process with the most popular techniquesfor contacting being packed, tray, or bubble columns, or agi-tated vessels [3]. Significant intensification can be obtained byusing either Higee technology or the tangential injection ofliquid into static vessels [108]. Kvarner recently evaluated amembrane absorption technology for removing CO2 from tur-bine exhaust gases in offshore applications [65]. Substantialdecreases in investment and operating costs relative to the con-ventional amine separation process are expected [109].
5 Conclusion
Economic and environmental considerations have encouragedmany separation processes in the chemical industry to befocused on PI-based innovative engineering solutions. The useof alternative energy sources, such as microwaves, centrifugalfields, electric fields, integration of multiple columns into oneunit forming a DWC, or an embedding reaction and separationinto a single unit forming reactive separation are the most radi-cal approaches of PI applications in distillation-based separa-tion processes. In particular, advances in reactive separationtechnology have many benefits in green processing technology,such as reducing the energy requirements, improving the reac-tion rate, and enhancing the productivity and selectivity, ulti-mately leading to high-efficiency separation processes. Thesetechnologies are promising and need to overcome several chal-lenges before their full potential can be realized. More inte-grated hybrid separation configurations combining RD orTCRDS or RDWC with other separation technologies also havegreat potential for future considerations of supplementing the
capability of each unit. The MRDWC, which combines RD,DWC, and membranes for azeotropic-mixture separation, is aninteresting and challenging example in this category. Further-more, different PI methods and different driving forces, such asmicrowaves, magnetism, gravity, and others, need to be consid-ered for integration in a unique system to achieve synergyeffects in PI research and development. To achieve the smoothdevelopment of separation processes, other issues, such as newcatalysts, membrane materials, new solvents, better rotarymachines, cheaper equipment fabrication, and reliable controlsystems, also need to be considered.
Acknowledgment
This study was supported by the Basic Science ResearchProgram through the National Research Foundation of Korea(NRF) funded by the Ministry of Education(2015R1D1A3A01015621), and by the Priority ResearchCenters Program through the National Research Foundation ofKorea (NRF) funded by the Ministry of Education(2014R1A6A1031189).
The authors have declared no conflict of interest.
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