Fluidized bed heat exchangers to prevent fouling in ice slurry systems and industrial crystallizers

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Fluidized Bed Heat Exchangers to Prevent Fouling

in

Ice Slurry Systems and Industrial Crystallizers

Proefschrift

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,

op gezag van de Rector Magnificus prof.dr.ir.J.T.Fokkema, voorzitter van het College voor Promoties,

in het openbaar te verdedigen op maandag 25 september 2006 om 15.30 uur door Pepijn PRONK

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Prof. dr. G.J. Witkamp Toegevoegd promotor: Dr.ir. C.A. Infante Ferreira

Samenstelling promotiecommissie: Rector Magnificus, voorzitter

Prof.dr. G.J. Witkamp, Technische Universiteit Delft, promotor

Dr.ir. C.A. Infante Ferreira, Technische Universiteit Delft, toegevoegd promotor Prof.dr.dr.-ing.habil. H. Müller-Steinhagen, Universität Stuttgart

Prof.dr.-ing. M. Kauffeld, Karlsruhe University of Applied Sciences Prof.dr.ir. P.J.A.M. Kerkhof, Technische Universiteit Eindhoven Prof.ir. H. van der Ree, Technische Universiteit Delft

dr.ir. J.S. van der Meer, Bronswerk Heat Transfer B.V.

Dit onderzoek is gedeeltelijk gefinancierd door Novem in het kader van het BSE-NECST programma.

ISBN 90-9020923-9.

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Summary

Fluidized Bed Heat Exchangers to Prevent Fouling in Ice Slurry

Systems and Industrial Crystallizers

Pepijn Pronk

The phase out of CFC and HCFC refrigerants and the restrictions to HFC refrigerants have led to a revival of natural refrigerants like ammonia and hydrocarbons in refrigeration systems. Since most natural refrigerants are toxic or flammable, indirect refrigeration systems are more frequently applied nowadays. The primary cycle of these indirect systems containing the hazardous refrigerant, is safely located in a machine room. The cold energy is distributed by a secondary refrigerant, usually an aqueous solution, to the locations where cooling is required. Ice slurry is an interesting secondary refrigerant for indirect systems, mainly because of its high heat capacity enabling cold thermal storage. A difficulty of ice slurry is however the marked tendency of ice crystals to adhere to cold heat exchanger walls, also referred to as ice scaling, which requires a mechanism to remove the ice crystals from the walls. In most ice slurry systems, scraped surface heat exchangers are applied for ice slurry production. The investment costs of these apparatuses are relatively high and therefore application of ice slurry as secondary refrigerant has been limited up to now. A new type of ice slurry generator using a liquid-solid fluidized bed may reduce the costs of ice slurry systems, which may lead to more widespread use of ice slurry as secondary refrigerant.

The main objective of this research is to study the capabilities of fluidized bed heat exchangers for ice slurry production in indirect refrigeration systems. The main focus is on the ability of liquid-solid fluidized bed to prevent ice scaling and on the physical mechanisms behind this phenomenon. Other objectives are to compare the fluidized bed ice slurry generator with competitive equipment and to investigate promising new industrial crystallization applications for the fluidized bed heat exchanger concept. A final objective is to study the behavior of produced ice crystals in other components of an ice slurry system, namely storage tanks and melting heat exchangers.

It is generally known that the ice scaling prevention ability of ice slurry generators is influenced by the solute of the aqueous solution. However, quantitative data on the role of solutes on ice scaling are lacking in literature and the physical mechanisms behind this phenomenon are not understood yet. Chapter 2 presents experiments with a single-tube fluidized bed heat exchanger in which ice crystals were produced from aqueous solutions of various solutes with various concentrations. The fluidized bed tube had a diameter of 42.7 mm and a height of 4.88 m, while a stationary fluidized bed consisting of stainless steel cylinders of 4 mm was operated at a constant bed voidage of 81%. The results reveal that ice scaling is only prevented when a certain temperature difference between the wall and the solution is not exceeded. This so-called transition temperature difference is approximately proportional with the solute concentration and is higher in aqueous solutions with low diffusion coefficients. The explanation for the observed phenomena is that ice scaling is only prevented when the mass transfer controlled growth rate of ice crystals on the wall does not exceed the scale removal rate induced by the fluidized steel particles.

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characteristics vary with fluidized bed conditions, such as the particle size, the bed voidage and the fluidization mode. Chapter 3 presents fluidized bed experiments in which a piezoelectric sensor was used to measure the impacts on the wall of both stationary and circulating fluidized beds. Impacts were measured for various fluidized bed conditions with particle sizes of 2, 3 or 4 mm and with bed voidages ranging from 69 to 96%. An analysis of the results shows two different types of impacts, namely collisions of particles on the sensor and impacts by liquid pressure fronts induced by particle-particle collisions close to the sensor. The impact measurements are used to formulate expressions for the frequency and the forces of both impact types. These expressions are subsequently used to analyze the total impulse and energy exerted by impacts on the wall for various fluidized beds. In stationary fluidized beds, both impulse and energy increase with increasing particle size and decreasing bed voidage. The impulse and energy exerted by particles on the wall of circulating fluidized beds increases as the circulation rate increases.

In Chapter 4, the influence of fluidized bed conditions such as fluidization mode, particle size and bed voidage on ice scaling and heat transfer coefficients during ice crystallization is experimentally studied. The single-tube fluidized bed heat exchanger was used to produce ice crystals from an aqueous 7.7 wt% sodium chloride solution. Both stationary and circulating fluidized beds were applied with various particle sizes varying from 2 to 4 mm and bed voidages ranging from 72 to 94%. The experimental results show that the ice scaling prevention ability of stationary fluidized beds increases with decreasing bed voidage and increasing particle size. Furthermore, the prevention of ice scaling appears to be more effective in circulating fluidized beds, especially at high circulation rates. A coupling of the results on ice scaling prevention and the impact characteristics shows that the prevention of ice scaling is realized by both wall collisions and pressure fronts induced by particle-particle collisions. The comparison reveals furthermore that the removal rate of ice crystals from the wall is proportional to the total impulse exerted by the impacts on the wall.

Besides the application of ice slurry production, fluidized bed heat exchangers may also be attractive for other industrial crystallization processes as is discussed in Chapter 5. From several industrial processes that suffer from severe crystallization fouling, two processes have been selected for an experimental study. First, experiments were performed on cooling crystallization of KNO3 and MgSO4.7H2O from their aqueous solutions showing that fluidized beds are able to prevent salt crystallization fouling. Next, eutectic freeze concentration experiments were performed from binary aqueous solutions of KNO3 and MgSO4, in which both salt and ice simultaneously crystallized. The experiments reveal that crystallization fouling during eutectic freeze crystallization is more severe than during separate salt or ice crystallization from the same solution. The explanation for this phenomenon is that the salt crystallization process eliminates the mass transfer limitation for ice growth resulting in an increased ice growth rate and more severe ice scaling. The addition of a non-crystallizing component strongly reduces crystallization fouling during eutectic freeze crystallization and enables to perform this process in fluidized bed heat exchangers at reasonable heat fluxes.

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investment costs per square meter are considerably lower for fluidized bed heat exchangers than for scraped surface heat exchangers. Due to the low ice scaling prevention ability at temperatures close to 0°C, fluidized bed ice slurry generators should be operated at ice slurry temperatures of about –5°C with a heat flux of approximately 10 kW/m2. Commercial scraped surface ice slurry generators are often operated with an ice slurry temperature of –2°C and a heat flux of 20 kW/m2. A comparison between these two systems for cooling capacities of 100 kW and larger shows that the investment costs of fluidized bed ice slurry generators are about 30 tot 60% lower than of scraped surface ice slurry generators. Furthermore, the energy consumption of ice slurry generators with fluidized bed is about 5 to 21% lower. It can therefore be concluded that the fluidized bed ice slurry generator is an attractive ice crystallizer concerning both investment costs and energy consumption.

One of the main advantages of ice slurry as secondary refrigerant is the possibility of thermal storage, which enables load shifting and peak shaving. During storage, ice crystals are subject to recrystallization mechanisms as attrition, agglomeration and Ostwald ripening. Storage experiments with ice crystals in various aqueous solutions are presented in Chapter 7 showing that Ostwald ripening is the most important mechanism inducing an increase in the average crystal size. The rate of Ostwald ripening strongly decreases as the solute concentration increases and the solute type and the mixing regime also play an important role. From these results is concluded that crystal growth and dissolution during Ostwald ripening are mainly limited by mass transfer, especially at higher solute concentrations. The obtained results are used to develop a computer-based dynamic model of Ostwald ripening in ice suspensions. Validation of this model with the experimental results shows that the model is able to predict the development of the average crystal size in time.

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Samenvatting

Wervelbed-warmtewisselaars ter voorkoming van ijsaankorsting in

ijsslurriesystemen en industriële kristallisatoren

Pepijn Pronk

Het uitbannen van CFK and HCFK koudemiddelen en de beperkingen voor HFK koudemiddelen hebben geleid tot een opleving van natuurlijke koudemiddelen zoals ammoniak en koolwaterstoffen in koel- en vriessystemen. Omdat deze natuurlijke koudemiddelen giftig of brandbaar zijn, worden indirecte koelsystemen tegenwoordig steeds vaker toegepast. De primaire kringloop van dergelijke indirecte systemen, die het gevaarlijke koudemiddel bevat, bevindt zich in een veilig afgesloten machinekamer. De koude wordt met behulp van een koudedrager, meestal een waterige oplossing, gedistribueerd naar plaatsen waar koeling nodig is. IJsslurrie, een suspensie van een waterige oplossing en ijskristallen, is een interessante koudedrager voor indirecte systemen. Het grote voordeel van ijsslurrie is de grote koudecapaciteit waardoor energieopslag economisch aantrekkelijk is. Een praktisch probleem van ijsslurrie is echter de sterke neiging van ijskristallen om aan de gekoelde wand van de warmtewisselaar te hechten, hetgeen ook wel ijsaankorsting wordt genoemd. Om dichtvriezen van de warmtewisselaar te voorkomen, is een mechanisme nodig dat de ijskristallen van de warmtewisselaarwand verwijdert. In de meeste ijsslurriesystemen worden hiervoor geschraapte warmtewisselaars gebruikt. De investeringskosten van deze apparaten zijn relatief hoog en daarom wordt ijsslurrie tot nu toe slechts op beperkte schaal toegepast als koudedrager. Een nieuw type ijsslurriegenerator die gebruik maakt van een vloeistof-vast wervelbed kan de kosten van ijsslurriesystemen beperken en kan daarom leiden tot bredere toepassing van ijsslurrie als koudedrager.

Het hoofddoel van dit onderzoek is het bestuderen van wervelbed-warmtewisselaars voor de productie van ijsslurrie voor indirecte koelsystemen. De focus is hierbij vooral gericht op de mogelijkheid van vloeistof-vast wervelbedden om ijsaankorsting aan de wanden van warmtewisselaars te voorkomen en de fysische mechanismen hierachter. Andere doelen zijn het vergelijken van wervelbed-ijsslurriegeneratoren met concurrerende apparaten en het onderzoeken van veelbelovende nieuwe toepassingen voor wervelbed-warmtewisselaars op het gebied industriële kristallisatie. Een laatste doel is het onderzoeken van het gedrag van geproduceerde ijskristallen in andere componenten van een ijsslurriesysteem, zoals buffertanks en smeltwarmtewisselaars.

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diffusiecoëfficiënt. De verklaring voor deze resultaten is dat ijsaankorsting alleen wordt voorkomen als de groeisnelheid van ijskristallen aan de wand, die wordt bepaald door stoftransport, niet groter is dan de verwijderingssnelheid die wordt bepaald door de deeltjes van het wervelbed.

Naast de invloed van de opgeloste stof wordt de mogelijkheid van wervelbed-deeltjes om ijsaankorsting te voorkomen ook sterk beïnvloed door de frequentie en sterkte van de deeltjesinslagen tegen de wand. Deze inslagparameters worden bepaald door wervelbedcondities, zoals de deeltjesgrootte, de bedporositeit en de wijze van fluïdiseren. Hoofdstuk 3 beschrijft experimenten met de enkelpijps wervelbed-warmtewisselaar waarin een piëzo-elektrische sensor is gebruikt om inslagen van deeltjes op de wand te meten in zowel stationaire als circulerende wervelbedden. De inslagen zijn gemeten bij verschillende wervelbedcondities met RVS deeltjes van 2, 3 en 4 mm en met verschillende bedporositeiten variërend van 69 tot 96%. De analyse van de resultaten laat twee verschillende soorten inslagen zien, namelijk botsingen van deeltjes op de sensor en inslagen door drukgolven als gevolg van botsingen tussen twee deeltjes vlakbij de sensor. De meetresultaten zijn gebruikt om empirische formules op te stellen voor de frequentie en de sterkte van de twee soorten inslagen. Deze formules zijn vervolgens gebruikt voor het analyseren van de totale impuls en de totale energie die door de inslagen worden uitgeoefend op de wand door verschillende wervelbedden. In stationaire wervelbedden blijken zowel de impuls als de energie toe te nemen als grotere deeltjes worden gebruikt of als een lagere bedporositeit wordt toegepast. De impuls en de energie uitgeoefend door de deeltjes op de wand van circulerende wervelbedden nemen toe als de circulatiesnelheid toeneemt.

In Hoofdstuk 4 worden de invloeden van wervelbedcondities, zoals de fluïdisatie modus, de deeltjesgrootte en de bedporositeit, op ijsaankorsting en warmteoverdracht tijdens ijskristallisatie experimenteel onderzocht. De experimentele enkelpijps wervelbed-warmtewisselaar is in dit kader gebruikt voor het produceren van ijskristallen in een waterige keukenzoutoplossing van 7.7 wt%. Voor deze experimenten zijn zowel stationaire als circulerende wervelbedden toegepast met deeltjesgroottes variërend van 2 tot 4 mm en met bedporositeiten tussen 72 en 92%. De resultaten van de experimenten laten zien dat de mogelijkheid om ijsaankorsting te voorkomen in stationaire wervelbedden toeneemt als de bedporositeit afneemt of de deeltjesgrootte toeneemt. Verder is de verwijdering aan ijsaankorsting effectiever in circulerende wervelbedden, vooral bij hoge circulatiesnelheden. Een koppeling van de resultaten over ijsaankorsting en de inslagkarakteristieken laat zien dat het voorkomen van ijsaankorsting wordt gerealiseerd door zowel de botsingen van deeltjes op de wand als ook door de drukgolven veroorzaakt door botsingen tussen deeltjes. De vergelijking laat verder zien dat de verwijderingsnelheid van ijskristallen van de wand evenredig is met de impuls die uitgeoefend wordt op deze wand.

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of ijskristallisatie vanuit dezelfde oplossing. De verklaring voor dit verschijnsel is dat zoutkristallisatie in de buurt van het ijsoppervlak de stofoverdrachtsweerstand voor ijsgroei opheft, waardoor de groeisnelheid van ijskristallen op de wand toeneemt en ijsaankorsting zeer moeilijk is te voorkomen. Het toevoegen van een niet-kristalliserende stof verkleint de neiging tot ijsaankorsting tijdens eutectische vrieskristallisatie aanzienlijk en maakt het mogelijk om dit proces uit te voeren met wervelbed-warmtewisselaars.

De prestaties van wervelbed-ijsslurriegeneratoren worden in Hoofdstuk 6 vergeleken met de prestaties van geschraapte ijsslurriegeneratoren. De laatstgenoemde apparaten gebruiken schrapers of roterende staven voor het verwijderen van ijskristallen van de wand en zijn momenteel de meest toegepaste ijsslurriegeneratoren. Experimenten met waterige KNO3 oplossingen laten zien dat het maximale temperatuurverschil voor het voorkomen van ijsaankorsting in geschraapte warmtewisselaars 7,5 maal groter is dan in wervelbed-ijsslurriegeneratoren. De warmteoverdrachtscoëfficiënt tussen wand en ijsslurrie is vergelijkbaar voor beide ijsslurriegeneratoren, terwijl de investeringskosten per vierkante meter aanzienlijk lager zijn voor wervelbed-warmtewisselaars. Door de geringe mogelijkheid om ijsaankorsting te voorkomen bij waterige oplossingen met vriespunten dichtbij 0°C, kunnen wervelbed-ijsslurriegeneratoren het best worden bedreven met ijsslurrie temperaturen rond –5°C en warmtestroomdichtheden van ongeveer 10 kW/m2. Commercieel verkrijgbare geschraapte ijsslurriegeneratoren worden vaak bedreven met een ijsslurrie temperatuur van –2°C en een warmtestroomdichtheid van 20 kW/m2. Een vergelijking van deze beide systemen voor koelcapaciteiten van 100 kW en groter laat zien dat de investeringskosten van wervelbed-ijsslurriegeneratoren ongeveer 30 tot 60% lager zijn ten opzichte van geschraapte ijsslurriegeneratoren. Daarnaast is het energiegebruik van ijsslurriegeneratoren met wervelbed zo’n 5 tot 21% lager. Samenvattend kan worden geconcludeerd dat de wervelbed-ijsslurriegenerator een aantrekkelijke alternatief is, zowel wat betreft investeringskosten als energiegebruik.

Eén van de grote voordelen van het gebruik van ijsslurrie als koudedrager is de mogelijkheid van koudeopslag, waardoor de koudeproductie kan worden verplaatst naar de nacht of pieken in de koudevraag over de gehele dag kunnen worden verdeeld. Tijdens opslag in buffervaten zijn ijskristallen onderhevig aan rekristallisatie mechanismen zoals attritie, agglomeratie en Ostwald rijpen. Hoofdstuk 7 beschrijft experimenten waarbij ijskristallen in diverse waterige oplossingen isotherm zijn opgeslagen. De resultaten laten zien dat Ostwald rijpen het belangrijkste mechanisme is dat zorgt voor een toename van de gemiddelde kristalgrootte. De snelheid van het Ostwald rijpen neemt sterk af met toenemende concentratie opgeloste stof. Daarnaast spelen de soort opgeloste stof en de mate van roeren een belangrijke rol. Uit de resultaten kan worden geconcludeerd dat het groeien en oplossen van kristallen tijdens Ostwald rijpen vooral wordt bepaald door stoftransport, vooral bij hoger concentraties opgeloste stof. De verkregen resultaten zijn gebruikt voor het opstellen van een dynamische model van Ostwald rijpen in ijsslurries. De validatie van dit model aan de hand van experimentele resultaten laat zien dat het model in staat is om het verloop van de gemiddelde kristalgrootte in de tijd te voorspellen.

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warmtewisselaar, waarin ijsslurrie door de binnenste buis stroomde en werd verhit door een waterige ethyleenglycol oplossing. De resultaten van deze experimenten laten een oververhitting zien die varieert tussen 0,5 en 5,0 K en afhangt van parameters zoals de snelheid, de gemiddelde kristalgrootte, de concentratie opgeloste stof, de ijsfractie en de warmtestroomdichtheid. De invloed van de diverse parameters wordt verklaard aan de hand van het smeltproces, dat kan worden beschouwd als een tweestaps proces. De eerste stap is het overdragen van warmte van de wand naar de vloeistof; de tweede stap bestaat uit het gecombineerde proces van stof- en warmteoverdracht tussen de ijskristallen en de vloeistof. Parameters als de kristalgrootte en de concentratie opgeloste stof hebben een sterke invloed op de tweede stap en daarmee ook op de mate van oververhitting. De gemeten trends voor de warmteoverdrachtscoëfficiënt tussen wand en vloeistof en voor de drukval zijn in overeenstemming met de trends die worden beschreven in de literatuur.

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1 Introduction

1.1 Recent Developments in Refrigeration

1.1.1 Reduction of Synthetic Refrigerants

In 1974, Molina and Rowland (1974) discovered that the emission of chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) into the atmosphere leads to the destruction of the ozone layer, which protects life on earth against too much ultraviolet solar radiation. From their introduction in the 1930s up to then, CFCs and HCFCs had been applied on a large-scale in refrigeration and air conditioning equipment. The findings of Molina and Rowland were confirmed by other researchers some years later and refrigerant producers began to search for alternatives (Powell 2002). After the discovery of the ‘ozone hole’ by Farman et al. (1985), governments agreed upon the Montreal Protocol in 1987, which prescribes the world-wide phase out of CFCs and HCFCs (UNEP 2003, IIR 2005a).

As alternative to CFCs and HCFCs, refrigerants without chlorine called hydrofluorocarbons (HFCs) were developed and successfully introduced in many different types of refrigeration equipment in the 1990s. Although some of these HFCs show good thermodynamic properties and are nonflammable and nontoxic, they appeared to be also strong greenhouse gases, just like CFCs and HCFCs. The emission of greenhouse gases into the atmosphere is believed to cause global warming and changes of local climates. The most important greenhouse gas in this respect is carbon dioxide (CO2), but also other greenhouse gases such as HFCs are believed to have a significant influence. Although the worldwide emissions of HFCs are relatively low compared to CO2 emissions, their contributions to global warming per unit of mass are considerably higher. In order to reduce global warming in the present century, governments drew up the Kyoto Protocol in 1997. In this agreement, industrialized countries agreed upon restrictions to greenhouse gas emissions by an average of 5.2% over the period from 2008 to 2012, compared to the period from 1995 to 2000. For the European Union the total reduction of greenhouse gases was set at 8% with respect to the emission level of 1990 (IIR 2005a, IPCC 2005, UNFCC 2005).

1.1.2 Revival of Natural Refrigerants

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-25.0% -20.0% -15.0% -10.0% -5.0% 0.0% 5.0% -35.0 -30.0 -25.0 -20.0 -15.0 -10.0 -5.0 0.0 5.0 Evaporation temperature (°C) Ammonia R407C R404A R134a Propane R el at iv e d if fer en ce in C O P com pa red t o a m m on ia

Figure 1.1: Comparison of COPs of propane, ammonia and some HFCs in a vapor compression system with 70% isentropic compressor efficiency and a condensation

temperature of 40°C

Besides the high energetic efficiencies, the application of ammonia as refrigerant shows additional advantages such as high volumetric refrigeration capacities, excellent heat transfer performance and the applicability in a wide temperature range (Lorentzen 1988). Despite these advantages, ammonia is not widely applied nowadays and its application is limited to installations in industrial environments such as food and beverage industry (Taylor et al. 2004). The main reasons for the limited use of ammonia are its toxicity and flammability. Ammonia gets toxic in air at concentrations of about 500 ppm. However, its smell is already noticeable at concentrations of 5 ppm and is intolerable at 50 ppm. Due to its distinctive smell, small leakages will be detected before dangerous situations will occur. Furthermore, ammonia gas is much lighter than air and is therefore easily vented away. Ammonia gets flammable in air at concentrations between 17 and 29 vol%, which is high compared to other flammable gases. These concentrations are not likely to occur in well-ventilated machine rooms and ammonia explosions are therefore unlikely.

The most promising hydrocarbons for refrigeration purposes are propane and iso-butane (Granryd 2001). Their only important disadvantage is the fact that they are combustible. The lower flammable limits of propane and iso-butane are only 2.1 and 1.3 vol% respectively, which means that relatively low amounts of hydrocarbon are sufficient to cause dangerous situations. Because of this threat, the application of hydrocarbons as refrigerant has been restricted to systems with low refrigerant charge or to systems located in well-ventilated machine rooms. For example, household refrigerators charged with iso-butane or propane are generally accepted by the public in many European countries (Radermacher and Kim 1996); in northern Europe the market is even dominated by these systems. For commercial installations however, the market share of hydrocarbons has been very small up to now (Granryd 2001).

1.1.3 Advance of Indirect Refrigeration Systems

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well-Introduction

ventilated machine room from where a secondary refrigerant distributes the cold energy to the applications where refrigeration is needed. The required primary refrigerant charge to operate the system is relatively small which also enhances the safety of the system. The secondary refrigerant is a safe and environmentally friendly fluid, for example an aqueous solution of potassium formate.

Figure 1.2: Schematic layout of indirect refrigeration system

An additional advantage of a secondary cycle is the possibility to store cold energy, which enables to shift electricity loads towards periods of the day with lower electricity tariffs. Another possibility of cold storage is peak shaving which results in a reduction of the required installed refrigeration capacity.

In principle, the extra heat transfer step in indirect systems reduces the energy efficiency compared to direct refrigeration systems. However, indirect systems can be operated with an energetic favorable refrigerant such as ammonia in the primary loop as a result of which the total system efficiency can be higher compared to a direct system with a synthetic refrigerant. The mentioned replacement for refrigeration of display cabinets in supermarkets is described by Presotto and Süffert (2001) and Horton and Groll (2003). According to these studies, both design calculations and measurements in practice show that indirect systems with ammonia use about 15% less energy compared to direct expansion systems using R22 as refrigerant. Furthermore, both studies report that the investment costs of indirect systems are comparable with direct expansion systems for cooling capacities of about 300 kW.

Apart from supermarkets, indirect refrigeration systems can also be applied in numerous other applications. The most widespread application is probably air conditioning in medium and large-sized buildings where chilled water is applied as secondary refrigerant. Similarly, secondary cycles can be applied for district cooling, such as in large warehouses for fresh foods. Other applications are found in industrial environments, for example in food and beverage industries.

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ethylene glycol, propylene glycol, and more recently potassium formate and betaine (Aittomäki and Lahti 1997, Jokinen and Willems 2004, Melinder 1997). The freezing temperature of these aqueous solutions depends on the type and concentration of freezing point depressant. An overview of freezing temperatures and thermophysical properties of several aqueous solutions is given in Appendix A.

In case of traditional liquid secondary refrigerants, only the sensible heat capacity is used and as a result relatively large amounts of liquid need to be circulated to provide enough cooling at the applications. The main disadvantages of these high circulation rates are the large pipe diameters and the high required pumping power. Moreover, large storage tanks are required to benefit from cold storage. In order to reduce these disadvantages, secondary refrigerants with phase change and therefore higher heat capacities have recently been investigated. The most important examples of these secondary refrigerants with phase change are carbon dioxide (CO2) and ice slurry.

In case of CO2, liquid refrigerant is pumped from the storage tank to the applications where it evaporates and takes up heat. Subsequently, the vapor flows to the evaporator of the primary cycle where it is cooled by the primary refrigerant and condensates. Finally, the condensate flows back to the storage tank. Indirect refrigeration systems with CO2 as secondary refrigerant have successfully been applied in supermarkets (Riessen 2004, Verhoef 2004). Disadvantages of CO2 as secondary refrigerant are the high pressures in the secondary cycle, especially at higher temperature levels. Application of CO2 in secondary cycles seems therefore more beneficial for freezing than for cooling purposes.

Ice slurry systems use the phase change of ice into water to take up heat from applications (Kauffeld et al. 2005). The heat capacity of ice slurry is therefore substantially higher than of liquid secondary refrigerants, which brings about energetic and economic advantages. A detailed description of the properties and possibilities of ice slurry is discussed in the next section.

1.2 Ice Slurry

1.2.1 Ice Slurry Properties

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Introduction

Figure 1.3: Microscopic picture of ice crystals

The temperature at which ice slurry can be applied ranges from 0°C down to approximately -30°C (Meewisse 2004). The initial freezing temperature, the temperature at which the original solution is in equilibrium with ice, depends on the type and concentration of freezing point depressant used (see Figure 1.4). Since produced ice crystals contain almost only water molecules, the concentration of solute increases as the ice fraction increases. As a result, the equilibrium temperature of ice slurry decreases as the ice fraction increases (see also Appendix B). Due to this phenomenon, ice slurries with low solute concentrations and initial freezing temperature close to 0°C show higher apparent heat capacities than ice slurries with higher solute concentrations. Therefore, ice slurries are most promising for temperatures between 0 and -10°C. -20.0 -15.0 -10.0 -5.0 0.0 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 Solute concentration (wt%) F ree zi ng t em per at ur e ( °C ) Ethanol Potassium formate

Sodium chloride Ethylene glycol

Figure 1.4: Freezing temperature as function of solute concentration

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Next to the heat capacity, also other thermophysical properties change as the ice fraction increases as shown in Figure 1.5 (Meewisse 2004). The most important property in this respect is the dynamic viscosity of the ice slurry, which increases by a factor of more than three as the ice fraction increases from 0 tot 25 wt%. The density of ice slurry slightly decreases with increasing ice fraction, while the thermal conductivity increases. More information on the thermophysical properties of ice slurries can be found in Appendix B.

0% 50% 100% 150% 200% 250% 300% 350% 0.0 5.0 10.0 15.0 20.0 25.0 Ice fraction (wt%) R el at ive p rop er ty c han ge Density Viscosity

Apparent heat capacity Thermal conductivity

Figure 1.5: Relative change of thermophysical properties at increasing ice fraction for ice slurry produced from a 9.2 wt% NaCl solution (Meewisse 2004)

1.2.2 Ice Slurry Systems

Indirect refrigeration systems with ice slurry as secondary refrigerant as shown in Figure 1.6, look very similar to systems with traditional secondary refrigerants. Ice slurry is produced in an ice slurry generator which is cooled by the evaporating primary refrigerant. The produced ice slurry flows to the storage tank from where it is pumped to the application heat exchangers. Here, the ice slurry melts and takes up heat from products or processes.

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Introduction

technique to prevent this is the application of scraped surface heat exchangers in which rotating scraper blades or orbital rods continuously remove the ice crystals from the walls (Stamatiou et al. 2005). Both investment and maintenance costs of these heat exchangers are relatively high. It is even generally believed that these high costs are one of the major factors that have limited a widespread application of ice slurry systems up to now. In this thesis, a new type of ice slurry generator is studied, which is based on a liquid-solid fluidized bed and has considerably lower investment costs especially at larger scales. The next section describes this fluidized bed ice slurry generator in detail.

After production, ice slurry is stored in a tank, which gives the opportunity to apply load shifting or peak shaving. In case of load shifting, ice slurry is produced during nighttime when electricity tariffs and outside temperatures are low resulting in economic and energetic efficient ice slurry production. In daytime, stored ice slurry is used in the application heat exchangers. In case of peak shaving, a constant amount of ice slurry is produced per unit of time while peaks in heat load are cooled by ice slurry from the storage tank. The main advantage of this control strategy is the possibility to install less primary refrigeration capacity than for the case without thermal storage. The best control strategy strongly depends on the load profile of the specific application, but also on external aspects as electricity tariff structures and local climate conditions (Meewisse 2004).

The latent heat of ice slurry is exploited in applications where the ice crystals melt. In most applications, ice slurry flows through heat exchangers, but it is also possible that the ice crystals melt in direct-contact with the products that need cooling. The heat transfer process taking place in melting heat exchangers can strongly differ from single-phase heat transfer processes in terms of heat transfer coefficients and pressure drop (Ayel et al. 2003). Furthermore, the melting process can operate far from equilibrium resulting in superheated ice slurry at the outlet of the heat exchangers (Frei and Boyman 2003).

1.2.3 Applications of Ice Slurry

Up to now, ice slurry systems have been applied for several applications in comfort cooling and in food processing and preservation (Bellas and Tassou 2005).

Some typical examples of realized comfort cooling projects can be found in Japan, such as the air conditioning systems of the Kyoto station building complex and the Herbis Osaka building in Osaka (Wang and Kusumoto 2001). In South Africa, ice slurry has been applied for cooling of gold mines with depths of more than 3000 meters where temperatures normally exceed 50°C (Ophir and Koren 1999). Drawback for air conditioning applications is the maximum temperature of 0°C at which ice slurry can be applied. Evaporation temperatures in the primary cycle are therefore around –5°C which is considerably lower than in standard air conditioning systems operated with water as secondary refrigerant where the evaporation temperature is normally about 2°C. The lower evaporation temperature induces higher energy consumptions for ice slurry systems.

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(Crielaard 2001, IIR 2005b). Refrigerating equipment accounts for approximately 40 to 70% of the total power consumption of supermarkets and daytime use of slurry produced at night generates considerable savings. Besides the mentioned applications, ice slurry might also be applied for several other applications such as fire fighting, instrument cooling and medical uses in the future (Davies 2005).

1.3 Fluidized Bed Heat Exchanger

The development of an efficient and inexpensive ice slurry generator is one of the key factors to make ice slurry technology more economically feasible. A promising ice slurry generator in this respect is the fluidized bed heat exchanger in which inert fluidized particles remove ice crystals from the heat exchanger walls.

1.3.1 Working Principle and Current Applications

The concept of a liquid-solid fluidized bed heat exchanger was proposed by Klaren (1975) for sea water desalination in the early 1970s. The proposed heat exchanger consists of one or more vertical tubes in which an upward flowing fouling liquid fluidizes inert particles (see Figure 1.7). The fluidized particles continuously impact on the heat exchanger walls and remove therefore possible deposits from these walls (see Figure 1.8). Moreover, the fluidized particles disturb the thermal boundary layer and increase therefore heat transfer coefficients. The overall result of the fluidized bed is that heat transfer rates are high and remain high, and that periodical cleanings are not necessary.

Fluidized bed

Inlet fouling liquid Outlet fouling liquid

Hot or cold fluid

Thermal boundary layer Fluidized bed Inert particle Hot or cold fluid Deposit Heat exchanger wall

Figure 1.7: Stationary fluidized bed heat

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Introduction

The heat exchanger in Figure 1.7 is a so-called stationary fluidized bed heat exchanger since the particles stay inside the tubes during operation. In the 1980s, a new fluidized bed concept was developed in which the particles are dragged out of the heat exchanger and are returned to its bottom via a downcomer (Klaren 2000). This concept is schematically represented in Figure 1.9 and is a so-called circulating fluidized bed heat exchanger.

Downcomer Fluidized bed Particle separation

Inlet fouling liquid Outlet fouling liquid

Hot or cold fluid

Figure 1.9: Circulating fluidized bed heat exchanger

The main advantage of the circulating mode is the higher design flexibility, since there is more freedom in choosing the velocity of the fouling liquid. Furthermore, the higher particle velocity may lead to a more efficient cleaning of the walls and higher heat transfer coefficients. Possible disadvantages are the higher required pumping power and the occurrence of wear in connections and curves induced by flowing particles.

Most installed liquid-solid fluidized bed heat exchangers in industry are operated in circulating mode (Klaren 2000, Rautenbach and Katz 1996). In most cases, fluidized beds are used for liquids that cause particulate fouling, which is the adherence of suspended particles to the heat exchanger wall. Typical examples of these liquids are oil emulsions in petrochemical industry, fruit juices in food industry, and waste waters in several branches. In other applications, fluidized bed heat exchangers are used to prevent crystallization fouling also referred to as scaling, which is the deposition of dissolved species on the heat transfer surface forming a crystalline layer. Typical examples are evaporation and cooling processes for example in desalination of seawater and cooling of geothermal brines respectively.

1.3.2 Fluidized Bed Ice Slurry Generator

In the early 1990s, Klaren and Meer (1991) proposed to use fluidized bed heat exchangers for ice slurry production. First experiments proved that the fluidized particles were indeed able to remove ice crystals from the heat exchanger walls.

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Most experiments were performed with a stationary fluidized bed which had a diameter of 54.8 mm and consisted of stainless steel cylinders of 2, 3 or 4 mm in both diameter and height. The fluidized bed was cooled by a liquid coolant which flowed countercurrently through an annulus around the fluidized bed tube. During the ice slurry production experiments, overall heat transfer coefficients were determined from the coolant flow rate and temperatures measured at the inlets and outlets. Subsequently, the fluidized bed heat transfer coefficients were determined from this overall heat transfer coefficient and a model for the coolant heat transfer coefficient.

The ice crystals produced in the fluidized bed heat exchanger appeared to be similar to those produced by other ice slurry generation techniques. Besides, the experiments showed that fluidized bed heat transfer coefficients just before and during ice formation are almost equal. From this observation was concluded that the heat transfer process near the wall is hardly influenced by ice formation.

Initially, fluidized bed heat transfer coefficients between 2500 and 4000 W/m2K were determined (Meewisse and Infante Ferreira 2003). However, during calibration experiments came to light that the tube sizes used were slightly different from what was stated in the drawings. Consequently, initially determined experimental fluidized bed heat transfer coefficients were up to 40% too low. The application of the correct dimensions to the measurements showed fluidized bed heat transfer coefficients between 3500 and 8000 W/m2_K (see also Pronk et al. 2005). An empirical heat transfer model proposed by Haid (1997) predicts heat transfer coefficients in a fluidized bed ice slurry generator reasonably well:

0 75 0 63 h 0 0734 h

. .

Nu = . Re Pr (1.1)

Haid’s heat transfer model overestimates measured heat transfer coefficients during ice generation with an average error of 9.4%.

The ice slurry production experiments also revealed that there exists a maximum allowable temperature difference for each set of fluidized bed parameters below which ice slurry can be stably produced. At higher temperature differences, the fluidized particles do not remove enough ice from the walls and as a result an insulating ice layer builds up. This phenomenon is often referred to as ice scaling. The maximum allowable temperature difference increases linearly with the solute concentration, but this linearity is different for various solutes. The observed phenomena are ascribed to mass transfer phenomena, but the physical mechanisms behind these phenomena are not fully understood yet.

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Introduction

temperature difference does not prevent its application at relatively high temperature applications, such as air conditioning systems (Meewisse 2004).

1.3.3 Promising New Applications

The fact that ice slurry production is feasible with a fluidized bed heat exchanger stimulates to inventory other applications where this apparatus might be successful.

A first promising application is freeze concentration, in which aqueous solutions such as beverages and wastewaters are concentrated by means of ice crystallization (Deshpande et al. 1984, Holt 1999, Verschuur et al. 2002). Main advantages of freeze concentration over concentration processes based on evaporation are the reduced energy consumption and the preservation of aromas and flavors. Up to now, the number of freeze concentration plants has been limited mainly because of the relatively high investment costs of the applied scraped surface heat exchangers. The introduction of fluidized bed heat exchangers may reduce these costs and makes this technology economically feasible for more applications.

A second interesting application for fluidized bed crystallizers is cooling crystallization of salts (Klaren 2000). In this process, salt is crystallized from its aqueous solution by cooling the solution below its solubility temperature (see Figure 1.10). A typical application in this respect is the crystallization of sodium sulfate (Na2SO4) from its aqueous solution. Conventional heat exchangers in which sodium sulfate is crystallized, are cleaned every 16 hours to remove the scale layer from the walls. Substitution of these heat exchangers by fluidized bed heat exchangers might make these costly maintenance stops redundant.

Salt concentration Te m pe ra tu re Aqueous solution Eutectic point

Salt + aqueous solution

aqueous solution

Ice + salt

Ice line

Salt solubility line

0°C Teut 0% weut Salt crystallization crystallization Ice crystallization Eutectic freeze Ice +

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1.4 Objectives

The first objective of this research is to unravel the mechanisms of ice scaling prevention in fluidized bed ice slurry generators. Previous work showed that ice scaling is prevented for certain conditions, but the key factors for successful ice scaling prevention are not understood yet. The first aim of this research is therefore to reveal the influence of liquid parameters such solute type and concentration on ice scaling prevention in fluidized bed heat exchangers. A second aim is to clarify the influence of fluidized bed parameters such as stationary or circulating operation, bed voidage and particle size. For both purposes, ice crystallization experiments are performed with a single-tube fluidized bed heat exchanger. An analysis of the experimental results is used to develop models to predict ice scaling in fluidized bed heat exchangers and to distinguish the most effective fluidized bed configuration for ice scaling prevention. A separate set of experiments using a piezoelectric sensor is performed to measure and analyze the collisions of fluidized particles on the wall to explain different ice scaling prevention characteristics for different operating conditions.

A second objective of this thesis is to compare the most efficient fluidized bed configuration with competitive ice slurry generators. An extensive comparison of this configuration in terms of ice scaling prevention, heat transfer, investment costs and energy consumption is made with the most commonly applied ice slurry generator type in practice, the scraped surface heat exchanger. Furthermore, promising new industrial applications for the fluidized bed heat exchanger concept are selected and investigated. Some of these promising applications, namely cooling crystallization and eutectic freeze crystallization, are tested in the experimental fluidized bed heat exchanger.

A final objective of this research is to study the behavior of produced ice crystals in other major components of an ice slurry system. Although an ice slurry system consists of a number of separate components, the processes taking place in these components strongly interfere with each other. In this respect, this research focuses on recrystallization mechanisms taking place in storage tanks and on melting processes in heat exchangers. For both topics, experiments are used to construct models that predict the development of the crystal size distribution during the storage or melting process.

1.5 Thesis Outline

Chapter 2 studies the role of the solute type and concentration on the ice scaling prevention ability of a fluidized bed ice slurry generator. For this study, ice slurry was produced in a experimental fluidized bed heat exchanger from six different types of aqueous solutions at various concentrations, while the fluidized bed conditions were constant. The results are analyzed and used to develop a model that predicts ice scaling for different aqueous solutions in fluidized bed heat exchangers.

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Introduction

The perspectives of fluidized bed heat exchangers for other industrial crystallization processes are studied in Chapter 5. The focus of this chapter is on cooling crystallization of salts and eutectic freeze crystallization. Both types of processes are tested in the experimental fluidized bed heat exchanger and results are compared to the findings of Chapter 2.

Chapter 6 compares the fluidized bed ice slurry generator with the most frequently applied ice slurry generator, namely the scraped surface heat exchanger. The comparison focuses on subjects as ice scaling, heat transfer, investment costs and energy consumption. For comparison on ice scaling, ice crystallization experiments are performed with an experimental scraped surface heat exchanger.

Chapter 7 describes recrystallization mechanisms that occur during long-term storage of ice slurry. On the basis of storage experiments with a 1-liter stirred tank crystallizer, a dynamic model is developed that predicts the evolution of ice crystal size distributions during adiabatic storage of ice slurry.

The melting of ice slurry in application heat exchangers is thoroughly studied in Chapter 8. Pressure drop, heat transfer coefficients and superheating are measured during ice slurry melting experiments in a tube-in-tube heat transfer coil. Subsequently, a model is developed to understand and predict superheating during melting of ice slurry in heat exchangers.

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Nomenclature

cp Specific heat (J/kg K) Greek

D Diameter (m) α Heat transfer coefficient (W/m2K)

Nuh Hydraulic Nusselt number, ε Bed voidage

α Dp ε/(λliq (1-ε)) λ Thermal conductivity liquid (W/m K)

Pr Prandtl number, cp,liq λliq/µliq µ Dynamic viscosity (Pa s) &

Q Heat (W) ρ Density (kg/m3)

Reh Hydraulic Reynolds number,

ρliq us Dp/(µliq (1-ε)) Subscripts

T Temperature (°C) eut Eutectic

us Superficial velocity (m/s) liq Liquid

w Weight fraction solute p Particle

Abbreviations

CFC Chlorofluorocarbon NaCl Sodium chloride

CO2 Carbon dioxide R134a 1,1,1,2-tetrafluoroethane COP Coefficient of Performance R22 Chlorodifluoromethane HCFC Hydrochlorofluorocarbon R404A HFC refrigerant blend HFC Hydrofluorocarbon R407C HFC refrigerant blend Na2SO4 Sodium sulfate

References

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Ayel, V., Lottin, O., Peerhossaini, H., 2003, Rheology, flow behaviour and heat transfer of ice slurries: a review of the state of the art, International Journal of Refrigeration, vol.26, pp.95-107.

Bellas, I., Tassou, S.A., 2005, Present and future applications of ice slurries, International Journal of Refrigeration, vol.28, pp.115-121.

Crielaard, G.A., 2001, IJsslurry bespaart energie (Ice slurry saves energy), Energietechniek, vol.79, no.3, 2001.

Davies, T.W., 2005, Slurry ice as a heat transfer fluid with a large number of application domains, International Journal of Refrigeration, vol.28, pp.108-114.

Deshpande, S.S., Cheryan, M., Sathe, S.K., Salunkhe, D.K., 1984, Freeze concentration of fruit juices, CRC Critical Reviews in Food Science and Nutrition, vol.20, pp.173-247.

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Introduction

Frei, B., Boyman, T., 2003, Plate heat exchanger operating with ice slurry, In: Proceedings of the 1st International Conference and Business Forum on Phase Change Materials and Phase Change Slurries, 23-26 April 2003, Yverdon-les-Bains (Switzerland).

Granryd, E., 2001, Hydrocarbons as refrigerants - an overview, International Journal of Refrigeration, vol.24, pp.15-24.

Haid, M., 1997, Correlations for the prediction of heat transfer to liquid-solid fluidized beds, Chemical Engineering and Processing, vol.36, pp.143-147.

Horton, W.T., Groll, E.A., 2003, Secondary loop refrigeration in supermarket applications: a case study, In: Proceedings of the 21st_{IIR International Congress of Refrigeration, 17-23}

August 2003, Washington, D.C. (U.S.A.), Paris: International Institute of Refrigeration.

Holt, S., 1999, The role of freeze concentration in waste water disposal, Filtration & Separation, vol.36, pp.34-35.

IIR, 2005a, Website of the International Institute of Refrigeration (IIR), http://www.iifiir.org. IIR, 2005b, French supermarkets turn to ice slurries, IIR Newsletter, no.21, Paris: International Institute of Refrigeration.

IPCC, 2005, Website of the Intergovernmental Panel on Climate Change (IPCC), http://www.ipcc.ch.

Jokinen, J., Willems, B., 2004, Betaine based heat transfer fluids as a natural solution for environmental, toxicity and corrosion problems in heating and cooling systems, In: Proceedings of the 6th IIR Gustav Lorentzen Conference on Natural Working Fluids, 29 August-1 September 2004, Glasgow (U.K.), Paris: International Institute of Refrigeration. Kauffeld, M., Kawaji, M., Egolf, P.W., 2005, Handbook on Ice Slurries: Fundamentals and Engineering, Paris: International Institute of Refrigeration.

Klaren, D.G., 1975, Development of a vertical flash evaporator, Ph.D. Thesis, Delft University of Technology (The Netherlands).

Klaren, D.G., 2000, Self-cleaning heat exchangers, In: Müller-Steinhagen, H. (Ed.), Handbook Heat Exchanger Fouling: Mitigation and Cleaning Technologies, Rugby: Institution of Chemical Engineers, pp.186-198.

Klaren, D.G., Meer, J.S. van der, 1991, A fluidized bed chiller: A new approach in making slush-ice, In: 1991 Industrial Energy Technology Conference, Houston (U.S.A.).

Kollbach, J.S., Dahm, W., Rautenbach, R., 1987, Continuous cleaning of heat exchanger with recirculating fluidized bed, Heat Transfer Engineering, vol.8, pp.26-32.

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Losada, V., Piñeiro, C., Barros-Velázquez, J., Aubourg, S.P., 2005, Inhibition of chemical changes related to freshness loss during storage of horse mackerel (Trachurus trachurus) in slurry ice, Food Chemistry, vol.93, pp.619-625.

Meewisse, J.W., 2004, Fluidized Bed Ice Slurry Generator for Enhanced Secondary Cooling Systems, Ph.D. Thesis, Delft University of Technology (The Netherlands).

Meewisse, J.W., Infante Ferreira, C.A., 2003, Validation of the use of heat transfer models in liquid/solid fluidized beds for ice slurry generation, International Journal of Heat and Mass Transfer, vol.46, pp.3683-3695.

Melinder, Ǻ, 1997, Thermophysical Properties of Liquid Secondary Refrigerants. Charts and Tables, Paris: International Institute of Refrigeration.

Molina, M.J., Rowland, F.S., 1974, Stratospheric sink for chlorofluoromethanes: chlorine atom catalysed destruction of ozone, Nature, vol.249, pp.810-812.

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Powell, R.L., 2002, CFC phase-out: have we met the challenge?, Journal of Fluorine Chemistry, vol.114, pp.237-250.

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Introduction

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Crystallization, Sorrento (Italy), pp.1035-1040.

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2 Influence of Solute Type and Concentration on Ice Scaling

*

2.1 Introduction

Previous experiments have shown that fluidized bed heat exchangers as described in Section 1.3 are able to prevent ice scaling during ice crystallization from aqueous sodium chloride solutions (Meewisse and Infante Ferreira 2003, Meewisse 2004, Pronk et al. 2005). However, during these experiments ice scaling was only prevented when the temperature difference between cooled wall and solution did not exceed a certain maximum. This transition temperature difference, ∆Ttrans, appeared to increase approximately proportionally to the sodium chloride concentration. A similar reduction of ice scaling with increasing solute concentration was also observed by Vaessen et al. (2002) for scraped surface heat exchangers. According to Stamatiou et al. (2005), solutes are generally applied to avoid ice scaling in this type of ice crystallizers. Despite the experimental results and the application of this phenomenon in practice, little is still known about the physical mechanisms that cause or prevent ice scaling in ice crystallizers and about the role of solutes on these mechanisms. The aim of this chapter is therefore to identify the influence of solution properties, such as solute type and concentration, on ice scaling in ice crystallizers. For this purpose, ice crystallization experiments were performed with several aqueous solutions in a liquid-solid fluidized bed heat exchanger. The experimental results are analyzed in order to unravel the physical mechanisms of ice scaling and to formulate a model that predicts the transition temperature difference.

2.2 Experimental Method

A single-tube fluidized bed heat exchanger as shown in Figure 2.1 was used for ice crystallization experiments. The heat exchanger was made of two stainless steel tube-in-tube heat exchangers connected by a transparent section. The fluidized bed consisted of cylindrical stainless steel particles 4 mm in diameter and height located in the inner tubes. The inner tubes had an inside diameter of 42.7 mm and the heat exchanger had a total length of 4.88 m. The fluidized bed was cooled by a 34 wt% potassium formate solution (see Appendix A.3.3), which flowed countercurrently through the annuli of the heat exchangers. The temperatures at inlets and outlets of the heat exchangers were measured by PT-100 elements, which had an accuracy of 0.01 K. Pressures were measured at the top and bottom of the heat exchanger to determine the bed voidage in the fluidized bed. The bed voidage was deduced from the measured pressure drop in the fluidized bed which consists of a hydrostatic term, a term for the liquid-wall friction and a term for the particle-wall friction (Garić-Grulović et al. 2004):

(

)

(

liq 1 p

)

fr,liq-w

p gh ερ ε ρ p

∆ = + − + ∆ (2.1)

The pressure drop by friction between the liquid and the wall was determined from experiments without particles.

Fluidized bed heat exchangers to prevent fouling in ice slurry systems and industrial crystallizers

Fluidized Bed Heat Exchangers to Prevent Fouling

in

Ice Slurry Systems and Industrial Crystallizers

Contents

Summary

Fluidized Bed Heat Exchangers to Prevent Fouling in Ice Slurry

Systems and Industrial Crystallizers

Samenvatting

Wervelbed-warmtewisselaars ter voorkoming van ijsaankorsting in

ijsslurriesystemen en industriële kristallisatoren

1 Introduction

2

Influence of Solute Type and Concentration on Ice Scaling

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