12
Pergamon Applied Thermal Engineering Vol. 17, Nos. 8-10. pp. 777-788. I997 C European Communities 1997. Published by Elsevier Science Ltd Printed in Great Britain PII: s1359-4311(97)00010-0 1359-431 l/97 $17.00 + 0.00 IMPROVEMENT OF MULTIFUNCTIONAL HEAT EXCHANGERS APPLIED IN INDUSTRIAL PROCESSES P. Bandelier Groupement pour la Recherche sur les Echangeurs Thermiques, I7 rue des Martyrs, 38054, Grenoble, France Abstract-This article concerns multifunctional heat exchangers. It is comprised of investigations into the improvement of failing film evaporators and rotating disc compact heat exchangers. The mechanism of heat and mass transfer during falling film evaporation of mixtures was studied to establish and improve heat exchange techniques for recycling diluted water-solvent-paint mixtures. Standard model was improved to take into account phenomena such as waves and turbulence in the film. It allowed to predict temperature and concentration profiles along the surface and in the film itself. This analysis was confirmed through experimental measurements with an accurate test facility composed of an electrically heated single vertical tube. Finally, recommendations about the heat and mass transfer surface geometry are suggested and tested with an industrial test rig. The improvement of heat and mass transfer during falling film evaporation of solutions was also studied. It consisted of the use of spiral fin graphite tubes. Modelling of heat transfer from heat carrier to film was done as well as the heat and mass transfer to the vapour, from the film flowing on an external or internal fin. The effect of obstacles was examined. The hydrodynamics of a film flowing along such a surface was studied. An industrial test rig was tested with water and a viscous fluid simulating the evaporation of phosphoric acid solution. With regard to reaction combined with heat and mass transfer, the use of a rotating disc as a reactor was studied. The hydrodynamics of a viscous melt was examined as a function of various parameters such as speed of rotation, flowrate, geometry, physical properties and type of distributor. A device, able to polymerise polystyrene was fabricated and tested. The results show that a spinning disc reactor enhances the reaction rate and reduces significantly the reaction time. Parallel to energy saving, the molecular weight distribution shows a better quality of the delivered polymer. :c European Communities 1997. Published by Elsevier Science Ltd. Keywords-Heat exchanger, heat transfer, mass transfer, falling film, evaporator, mixture, chemical reaction, rotating disc, NOMENCLATURE h HTU Y* R B F total film thickness (m) height of a transfer unit (m) total tube length (m) film flowrate (g/s) pressure (bars); spiral fin pitch (mm) Prandtl number (-) thermal flux density (W/m’) external radius of tube (mm) internal radius of fin (mm) external radius of fin, internal radius of tube (mm) position along the tube length (m) position in the film thickness (m)Greek letters upper face fin angle (‘); heat transfer coefficient (kW/m’.K) sector angle of fin (“) specific liquid flowrate (kg/ms) fin thickness (mm) bottom face fin angle (‘) temperature (“C) interface liquid/vapour temperature (“C) mass water concentration (-) 777

Improvement of multifunctional heat exchangers applied in industrial processes

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Page 1: Improvement of multifunctional heat exchangers applied in industrial processes

Pergamon Applied Thermal Engineering Vol. 17, Nos. 8-10. pp. 777-788. I997 C European Communities 1997. Published by Elsevier Science Ltd

Printed in Great Britain

PII: s1359-4311(97)00010-0 1359-431 l/97 $17.00 + 0.00

IMPROVEMENT OF MULTIFUNCTIONAL HEAT EXCHANGERS APPLIED IN INDUSTRIAL

PROCESSES

P. Bandelier

Groupement pour la Recherche sur les Echangeurs Thermiques, I7 rue des Martyrs, 38054, Grenoble, France

Abstract-This article concerns multifunctional heat exchangers. It is comprised of investigations into the improvement of failing film evaporators and rotating disc compact heat exchangers.

The mechanism of heat and mass transfer during falling film evaporation of mixtures was studied to establish and improve heat exchange techniques for recycling diluted water-solvent-paint mixtures. Standard model was improved to take into account phenomena such as waves and turbulence in the film. It allowed to predict temperature and concentration profiles along the surface and in the film itself. This analysis was confirmed through experimental measurements with an accurate test facility composed of an electrically heated single vertical tube. Finally, recommendations about the heat and mass transfer surface geometry are suggested and tested with an industrial test rig.

The improvement of heat and mass transfer during falling film evaporation of solutions was also studied. It consisted of the use of spiral fin graphite tubes. Modelling of heat transfer from heat carrier to film was done as well as the heat and mass transfer to the vapour, from the film flowing on an external or internal fin. The effect of obstacles was examined. The hydrodynamics of a film flowing along such a surface was studied. An industrial test rig was tested with water and a viscous fluid simulating the evaporation of phosphoric acid solution.

With regard to reaction combined with heat and mass transfer, the use of a rotating disc as a reactor was studied. The hydrodynamics of a viscous melt was examined as a function of various parameters such as speed of rotation, flowrate, geometry, physical properties and type of distributor. A device, able to polymerise polystyrene was fabricated and tested. The results show that a spinning disc reactor enhances the reaction rate and reduces significantly the reaction time. Parallel to energy saving, the molecular weight distribution shows a better quality of the delivered polymer. :c European Communities 1997. Published by Elsevier Science Ltd.

Keywords-Heat exchanger, heat transfer, mass transfer, falling film, evaporator, mixture, chemical reaction, rotating disc,

NOMENCLATURE

h HTU

Y* R

B F

total film thickness (m) height of a transfer unit (m) total tube length (m) film flowrate (g/s) pressure (bars); spiral fin pitch (mm) Prandtl number (-) thermal flux density (W/m’) external radius of tube (mm) internal radius of fin (mm) external radius of fin, internal radius of tube (mm) position along the tube length (m) position in the film thickness (m)Greek letters upper face fin angle (‘); heat transfer coefficient (kW/m’.K) sector angle of fin (“) specific liquid flowrate (kg/ms) fin thickness (mm) bottom face fin angle (‘) temperature (“C) interface liquid/vapour temperature (“C) mass water concentration (-)

777

Page 2: Improvement of multifunctional heat exchangers applied in industrial processes

778 P. Bandelier

INTRODUCTION

This present work concerns all industrial processes such as reaction, combustion, separation and absorption, combined with heat transfer in a single device. The efficiency of such operations may be increased largely if they are carried out in improved heat exchangers. This type of exchanger, combining other unit operations, is called multifunctional. For example, a better heat transfer combined with mass transfer in an evaporator will allow one to operate a separation of mixtures. If efficient transfers are combined with a chemical reaction, an improved chemical reactor is obtained. The activities presented here comprise investigations with respect to the improvement of falling jilm evaporators and rotating disc compact heat exchangers.

The different partners involved in this project are: Combustion Environment Engineering (CEE), France; Center for Renewable Energy Sources (CRES), Greece; Eisenmann Maschinenbau KG (EISEN), Deutchland; Groupement pour la Recherche sur les Echangeurs Thermiques (GRETh), France; Le Carbone Lorraine (LCL), France; Laboratoire des Sciences du Genie Chimique (LSGC), France; University of Newcastle upon Tyne (UNE), UK; Universitat Politechnica de Catalunya (UPC), Spain; Technical University of Berlin (TUB), Germany.

FALLING FILM EVAPORATORS

Falling liquid film evaporators are encountered in several heat and mass transfer processes, particularly in low pressure evaporation. As a result of a continuous vapour space, the pressure drop associated with the process is very low compared with a pool boiler which is a competing evaporation process. Falling liquid film systems are suitable for processing viscous and heat sensitive fluids and are thermodynamically efficient since high heat flux can be obtained with a low temperature driving force. This work describes hydrodynamic and heat and mass transfer models during falling film evaporation of mixtures on plain and enhanced surfaces. Results are used to design industrial pilot rigs which are tested.

Heat and mass transfer in industrial evaporation processes

In recent years the legislator has enacted stricter environmental regulations, which have to be put into practice now. Therefore, recycling processes must be introduced allowing for low emission production and avoiding expensive reprocessing devices for industrial effluents.

An example for such recycling processes is the separation of water-solvent-paint mixtures occurring in the spray application of water-based coatings. The aim was to reduce the water content by evaporating the formed water-solvent paint-mixtures into reusable water paint.

Modelling. The basic problem was to describe axial and transverse concentration and temperature gradients in a two-component falling liquid film resulting from preferential evaporation of the more volatile component and diffusive resistance in the liquid film.

Governing equations and boundary conditions were provided for an evaporating, two-component falling liquid film driven by gravity down a vertical surface. For the case of evaporation to a laminar vapour-phase, mass-transfer resistance in the liquid phase is significant, whereas the vapour-phase resistance is negligible. The governing equations were solved numerically using finite difference techniques.

Figs 1 and 2 show an example of calculated concentration and temperature profiles in the thickness of the film and along the tube length.

The example chosen is a 50% mass ethylene glycol-water mixture with a mass-flow rate of F = 0.08 kg/ms, and a wall heat flux of’g = 10000 W/ m*, a system pressure ofp = 1 bar and a tube length of L = 1.0 m. As expected, the interface composition of the more volatile component decreases as the film progresses down the vertical surface.

Comparison of calculated results with experimental data [l, 21 shows good agreement in the case of low water concentration and low Reynolds numbers (Fig. 3) but an unacceptable .deviation when the water concentration and the Reynolds number increases (Fig. 4).

This deviation is mainly due to the wave effect at the surface of the flowing film and the turbulence inside the film thickness. Theses effects are a function of the Reynolds number and the tube length. Work is still continuing to include them in the model. To this effect, a hydrodynamic

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Improvement of multifunctional heat exchangers 779

Fig. 1. Calculated concentration profile in a falling liquid film of water-ethylene glycol

model of the film behaviour was developed and validated with experimental data. In this way, a good knowledge of heat and mass transfer coefficients and temperature and concentration profiles was possible.

Experimental rig. A suitable test mixture (water solution of ethylene glycol) was selected and an analytical investigation of heat and mass transfer in fatling film evaporation was carried out. These data were used to design the test facilities. The aim was to develop a facility for falling film evaporation of mixtures on smooth and enhanced heat exchanger surfaces. Then, theoretical and experimental results were used to develop recommendations for the prediction of failing film

evaporation of wide boiling mixtures. A vertical falling film evaporator working under subatmospheric pressure which is especially

advantageous for temperature sensitive solutions was used for the experiments. The first step was

’ =lOOOO W/m2 ? “= 0,08 kg/ms Water - Ethylene Glycol System

adi =108,6”C

Yh 9 =I07 1°C mu1 3

Fig. 2. Calculated temperature profile in a falling liquid film of water-ethylene glycol

Page 4: Improvement of multifunctional heat exchangers applied in industrial processes

780 P. Bandelier

2

1.8

1.6

a 1.2

kW/m’K

0.8

0.6

0.4

-+- measured values by Palen [I] (uncertainty bars of +/- 15 %)

-----$-----_--~-_-_-__~__.__---------_____

I I I

0 0.2 0.4 0.6 0.8 1 relative tube length, x/L

ijw = 11700 W/m2

lY=0.247kg/ms (Re=275)

& = 0.111

Pr= 10.11

p=lbar

L=1.45m

Fig. 3. Heat transfer coefficient versus tube length ( c’,,+ = 0.1 I I).

to use the facility for basic validation tests. The second step was to integrate the same or improved types of heating surface geometry in the industrial test rig available at Eisenmann. The design of the test facility at the University of Berlin is shown in Fig. 5.

Spiral fin tubes evaporator Evaporators having a thin film flowing down a vertical smooth heated wall have some decisive

theoretical advantages with respect to more conventional designs. However, they are not used widely in industry as they are not reliable. This is because it is very difficult to make a thin film flow uniformly over a large surface as there is a spontaneous tendency for the liquid to concentrate in rivulets and leave vast dry areas.

2.2

2

1.8

a

kWlm’K1.6

1.4

I + ulculated vdues __ _t measured values by P&n [l] (unc~tainty ban of +I- 10 %) __

&,, =11720 W/m*

r = 0.130 kg/ms (Re = 225)

5’ H,O = 0.35

Pr= 5.97 p=lbar

L=t45m

1.2 ___ i

t I

,-_+___++__ ! /

I ! t ! I 1L I I I I

0 0.2 0.4 0.6 0.8 1 relative tube length, x/L

Fig. 4. Heat transfer coefficient versus tube length (IY~+ = 0.35).

Page 5: Improvement of multifunctional heat exchangers applied in industrial processes

Improvement of multifunctional heat exchangers 781

Vapour flow in contercurrent flow confouration

---Jz- recirculatio

I tilter

\

-0

&uid-vapour separator

\ Vapour flow in cocurrent flow configuration

/

tank

Fig. 5. Evaporation test facility.

To eliminate this problem, the technique proposed by the Laboratoire de Sciences du G&nie Chimique [3-51 uses a vertical spiral fin tube. The fin surface remains uniformly wetted even at the highest heat transfer fluxes.

The liquid to be evaporated flows over a slight slope fin wound round a vertical tube heated on the other side. This fin can be either one single spiral or several parallel spirals wound around the base of the cylinder. The spiral fin can be internal or external to tlie tube.

In this work, the tube and the fin were made of metal (copper alloy) or impregnated graphite to resist corrosion at high temperature caused by phosphoric acid or other chemicals. Figure 6 shows a cross-section of a graphite single external spiral fin tube manufactured by the Carbone Lorraine company.

Fig. 6. Cross-section of a spiral fin tube.

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782 P. Bandelier

In comparison with plain tubes, the advantages of this technique are: a higher evaporation surface ( x 2.5); an increased residence time ( x 24); a better specific rate of evaporation ( x 8) and an enhanced heat transfer coefficient ( x 2).

This geometry was tested in the laboratory and then on an industrial scale. The results are presented later.

In parallel to the thermal characterisation with external fin tubes, the characterisation with internal spiral fin tubes was also examined.

Hydrodynamic modelling. At the surface of the film, the liquid moves outwards by centrifugal force while near the film-fin interface, the liquid moves towards the base of the tube by gravity acting on the angle of the fin which depends on the distance from the tube axis. This means that the liquid does not flow as a set of parallel and independent streams but that the streams undergo transverse mixing by oscillating in quasi-sinusoidal trajectories between the wall of the base tube and the outside edge of the fin.

The heat exchanger fins were divided in small cells where the velocity profiles were assumed to be uniform. That is, the velocity of every cell was given a polynomial with constant coefficients. Cylindrical coordinates were chosen. The cell size was selected to reach good results without an excessive computational effort. Velocity profiles were divided in two components. Mass and momentum equations were derived from the integral analysis of every cell. Boundary conditions considered the fluid flowrate coming from previously calculated cells. For the geometry and a given flowrate range, it was assumed that the fluid carried over the fin and ran back to the tube wall through the bottom side of the fin. Then the fluid fall on the tube wall to the next level.

An example of a numerical result is given in Fig. 7. The velocity angle was calculated for different fin slopes as a function of the fin turn number along the tube. If its value became constant, the flow was stable and the fin remained wetted; when its value increased up to 90”, the flow was unstable, the film left the fin surface and dry points appeared.

Laboratory scale experiments. The work carried out concerned internal finned tubes. The geometry of such tubes is shown in Fig. 8. The process fluid flew as a thin film on spirally wound fins formed on the inside of a vertical circular tube. This spiral fin heat exchanger was not a single piece. It was built of individual discs of which one, two or three sections were cut out. The discs were assembled with respect to each other so as to make one, two or three spiral stair-cases. The average slope of each stair-case was changed by varying the overlapping of the discs. The average slope was measured by its angle.

The experimental work concerned the hydrodynamics and the heat transfer of the flowing film with the vapour phase flowing either counter-currently or co-currently.

Pressure drops and liquid hold up were measured; thermal performances are given as the Height of Transfer Unit. The smaller the HTU, the more efficient the tube is. Results are given in Fig. 9.

8.00E+ol

7.00E+Ol e B 6,00E+Ol

d z 5.00E+Ol

i? m 4.00E+Ol E = E

3.00E+Ol ‘Z rrt 2.00E+Ol E

l.OOE+Ol

-. ,:,_ ,. . . . . . . . . . . . . .

Slope = 3.1”

1

O.OOE+OO 0 20 40 60 80 100 120 140 160

Number of turns: height = 3~. 0.03 tg(slope).number of turns

Fig. 7. Maximum velocity angle on internal spiral fin tube.

Page 7: Improvement of multifunctional heat exchangers applied in industrial processes

Improvement of multifunctional heat exchangers 783

monospire bispire trispire

The geometric characteristics of the three types of disc studied

P (“1

rp (mm)

rt (mm)

R (mm)

6 (mm)

monospire bispire

120 90

25 25

7,5 9

30 32

4 10

Fig. 8. Three types of disc studied.

trispire

60

25

9

32

10

These results show that we have to look for the optimal geometric structure which results from the best compromise between the following two factors:

An enhanced heat transfer coefficient caused by a high step density (strong slope). A large surface area per unit volume caused by fins with a slight slope.

But there are still a number of geometric and operational parameters which have to be studied before a final conclusion is reached.

0.30 -

0.25 -

E 2

Fi 0.20 - 3

z

0.15 -

0.10 -

_.-- _..* mono-30” _..*

._-* .*- . ,.-- mono- 13”

_.--

. 9’ mono-8”

o bi - 0 = 47” A bi - 0 = 23” 0 mono - 0 = 30” xmono-8=13” + mono - 0 = 8” 0 tri - 8 = 47”

I I 1 I I I 10 20 30 40 50 60

mf (g/s)

Fig. 9. Variation of the height of transfer unit HTU (wall-film) in function of the film flowrate for water heating.

Page 8: Improvement of multifunctional heat exchangers applied in industrial processes

P. Bandelier

Parameters

Table I.

Tube reference: G6

Internal diameter 40 mm External diameter 140 mm Fin width 40 mm Base-tip thickness 2C-8 mm Spiral pitch p 35 mm Slope angle 0 26.5 Inclination of fin in a radial cut OL 11.3

Industrial pilot rig experiments. The geometry of the graphite spiral fin tube tested is given in Table 1. It conforms with the drawing in Fig. 6.

As was observed, for high flowrates (greater than a critical value depending on the fin geometry, c( and 0), the film may flow over the fin edge and, in the case of a graphite fin with excellent surface wettability, may remain on the lower part of the fin. This unusual hydrodynamic regime gave an increased fin to film heat exchange area, of more than two.

So, basically two flow regimes occured on a spiral fin graphite tube: spiralling film (only the upper face of the fin was wetted) and spiralling film plus overflowing film (both faces of the fin were wetted). The regime changed when a critical Reynolds number was reached. The Reynolds number of a fluid depends on the liquid viscosity which varies strongly with temperature.

From an experimental point of view, the knowledge of the optimum flowrate was necessary for the sizing of the evaporator recycling pump.

Preliminary hydraulic tests at room temperature and atmospheric pressure were carried out with water and then with an ethylene glycol water solution (94% of mass concentration and viscosity of 15 x 10 - 3 Pa.s) in order to determine the optimum flowrate for sizing the recycling rate, taking into account the viscosity effect. A small rig comprising of a single tube element (0.4 m) was built. For a fluid of low viscosity (water, 10m3 Pa.s) the flowrate had to remain below 600 l/h to prevent the flow from excessive centrifugation; for higher viscosity fluids (of 15 x 10e3 Pa.s) this value rose to 1400 l/h. The optimal flowrate values were respectively 200 and 400 l/h/tube.

Later, a full scale pilot rig supplied by LCL was connected to the GRETh facilities. It was composed of seven G6 graphite spiral fin tubes of 1.6 m length (internal surface, 1.4 m2; external surface, 10.7 m’). The heat carrier fluid was steam, as in most of the industrial processes. The heat transfer was measured as a function of different parameters such as the feed flowrate and the temperature difference. Two process fluids were evaporated: water to simulate a low viscosity fluid and ammonium lactate solution (up to 60%) to simulate a viscous fluid (of 10m2 Pa.s).

It was observed that the flowrate had no significant effect in the range of 200-800 l/h/tube. Figure 10 summarises the main thermal results. It shows the comparison of plain vertical tubes [6, 71 with G6 spiral fin tubes. In the case of a low viscosity fluid evaporation (water), vertical plain tubes are more efficient than fin tubes. This is because of the relatively good filmside heat transfer

0 5 10 15 20 25 30 35 Difference of temperature between heat carrier and liquid (‘C)

Fig. 10. Film heat transfer coefficient for G6 and plain vertical tubes.

Page 9: Improvement of multifunctional heat exchangers applied in industrial processes

Improvement of multifunctional heat exchangers 785

To collecting tanks

Fig. 1 I Rotating discs experimental set up

coefficient. This leads to a poor fin efficiency (0.2 to 0.4) especially for important temperature differences because nucleation appears at the base of the fins. The fin effect is very attenuated. In the case of viscous solution (of lo-’ Pa.s) fin tubes become more efficient than plain tubes because the heat transfer coefficient in the film drops and the fin is effective. This drop and the decrease of the curve in Fig. 11 can be explained by the absence of nucleation and the much higher film thickness. But, this was not the case with water evaporation at wide temperature differences.

Study of market. The market analysis showed that the use of specific materials such as graphite were only required when the solution had corrosive properties. Intensification of evaporation by use of spiral fin tubes will be attractive if the properties of the solution reduce significantly the heat transfer coefficient at the evaporation side. This occurs, for example, when the viscosity increases, because of high ratios of viscous components. This is rarely the case for waste water treatment, where saline solutions are often encountered. However this situation is common in chemical processes.

ROTATING HEAT EXCHANGERS

Batch reactors are often used to operate reactions of polymerisation. In the case of catalysed reactions, the degree of conversion of the monomer to the polymer is limited by heat and mass transfer inside the mass of the mixture caused by the bad mixing. This leads to an increase in the reaction time, the energy to be used during a longer and inefficient operation and the quality of the delivered polymer is less than expected (wide molecular weight distribution). The use of a multistage spinning discs reactor cancels theses problems by the enhancement of heat and mass transfer inside a very thin film of mixture created by the centrifugal on the discs surface.

The reaction is quite immediate on each disc and the concentration and temperature conditions are well controlled.

Hydrodynamic characterisation A compact multifunctional heat exchanger, based on intensified heat and mass transfer in a

centrifugal field, consisted of a multiple matrix of closely spaced discs. When a liquid flows over a rotating disc surface, a thin film is formed under the centrifugal force effect. The fluid dynamics of such films were not studied. The formation of the film on the bottom side of the top horizontal disc required examination. The experimental study of film flow between co-rotating discs was the subject of the investigation here.

The experimental set-up shown in Fig. 11 was used. The main unit consisted of two flat discs of 30 cm diameter and 20 mm thickness, co-rotating on a central vertical shaft. The clearance between the two discs was adjustable from 2 to 5 mm. A liquid film distributor located at the centre between the two discs ensured liquid feed on both discs. A stainless steel sheet-ring, 1 mm thick

Page 10: Improvement of multifunctional heat exchangers applied in industrial processes

786 P. Bandelier

and 6 cm wide, with Di = 29 cm and D, = 35 cm, was placed between the two discs (in the outer diameter) to ensure that the two films were collected separately and were not mixed. The stainless steel ring was held in place by two Teflon rings which were bolted on the periphery of the discs and to each other. The Teflon rings were machined so that the liquid films were guided to two separate collection tanks which were connected to the main unit. These tanks were equipped with sight glasses in order to measure the volume of the liquid collected during each test.

The metallic parts of the system were made of stainless steel. The top disc was made of Lexan polycarbonate, to allow visual observation of the film, while the bottom disc was made of aluminium. Dye injection holes were drilled into the top disc in order to observe the flow regime through visual inspection of photographs and video of dye filaments.

All the experimental runs were carried out at atmospheric pressure. Design and operational parameters which were studied were: distance between the discs (2-5 mm); type of liquid (water, water-glycerol mixture); liquid flow rate (l-5 ljmin) and speed of rotation (100-1000 rpm). An example of results is given in Fig. 12. For a low viscosity fluid (water), the effect of an accurate feed distributor on the flowrate distribution between the top and the bottom disc is shown. The results look good even without special devices to improve distribution.

Characterisation of prepolymeriser A multifunctional rotating exchanger studied the polymerisation of high impact polystyrene. The

aim was to carry out the reaction in the best possible conditions: energy and time were simultaneously saved while polystyrene resin was produced with a narrow molecular weight distribution (improved quality of the final product).

To this effect, a batch reactor and a rotating disc reactor were used and compared. The batch reactor; allowed the reproducibility of the exotherm of polymerisation to be checked, provided a calibration curve (viscosity and molecular weight vs time) and bench marked the performance of the spinning disc reactor.

Top disc (% of total flow rate) 100 ,

i

Deionized water 760 mmHg, 16 OC Lexan upper disc

without feed distributor A _______

0 100 200 300

Volumetric flowrate (I/h)

Q- 200 rpm -A- 600 rpm * 1000 rpm

Fig. 12. Flowrate distribution between top and bottom disc.

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Improvement of multifunctional heat exchangers 787

J 0 20 40 60 80 100 120

Time (minutes)

Fig. 13. Time savings on spinning discs reactor at high conversion.

140

The rotating disc reactor resembles the one described before, but was adapted for operating a chemical reaction. The effect of rotational disc speed on conversion and molecular weight distribution was studied.

Comparison between batch and spinning disc reactors was carried out. A first step has shown that for low conversion value, the time saving was considered unsatisfactory ( < 15 min, > 45 min) Then, the spinning disc reactor was fed with a 36% conversion mixture for different rotational speeds (300 to 1000 rpm). Figure 13 shows the results. It compares characteristics of the batch reactor with the spinning disc reactor. For low rotation speed (below 750 rpm), the time saving was still unsatisfactory ( < 30%). Above 750 rpm, the efficiency of the spinning disc reactor increased quickly, the reaction was quite instantaneous. At 1000 rpm, the total reaction time from 0% to 70% droped from 100 min (with the batch reactor) to 40 min (with the batch reactor from 0% to 36% and the spinning disc reactor from 36% to 70%).

The extrapolation to higher conversion value (up to 0.9) with an optimised multistage spinning disc reactor allows to expect that the reaction time will drop from more than 130 min to 5-10 min. Further, the process time of a conventional batch reactor was 14 h while the spinning disc reactor process time was about 1.5 h in total including the prepolymer stage which may not be required in industry. Such reduced process time would result in even greater energy savings.

CONCLUSION

Studies were carried out in the field of multifunctional heat exchangers applied in industrial processes.

Modelling of falling film evaporation along plain or enhanced surfaces allowed one to correctly design equipment when the lack of data and models prevented the use of this technique. Phenomena caused by wide boiling range mixtures and hydrodynamic effects were considered for an improved model. This model was tested in a pilot rig before being used in industrial scale equipment.

A novel design of falling film evaporator using vertical spiral fin tubes was proposed. Hydrodynamic modelling and laboratory scale experiments were completed with industrial scale tests. In addition to a market analysis, the results have shown the field and the operating range where this technique can be used successfully. Data are available to determine the optimal geometry for a given application.

The hydrodynamic studies of co-rotating heat exchangers have shown how to get a good distribution and mixing of the liquid flow. The use of co-rotating discs during chemical reaction decreases considerably energy consumption and time of reaction, while it significantly improves the quality of the final product.

Acknow/edgemen/s-The present work was partially financed by the Commission of the European Communities. It is the result of the collaboration between research teams and industrial companies from all over Europe. Contributors are Professors’ Auracher (TUB), Le Goff (LSGC), Ramshaw (UNE), Sigales (UPC), Valachis (CRES) teams and Mr Bauer (LCL), Grimm (Eisenmann) and Richelmy (CEE) for the industrial aspects.

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188 P. Bandelier

REFERENCES

1. Palen, J. W., Falling Film Evaporation of Wide-Boiling-Range Mixtures Inside a Vertical Tube. PhD Thesis. Lehigh University, 1988.

2. Rau, S., Energietransport und Oberfllcheneffekte bei der Fallfilmverdampfung von bit&en Gemischen (Heat transfer and surface effects of the falling film evaporation of binary mixtures), Diplomat&it (Diploma Thesis), TU Berlin, 1995 (in German)

3. Le Golf, P, Brevet d’lnvention Francais, no. 92-l l-234 du 15.9.1992, Contacteur gaz/liquide a filmruisselant. 4. Soetrisnanto, A.Y., Un nouvel Cvaporateur a film ruisselant sur tubes a ailettes spiralees en graphite. Doctoral Thesis,

Institut National Polytechnique de Lorraine, Nancy, France, 1992. 5. H. Le Gaff, A. Y. Soetrisnanto, B. P. Schwarzer and P. Le Golf. A new falling film evaporator with spiral fins. C/rem.

Eng. J. 50, 169-171 (1992). 6. P. Bandelier and P. Gerard, High performance grooved tubes for heat exchangers. Seminar Eurotherm 33. October

13-14, 1993. Editions Europeennes Thermiques et Industrie. 7. P. Bandelier, R. Casset and A. Gillot, Essais de tubes cannel& sur un concentreur de jus sucre-Comparaison avec

des tubes lisses, Note technique. GRETh ESTHER 90/33, 1990.