Sugar Cane Juice Evaporator

 

J.J. Bhagat FALLING FILM EVAPORATION N THE CANE SUGAR INDUSTRYAN INDI N EXPERIENCE J.J.Bhagat Sugar Technology Mission, D-5 Apt., Qutab Hotel, New Mehrauli Road, New Delhi 110016

ABSTRACT Among tthe Among he various designs of evaporators operating in the cane suga r industry, indust ry, the Fallin g Film Eva porator pora tor FFE ) is a recent one. The FP s claimed to have vario us advantages over the conventional Robe rts design or the climbing climbing film evapo evaporato rator. r. Juice Juice distribution distribution is an important design feature in an FF E and has to b e arra nged carefu lly. A thin film of juice and abs ence of a hydrostatic head head in the FFE tubes should should lead to better heat trans fer. Scale formation in the FF is minimal and there is a very gradual loss in efficiency efficiency over time. Various problems including the the tube deforma deformati tion, on, juice juice caramelization and entrainment in F FE have been overcome through suitable design modification. Sugar inversion is lo low w du e to short juice residence time. time. The rate of evaporation , how ever, continues to remain low and requires further des ign improvements to achieve cost economy, Keywords: Sugarcane, falling film, inversion, climbing film, film, evaporato rs, India.

INTRODUCTION The use of falling film evaporato rs FFEs ) in the sugar industry industry in India India is recent, with a number of cane sugar existing evaporator arrangem ents. A number of factories having introduced falling film vessels in conjunction with existing equipme equ ipment nt manufacturers have develop ed differen t designs-the main differences being in the arran gem ents for juice distr ibutio n. Factories using FFE s have faced faced a number of problems including caram elization, tube deformation, inadequate wetting and juice distribution; but a common advant advantage age claimed claimed by the u sers is the short shor t juice-retention time with minimal sugar losses due to to inversion. Theree are di ie te nt options Ther options for the installation installation sequence sequence of of the F FEs. In a number of case s, they they ar e installed as the first or second vessels of an evaporator arrangement. This paper deals with the operating aspects of the equipment; discussion of theorical aspects of falling film evaporation has has been deliberately avoided. INSTALLATION STATUS The first fi rst installation installation of FFEs FFE s was at Dhampu Dhampurr Sugars Sugars Limited Limited,, UP. The vessels, wh ich were of indigenous design, were operated operated as second second and third effects in a sextuple system . Wiegand Germany), through collaborato rs in India, installed install ed FFE s as first ef fects at Ponni Sugars Ltd. 1988) and at Erode and A r m Sugars Ltd ., Pennadam 1990). To date, numerous factories have installed FFEs in various configurations. All are manufactured locally and are generally for frrst- or second-e ffect duties Table 1). DESIGNS The different designs in the Indian sugar industry can be classifi classified ed generally into the following categories: Those that operate on a once-throu gh system. No common juice recirculation is provided. The heating surface is divided into two segments on the juice juice side. After being heated in a plate heat exchanger to 104OC, the juice juice is pumped to the first juice juice com partment at the top top of the juice distributo r llocat ocated ed above the top tube plate. The juice is distributed through a five-tier distributor, common to both juice compartments but with partitions for segregati segr egating ng the juice juice between the comp artmen ts Fig. 1A 1B). After falling through the first compartment, the juice is collec collecte tedd in the concentric cylinder located bbelow elow the tubes. This juice is pu pumped mped throu gh a recirculation pump back to tthe he distributer. The juice collected collected from the second com partme partment nt is pumped to the t he next vesse l. The distributor has five stages, the first being the juice juice receiving and primary d istrib utor to the su bsequen t section. The second nd third stage distributi distribution on trays trays have a hexag hexagonal onal shape; wh erea ereas, s, th e fo urth an d fifth stages are triangular. In the final stage the distribution tray is zig-zag in shape shape,, with with finger-type guides to discharge the juice in the center of a set of three tubes.

 

Factory: Evaporation

Table 1

Details of installations

F act act or or y Dhampur

Mansurpur

No . of fallin fallingg eff eff ect ectss 1

FFE eff eff . 1 2

Tub e length length (m )

OD (m )

1607 1858

8230 8534

40 40

HSA (m 22))

Thickness (S WG) WG)

Mat Mater erii al al 55304

SS304

1 1

2 J

Rouzagaon

2

1

1150

8535

42

18

SS304

2

2

1050

8230

42

18

SS304 SS304

1

3

1100

8050

42

18

2

1

1150

8535

42

18

55304

2

2

1050

8230

42

18

SS304

1

3 1

1000 1800

8050 8000

42 45

18 18

55304 55304

1 1

2

1000 2300

8000 10000

45 45

18 18

SS304

1

1 1

1600 2000

7995 7995

42 42

18 18

1

1

1664

7995

42

18

SS304 55304

2

1664

7995

42

18

SS304

Jawahar Shamli

1 1

2

2000

7995

42

18

SS304

urna

1

Asmoli Bhogawati Dyaneshwar Manjara Mula Malegaon

Vighnahar Aruna Ponni hrigonda alna adhukar

1

1

55304 55304

2500

10000

42

18

SS304

1

900

6000

42

18

1

1

1040 2300

6000 10000

40 38

18 18

SS304 55304

1

1

1400

7500

38

18

1

1

2300

1

1

2000

1

1

2300

1

SS304

SS304

 

Factory: Evaporation In the second F FE design, the d istribution system at the top tube plate consists of specially desi designed gned individual

distributor nozzle distributor nozzless for each tube p ig . 2 . The Theyy are inserted into the tubes an d distribute the juice to the walls with a spiraling spiraling motion, forming a film. A juice-distribution tray fo r receiving inlet and re circu latio n juice is provide d above the distributor nozzle s. The nozzles are easily removed removed for cleaning. In the the third F FE d esign juice enters from the top of the caland ria. A juice d istribution device rests on the top of juice leaving the distribut distribution ion box the tube plate. The distribution box can be removed when requ ired. The juice descends in a filmlike filmlike layer in each tube Fig. 3). There is a collecti collection on chamb er for juice with a cen tral funnel. The juic e captured by the funnel is transferred to the next vessel and the juice that falls outsi outside de the funnel gets recycled. Additional distributor designs are show n in Figures 4, 5 6. The heating su rface, tube length and size, and other dimensional details are indicated in Table 1. STE

M ECONOMY

The variou s prime movers used in the Indian industry industry exhaust steam at 0.4-0.5 kg/cm2 g) pressure. Therefo re, in most installalions an attempt h s been made to achieve steam economy through through intelligent vapor bleeding. The use of FF n the first and second effects has enabled the factories to to bleed vap ors from these vessels to vacuum pans at 0.1-0.2 kg/cm2 g) pressure. Pressure lower th n this is inadequate for rapid boiling in m ost factories because they they have only only batch batch pans. The FFEs have therefore therefore helped in providing providing the the nec essary v apo r pressu re for the vacuum pan s. Substantial steam economy has been reported by various factories through use of FF Es, leading to a drop in steam consumption of 0.4-0.5 kglkg of sugar. The pressure and and temperature drops acro ss the FFEs compared to conventional vessels for various duty a pplications pplications are indicated in Table 2A B. CLE

NING SHUTDOWNS

It is difficult difficult to to remove the cumbers cumbersome ome juice juice dist distribu ributio tionn arrangement for m e c l d c a l cleaning cleaning of tubes. tubes. Fortunately most factories lave reported only a minor drop the in overall heat transfer co-efficient n comparison w ith conventional convent ional vessels vessels after similar through put and dura tion. In addition the sc ale forme d in falling film vessels is very soft a nd is easily removed by chemical cleaning. All factories factories have therefore resorted to chem ical cleaning cleaning of of the falling film vess vessels els wi with th the the dis distributi tribution on system in situ during the crushin g cam paign. Mec hanica l cleaning is required and done only in the off campaign. RESIDENCE TIME

A comm on feature reported by all factories iiss very low residence time time of juice in comparison with conventional vessels with similar heati heating ng surf surface ace area . Retention time has been e stimated to be about 30 sec vs 15 min for Rob erttype vessels, and 5 min for climbing film vessels Table 3). SUG R LOSSES

The sugar loss due to inversion ooff sugars and o r destruction destruction of the reducing su gars has been reported to be very low in falling film vessels. A detailed study study was done by Lonk ar, Bhojraj Gava nde. The difference in reducing sugar per 100 Brix between entry and exit of of FF E was from 0.07-0.39 vs 0.21-0.67 for conventional vessels. This is primaril primarilyy du e to the low low residen ce time . A compa rison of of loss due to inversion in FFE s and other types is shown in Table 4. OPER

TING PROBLEMS

The factories factories have reported a number of of operating problems such as caram elization in tu bes, tube defo rmation and insufficient insuffi cient distrib utio n and wetting . Initially there were a num ber of incidents of chokin g due to inadequ ate wetting through increased recirculation of juice. The number of of the tubes. This problem has largely been overcome through recirculation is kept at about 1.5; howe ver, this has led to use of substantial powe r and increased pump costs. Tube deform ation has been a problem in numerous factories. This has been solved iinn many cases by keeping keeping the

 

.Jojnq gqp alz alzzou zou pue alqmnqj alqmnqj JO m v d ? ~ a s a ~ w d Lpoq

m

ap sq

 

J.J. Bhagat

Evaporator body shell uice inlet pipe uice overflow overflow tr y Deflectorplat e

-

Hole 2110) Hole 15 mm

hole)

Tube plate Tube

15m 15 mm s hole

0

Top superimposition of 15mm 15 mm h o l s onthe tubes of the tube plate

Figure 6

Table 2A

Pressure and temperature drop

Vessel effect Type 1 2 3 4 5 6

Juice distributor system

Climbing film Falling film Falling film Falling film Falling film Robert Robert Robert

Caland ria

Pressure/V acuum Vapor space

0.7-0.8 kg/cm2g 0.7-0.8 kg/cm2g 0.20-0.35 kg/cm2g 0.20-0.35 kg/cm2g 0-50 mm (vac) 200 mm (vac) 380 mm (vac) 530 mm (vac)

Difference

Tem perature 'C Caland ria Vapor

Diff.

0.15-0.35 kg/cm2g 0.40-0.40 kg/cm2g 50 mm (vac) 50 mm (vac) 200 mm (vac) 380 mm (vac) 530 mm (vac) 640 mm (vac)

Table 2B Press Pressure ure and temperature drop

Vessel effect

Type

Caland ria

1 2 3 4 5

Falling film Robert Robert Robert Robert

0.65-0.7 kg/cm2g 0.25 0.00 175 vac) 400 (vac)

Pressure/Vacuurn Pressure/Vacuurn Vapor space 0.25-0.28 kg/cm2g 0.00 175 mm (vac) 400 mm (vac) 660 mm (vac)

Difference (kg/cm2) 0.40 0.25 0.23 0.29 0.34

Tem perature C Caland ria Vapor Diff Diff 125 105 100 90 77

105 100 90 77 55

20 5 10 13 22

 

Factory: Evaporation

Table 3 Residence times in different evaporators

Surface m2) Tube length m) Shell diam. m)

Falling film

Rising film

Rob ert

3000 15 2.586

3000 7.0 3.786

3000 2.5 6.449

Flow rate tlh) 250 250 Brix in 11.0 11.0 Brix out 16.6 16.6 Retention min) 0.548 4.962 Courtesy: Vasant Data Sugar Institute, Pune.

Table 4 Sugar loss. Crush rate TCD )

Sample details

1850

250 11.0 16.6 15.884

pH

Brix

Pty

RsIlO 0 Brix

Clear juic juicee FFE Vapor cell Robert)

7.0 6.9 6.8

17.06 22.80 23.51

79.69 79.64 79.07

13.66 14.05 14.72

0.39 0.67

1850

Clea rjuic e FFE Vapor cell cell Robert)

7.0 6.9 6.8

16.96 22.29 24.50

78.18 77.40 76.80

9.72 10.02 10.52

0.30 0.50

1640

Clearjuice FFE Vaporcell Robert)

6.9 6.8 6.8

17.78 22.40 24.40

65.64 64.98 64.50

9.69 10.01 10.54

0.32 0.53

1720

Clearjuice FFE Vapor cell cell Robert)

6.9 6.8 6.7

17.82 25.60 23.60

67.06 66.62 65.68

10.94 11.14 11.39

0.20 0.25

1250

Clear juic juicee FFE Vapor cell cell Robert)

6.9 6.8 6.7

16.14 19.34 21.00

72.88 72.75 72.60

1.47 11.54 11.85

0.07 0.21

Diff.

 

 

J.J. Bllagat

1ei1gt 1ei1gt.h .hof tub es < 1 aid also by arrangulg baffle plates uiside the calatldria to give adequate support to the tubes. Suppost Supp ost bel below ow th thee tube plate is also pr provided ovided to prevel prevelit it sagguig due to the weiglit of of tlie tlie tubes a i d plate. E xp a ns io ~ ~ joiiits ul the calalldria sllell also help to prevei~t eformatiol~ ue to differential thermal expalsion. The elurin elurinme1g me1g of juice is a co ~nm oil roble n~ t the vapor- separation stage; therefore, therefore, asrangemel asrangemelits its la ve been made UI many cases to provide se parate vapor-separati vapor-separation on vessels. These are fitted with efficient poly baffle-type separators of 304SS Table 5 Rate of evaporation selected factories). quality. EVAPORATION RATE Location of FF E Evap.rate Evap.rate The rates of evaporatioa hi all falluig film vessels ui~der coi~sideratiol~re low. Ev Evee11 the press ure aiid tempera ture drop across Fac toV (effect) (k (kg/ g/m2/ m2/11 11)) tlie FFE is UI many cases Ili Iliplie plierr tlian iii conv ent ioi al vesse ls. Gi Give ve11 11 the 1 24.65 Malegaol1 tl hi fihn a ld absei abseilce lce of any liydros tat~c ress ressure. ure. it was expected tllat tllat 2 20.80 FF s shoul shouldd result result in a lligli overall heat tra nsfer co-efficient and hence 29.60 MaIljara rate of evaporatiou; but tlus tlus was iiiot iot so. The reasolls for low evapora1 20.56 Mula tioil rates inay be the result of mefficient juice distribution a~id Porulii Porul 1 24.12 ilo iiu ~ ~ if o n nill ill11 tluckness ai aiid id tube wettmg. hi ot li er possibili possibility ty is Rou zaga o~~ 1 2 44.6 .611 ii~ e f f ~ c ~ e uistribu istt ributioi tioiii of stem1 in the the calaid ria. To ensure proper 2 21.60 distribut~on, in factories llave arranged for a sweepuig effect of stean 3 10.54 ui the calaldsia rxl Imve Imve reported superi or perforinN lce. A colliparis colliparisoii oii 1 21.52 Aruna of evaporation rates ui differelit co~lf igur atio iis s give1 give111 ui Table 5 . CONCLUSIONS TI]; study of FF p e r f o r ~ r wlc e ii the cane sugar uidustry UI hidla, ptoducu~g la~itatloilwlute la~itatloilwlute sugar for d~rect coi~smrg tion, hows ttll llat at there there is si pt ic at it advatitage advatitage UI tellns tellns of lower juic e-ret e~it~ onune u ne and sugar loss. A nnumber umber of jmce jmce dls dlstnbut tnbutloii loii systelrisare employed, but they are far from perfect. The probleiri probleiriss ooff c ara me l~z atl o~if juice UI tl tlie ie tubes, tubes, tube defonratioii defonratioii aii aiidd jui juice ce e ii tra uu ~~ e~ave it been largely mrwe mrwerrcom omee tl ~ o u g l i arious U uiova uiovatioI tioIis. is. The mnauit mnau itelwi elwice ce costs ooff FF E vessels vessels were also low. Ho we ver. overall heat tr;uisf tr;uisfer er co-efficie~its i d lie~i ce lie rate of evaporati evapo ration on aclueved at ttlie lie various uist all at~ o~ is isere ere low, r equ~ ruig rtlie r study ai d Improv Improvemen ements ts.. REFERENCES Loilkar, M.Y.; Bliojraj, S.K.; GavxKle, C . N . ; Fallu Falluig ig F ~lt li vaporat vaporator, or, 54t 54thh hm u al Conveiit~oii f S.T.A .I. (1992).