Factory Engineering

FOAMY TWO-PHASE FLOW EVAPORATION# APPLIED TO SUGAR CANE PROCESSING

Ogbemi Ola Omatete* and Hugo H. Sephton*"

"Chemical Engineering Dept., University of Lagos, Nigeria; **Sea Water Convertion Laboratory, University of California, Berkeley, California, USA

ABSTRACT

The evaporative heat transfer performance in the preconcen- tration of cane sugar solution before crystallization in vacuum pans has been measured in a downflow vertical tube evaporator with a vapor compressor. It has been shown that the addition of a selected surfactant, sodium dodecyl sulfate, in parts per million to the cane sugar solution resulted in an increment of 70% on the average in the performance (overall heat transfer coefficient, U values) over the concentration range when compared to evaporation without surfac- tant addition. This enhancement results not merely because of the addition of the surfactant but also because of the special arrangement of the entrance orifice in order to initiate a continuous foamy two- phase flow throughout the tubes.

INTRODUCTION

The importance of evaporation in the processing of cane sugar cannot be overemphasized. Hugotl, Jenkins2 and Baikow3 all show that, in concen- trating sugar juice from about 15%rix to about 70° Brix by evaporation, the typical evaporation load is 80% of the clarified juice. This approximates to 80% of the cane processed since its weight is about equal to that of the clarified juice. The steam requirement for such a large evaporation duty would be prohibitive, consequently, multiple effect evaporation and vapor bleeding had been devised to save on the steam consumption. Other devices such as the turbo-compressor which is similar to the system used in this study, and thermal (steam) compression have also been employed to reduce energy costs. Thus any processing innovation that increases the evaporation rate at minimal cost is important to the industry.

In the sugar cane industry, evaporation is generally carried out in tubular calandria type multiple-effect evaporators consisting of 1 . 2 m' to 2 - 2 m (4 to 7 ft) long having 25.4 mm to 50.8 inm (1 to 2 in( diameter tubes in bundles, with a large head space for entrainment separation. Kest- ner evaporators based on the "climbing film" principle and containing long tubes of up to 7 m (23 ft) by 25.4 mm to 38.1 mm (1 - 1% in) diameter are also used. Although different methods of feeding the sugar solution

# U.S. Patent No. 3,846,254, PJov. 1972 & Foreign Patents.

2278 FACTORY ENGINEERING

from one effect to the other are used, the upflow mode with level regulation is prevalent.

The use of surfactant is common practice in the industry but not for the purpose described here. Bergefi in a detailed review article on the subject discussed the use of surfactants in inhibiting foaming, corrosion and scale formation, in enhancing flocculation and dispersion, and in regulating viscosity and controlling crystallization and evaporation rates. He emphasized the application of surfactants as antifoamers and defoamers in both evap- orators and crystallizers and that this application must be done carefully so as not to interfere with normal evaporator efficiency. Baikow3 reports that inore efficient evaporation and reduction in scale formation in tubes are achieved by using surface-active additives which lowers surface tension and enhances the wetting power of the sugar solution. Neither of these authors reports quantitatively what these improvements in evaporation rates are. Undoubtedly, the primary concern in the sugar industry is the sup- pression of foaming and surfactants are used for that purpose rather than for the enhancement of the rate of evaporation. The work reported here demonstrates that substantial improvement in overall heat transfer coefficient can result from induced foamy flow evaporation after the addition of a selected surfactant to the feed in the parts per million range.

In the desalination of sea water foaming was considered a nuisance until Sephton5,8 demonstrated "* that such induced foamy flow (interface enhancement) increases the overall heat transfer coefficient in vertical tube evaporators (VTE) substantially. His work has been corroborated by other workers9J0. Sephton showed that for vertical-tube foamy evaporation (VTFE) in the upflow mode, the overall heat transfer coefficient is increased by 50 to 200% and the pressure drop through the tubes is reduced by a factor of 3 to 10 for double-fluted tubes5 under pilot plant conditions. The overall heat transfer coefficient was increased by 30 to 60% for downflow VTFE. In a detailed study, Sephton and coworkersll-'3 have elucidated the mechanism for both the increased heat transfer coefficient and the large pressure drop in the upflow VTFE.

They showed (1 ) that the reduction in pressure drop after the addition of surfactant to induce foamy flow, is due to a reduction in liquid holdup in the tube and that the high heat transfer coefficient is associated with the wiping of thin films of liquid present in the foamy structure over the tube wall; (2 ) the coeff~cient can be further increased by addition of a foam builder; (3) the high surface tension depression effect when surfactant is added is necessary, but not a sufficient condition for maintaining a stable foamy flow since the foam-enhanced performance is cancellable by the addition of anti-foam and (4) the pressure drop induced by foamy flow contributes to the heat transfer enhancement and improved upflow VTE stability, permitting the use of relatively low AT values and a proportionate increase in the number of effects allowable for a multieffect upflow VTE plant of increased gain ratio. Sephtonl43l5 has extended the interface enhance- ment studies to power plant cooling tower blowdown, purification of industrial waste waters, irrigation drainage water, and other saline waters.

0.0. OMATETE AND H.H. SEPHTON 2279

The primary objective of this work was to demonstrate that the overall heat transfer coefficient is increased when sugar solution is evaporated under the conditions of foan~y two-phase flow through the tubes. It is proposed that this novel method of heal transfer enhancement may be applied to the sugar industry as follows. A selected surfactant is added in parts per million to the mixed juice from the clarifiers before evaporation commences. The evaporators will be designed so as to ensure loamy two-phase flow through the distillate tubes. The concentrated syrup from the last effect is stripped to remove the surfactants. Valdes-Krieg et al.16717 have shown that this is feasible and leads to the removal of several other surface active components. Probably, in the case of raw sugar solution, some of the coloured compo- nents may be removed in the stripping process. This extra process step may not be necessary since surfactants and antifoainers are now added to the sugar solution in the current processing procedure and the surfactant added to enhance heat transfer may be treated as the other additives are now treated. Finally the concentrated solution, the syrup, is sent to the vacuum pan for further concentration and crystallization.

EXPERIMENTAL PROCEDURE

The experiment was carried out in a vertical-tube evaporator (VTE) shown in a flow diagram in Figure 1 and in a photograph in Figure 2. The WTE designed for use in sea water desalination and for such operations

I FIGURE 1 . Downflow vertical Lube evaporator with vapor compressor.

ORIFICE PLATE

COMPRESSOR

CONDENSATE TAKE-OFF

CONDENSATE RECYCLE

MEASUREMENT

FEED RECYCLE

1 i

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as power plant cooling water blowdowil and waste water treatment, was not well suited for evaporation of sugar solution. However, though of limited flexibility, it has been used for this study without significant modification.

The VTE contained a 49-tube bundle of double-fluted, aluminum- brass distillation tubes (Yorkshire Imperial Metals Ltd.) of 38 .1 mm ( 1 . 5

in) diameter and 1 . 8 3 in (6 It) heated length. Although the bundle can be run with upflow or downflow feeding it was set for the downflow mode, using an orifice plate distributor for dispensing the feed through 0.51' mm ( .20 inch) holes on to the top-tube sheet at three positions adjacent to each tube inlet end, as shown ~n Figure 3. The feed was thus bounced off the top-tube sheet, and directed downward into the tubes by means of short cylindrical deflectors attached to the orifice plate and entering the distillation tube inlet ends by about 1 2 . 7 mm ( 0 . 5 in) to provide an annular feed distribution for each tube. Provision was made for preheating the feed as needed with a steam coil located in the inlet distributor vessel, immedi- ately above the orifice plate.

The 49-tube bundle discharged from its open ends directly with a large stainless steel vessel for separating the vapor produced from the feed solution. This vessel was provided with three windows to permit observation

0.0. OMATETE AND H.H. SEPHTON

FIGURE design.

Inlet orifice

of the mode of flow at the tube outlet ends. A pair of windows located in the inlet distributor vessel similarly permitted observation of the mode of flow between the orifice plate and the tube inlet ends. The vapor-liquid separation vessel was provided with a stainless steel mesh mist eliminator located in the upper part of the vessel as a collar surrounding the tube bondle to remove entrained droplets of liquid phase from the vapor produced. Vapor was conducted from the mist eliminator to a vapor compression unit.

The vapor compression unit consisted of a Sutorbilt, Roots type com- pressor constructed of bronze having a 26 m3/inin (920 CFM) displacement, capable of producing 1,135 litres (300 gal) of distillate per hour. A 18 . 6 Kilowatts (25 hp) electric motor was used to drive the compressor at 918 rpm. This provided for about a .34 bar ( 5 psi) compression of the vapor from the vessel; the compressed vapor was conveyed to the shell-side of the VTE through stainless steel pipes of 152.4 mm (6 in) diameter. Two bypass lines of 50 .8 mm ( 2 in) diameter were provided, for the optional bypassing of some compressed vapor to the suction end of the compressor, to provide an experimental variation of its capacity.

The compressed vapor*was sent to the shell-side of the tube bundle where some of it was vented in.order to remov,e the noncondensable gases.

TABLE I. Summary of data.

Run No. Feed Surfactant Solution Average Average Overall Heat Conc. (ppm) Conc. OBrix Steam-side Feed-side Transfer Coeff. U.

Temp. OC (OF) Temp. OC (OF) W/m% (Btu/hr-ft2-OF)

1 Tap water

4 Sugar - 14.2 109.7 (229.4) 102.1 (215.8) 5,750.4 (1,012.7) Soln.

Sugar Soln.

7,

2,

> I

,, ,,

'Compressor tripped off due to overload before data were completely measured. ZCompressor tripped off due to overload before any data were taken.

0.0. OMATETE AND H.H. SEPHTON

This steam condensed on the shell-side of the tube bundle and was contin- uously removed by a pump and its rate of flow was measured. This condensate flow rate provided the heat flux data (Q) needed to measure the evaporator performance. Additional data required were the steam temperature (Ts) and the feed temperature (Tf) measured with platinum resistance probe and converted electronically into degrees Fahrenheit in a digital display. From these data the overall heat transfer performance for the VTE was determined. To avoid condensation in the vapor compressor, the experiments were run with Tf just above the boiling point of water at atmospheric pressure, and Tf never exceeded about 104OC ( 219*F).

Although the VTE could be run upflow or downflow it was operated in the downflow mode because there was less likelihood of fouling the tubes. Furthermore, since Sephton and coworkerss-8. "-I3 have shown that the heat transfer enhancement is more pronounced for the upflow mode than for the downflow, running in the downflow mode was being conservative. If there is any enhancement in the downflow mode, it will be increased in an upflow operation which is most common in the sugar industry.

The holdup capacity of the V'rE limited the amount of the solution that can be held in it to about 200 litres. In order to concentrate sugar solution from lSOBrix, to about 60°Brix, and still have enough solution so that the feed pump would not cavitate, about 400 litres of solution was necessary. This had to be fed into the VTE in stages.

To check out this stage-wise feeding procedure, the first runs were with tap water. 200 litres of tap water were measured into a stainless steel drum and its temperature was raised to about 90°C (194OF) with steam passed through copper coils, before it was pumped into the VTE. The water was circulated through the tubes by the feed pump while it was heated by passing steam through the coil above the orifice plate. At the feed temper- ature of about 102OC (215OF) the compressor was started and the steam turned on. When steady-state was achieved, the feed temperature Tf, the steam-side temperature Ts and the rate of condensation were measured. Generally, four readings were taken at each steady-state and the average value was used to calculate the overall heat transfer coefficient.

While the tap water was being circulated through the VTE, another 200 litres of fresh tap water was heated in the stainless steel drum. After the first set of data was taken, 100 litres of the condensate were rejected, then 100 litres of tap water was pumped into the VTE. The system was again brought to steady-state and data were taken. 100 litres of condensate were again rejected and the remaining 100 litres of tap water was pumped into the VTE. The system was brought to steady-state and data were taken. These data are shown as the Runs 1 to 3 in Table I. The average of the three values is plotted as the square point in Figure 4 which shows the overall heat transfer coefficient plotted versus s&tion concentration (OBrix) and surfactant concentration (ppm) . , ( "

' I - ' < , ( ,

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FIGURE 4. Overall heat transfer coefficient versus solution concentration (OBrix) and surfactant concentration (ppm).

SUGAR SOLUTION CONCENTRATION ( O Brix)

SOLUTION NO SURFACTANT

0 SOLUTION (ppm) SURFACTANT

I I 1

Raw sugar which was obtained from the C & H Sugar Refinery in Crockett, California, was dissolved in tap water to make 200 litres of about 14OBrix solution in the stainless steel drum by using a large stirrer. It was heated and pumped into the VTE as was done with tap water. Data were taken at each concentration level as condensate was rejected while more sugar solution was added. After all the 400 litres of 14OBrix solution were fed into the VTE, condensate was rejected and data taken at the new con- centration level. These data are shown as Runs 4 to 7. At the end of the run with sugar solution, the concentrate was pumped out of the system through the lowest part of the system so that nearly all the concentrated sugar solution was recovered. The tubes were then washed thrice with tap water heated to the feed temperature and circulated for about fifteen minutes each time. Finally the system w'as steamed out for about an hour.

I I 0 10 20 30 40 50 6(

0 . 0 . OMATETE AND H . H . SEPHTON 2285

The runs with raw sugar solution and surfactant were carried out in a manner similar to that for the sugar solution without surfactant. After the first 200 litres of the approximately 14OBrix was fed into the system, it was brought to steady state and data were taken. Then a solution of sodium dodecyl sulfate (SDS) also called sodium lauryl sulfate (n-CH3 (CH2) lo CH20S03Na+) an FDA-approved surfactant was added by syringe through a special septum on the flow meter until the amount calculated to provide 50 ppm was added. Data were taken and more surfactant was added to give 75 ppm and performance data were taken. The rejection of condensate and the addition of .sugar solution to change the solution concentration was sim- ilar to that for the sugar solution without surfactant. The final blowdown sugar solution and the final condensate were analyzed for sugar content.

In all the runs the flow meter was set so that the flow rate through each tube was 3.785 litres per minute (1 US gallon/min). Both the steam side and the feed side were vented to remove aoncondensables. Although there was some foaming while the sugar solution was b&ng prepared, the foaming did not appear in the VTE. It was only after about 50 pprn of the SDS was added that there was persistent foaming in the system.

R E S U L T A N D DISCUSSION

The overall heat transfer coefficient was calculated by the usual equation

Q = U A A T (1

where Q is obtained from the condensate flow rate (AT = Ts - Tf) where Ts and Tf were measured as indicated. Tf is taken after the feed has flashed down already to obtain the largest AT which gives conservative value of the overall heat transfer coefficient, U. A is the outside surface area of the 49 tubes, 1.38 m by 38.1 Elm.

Table I is a summary of the data where Runs 1 to- 3 are tape water without surfactant; Runs 4 to 8 for sugar solution without surfactant, and Runs 9 to 14 for sugar solution with surfactant. Figure 4 is a plot of the overall heat transfer coefficient versus sugar solution concentration, with and without surfactant. In this figure the triangles represent runs without surfactant, the circles, those with surfactant, and the rectangle, the average of the three runs for tap water. The surfactant concentration (ppm) is shown in brackets near the circles.

Figure 4 shows the large increment in the overall heat transfer coef- ficient as surfactant is added to the feed. The data at 14.2OBrix show the effect of the addition of surfactant alone before concentrating. Although run on different days, the two points with0,utusurfactant on the 14.2rJBrix line agree within the experi~nental errors. When 50 ppm of surfactant was added to the feed solution there was continuous foaming and, as is shown

2286 FACTORY ENGINEERING

in Table 11, AT decreased. The heat transfer coefficient was increased by about 65% as shown in Figure 4. After 75 ppm of surfactant was added, the enhancement went up to 80%.

As the solution was concentratcd the surfactant concentration increased slightly although none was added. The U value was increased by more than 100% (i.e., double) at about 22O and 30°Brix. Further concentration re- sulted in lower enhancement of the heat transfer coefficient, although in all cases the coefficient was substantially higher with surfactant than without. The average enhancement is about 70%. This agrees with the results Sephton et al, reported earlier.

The overall heat transfer coefficient U is related to the sum of the heat transfer resistances of the steam-side condensate film, the tube wall and the feed-side film. Since the latter is usually limiting, a doubling of the overall U, implies 2 to 4 times increment in the feed:side heat transfer coefficient as shown by Sephton18. As mentioned earlier, this several-fold factor in- crement is due to thin film wiped-film mode of heat transfer resulting from foamy two-phase flow over the feed-side surface.

11 would have been desirable to obtain data at around 60°Brix with and without surfactant to confirm the trend at the highest concentration typical of evaporators in raw sugar processing. However, the compressor which was not designed for the high loads at the higher concentration always tripped to off at about 50°Brix. The condensate from the highest concen- trated solution run with surfactant was analyzed and it contained 500 ppm of sugar. This high entrainment is because the vapor dome and stainless steel demister were not designed adequately for concentrated sugar solution evaporation. With sea water, for which it was designed, the carryover usually is less than 10 ppm of salt.

CONCLUSION AND RECOMENDATION

The addition of surfactant to cane sugar solution during evaporation has resulted in about a 70% increase in the overall heat transfer coefficient U. This means that less evaporation area is needed for the same load and there is a concomitant savings on equipment cost. Also this effect manifests itself in lower AT which allows for more effects to increase the gain ratio, if this were desired.

However, it must be pointed out that the beneficial effect of surfactant addition is due not just to its addition but more importantly to imposing of two-phase foamy flow throughout the tubes. This requires an arrangement of the feed orifice to ensure that loamy flow begins from the inlet of the tubes.

This has merely been a demonstration of the heat transfer enhancement effects by the addition of a surfactant to cane sugar solution. As mentioned in the paper, the facility used was not designed for sugar solutions. Work

0 . 0 . OMATETE AND H.H. SEPHTON 2287

needs to be done with real clarified juice which is not easily available in California and with systems designed for cane sugar evaporation. Further- more, since the upflow mode of operation is the norm in the sugar industry, this should be examined. If the result with sea water holds true there, the enhancement effect in the upflow mode will be dramatic. Such further tests are planned in this Laboratory.

ACKNOWLEDGEMENT

The authors wish to thank C & H Sugar Refinery for their interest shown, in supplying raw sugar and analysing samples. Dr. Omatete is grateful to the University of Lagos for supporting him while this study was being carried out.

REFERENCES

1 . E. Hugot and G.H. Jenkins (Translator). (1960). Handbook of Cane Sugar Engineering. Elsevier Publishing Co., Amsterdam, 872 p.

2 . G.H. Jenkins. (1966). Introduction to Cane Sugar Technology. Elsevier P~iblishing Co., Amsterdam, 478 p.

3 . V.E. Baikow. (1967). Manufacture and Refining of Raw Cane Sugar. Elsevier Publishing Co., Amsterdam. 453 p.

4 . P.D. Berger. (1975/76). Surfactants and Surface Act~vity in Sugar Manufacturing. Sugar Technology Review. 3, 241-273.

5 . H.H. Sephton. (1970). "Vertical Tube Evaporation Utilizing Vortex Flow and In- terface Enhancement." Office of Saline Water, Research and Development Progress Report, No. 574, May 1970.

6 . H.H. Sephton. (1971 j . "Interface Enhancement of Vertical Tube Evaporators: A Novel Way of Substantially Augmenting Heat and Mass Transfer." Presented at the ASME Heat Transfer Conference, Tulsa, Oklahoma, August 1971. ASME Publication 71-HT-38.

7. H.H. Sephton. ( 1974). "Interface Enhancement Applied to Evaporation of Liq- uids." US Patent No. 3846254, and Foreign Patents.

8 . H.H. Sephton. (1975). "Upflow Vertical Tube Evaporation of Sea Water with Interface Enhancement: Process Development by Pilot Plant Testing." D~~sulirtutiort, Vol. 16, NO. 1, 1-13.

9 . L.G. Alexander & H.W. Hoffman. (1971). "Performance Characteristics of Ad- vanced Tubes for Long Tube Vertical Evaporators." Office of Saline Water Research & Development Progress Report, No. 644, January 1971.

10. R.J. Kedl. (1972). ORNL Task Report No. 8, April 1972. 11. H.L. Fong, C.J. King & H.H. 5,ephton. (1975). "Upflow Vertical Tube Evapora-

tion with Interface Enhancemenl: Pressure Drop Reduction and Heat Transfer Enhancement by Addition of a Surfactant." Descrlintrtiott, Vol. 16, No. 1, pp. 15-38, 1975.

12. H.L. Fong, C.J. King & H.H. Sephton. (1975). "The Mechanism of Heat Transfer Improvement in Upflow Vertical Tube Evaporation by Induced, Foamy Two-Phase Flow." Paper presented at Boiling Heat Transfer Session, 15th National Heat Transfer Conference. Paper No. AlChE/CSME-75-HT-I. San Francisco, August 1975.

13. H.L. Fong, C.J. King & H.H. Sephton. (1976). "An Experimental Study of Heat Transfer in Upflow Vertical* Tube Evaporation." Presented at the AIChE-ASME Heat Transfer Conference, ASME Publication 7 5 - ~ ~ - 4 6 . San Francisco, August 1975. k v $

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14. H.H. Sephton. (1976). "Renovation of Power Plant Cooling Tower Blowdown for Recycle by Evaporation-Crystallization with Interface Enhancement." Final Report on Environmental Protection Agency Project No. R-803257, in press.

15. H.H. Sephton & G. Klein. (1976). "A Method of Using Irrigation Drainage Water for Power Plant Cooling." Proceedings of the First Desalination Congress of the American Continent. Mexico City, October 1976.

16. E.C. Valdes-Krieg, C.J. King & H.H. Sephton. (1973). "Foam and Bubble Fractionation for Removal of Trace Metal lons from Water." Paper presented at EPA Conference on Traces of Metals in Water, Removal and Monitoring. Prin- ceton, N.J., (1973).

17. E.C. Valdes-Krieg, C.J. King & H.H. Sephton. (1975). "Removal of Surfactants and Particulate Matter from Seawater Desalination Blowdown Brines by Foam Fractionation." Desulirlation (1975).

18. H.H. Sephton. (1976). "New Developments in Vertical Tube Evaporation of Sea Water." 5th International Symposium on Fresh Water from the Sea, Vol. 2, pp. 279-287. Athens, 1976.

FLUJO EVAPORATIVO DE DOBLE FASE ESPUMOSO APLICADO AL PROCESAMIENTO DE CARA

0.0. Omatete y H. Sephton

RESUMEN

Se ha medido en un tub0 vertical de flujo descendente, con compresor de vapor, la transferencia evaporativa de calor en una solucion azucarada pre-concentrada antes de su cristalizacicin en tanques de vacio. Se ha mostrado que la adici6n de un surfactante seleccionado, sulfato dodecil de sodio, en partes por millon a la solucion azucarada de cafia, resulta en un increment0 de 70% sobre el promedio de desempefio (coeficiente total de transferencia calo- rica, valores U) encima del rango de concentration, cuando se com- para a la evaporacion sin adicion de surfactante. Esta mejora resulta no solo de la mera adici6n de surfactante, sino tambib del arreglo especial del orificio de entrada para iniciar un flujo continuo de doble fase espumoso a traves de 10s tubos.