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Synoptic of the History of Thirty Years of Developments and Achievements inSelf-Cleaning Fluidized Bed Heat Exchangers

(Including the Citation of many Important References)

Dr.Ir.Ing. Dick G. Klaren M.Sc.President and Chief Scientist KLAREN BV

and

Ing. Eric F. de BoerProcess and Development Engineer KLAREN BV

Internal Report KBV Nr. 7March 2004

KLAREN BV, HILLEGOM, THE NETHERLANDSPhone: (31) 252 530606; Fax: (31) 252 530605; E-mail: info@klarenbv.com

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1. Introduction.

Dr.Ir. Dick G. Klaren, currently President andChief Scientist of the Dutch based companyKLAREN BV, started the development of theself-cleaning fluidized bed heat exchanger forseawater distillation plants in 1971, by findinga practical solution for the hydraulicstabilization of multi-parallel fluidized bedsand recognizing the excellent heat transferfilm coefficient in a fluidized bed at very lowsuperficial liquid velocities. Then onlystationary fluidized beds were consideredwhich did not apply circulation of thecleaning particles. See figure 1.

Figure 1: Self-cleaning heat exchanger withstationary fluidized bed of cleaningsolids.

For the purpose of designing seawaterdistillation plants equipped with liquid-solidself-cleaning fluidized bed heat exchangers,Dr. Klaren made use of the very limitedinformation available at that time about thehydraulics and heat transfer in liquid-solid

fluidized beds as presented in references [1]upto and including [5], and added the resultsof his own research and development work aspresented in reference [6].

In 1976, the Dutch Government supported theinnovative ideas of Dr. Klaren and subsidizedthe design and construction of a multi-stage-flash (MSF) seawater distillation plant with aproduction of 500 ton/day and equipped witha self-cleaning fluidized bed heat exchangerof 10,000 ft². This exchanger, with astationary fluidized bed in the tubes, consistedof 1600 parallel aluminum brass tubes with alength of 40 ft.

Figure 2: Multi-stage-flash (MSF) seawaterdistillation plant with stationaryfluidized bed heat exchanger.

In 1978, this evaporator, shown in figure 2,was put into operation and it convincinglydemonstrated that chemically untreatednatural seawater could be heated totemperatures of 115 °C without sufferingfrom fouling of its heat transfer surface due tothe precipitation of calcium carbonate scaleswhile using glass spheres with a diameter of

Outlet channel

Liquid

Cleaning solids and liquid

Heat exchangers tubes withupward flow and stationaryfluidized bed of cleaning solids

Tube plate

Shell

Distribution plate

Inlet channel

Inlet fouling liquid

Outlet fouling liquid

Inlet

Outlet

Tube plate

Liquid

3

only 2.0 mm as the cleaning particles.Moreover, this fluidized bed heat exchangershowed that very high heat transfercoefficients (3,000 W/(m²·K)) could beachieved at very low liquid velocities(0.12 m/s), which creates complete newdesign possibilities for large industrial heatexchangers.

For more information about this fascinatingdesalination plant technology, also referred toas Multi-Stage-Flash / Fluidized BedEvaporator (MSF/FBE), see references [7]upto and including [18].

2. The change from stationary fluidization tointernal circulation of the fluidizedcleaning particles.

In 1980, Dr. Klaren extended the design of thefluidized bed heat exchanger to a configu-ration which applies internal circulation of thecleaning particles. The principle of thisexchanger is shown in figure 3.

An argument in favor of this new innovationand improvement the very low liquid velocityin the tubes of the exchangers with astationary bed, which limited the freedom indesign of the exchangers and often resulted inunfavorable dimensions of the exchangers. Asa matter of fact, the introduction of internalcirculation was a great success during theperiod 1980 till 1992. Dr. Klaren and hiscollaborators were responsible for theinstallation of more than 60 self-cleaning heatexchangers with internal circulation in ninecountries with a total surface of 25,000 ft². Ontop of that 30 experiments were carried out allover the world.

For more information about the possibilitiesof this self-cleaning fluidized bed heatexchange technology applying internalcirculation of the cleaning particles, one isreferred to references [19] and higher.However, a substantial number of thesereferences also pay attention to theoreticaland/or experimental work dedicated to heat

transfer, hydraulics and fouling removalmechanisms and pilot plant testing, very oftenonly investigated for stationary fluidized beds.

Figure 3: Self-cleaning heat exchanger withinternal circulation of cleaning solids.

Operating experience with the manyexchangers proved the advantages of theinternally circulating fluidized beds oftenequipped with tubes of 1½” and cleaningparticles consisting of 2 mm chopped metalwire. However, there were also disadvantages.

Largest advantage is the perfectcleaning of the heat exchanger tubes.

Largest disadvantage is the ‘black box’effect, which means that operators couldnot easily monitor the rate of internalcirculation.

Or said otherwise:

The internal circulation of the particles

Outlet channel

Liquid

Cleaning solids and liquid

Risers (heat exchangers tubes)with upward flow of liquid andcleaning solids

Tube plate

Downcomer with downward flowof liquid and cleaning solids

Shell

Distribution plate

Inlet channel

Inlet fouling liquid

Outlet fouling liquid

Inlet

Outlet

Tube plate

4

through multiple parallel downcomerscannot be seen, hardly be measured,controlled or influenced. On top of that,there is the problem that malfunctioningof the internal circulation willimmediately cause problems with theperformance of the exchanger.

Other serious disadvantages are:

Internal circulation applies to theparticles and to the liquid. The amountof the naturally circulating liquid flowcan be as high as 20% to 60% of thefeed flow supplied to the exchanger.This causes a mixing temperature in theinlet channel of the liquid entering thetubes and has a detrimental effect on theΔTlog of the heat exchanger. In case of asaturated feed flow, the mixingtemperature would create a state ofsupersaturation which could produceheavy deposits on parts of the inletchannel and distribution system not incontact with the scouring action of thefluidized cleaning particles. Duringoperation, such deposits might breakloose and cause plugging of parts of thedistribution system.

A strong internal circulation may alsocause serious wear of parts of the inletchannel.

3. The change from internal circulation of thefluidized cleaning particles to externalcirculation.

In 1990, Shell, DuPont de Nemours andPechinay contacted Dr. Klaren to startdiscussions regarding the possibility ofreplacing internal circulation by externalcirculation. Shell expressed serious interestfor a wide range of applications of the self-cleaning heat exchange technology, butinsisted on a better controllability of thecirculating cleaning particles, which,according to Shell, could only be realized byexternal circulation. DuPont de Nemours and

Pechinay supported Shell’s ideas.

A study of the possibilities of externalcirculation by Dr. Klaren and co-workersresulted in a very favorable advice, as manymore advantages could be expected thanoriginally envisioned.

As a consequence, a large research anddevelopment program was proposed in whichthe above-mentioned companies and theDutch Government participated to a totalamount of approx. € 600,000.- to $ 700,000.-.This development program started with thedesign and construction of a large plexiglasstest installation for the investigation of thehydraulics at the end of 1993. One year later,the test results fully confirmed the highexpectations and even more. These verypositive results made it possible to shorten thetest program and already at the beginning of1995, it was decided to start with themarketing of this new technology.

The principle of this new self-cleaning heatexchange technology with external circulationof the cleaning particles through one externaldowncomer is shown in figures 4 and 5. Forthe disengagement of the cleaning particlesfrom the liquid, figure 4 shows aconfiguration which applies a cyclone. Acouple of years later, for a number of reasons,the configuration employing the cyclone wasreplaced by the configuration shown infigure 5, which employs a widened outletchannel.

The reactions received from the market forthis new technology employing externalcirculation were encouraging as will beexplained in more detail later. At thismoment, even operators of self-cleaning heatexchangers with internal circulation showinterest to have their system retrofitted into aheat exchanger with external circulation. Oneinstallation in the Netherlands, which hadbeen operating with internal circulation foralmost 15 years, was modified and has alreadybeen operating successfully for a couple ofyears employing external circulation.

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Figure 4: Self-cleaning heat exchanger withexternal circulation of cleaning solidsand cyclone.

A second installation in the United States isunder consideration.

The advantages of the self-cleaning heatexchange system employing externalcirculation can be summarized as follows:

This system has only one externaldowncomer, which allows for anaccurate measurement of the packed bedvelocity of the particles in the lower partof the downcomer. For this measure-ment, the acoustic Doppler Shiftmeasuring principle may be used. Sucha measurement is carried out from theoutside, i.e. through the tube wall,which has the advantage that themeasuring device cannot foul orcorrode. In a number of cases, i.e. atrather low pressure and temperature, asight-glass can be mounted in thedowncomer. For the operators, thismakes it possible to observe visually the

Figure 5 : Self-cleaning heat exchanger withexternal circulation of cleaning solidsand widened outlet channel.

flow of particles moving as a packedbed through the lower section of thedowncomer.

The packed bed velocity in thedowncomer can be influenced by asmall liquid flow supplied to the controlchannel. Varying this control flow alsovaries the particle flow. As a conse-quence, the cleaning action exerted bythe particles can be varied andcontrolled between a maximum andminimum value. The latter valuecorresponds to zero control flow and,consequently, to zero packed bed flowin the downcomer. This then results inthe situation where all particles arestored in the downcomer and wherethere are no particles in the heatexchange tubes.

External circulation of particles bymeans of a moving packed bed excludes

Outlet

Inlet channelwith distributionsystem

Heat exchanger

Inlet

Outlet channel

Outletfoulingliquid

Support

Cleaning solids

Downcomer withdownward flowof cleaning solids

Liquid +cleaning solids

Control channel

Inletfoulingliquid

1

1A 1B

2

4

3

1A = Main flow

1B = Control flow

Separator(cyclone)

Risers(Heat exchanger tubes)with upward flow of liquidand cleaning solids

1

1A 1B

2

4

3

1A

1B

Outlet channel

Inlet

Heat exchanger

= Main flow

= Control flow

Outlet

Inlet channelwith distributionsystem

Control channel

Liquid +particles

Downcomer withdownward flow ofcleaning solids

Cleaning solids

Support

Risers(heat exchanger tubes)with upward flow of liquidand cleaning solids

Outletfoulingliquid

Inletfoulingliquid

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circulation of liquid. This prevents wearof components in the inlet channel andalso avoids the formation of deposits onparts of the inlet channel anddistribution system caused by thepossible supersaturation of the liquid inthe inlet channel, since mixing of liquidflows at different temperatures does nottake place.

By very ‘difficult’ liquids, i.e. with ahigh viscosity and/or a high solidscontent, the packed bed flow of cleaningparticles can be ensured by the installa-tion of proprietary internals in theexternal downcomer.

The configuration shown in figure 5makes it possible to revamp existingconventional fouling vertical shell andtube exchangers in reboilers, evapora-tors and crystallizers into a self-cleaningconfiguration.

From the above, the conclusion seems to bejustified that the self-cleaning heat exchangetechnology with external circulation is to bepreferred over the system with internalcirculation. This is also reflected by the factthat in a period of only five years more than80,000 ft² of self-cleaning surface withexternal circulation have been ordered,installed, or already put into operation. That isthree times more surface than all the surfaceemploying internal circulation sold in a periodof twenty years.

What can be achieved with self-cleaning heatexchangers in comparison with conventionalexchangers can be best explained by means ofthe following example. In the 90s, a chemicalplant in the United States compared for theirseverely fouling application a conventionalsolution versus the installation of self cleaningheat exchangers employing external circula-tion of the cleaning particles. The result ofthis comparison is shown in table 1. As couldbe expected, but also convinced by a test,plant management decided in favor of theself-cleaning configuration. During operation,

Self-cleaningHEX

ConventionalHEX

Heat transfersurface (m²) 4,600 24,000

Pumpingpower (kW) 840 2,100

Number ofcleanings peryear

0 12

Table 1: Comparison of self-cleaning heatexchangers versus conventionalheat exchangers (HEX).

the expectations for the self-cleaning heatexchangers were fully met and were evenbetter. After 26 months of continuousoperation, the self-cleaning heat exchangersstill have not been cleaned. Figure 6 gives animpression of this unique installationconsisting of four parallel heat exchangers,each with a surface of 1,150 m² and a totalheight of the installation of approx. 20 m.

Figure 6: 4,600 m² Self-cleaning heatexchanger surface replacing24,000 m² conventional heatexchanger surface.

For the references presenting external circula-tion of the cleaning particles as shown in thefigures 4 and 5, one is referred to thepublications by Dr. Klaren and co-authorswhich were published in 1999 or later, such as[40], [41], [42], [43], [44], [45], [46], [49],[50], [51] and [52].

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4. More innovations, improvements and newinteresting applications.

Although the future for self-cleaning heatexchangers based on the above describedtechnology looks fascinating, there arereasons to believe that an even morefascinating and brighter future can beforeseen. Just a couple of years ago, acompletely forgotten method to separate theparticles from the liquid caught renewedattention. After some innovative modifica-tions the method has been improved and canbe considered as the superior separationmethod for this type of heat exchanger.Figure 7 shows this self-cleaning heatexchanger with the KLAREN separator on topof the downcomer.

Figure 7 : Self-cleaning heat exchanger withexternal circulation of cleaning solidsand KLAREN separator.

Next to this major improvement regarding theseparation of particles, another important

discovery has been made. It has beendemonstrated that self-cleaning heat ex-changers consisting of multi-parallel tubescould be operated with steel particles with adiameter dp equal to 2 mm in tubes with aninternal diameter Di of only 10 mm. Thisrequired a new design for the inlet channeland the distribution system. Existingtechnology could only use 2 mm steelparticles in tubes with an internal diameterequal or larger than 30 mm. As a result of thisvery important discovery, for steel particles aratio Di / dp ≥ 5 can be used , while withoutthese improvements this ratio should be 15 orhigher.

Using small thin-walled diameter tubes incombination with the new separator makes itpossible to design very compact self-cleaningshell and tube exchangers. From the point ofview of heat transfer, these exchangers shouldperform as good as plate exchangers, althoughthey have the advantages that they do not needgaskets, can be used for high pressures andtemperatures and are self-cleaning at the tube-side.

If we apply all these new ideas on the designof the already so successfully self-cleaningexchangers in comparison with conventionalexchangers as referred to in table 1, theadvantages of the self-cleaning exchangerswould even become more pronounced,because heat transfer surface would be furtherreduced by 30%, pumping power by 15% andinstallation height from 20 m to approx. 11 m.

Another consequence of the new develop-ments are the better possibilities to revampexisting severely fouling conventional verticalheat exchangers in reboilers, evaporators andcrystallizers. Many of such existing exchan-gers employ rather small diameter tubes and,for that reason, cannot that easily berevamped. This has been changed now, andalso ‘small tube diameter’ equipment can berevamped into a self-cleaning configurationand even operated with large diameterparticles which saves on the cost of therevamp. Figures 8 and 9 explain what is

Liquid +cleaning solids

KLAREN separator

Liquid

2

= Control flow1B

= Main flow1A

3

4

1B1A

1Inletfoulingliquid

Control channel

Liquid +cleaning solids

Downcomer withdownward flow ofcleaning solids

Support

Outletfoulingliquid

Outlet channel

Inlet

Heat exchanger

Inlet channel withdistribution sytem

Outlet

Cleaning solids

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Figure 8: Existing vertical conventionalreboiler.

Figure 9: Existing conventional reboilerrevamped into self-cleaningconfiguration.

meant by revamping a conventional reboilerinto a self-cleaning configuration.

Taking into account thirty years ofdevelopment and particularly including thelatest innovations and improvements, thescope of applications for the self-cleaning heatexchange technology has been broadenedtremendously. A few examples:

Application of self-cleaning heatexchangers for the production of slush-ice or ice-slurries for cold thermalstorage purposes in a.o. air-conditioningsystems. See also references [28] and[53].

Application of self-cleaning heat ex-changers in foul water steam generationplants. See reference [44].

Application of self-cleaning heat ex-changers in ‘mother-coolers’ for largechemical plants. See also reference [45].

Self-cleaning fluidized bed slurryheaters for the processing of a slurry ofhighly viscous non-Newtonian lateriteore for the extraction of nickel andcobalt in high-pressure-acid-leach(HPAL) plants to replace direct heatingby steam injection. See references [49]and [52].

Concept proposal for a self-cleaningreboiler with forced circulation andevaporation in the tubes, employing anew innovative method to monitor tubewear in combination with the inter-mittent use of the cleaning particles.Application is an oil-stabilisation plantin Saudi Arabia. See reference [54].

Compact self-cleaning shell and tubeheat exchangers as an alternativesolution for plate exchangers with theadvantages of ‘self-cleaning’performance gasket-free design, andsuitable for high pressures and hightemperatures. See reference [55].

Steam

Reboiler

Condensate

Outletchannel

Throttle plate

Column

Vapor

Liquid

Recirculatedliquid

Circulationpump

Inletchannel

Particles

Condensate

Existing outletchannel withinternals

Steam

Reboiler

New inletchannelsection

Throttle plate

Column

Vapor

Liquid

Recirculatingliquid

Circulationpump

Control channel

Downcomer

Liquid +particles

Existinginlet channel

Liquid

Liquid

KLARENSeparator

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Compact self-cleaning heat exchangersfor the ‘Standard Combined Feed /Effluent’ heat exchange application inthe Platforming Process. A comparisonbetween shell and tube Texas Tower,Packinox plate exchanger and shell andtube KLAREN Tower. See reference[56].

Concept proposal for a revamp of aseverely fouling conventional glycerineevaporator into a self-cleaningconfiguration applying the latest designprinciples and innovations. Location is achemical plant in the Netherlands. Seereference [57].

Concept proposal for a revamp ofseveral severely fouling conventionalmulti-effect evaporators / crystallizersinto a self-cleaning configurationemploying the latest design principlesand innovations. The conventionalevaporators suffer from Glauber’s saltdeposits and require weekly cleanings.Location is a chemical plant in Mexico.See reference [58].

Proposal for a revamp of severalseverely fouling conventional multi-effect phosphoric evaporators / concen-trators with carbon tubes into a self-cleaning configuration employing thelatest design principles and innovations.Location of this major phosphoric acidproducer is in the United States. Seereference [59].

Compact self-cleaning direct seawatercooled heat exchangers (with seawaterand cleaning particles in the tubes), toreplace air-coolers on offshore plat-forms. Under development.

Compact self-cleaning direct seawatercooled heat exchangers (with seawaterand cleaning particles in the shell andhigh-pressure natural gas in the tubes),to replace air-coolers on offshore plat-forms. Under development.

Gas / gas-cooled self-cleaning heatexchangers with cleaning particles in thetubes to prevent fouling and cloggingdue to hydrate formation. Underdevelopment.

5. Conclusions.

Developments of the kind as described abovecan take a very long time. For the self-cleaning heat exchange technology employingfluidized beds, it shows that it took more thantwenty years before major companies showedinterest. We believe that now the ban has beenbroken and then we refer to interest shown bya number of large international oil companiesand a variety of internationally operatingmining companies. It is regrettable that somany initially interested Research Instituteswith a reputation like the Technical Universityof Delft in the Netherlands and the TechnicalUniversity of Aachen in Germany gave uptheir early efforts of the 70s and 80s.

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[38] Liang, W.G., Zhu, J.X., Jin, Y., Yu,Z.Q., Wang, Z.W. and J. Zhou (1996);Radial non-uniformity of flow structurein a liquid-solid circulating fluidizedbed. Chemical Engineering Science 51(10), p. 2001-2010.

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[41] Klaren, D.G. and D.W. Sullivan (1999);Nonfouling Heat ExchangerPerformance in Severe FoulingServices: Principle, IndustrialApplications and OperatingInstallations. Spring National MeetingAIChE, Houston, Texas, April.

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[43] Klaren, D.G. and D.W. Sullivan (2000);Application of Self-Cleaning HeatExchange Technology in the Pulp andPaper Industry. Spring NationalMeeting AIChE, Atlanta, Georgia,March.

[44] Klaren, D.G. and D.W. Sullivan (2000);Self-Cleaning Heat Exchangers in FoulWater Steam Generators. 34th NationalHeat Transfer Conference, Pittsburgh,Pennsylvania, August.

[45] Klaren, D.G. and D.W. Sullivan (2000);Self-Cleaning Heat Exchangers in VeryLarge Closed-Loop Coolers. 34thNational Heat Transfer Conference,Pittsburgh, Pennsylvania, August.

[46] Klaren, D.G. (2000); Self-CleaningHeat Exchangers: Principle, IndustrialApplications and OperatingInstallations. Industrial Heat TransferConference, Dubai, UAE, September.

[47] Klaren, D.G. and D.W. Sullivan (2001);Improvements and Achievements in Self-Cleaning Heat Transfer. SpringNational Meeting AIChE, Houston,Texas, April.

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[49] Klaren, D.G. (2002); Self-CleaningFluidized Bed Slurry Heat Exchangers:A Breakthrough in the Hydrometallurgyfor the Processing of Laterite Nickeland Cobalt in High-Pressure-Acid-Leach (HPAL) Plants. 2002 SMEAnnual Meeting, Phoenix, Arizona,February.

[50] Klaren D.G. (2003); Principle andApplications for Self-Cleaning FluidizedBed Heat Exchangers and theRevamping of Severely FoulingConventional Heat Exchangers into aSelf-Cleaning Configuration. 2003SME Annual Meeting, Cincinnati, Ohio,February.

[51] Klaren D.G. (2003); Improvements andNew Developments in Self-CleaningHeat Transfer Leading to NewApplications. 5th InternationalConference on Heat Exchanger Foulingand Cleaning, Santa Fe, New Mexico,May.

[52] Klaren, D.G. and E.F. de Boer (2004);Revamp of Existing Directly HeatedLaterite Nickel Processing Plants intoan Indirectly Heated ConfigurationApplying Self-Cleaning HeatExchangers. 2004 SME Annual

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Meeting, Denver, Colorado, February.

[53] Meewisse, J. (2004); Fluidized Bed Ice-Slurry Generator for EnhancedSecondary Cooling Systems. Ph.D.Thesis, Delft University of Technology,Faculty of Design, Engineering andProduction.

[54] Klaren, D.G. and E.F. de Boer (2003);Proposal for a self-cleaning reboilerwith forced circulation and evaporationin the tubes employing tube-wearcontrol. KLAREN BV, Internal ReportKBV Nr. 1, September.

[55] Klaren, D.G. and E.F. de Boer (2003);Compact self-cleaning shell and tubeheat exchangers as an alternativesolution for severely fouling plateexchangers. KLAREN BV, InternalReport KBV Nr. 2, September.

[56] Klaren, D.G. (2003); Compact self-cleaning heat exchangers for the‘Standard Combined Feed / Effluent’heat exchange application in the

Platforming Process: A comparisonbetween shell and tube Texas Tower,Packinox plate exchanger and shell-and tube KLAREN Tower. KLARENBV, Internal Report KBV Nr. 3,October.

[57] Klaren D.G. and E.F. de Boer (2003);Proposal for a revamp of a severelyfouling conventional glycerineevaporator into a self-cleaningconfiguration. KLAREN BV, InternalReport KBV Nr. 4, October.

[58] Klaren, D.G. (2003); Proposal for arevamp of several severely foulingconventional multi-effect evaporators /crystallizers into a self-cleaningconfiguration. KLAREN BV, InternalReport KBV Nr. 5, October.

[59] Klaren, D.G. (2003); Proposal for arevamp of several severely foulingconventional multi-effect phosphoricevaporators / concentrators with carbontubes into a self-cleaning configuration.KLAREN BV, Internal Report KBV Nr.6, November.

Synoptic of the History of Thirty Years of Developments and Achievements inSelf-Cleaning Fluidized Bed Heat Exchangers(Including the Citation of many Important References)Introduction.The change from stationary fluidization to internal circulation of the fluidized cleaning particles.The change from internal circulation of the fluidized cleaning particles to external circulation.More innovations, improvements and new interesting applications.Conclusions.References.