39 Glimpses of the paper on of evaporator

 

D B Jambhale 

Consulting sugar technologist,

Dear friends -

In the last fortnight I presented a paper on  " Mathematical model of evaporator for a raw sugar factory attached refinery for a two massecuite boiling system" at DSTA annual convention  held on 18th 19th September 2022.  Abstract of the same is giving hereunder for your studies.  For the original paper you may contact to DSTA office. 

    Now in India, in many factories  two massecuite system is adopted and high purity  B heavy molasses is diverted to distillery for ethanol production. 

        Considering this, a mathematical model of evaporator set with ultimate steam economy for a modern raw sugar factory with attached refinery has been discussed in this paper.

The steam consumption in boiling house is divided in 

1                     Vapours used for juice heating,

2                     Exhaust steam used at evaporator,

3                     Vapours used at pan station,

4                     Miscellaneous use of vapors.

·         Use of hot condensate water for heating the raw juice has started from 2006 in Indian sugar industry. This can be brought down the steam consumption at juice heaters from 18-19 % to about 13-14 % or even lower.

·         The use of   noncondensable gases has also been tried successfully in some factories for heating the juice, melting or molasses conditioning. Use of vapors from clear juice flash tank also has been suggested; however care has to be taken in controlling of it.

·         At evaporator in many modern factories with evaporators (quintuple) and profuse vapour bleeding, the steam consumption at evaporator is 1% or less.

·         An idea using vapours from last effect of evaporator to continuous pans has been implemented in Indian sugar industry some 10 to 12 years back. Spray engineering are pioneers of using this system in cane sugar industry.

·         At evaporator a quadruple set of falling film evaporator is recommended for this model.  High exhaust pressure (2.03 kg/cm ab) is to be maintained at the first effect.

·         Further profuse vapor bleeding is to be adopted for heating the juice and miscellaneous use.

·          The vapours from the last effect - of absolute pressure 0.71 kg/cm and temperature 90°C, are to be used at pan station for boiling the massecuite in vertical continuous pans (VCP). Modern continuous pans are designed to work at negative pressure at calandria with vapour temperature of 88-89°C.

·         At pan station maintaining high brixes of syrup and molasses and controlling intake of water, also by tight boiling and proper cooling can control the massecuite percent and the vapour consumption. Vapour demand at pan stations (raw and refined) with the described boiling scheme is 16 % on cane and vapours provided from evaporator and refinery liquor concentrator  to pans can exactly meet the requirement.

·         With the adoption of two massecuite boiling system for raw sugar manufacture,  with B sugar melt to refinery, the total massecuite percent in raw house can come down to 27 to 30% on cane.  Consequently the vapour consumption is also reduced.

·         Now in many factories with modern machinery and proper control, the miscellaneous vapor consumption has been brought down to 1.5 to 2.0 % on cane.

In the model evaporator set all the evaporator bodies - falling film evaporator are to be used of equal heating surface with additional one as standby FFE so that each FFE can be isolated for cleaning.

 

 

As defecation process is to be followed scale formation at evaporator will be considerably less and soft. The scale can be easily removed only by chemical cleaning.  Thus by circulation hot acid followed by circulation of hot caustic soda solution is to be followed and each body should be cleaned weekly in the season. No mechanical cleaning is required and hence to evaporator cleaning stoppages are not required. 

The main advantage of using FFE evaporator set is that there is no hydrostatic head. Further there is relative high velocity of falling juice with thin layer of juice so that the distance between the juice particles and heating surface / media is less. Therefore high coefficient of heat transfer is maintained.

The vapor bleeding recommended is as follows,

	Juice heating	increase in temperature
	Raw juice heating  in three stages
1	Raw juice 1st heating  in VLJH  B massecuite continuous pan	30 to 44°C
2	Raw juice 2nd heating   in liquid- liquid  heat exchanger - heating by condensate	44 to 60°C
3	Raw juice 3rd  heating in JH by vapours from 4th effect	60 to 74°C
	Limed juice heating in three stages
4	Defecation  juice 1st heating  by vapors from 3rd effect in juice heater	72 to 88°C
5	Defecation  juice 2nd heating  by vapors from 2nd  effect in JH	88 to 98°C
6	Defecation juice 3rd heating by vapors from 1st  effect in JH	98 to 102°C
	Clear juice heating in two stages
7	Clear juice  1st  heating by vapors from in effect 1st effect	97 to 108°C
8	Clear juice  second heating	108 to 115°C
	Exhaust pressure kg/cm2ab.	2.03
	exhaust temperature °C	121-125
	water to be evaporated % on cane	85 to 88
	syrup brix	60-65

 

There will be only two vertical continuous pans (VCP) in raw house, one for A massecuite and one for B massecuite. Advantage of continuous pans / VCP is that – that can work at lower absolute pressure vapors of 90°C. The raw house with two pans station and 3 -5 centrifugals will be less congested.  

 

·         The exhaust steam will be only used at first effect of evaporator and at one clear juice heater.

·         Vapours from the second effect will be send to refinery for refinery liquor concentrator.

Evaporator working conditions and heating surface calculations:

Clear juice	Brix	Pol	Purity	% on cane	Bx n CJ % cane	Pol in C J % cane
	13.40	11.12	83.00	110	14.74	12.22

	Evaporator Body	FFE  1	FFE  2	FFE  3	FFE  4
	Calandria vapors working conditions
1	steam / vapor pressure  kg/cm2 ab	2.03	1.70	1.37	1.04
2	Steam/vapor temperature  °C	120.07	114.55	108.08	100.16
3	steam / vapor total heat Kcal/kg	646.45	644.51	642.17	639.23
4	vapor latent heat  Kcal/kg	526.03	529.69	533.90	538.95
	Vapor space working conditions
5	pressure kg/cm ab	1.70	1.37	1.04	0.71
6	vapors temperature °C	114.55	108.08	100.16	89.79
7	steam / vapor total heat Kcal/kg	644.51	642.17	639.23	635.26
8	vapor latent heat  Kcal/kg	529.69	533.90	538.95	545.41
11	Condensate water temperature °C	117.93	112.07	105.09	96.56
9	BPR	0.18	0.27	0.45	0.96
10	Juice boiling temp. °C  ( row no. 6+9)	114.73	108.35	100.61	90.75
	Heating surface calculations
11	Temperature diff. In heating vapors and boiling juice °C (row no.2 -10)	5.34	6.20	7.47	9.41
12	Coeffi. of heat transfer   Kcal/m2/°C/hr (assumed)	2400	1800	1300	800
13	evaporation rate (kg/m2/hr)	24.20	20.90	18.02	13.80
14	Water to be evaporated kg/hr	26361	23424	19036	16980
15	heating surface required m2	1089	1120	1056	1230
16	Heating surface recommended for 100 tch sq. mtrs.	1250	1250	1250	1250
17	Heating surface for 230 tch / 5000 tcd plant sq. mtrs.	3000	3000	3000	3000

 Calculations of refinery concentrator are not included here.

Flashing of Condensate:

·         Condensate of the first effect to be flashed to the calandria of the second effect and the same of temperature 115°C will be send to boiler.

·         The condensate of second effect will be flashed in the third effect calandria and with the third effect condensate will be send to flash tank to open to the fourth effect calandria.

·         The fourth effect condensate will be send to flash tank to open to vapour space of the fourth effect.

·         All the condensate from juice heaters (expect VLJH), FFE (except first effect) and pans are collected together (of temp.93°C) to use it to heat the raw juice. ( RJ2 from 44 to 60°C)

·         B sugar melt and A sugar will be send to refinery for process

·         There will be two pans at refinery one for R1 massecuite and one for R2, R3 R4 massecuite. The R4 run off will be returned to raw house A massecuite.

·         Vapours from the second effect of Refinery concentrator will be used for refinery pans. As well as vapors from the fourth effect of evaporator will be provided to refinery pans.

·      From the above table it can be seen that for a 5000 TCD /230 TCH plant with equal heating surface for all effects, the heating surface required will be 12000 sq. mtrs.

o  When juice is heated in direct contact heaters (DCH), the juice quantity goes on increasing at every step. In such case the clear juice % cane may be increased up to 122-123% on cane and water to be evaporated may be 97 to 100 % on cane. In such case evaporator heating surface required will be higher.

 

Conclusion:

Establishing raw sugar factory with two massecuite boiling scheme with back end refinery will be next road for Indian sugar industry.

 

 Exhaust applied:

Sr. no.	particulars	percent on cane/  MT /hr
1	evaporator	27.00
2	clear juice second heating	1.35
3	Total	28.35

·         The factory including refinery can work with steam consumption 28.0 to 28.50 % on cane.

·         The factory can work on 11 to 12 % bagasse.

·         Thus 16 to 18 % bagasse can utilized for cogeneration and /or sale.

Vapours used:

Sr. no.	particulars	percent on cane  MT /hr
1	At  juice heaters	13.90
2	At pans 16.0 -2.55(used in VLJH)	13.45
3	Miscellaneous use	1.50
4	Total	28.85

 

Vapour recovery by flashing of condensate at evaporator:

Sr. no.	particulars	percent on cane  kg/hr
1	flashing of 1st effect condensate  to 2nd effect	168
2	flashing of 2nd effect condensate to 3rd effect	172
3	flashing of 2nd + 3rd  effect condensate to 4th  effect	383
4	flashing of 2nd + 3rd + 4th effect condensate to pans	727
5	Total	1450

 

·         Thus the percentage of vapours recovered by flashing of condensate to exhaust applied to evaporator comes out to be 5.37%

General advantages:

·          The factories can make and establish a brand of refined sugar which will be sold more easily and at higher prices. The interest because of long time storage sugar in godowns will be reduced. Thus the factory will make more profit.

·         The refined sugar also can be exported at higher price in International market.

·         When there can be order for raw sugar from international market, factory can produce VHP or VLC raw and can export.

·         With diversion of B heavy for the production of ethanol, factories get money within month’s time for day today transaction.