Carbonation Clarification Followed by Candle filtration in Modern Standalone Sugar Refineries

DOI : 10.17577/IJERTV11IS080023

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Carbonation Clarification Followed by Candle filtration in Modern Standalone Sugar Refineries

Sanjay Awasthi, Rajesh Srivastava, Awanish Kumar ISGEC Heavy Engineering Ltd.

Address: A-4, Sector-24, Noida-201301, UP, India

Abstract India is one of the biggest sugar cane producers and always remains at first or second position in world for sugar cane and cane sugar production. This year also India produced 335 LMT of sugar discounting diversion of 35 LMT for ethanol and becomes largest producer of sugar in the year 2022-23 and second largest exporter of sugar after Brazil. A very few standalone refineries are working in India due to the reason of abundant sugar cane availability.

Standalone sugar refineries are installed in countries like Arabian Countries, Indonesia, and USA etc. having non- availability of required quantity of sugar cane.

Operations of standalone refineries are quite different from cane sugar plants especially in clarification. Generally these factories use carbonation process of clarification followed by different types of filtration system. Lots of different machines and technologies are used by refineries for filtration. Selection of best combination is important to run refinery smoothly and efficiently.

This paper describes a generalised view of standalone refinery in term of design basis, clarification technology, efficiency parameters and our practical experiences for the same.

Keywords Raw sugar, EEC-2 Grade, Carbonation, Available CO2 Gas, flue gas, CO2 gas, TPD (Tons per Day)

INTRODUCTION

ISGEC has been appointed to design; supply installation and commissioning of a standalone refinery by Durrah advanced Development Company, Saudi-Arabia (KSA).

The refinery plant is design for processing raw sugar to produce 2,500 TPD EEC-2 grade refined sugar. Our team is residing in factory and is involved in day to day working with operating team. Factory is achieving all performance parameters which are one of the best in industry.

Factory used Carbonation clarification process with pressure filtration (Candle filter) followed by Press Filter as primary clarification process.

ICUMSA = International Commission for Uniform Methods of Sugar Analysis, IU = ICUMSA Unit

Target to get efficiency of Carbonation and filtration system are as per table-1 and same was achieved under normal operating condition of refinery.

Table-1: Design v/s Achieved Carbonation and filtration parameters

Description

UOM

Target parameters

Achieve Parameters

Carbonation

Lime (as CaO) dosing on Refined sugar output (RSO)

%

0.8 to 1.0 %

0.8 to 1.0 %

CO2 concentration in Flue gas

%v/v

8.5-9.0 %

>8.5

CO2 absorption efficiency of saturators

%

30

>30

pH before First Saturator

pH

10.3-10.5

10.3-10.5

pH after Second Saturator for design & operating performance

pH

8.1 -8.50

8.1 -8.50

Color Removal (after Filtration)

> 50 %

>50 %

Pressure (Candle) Filters

Filtration rate

m3/m2/h

0.07 m3/m2h

0.07

Suspended particles in filtered liquor

mg/L

5-10

2-10

Sludge (Press) Filters

Cake Moisture

%

<30

<30

Cycle

per day

18

>18

Filter Cake Thickness, before squeezing

mm

40 mm

40

DURRAH SUGAR REFINERY:

Plant is located at in King Fahad Industrial area Yanbu, About 300 km north Jeddah at west-coast of Saudi Arabia on the port of Red Sea. Plant actual overview is shown in figure 1.

Figure 1: Durrah Refinery Actual view

This is a standalone refinery with primary clarification as carbonation and secondary clarification as Ion exchange process.

Figure 2: Raw Sugar Carbonation & Filtration Process

Raw sugar quality and required refined sugar specification are in table 2 & 3 respectively.

Table-2: Very High Pol (VHP) grade Raw Sugar Specification

Description

Unit

VHP Raw sugar

Polarization

0 Z

Min 99.2

Colour

IU

<1500

Moisture

%

Max 1

Starch

ppm

<330

Dextran

ppm

<170

Insoluble matters

mg/kg

<200

Table-3: European Economic Community (EEC) 2 grade Refined Sugar Specification

Description

UOM

EEC 2 Refined Sugar

Pol

%

99.9

Moisture (Loss on drying)

%

Max 0.04

Colour

IU

< 45

Ash

g/100g

0.02

Reducing Sugar

%w/w

0.01

Sulphite

ppm

6

Sediment

mg/kg

< 6

Floc Potential

non

RAW SUGAR MELTING:

Raw sugar is reclaimed from warehouse and after weighing, it is melted in a magma mingler followed by melter using sweet water recovered from Filtration & decolourization; this water is used on sugar to water mass basis to maintain raw sugar Melter outlet Brix 62-63.

As shown in figure 4, liquor is taken from 2nd compartment of melter and re-circulates through heater to increase the temperature of the liquor and other pump used to transfer the melt from melter to rotary screen.

Figure 3: Raw Sugar Melting area 3D view CARBONATION:

Factory has a 100 TPH, 45 kg/cm2 pressure gas fired boiler and 12 MW back pressure turbine. Flue gas contains approx. 9-10 % v/v Carbon Di Oxide. Gas compressor has been designed considering this availability of CO2 in flue gas and this flue gas is scrubbed in a wet type scrubber before using in carbonation. Flue gas duct from chimney to Scrubber is shown in figure 4 and its CO2 pumps are shown in figure 5.

Figure 4: 100 TPH, 45 Kg/cm2, Gas fired Boiler & Flue gas duct going to CO2 scrubber

Figure 5 : 5 Nos. CO2 compressor, Capacity 6000 Nm3/h each, 1.10 bar g pressure

Carbonation is a robust process of clarification, simple to operate, requiring no chemical other than Lime and removes colour and other impurities efficiently. Generally colour reduction more than 50%is obtained. Carbonation efficiency mainly depends on following equipments design and its working.

  • In line static mixer, this is responsible for uniform mixing of limed and melt and thus to maintain uniform pH of limed melt before Carbonation.

  • Design of saturators (Carbonation vessels), for good recirculation and less temperature drop.

  • Selection and sizing of CO2 compressors, based on CO2 requirement.

  • CO2 distribution in saturators, responsible for better uniform reaction and calcium carbonate formation.

  • Self-cleaning type CO2 distributors known as Richter tubes, for continuous effective cleaning of CO2 distributors for better mixing efficiency.

Carbonation process consists of two numbers carbonators working in series as 1st carbonator and 2nd Carbonator having 40-45 and 20-25 minutes retention time respectively. Lime dosing in raw melt is doing in Inline static mixer having 15-20 Second reaction time before CO2 gassing. Estimated carbonation efficiency 38% & 33% of carbonation 1st stage and 2nd stage respectively, considering CO2 content 8.5 % by volume.

CALCULATION OF FLUE GAS REQUIREMENT FOR MELT CARBONATION

CaO + H2O Ca(OH)2 Ca(OH)2 + CO2 CaCO3 + H2O

CaO +CO2 CaCO3 56 44 100

Assume 100 Ton of Brix in Melt per hour.

Case I:

Lime dose = 0.80% Melt solids

Weight of CaO = 0.80 x 100/100 = 0.8 T/ h Stoichiometric weight of CO2

= 0.8 x 44 x 1000/56 = 629 Kg/h

Actual CO2 required @ 30% efficiency

= 629/0.3 = 2095 kg/h

= 2095/44 = 47.62 kg moles/h Volume of CO2 at STP

= 47.62 x 22.4 = 1066.67 m3/h

Volume of flue gas considering (CO2% in flue gas = 9%)

= 1066.77 / 0.09 = 11851.85 m3/h at STP

Temperature of flue gas

= 700C = 70 + 273 = 3430K

Pressure in suction of compressor = 0.92 kg/cm2 (a) Volume of flue gas at 3430K & 0.92 kg/cm2 (a)

= 11851.85 x 1 x 343 / (0.92 x 273)

= 16185.64 m3/h

For 113.14 TPH solid in melt

= 16185.64 x 113.14/100

= 18312.43 m3/h

Density of flue gas at 700C = 1.0 kg/m3 Quantity of flue gas

= 18312.43 x 1

= 18.3 T/hr.

P1 = 0.92 kg/cm2 (a) & P2 = 1.70 kg/cm2 (a)

Case II:

Similarly we can calculate for lime dose @ 1.0% on melt solids-

Quantity of flue gas

= 18312.43/0.8

=22890.54 m3/h.

= 22.89 T/h.

After reaction in first carbonator, it flows to second carbonator by gravity and where all the carbonated crystals grow. Each carbonator is equipped with Richter tubes for better mixing of CO2 with the liquor and facilitates the on line cleaning of CO2 nozzles, refer figure 8 & 9. This results better colour reduction in the carbonation system. Carbonated liquor from the second carbonator tanks flows by gravity into carbonated liquor tank. The carbonated liquor heated to 85 0C in direct contact heater before filtered in pressure filters to separate the calcium carbonated precipitate from carbonated liquor. Clarified melt from the carbonation system will be subjected to 2nd decolonization process in Ion exchange columns.

Figure 6: Carbonation station

Figure 7: Saturators inside view with its working

CO2 GAS DISTRIBUTION:

Richter tubes are installed at the bottom of the saturators for the efficient gas distributors for carbonation vessels. Total 6 and 5 numbers of Richter tubes are installed in 1st and 2nd carbonation vessel respectively, refer figure 8.

Figure 8: Richter tubes installation view in Carbonator-1

The reaction in the saturator takes place through contact between the CaO in the melt and the CO2 in the flue gas bubbles, it is through the surface area of the gas bubbles, the gas inside the bubble does not react and actually escapes unused through the gas exhaust line. It is therefore essential to maximise the surface area of the gas bubbles. This is achieved by the Richter tubes system, which has precisely dimensioned gas outlet slots and its constant shear cleaning pins. The aim is for small bubbles because geometrically the smaller the bubbles the greater the surface area for the same volume of gas as illustrated in the figure 9 & 10.

Figure 9: Diameter of gas bubbles

Figure 10 : Typical Calcium carbonate precipitate magnified view has very a large surface area on to which the colouring matters & impurities are absorbed

/ adsorbed.

CARBONATION PROCESS CONTROLING

  • The process consists of adding slurry of calcium hydroxide into the raw melt solution in an inline static mixer to increase the pH to 10.3 10.5.

  • Addition of CO2 on controlled basis as per carbonated liquor pH as per table-4.

  • All above operations to be controlled using automation through DCS

Lime & Liquor in static mixer

After Static mixer

Exit 1st Saturator

Exit 2nd Saturator

CaO %

Brix 0.8-1%

pH

10.3-10.5

9.3-9.5

8.2-8.3

Temp

85 to 87 oC

74-75 oC

73-74 oC

Table-4: Process operation parameters of Carbonation

Carbonation efficiency depends on the limed liquor pH but this pH should not be increased for a long time to avoid any sugar loss due to destruction. A typical CO2 absorption efficiency is shown figure-11 with its different pH.

Figure 11:CO2 absorption efficiency at different pH

Carbonation process clarification efficiency is depends on the lime dose rate but it should be limited to 0.8 to 1.0 % CaO on Brix above this there is not gain in clarification efficiency, which is shown in figure-12.

Clarification Achieved at Different CaO% Brix in Melt

% Removal

Figure 12: Clarification achieved at different CaO % Brix in melt

The above graph by a different author also confirmed by Moodlys, however it is interesting to note that there is little advantage of liming much more than 0.8 % CaO on Brix in melt. It is necessary to provide the necessary ingredients, calcium oxide (CaO), and carbon dioxide (CO2) and the right conditions to optimise the reaction.

From results collected from different refineries, the degree of colour removal was extrapolated and was found to be proportional to the colour of the incoming raw sugar. The figure-16 below illustrates this and shows Durrah Refinery can probably expect to get about a 50% decolourisation with its sugar colour sent to the refinery.

liquor outlet; this is provided with inline glass for monitoring of liquor quality. In figure 14, ramps are shown with its cloth and assembly of candle filter. Filtering elements/cloths are made up of polyamide of 40-50 µm pore size having monofilament woven fabric.

70 67

Sugar Colour & Expected Decolourisation with Carbonation

65 63 y = -4x + 71

59

60

55

55 51

Expected % decolourisation

50 47

45

% decolourisation

43

Figure 14: Candle filter top and inside view with ramps Figure

39

40

35

Figure 13 : Carbonation decolourisation % with different input sugar colours – results extrapolated from actual results of Carbonation refineries internationally

In Durrah Sugar Refinery Yanbu-KSA, we have received approximately 50 to 55% clarification efficiency with respect to inlet colour of raw melt at stated CaO dose rate. Following table -5 shows the results of Carbonation efficiency.

Figure 15: 3D View of Clarification Section

WORKING OF RESSURE FILTRATION

Carbonated liquor is pumped to each filter from pumps. The filtering period consist of following processes.

  • Maximum flow rate at low pressure.

  • Decreasing flow rate at constant pressure. Decreasing flow rate at increasing pressure.

Before carbonated liquo entering the filter temperature of liquor is maintained at about 82 85 oC in a direct contact heater for better filterability. Filter cycle and its timer setting are as per table 6 and for operation diagram refer figure 21.

Date

Raw melt Colour

CaO dose rate

Filtered liquor Colour

Colour Reduction

IU

% DS

melt

IU

%

25-05-2021

1654

0.85

850

49

26-05-2021

1591

0.86

701

56

27-05-2021

1547

0.87

729

53

28-05-2021

1566

0.85

668

57

29-05-2021

1579

0.87

658

58

30-05-2021

1614

0.83

705

56

31-05-2021

1533

0.84

595

61

01-06-2021

1523

0.87

728

52

Average

1576

0.854

704

55

Table-5: Clarification efficiency in Durrah Refinery at different CaO dose rate

PRESSURE FILTRATION OF CARBONATED LIQUOR BY CANDLE FILTER:

We are using 8 candles filter of 311 m² filtering areas and 41 m3 working volume of each. Which have 2 lines of 4 filters in each line, refer figure 19 & 20. Each line is fed by an open impeller type centrifugal pump. Installed capacity of filters is designed so as to handle of raw sugar liquor having filtration flux as 0.07 m3/m2/hr. Each filter has 31 ramps for filtered

Step

Process

Duration , minutes

1

Filling

17

2

Recirculation

30

3

Filtration

1200

4

Concentration

10

5

Draining

2.50

6

Steam wash

2.0

7

Acid wash

180

Cycle time excluding acid wash

Minutes

1262

Hours

21

Table-6: Candle filter timer with Sequence of operation

Figure 16: Operational diagram of candle filter

As per filter manufacturer, Choquenet recommendation, filtered liquor volume of candle filters should be maintain such way that the mud slurry concentration inside the filter should not go beyond 200 g/L. With this limitation, we have decided to provide logic with limed liquor alkalinity v/s filtered volume as shown in figure 17. This is necessary for the safe operation of the candle filter, filter press and also to get good clarification results.

igure 17: Performance of Candle filters as per lime dosing

To review the performance of filter, we have recorded filter no 80 filtered volume and its total filtration time during normal working days. This is as per following table -7

Table-7: Performance of Candle filter

We have checked a filter # 80 for turbidity removal under recirculation phase of the filter cycle and found that clear filter will come in the range of 15 to 30 minutes. A test report of filter # 80 is shown is figure-18.

Figure 18: Performance of Candle filters #80

EEFFECT OF STEACH CONTENT ON FILTRATION

Starch levels in the sugar will have an adverse effect on filterability of the final product also, see graph below. Fortunately the Carbonation process most of the starch is removed as mentioned in figure-19.

Effect of Starch on Filterability

70

60

50

40

30

20

10

0

Figure 19: Effect of starch content on filterability

CONCLUSION

In standalone refineries, most of the factories use carbonation process. In India, generally all backend refineries are using Phospho-flotation for clarification. Only few standalone refineries are using carbonation. Isgec got a very good experience of designing and assisting the operation for such a big standalone refinery. During the designing, execution and operation stage, we experienced many new things and summarised that carbonation clarification and filtration is the area of focus as other parts of process are almost same that being followed in other backend or standalone refineries.

If a refinery can achieve required efficiencies in clarification, the whole process runs very smoothly. And this is easily possible if related equipments are well designed, operation is well understood and controlled.

ACKNOWLEDGEMENT

Authors are thankful to the Durrah advanced Development Company, Saudi-Arabia (KSA)., Choquenet France and ISGEC team for their support in data record and analysis too.

REFERENCES

[1] Functional description of Carbonation & Candle filters by M/s Choquenet SAS, France.

[2] Sugar Technology for student by G R E Lionnet

[3] Handbook of Sugar Refining, A Manual for the Design and Operation of Sugar Refining Facilities by Chung Chi Chou