DIRECT CLEAR JUICE: A FEASIBILITY STUDY AND ... - … · refereed paper direct clear juice: a feasibility study and piloting investigation into the production of clear juice in a

REFEREED PAPER

DIRECT CLEAR JUICE: A FEASIBILITY STUDY AND PILOTING

INVESTIGATION INTO THE PRODUCTION OF CLEAR JUICE

IN A SUGARCANE DIFFUSER

JENSEN PS

Sugar Milling Research Institute NPC, University of KwaZulu-Natal, Durban 4041, South Africa

[email protected]

Abstract

Since the establishment of mud recycling to the diffuser in 1998, the shredded cane bed has

proved itself as an efficient filter of mud solids in most South African diffuser factories. This

has raised the question of whether, under the right circumstances, these solids could be

filtered directly in the diffuser, thus bypassing the need for settling clarification. A pilot

diffuser-clarifier was configured at the Sugar Milling Research Institute NPC (SMRI) to

investigate the quality of juice that could be obtained, and the conditions required for Direct

Clear Juice (DCJ) production. This paper looks at previous attempts at DCJ production, the

potential benefits of replacing settling clarification with DCJ production and some results of

piloting work performed at the SMRI.

Keywords: diffusion, clarification, clear juice, mud recycling

Comparison of a DCJ factory with a typical diffuser factory

Before commenting on the literature surveyed, it is helpful to look at the difference between

the front ends of a conventional factory and a proposed DCJ factory, as illustrated in Figures

1 and 2. Typical temperatures of juice in various locations have been included.

Figure 1. Front end configuration of a typical diffuser factory with mud recycle.

Figure 2. Proposed front end configuration of a DCJ factory.

Jensen PS Proc S Afr Sug Technol Ass (2012) 85: 344 - 367

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Lime and flocculant could be added to the front of the diffuser (Figure 2), and the juice

directed through the cane bed in order to filter out the resulting mud particles. In doing this,

at least one stage in the diffuser would be ‘sacrificed’. For example, a 12 stage diffuser would

be reduced to 11 stages, but the first stage would double in size.

From Figures 1 and 2 it can be seen that there is significantly less equipment in the proposed

DCJ factory than the typical factory. Under DCJ configuration, the following would no

longer be required:

Mixed juice (MJ) tank

Flash tank

Clarifier

Mud pumps and scales

Some heating capacity.

In a new factory, the front end structure could also be significantly smaller due to the large

footprint and height of most clarifiers.

The clear juice leaving the diffuser in Figure 2 is at a higher temperature than the draft juice

leaving the diffuser in Figure 1. This implies that a DCJ diffuser requires greater scalding

juice heater capacity than a conventional diffuser. The juice would be suspended solids ‘free’,

and be conducive to heating in plate heat exchangers, or possibly fed directly into the

evaporator train if there was sufficient capacity to both heat and evaporate the juice in the

first effect evaporators (Peacock and Love, 2003).

Introduction and history

The earliest discovered reference to the idea of performing clarification within a cane diffuser

is Payne (1965). He recorded that “considerable interest was raised” when operating their

pilot plant diffuser at the possibilities of clarification within the diffuser. Dry lime was

pumped onto the cane between the buster and fiberiser to promote mixing of lime and cane

external to the diffuser. Laboratory heating of the treated mixed juice resulted in no

secondary precipitate.

After five years of operating cane diffusers, Payne (1968) noted that clarification within the

diffuser was best achieved by:

Adding lime slurry to the cane entering the diffusion vessel;

Passing the recycled juice through the scalding juice heaters;

Filtering the resulting floc out of the juice by a second pass through the cane bed.

He also commented that:

The fibre increased the effectiveness of flocculation and the time and temperature

necessary were substantially less than in usual clarification processes;

Higher liming levels produced juices of lower turbidity;

Clarified juice from this treatment averaged lower turbidity than conventional clear juice

from milling;


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Conditions affecting the quality of clarification were pH, temperature, dilution, recycle,

and cane quality including trash content;

Higher pH levels accelerate colour formation, especially with a significant proportion of

leafy material. Pioneer mill in Hawaii averaged 10% trash during the trials.

In hindsight, one might expect the colour increase reported to be largely a result of the lime

slurry being added directly to the shredded cane.

Chen (1972) recorded results from trials performed in 1969 where diffuser clarified juice was

compared with milling-defecation juice. He noted that:

Juice could be clarified at 70-80°C and at a pH of 7.0-7.2;

No secondary precipitation was found in diffuser clarified juice, i.e. clarification was

complete;

Diffuser clarified juice contained, almost without exception, less turbidity than clarified

juice by conventional milling defecation.

Lamusse (1981) carried out tests on liming in a De Smet cane diffuser in Venezuela. He

found it possible to obtain a juice requiring no further clarification by liming to a pH of 7.0 to

7.3 in the first stage of the diffuser. There was, however, heavy precipitation from the juice in

the first vessel of the evaporator. The diffuser, in these experiments, was operated below

70°C, which would not have been enough to precipitate all the protein in the juice.

Lamusse remarked that, “The absence of recent references on clarification in diffusers seems

to indicate that the process is no longer favoured.”

In addition to his comments on the Venezuela trial (Lamusse, 1980), the following was

discovered through personal communication1:

The trial was performed during the commissioning of the first diffuser installed in South

America;

Lime was dosed into the front stage of the diffuser under normal configuration;

There was no pH measurement or control of the juice;

No flocculant was dosed into the diffuser.

The trial was stopped after a short while due to severe flooding in the front of the diffuser.

It was also discovered (Lamusse, 1981) that there were no lifting screws installed in the

diffuser at the time of the trial. The conditions and results of the three previous trials

mentioned above are summarised in Table 1.

Although not a direct reference to DCJ, Meadows et al. (1998) showed the effectiveness of

the diffuser bed at filtering suspended solids from clarifier muds, with no adverse effects on

extraction, or evidence of increased sucrose destruction in the diffuser. Since then, mud

recycling has been adopted as the preferred method of treating clarifier mud in South African

diffuser factories.

Lionnet (2000) observed that based on the low suspended solids of diffuser juice, the proven

ability of the diffuser at filtering clarifier muds, and the proven practice of using saccharate to

1Mr JP Lamusse, Mauritius (2012), Telephonic communication.


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facilitate liming in the diffuser, it should be possible to produce a treated diffuser juice which

would be suitable as a feed for plants based on the new separation technologies.

Table 1. Summary of results of previous direct cane juice (DCJ) trials

compared with conventional clarification.

Author Positives Negatives Conditions/observations

Payne

(1968)

Lower turbidity. Higher colour. Lime slurry added to shredded cane.

Trash in cane 10%.

1 stage co-current.

pH ~7.

No flocculant addition mentioned.

No lifting screws.

Chen

(1972)

Lower turbidity.

Lower colour.

Temp 70-80°C.

pH 7.0-7.2.


No lifting screws.

Lamusse

(1980)

Adequate turbidity. Precipitation in

first evaporator.

Flooding in

diffuser.

Lime added to first stage.

Temp below 70°C.


No lifting screws.

Since the Lamusse trials, Ivin et al. (1987) and Schäffler (1988) showed that liming in a

diffuser can lead to increased acetic acid concentrations in mixed juice. This leads to various

downstream complications. They both commented, however, that where liming was well

controlled, the increase in acetic acid would be lessened. Schäffler compared four factories

which were liming in their diffusers, and found only one (Felixton) to have an increased

concentration of acetic acid due to lime addition in the diffuser. It is also worth noting that at

Felixton the lime was added at 8 points, compared with one or two points in the other three

factories.

Reasons for previous non-implementation

Almost 50 years after the DCJ idea was first proposed, there are still no known installations

in operation. Upon examination of the literature referencing the idea, all three authors were

positive about the concept, and there are no clear reasons for its non-implementation. The

reasons may be one or a combination of the following suppositions:

1. The step change from milling to diffusion was large enough in itself, and the industry was

not ready for two steps in one;

2. The operational benefits of DCJ were not enough to justify its implementation;

3. Concern over increased sand being sent to the boilers;

4. Concern over decreased extraction due to loss of stages in the diffuser;

5. Concern over decreased percolation in the diffuser;

6. Concern over poorer juice quality.

If DCJ is to be adopted by the industry, the above six concerns need to be addressed, and be

shown to be less costly to a mill than the benefits of DCJ.


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Reasons for reconsidering DCJ

1. The step change from milling to diffusion was large enough in itself, and the industry

was not ready for two steps in one.

The step from milling to diffusion has now been widely adopted by the sugarcane industry.

The advantages of diffusion have been dealt with in detail by Rein (1995). The cane diffuser

has proved itself to be easy to operate, robust and, since the onset of mud recycling in 1997

(Meadows et al., 1998), an effective filter of clarification mud solids. The step from

conventional clarification to DCJ is now significantly less than it was in 1968.

2. The operational benefits to DCJ were not enough to justify its implementation.

This raises the question: “What would be the benefit to a mill today if DCJ could be

successfully implemented?” A 300 tcph factory was modelled using the computer program

Sugars™, and the basis for the calculations can be found in Appendix 1. The benefits are

summarised in Table 2. The advantages of a DCJ factory would include:

2.1 Reduced heat losses

In the typical configuration, juice exits the diffuser at 65°C. Due to the fact that settling

clarification requires the juice to be ‘air free’, it needs to be heated to above its flash

temperature. The vapour is flashed to the atmosphere. Under DCJ, flashing is not required,

and this evaporation would instead take place in the multiple effect evaporator train. The

energy in the flashed vapour would then be mostly recovered in the evaporator train rather

than being lost to the atmosphere.

After lime and flocculant addition, the juice flows by gravity into the clarifiers. Depending on

the type and number of clarifiers in operation, the residence time is usually between 30 min

and 2 h. A significant amount of heat is lost through the walls of the clarifier during this time,

such that by the time it enters the CJ heaters it is at approximately 98°C (Jensen, 2001).

According to Wright (2006), mud temperatures as low as 93°C are not uncommon.

2.2 Reduced inversion losses

A significant amount of sucrose is inverted while the juice is in the clarifier. This is mainly

due to the high temperature of the juice and the long residence time. Vukov’s model (Vukov,

1965) was used to predict the amount of sucrose which is inverted in the clarifier under the

conditions specified in Appendix 1.

2.3 Removal of mud

Under DCJ configuration, all equipment usually associated with the handling of mud would

be eliminated. Mud would no longer be a sugar factory stream. The benefits would be

reduced sampling and analytical load, removal of mud scales, improved hygiene and reduced

sucrose losses.

2.4 Improved juice quality

During the previous DCJ trials, Payne (1968), Chen (1972) and Lamusse (1980) all noted

lower turbidity juice than CJ from conventional clarification. Trials performed at the SMRI

and reported on here also suggest that this was likely. Settling clarification is susceptible to

mud carryover if conditions in the clarifier are disturbed (Mkhize, 2003). The negative

consequences of mud carryover include increased evaporator fouling, increased sugar colour,


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and expected reduced sugar filterability (Sahadeo et al., 1998). DCJ would be of particular

advantage where downstream operations consisted of new technologies such as membrane

filtration or resin-based separations. Mud carryover in clear juice was a challenge often

encountered by the White Sugar Milling (WSM) (Jensen and Kitching, 2007) slipstream plant

(personal communication2).

2.5 Reduced heating area required and option for cheaper heaters

Less heating area is required in a DCJ factory than a conventional factory. In a conventional

factory, it is essential to heat the juice to above its flash temperature in order to expel the air

from the juice before settling clarification. A DCJ factory, by comparison, does not need the

heating area required to heat the juice from 98 to ~104°C. A slightly higher load is, however,

placed on the evaporators. Furthermore, all heating in a DCJ factory, except the first pass

through the scalding juice heaters, is performed on ‘clear juice’. Fouling is expected to be

reduced as a result. Plate heaters, which have a Heat Transfer Coefficient (HTC) of two to

three times that of tubular heaters (Rein, 2007) are worth considering, if the juice to be heated

is low in suspended solids.

2.6 Improved energy efficiency by using low grade steam for final stage heating

The vapour to the final stage heating before a clarifier is usually required to be vapour bleed

from the first effect evaporator (V1), in order to heat the juice to above its flash temperature.

Under DCJ this is no longer a requirement, and second effect vapour bleed (V2) could be

used instead. If the final juice heaters raise the temperature from 89 to 98°C and V2 is used

instead of V1, a significant steam saving can be achieved, as is described in Appendix 1 and

Table 2.

Table 2. Summary of benefits to a 300 tons cane/hour

direct clear juice (DCJ) factory over a conventional factory.

Description of operation Annual

savings

2.1 Elimination of vapour flashing R703 248

2.1 Elimination of clarifier heat losses R602 784

2.2 Elimination of clarifier inversion losses R1 307 411

2.3 Removal of mud ?

2.4 Improved juice quality ?

2.5 Reduced heating area required and option for cheaper heaters ?

2.6 Use of V2 instead of V1 in final juice heaters R659 568

R3 273 011+?

3. Concern over increased sand being sent to the boilers

Sand in bagasse is undesirable as it lowers the calorific value of the bagasse slightly, and may

lead to increased wear in the boilers. Meadows et al. (1998) claimed that the increase in ash

% bagasse when moving from mud filtration to mud recycling is less than 10%. The success

of mud recycling in South Africa suggests that the benefits gained outweigh the negative

effect it has on the boiler. Under DCJ the extra ash load on the bagasse is equivalent to the

ash load on bagasse under mud recycling. For mills still operating mud filters, the transition

to DCJ would contain all the benefits associated with mud recycling (Jensen, 2001), as well

as those associated with DCJ.

2Mr CRC Jensen, Tongaat (2012), Telephonic communication.


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4. Concern over decreased extraction due to loss of stages in the diffuser

Under DCJ configuration, a diffuser could ‘lose’ between one and three extraction stages. To

determine the loss of extraction which might be associated with this combining of stages, a

simplified diffuser mathematical model was constructed. By adjusting the stage efficiency

parameter for each stage in the model, the brix curve could be fitted to that of a typical profile

obtained from a diffuser with mud recycling (Figure 3).

Figure 3. Diffuser model fitted to typical diffuser

(with mud recycle) brix curve.

The stage efficiencies (defined as the percentage of brix entering a stage that exits in the

underflow into the tray below) required to fit the model to the Brix curve were higher in the

front of the diffuser. This is to be expected, as percolation rate is highest in the front of the

diffuser. From the model, a brix extraction of 98.7% was calculated. Stages were then

sequentially removed from the model to estimate the effect this would have on extraction.

The results are shown in Figure 4. It can be seen that removing three stages in the model

results in a predicted loss of extraction of less than 0.5%. The monetary (Rand) value of the

loss in extraction was also calculated, and is shown in Figure 4.

In practice, the loss in extraction due to the combining of stages is expected to be even less

than the numbers shown in Figure 4, as the overall length of the diffuser, and hence cane

residence time, is still the same. The fact that a stage is double the length should mean the

extraction efficiency of the stage is increased, thus offsetting somewhat the negative effect of

reducing the number of stages. Rein (2007) alluded to the fact that the choice of 12 stages in

a diffuser is also influenced by structural constraints and not extraction theory alone.

It is also worth noting the ‘kink’ in the typical diffuser brix curve in Figure 3 as a result of

mud recycling to the diffuser. This is clearly not ideal from an extraction point of view, but

has been shown to have minimal impact on extraction (Meadows et al., 1998). Under DCJ the

mud is essentially recycled to the front of the diffuser which, from a brix point of view, is the

best location.

In light of these observations, it is expected that the negative impact on extraction, through

‘losing’ up to three stages in a 12 stage diffuser, should be less than the benefits of a DCJ

configuration described in Table 2.


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Figure 4. Mathematically modelled extraction

as a function of number of stages.

5. Concern over decreased percolation in the diffuser

Although there is no mention of flooding in any of the previous DCJ investigations found in

the literature, personal communication1 revealed that a trial was stopped due to flooding in

the diffuser. Love and Rein (1980) and Lionnet (2000) all found percolation rate to decrease

with an increase in pH. After conducting DCJ piloting at the SMRI it is suspected that the

decreased percolation rate reported by these authors was more a result of calcium

precipitation than a reduced concentration of hydrogen ions. No tests were performed with

bases other than lime to test this speculation.

The diffuser used by Lamusse (1980), and the columns used by Love and Rein (1980) and

Lionnet (2000), contained no lifting screws. The trials performed by Payne (1968) were on a

Silver Ring diffuser which also did not contain lifting screws. Lifting screws are critical to

the implementation of mud recycling to the diffuser, and are therefore also expected to be a

requirement in a DCJ diffuser. One of the reasons for relooking at DCJ is the success of

lifting screws in diffusers since they were first tested by Huletts Sugar Ltd at Empangeni in

1968 (Van Der Riet and Renton, 1971).

DCJ piloting at SMRI in 2011

1. Piloting objectives

The four objectives of DCJ piloting at the SMRI in 2011 were:

1. To determine whether DCJ of similar quality to mill CJ could be made.

2. To assess the filtration time required for DCJ production.

3. To assess the effect on percolation rate of DCJ production.

4. To observe factors which possibly affect DCJ production and should be further

investigated.

2. Equipment

A schematic view of the rig is shown in Figure 5, and major equipment is described.


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Heated tank – A 200 litre tank equipped with:

Steam heating coil.

Thermocouple used to control the tank temperature.

pH probe for monitoring the pH of the juice.

Stirrer and stirrer baffles.

Overflow valve for maintaining a level in the tank above the stirrer and heating element.

Diffusion column – A 300 mm diameter by 1.3 m high glass column featuring:

A Perspex distribution plate to ensure even distribution of the juice into the column.

A tank below placed on a weighing scale for measurement of percolation rates.

Turbidity meter (TM) – An inline absorbance meter developed by the SMRI and marketed by

Sugarequip (Pty) Ltd. The unit, as described by Mkhize (2003), was used to measure the

absorbance of mill CJ, as well as for continuous monitoring of DCJ absorbance during each

run.

Bubble tank (BT) – A 1 litre tank which allows air bubbles to be released from the juice

before it flows by gravity through the turbidity meter. Temperature and pH of the juice was

logged in the tank.

Figure 5. Graphical representation of the Sugar Milling

Research Institute direct cane juice (DCJ) pilotting rig.

3. General procedure

Each of the runs performed followed the general procedure detailed below.

110 kg juice, 25 kg shredded cane, milk of lime and blended flocculant were collected

from a mill on the day of each run.

The juice was added to the tank, and the stirrer was started.

The cane was dropped into the diffusion column.


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The juice was heated in the tank to 95°C.

Lime was added until the desired pH setpoint was reached.

Flocculant was usually added directly after the pH setpoint was reached.

The overflow valve was opened to allow juice from the tank to flow into the diffusion

column.

Flocculant was continuously dosed into the juice at the outlet of the tank, before it entered

the column.

The juice exiting the column was pumped back into the heated tank, where the

temperature of the recirculating juice was maintained at a maximum of 80°C (higher

temperatures could not be reached due to heat losses and an inadequate heating coil).

Lime was continuously added to the tank during the run to maintain a chosen pH.

A side stream of juice exiting the column was passed through a turbidity meter to monitor

the clarity of juice over time.

Temperature and pH were also logged during each run.

The run was stopped after about 40 minutes, or when the turbidity had stabilised.

At intermittent times, the flow rate of juice through the column was measured (by shutting

valve V2 after the pump for a few seconds, and observing the increase in mass measured by

the scale), allowing percolation rate to be calculated in m3 juice per m

2 of cross-sectional area

per minute. A printout and commentary of one full run is given in Appendix 2.

4. Design of experiment

The initial goal of piloting was to check the possibility of producing DCJ of similar quality to

mill clear juice. This goal was achieved after run 1! It was clear that simply filtering treated

juice through the cane bed for long enough could reduce its turbidity to below that of mill CJ.

The focus then became to ‘tweak’ the procedure to produce DCJ of similar quality to mill CJ

in as short a time as possible, while maintaining high percolation rates.

A multitude of choices are available when deciding on an experimental procedure of this

nature. Some of these choices are shown in Table 3 below. The highlighted cells represent the

‘base levels, which were chosen based on the experience gained in the preceding runs.

Table 3. Factors and levels available in the design of the

direct clear juice (DCJ) piloting experiments.

Factor Choices/levels

A B C D

1. Starting juice Water Scalding juice Clear juice Mixed

Juice

2. pH juice initially limed to: No lime (±5) 5-6.8 6.8-7.4 7.4-7.6

3. Type of base used Milk of lime Saccharate Other

4. Initial floc added (ppm) No initial floc <1.5 1.5-5 >5

5. Total floc added within

15 minutes (ppm) No floc <1.5 1.5-5 >5

6. Percolating temperature (°C) <70 70-85 >85

7. Mud from initial clarification

All mud

filtered

through cane

5 L removed

from system

Mud mixed with

cane when

loading column

8. Bed manipulation (lifting

screw)

No

manipulation

Manipulation

within

15 minutes

Manipulation

after 15 minutes


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Table 4 shows the difference in operating conditions between the runs.

Table 4. Levels chosen for each of the piloting runs (blank cells indicate base values).

Run

Factor Base 22/6 19/7 02/8 04/8 23/8a 23/8b 14/9 20/9 23/9 30/9 5/10 25/10

1 B C A

BA

SE

BA

SE

D D D

2 D C

C C A A B C C C

3 A

B

4 C

A A

5 C D D

6 B A

7 B

A A A

8 C A A A A A A

B B

5. Results

5.1 Mill CJ absorbance

Upon collecting cane on the morning of each piloting run, a sample of mill CJ was also

collected from the mill. The sample was passed through the TM in order to determine its

absorbance. Its turbidity was also tested by the ICUMSA method (Anon, 1985) using a

wavelength of 420 nm. The results (Table 5) were used as the basis on which to assess the

quality of DCJ produced during piloting. A large range, in the turbidity of CJ collected from

the mill, was observed.

Table 5. Online absorbance and turbidity measurements

from mill clear juice (CJ) samples.

Online absorbance ICUMSA turbidity

No. of samples 13* 14

Average 1.15 8960

Max 2.70 16093

Min 0.25 2523

Standard deviation (σ) 0.71 4108

*Online absorbance was not measured for one of the samples.

It was decided to compare the DCJ absorbances for each of the runs to the average of the mill

CJ absorbances for three reasons:

1. It allowed a comparison between individual DCJ runs with generally encountered mill CJ

turbidities.

2. The mill CJ samples did not originate from the same cane consignment as the cane and

juice used in the DCJ trial, and thus a direct comparison is misleading.

3. The DCJ absorbance trends tended to be more uniform than the mill CJ results. Mill CJ

quality is largely influenced by the conditions in the clarifier at the time of sampling, and

averaging the results gives a better indication of the quality of typical mill CJ.


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5.2 Filtration time

As the limed and flocculated juice percolates through the shredded cane bed, mud particles

are trapped between the fibres. At the same time, new impurities are washed out of the ‘fresh’

cane in the column. Most of these impurities are too small to be trapped by the cane fibres,

and they exit the column with the percolating liquid. Continuous monitoring of the turbidity

at the exit of the column shows it to decrease with time. This decrease is a result of three

factors working together:

1. The amount of impurities left in the cane decreases with time.

2. Bed compaction and a mud layer in the bed results in a tighter filter with time.

3. The clarity of the feed to the column improves with time as the juice exiting the column is

recycled through it.

The turbidity reduction over the first 20 minutes of the two base runs is shown in Figure 6,

where the average of the mill CJ absorbances and the average plus or minus one standard

deviation are plotted as horizontal lines. It can be seen from Figure 6 that after ±9 minutes of

filtration, the DCJ turbidity was similar to the average mill CJ turbidity.

Figure 6. Effect of filtration time on turbidity for the two base runs.

5.3 Percolation, and the effect of bed agitation (lifting screw)

Lionnet (2005) showed lab percolation rates to be largely dependent on cane packing density.

For a density of 450 kg/m3 he found the percolation rate to be ~0.3 m

3/m

2/min. The column

used for his experiments was 450 mm in diameter and the packed cane bed heights were

~0.45 m deep.

Love and Rein (1980) recorded lab percolation rates of ~0.23 m3/m

2/min with a cane packing

density ~450 kg/m3. The column used for their experiments was 320 mm in diameter and the

packed cane bed heights were ~1.5 m deep.

The SMRI DCJ piloting rig used a 300 mm diameter column, with bed heights between 0.6

and 0.8 m. The plot of percolation rates with time for the base level runs are shown in

Figure 7. The rates were initially different due to a difference in the rate of opening the outlet

valve from the tank, but were thereafter similar to each other, and to percolation rates

recorded by previous authors.


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*Cane packing density was not measured for run 14/09.

Figure 7. Trend of percolation rates with time for the two base runs.

It can be seen that the percolation rate tends to decrease exponentially with time, in a similar

way to the turbidity curves in Figure 6. Although the percolation rates of the above two runs

appear normal, with time, percolation rates as low as 0.05 m3/m

2/min were observed. For this

reason it was decided to insert the equivalent of a diffuser lifting screw (see Figure 8) into the

column before adding the shredded cane.

Figure 8. The ‘lifting screw’ used for bed agitation.

After a certain time, the ‘screw’ was removed from the column in such a way as to

redistribute the mud within the cane. Agitating the bed had a significant effect on percolation

rate. The rate would immediately increase, but also remain at a much higher value than the

stabilised percolation rate before agitation.

In Figure 9, the time at which the bed was agitated is indicated by the vertical section of the

percolation curve (inserted manually into the data for clarity). The only run in which the

percolation rate decreased again to low levels was the run on 05/10. During this run the bed

was agitated after just 6 minutes. This indicates that a significant portion of the mud had still


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not been filtered by the bed and was thus still present to form a layer in the cane after

agitation. A second agitation was performed on this run after 38 minutes, and the percolation

rate increased and thereafter stabilised. In light of these results, the lifting screws in a full

scale DCJ diffuser should not be located too close to the front. Percolation problems seem to

be the main reason for discontinuing DCJ trials in the past (personal communication1). The

fact that lifting screws are now standard features in most diffusers is a reason to relook at the

idea on a full scale, as bed agitation significantly increases percolation rate. The design of the

lifting screws should be examined more closely, as it is desirable to distribute the mud solids

within the bed while still minimising the passage of fines completely through the bed during

agitation.

Figure 9. Trend of percolation rates after bed agitation.

5.4 The choice and effect of type of juice used during each run

One of the differences between the DCJ piloting rig and a full scale diffuser is the ratio

between liquid and fibre in the plant. The SMRI DCJ column contained approximately 25 kg

cane. At an imbibition % fibre of 350%, this equates to 11 kg of imbibition which should be

added to the cane in order to replicate a typical full scale diffuser. Due to the requirement of a

liquid holdup in the column and the tanks associated with the rig, 110 kg of juice was

required to be in circulation during each run. This raised the question of the best choice of

juice with which to start each run. On a full scale, each kilogram of cane is required to filter

out only the suspended matter originating from itself. In the pilot plant, a far higher solids

load could be imposed upon the cane than would be encountered on a full scale. The base

choice was to start with scalding juice, and to remove ~5 L of mud after the initial

clarification in the heated tank in order to reduce the filtration load on the cane in the column.

CJ and MJ were also tried as starting materials, and the results are shown in Figure 10.

The CJ run showed the most rapid decrease in turbidity of all the runs. Within three minutes

the turbidity originating from the cane in the column could be reduced to below the average

mill CJ absorbance. Using MJ as the starting material resulted in a turbidity reduction very

similar to the base runs.

The percolation rate in the CJ run was extremely low. This result was surprising, as the lower

filtration load was expected to result in higher percolation rates. The packing density of this

run was high, as blinding of the bed tends to result in it becoming more compacted. The

percolation rate for the MJ run was also low, and the reasons for this are unclear.


357


Figure 10. Turbidity and percolation results for three different starting juices:

30/09 mixed juice, 04/08 clear juice, 14/09 and 23/09 scalding juice.

5.5 The effect of initial juice pH

The two base level runs were limed to ~7.5 pH before filtration through the bed began. The

only difference between these runs and run 20/09 was the initial pH. The lime addition and

juice pH for the three runs are shown in Table 6.

Table 6. Lime addition and initial pH of four piloting runs.

Run Initial clarification 10 minutes into run

pH MOL added (kg) pH MOL added (kg)

14/09(base) 7.5 0.5 6.3 0

23/09(base) 7.6 0.5 6.9 0

20/09 6.5 0.2 6.5 0.2

MOL = milk of lime

The turbidity and percolation trends are shown in Figure 11. The starting pH appears to have

little effect on the rate of turbidity removal. It is not clear why the base run percolation rates

were significantly higher than run 20/09 which was initially limed to a lower pH. It is highly

unlikely that it is due to the lower initial pH, especially as previous authors (Love and Rein,

1980) report higher pH as contributing to lower percolation rates.


Figure 11. Turbidity and percolation trends investigating

the effect of initial pH on direct cane juice (DCJ) production.

5.6 Individual effect of heating, lime addition and flocculant addition


358

The juice was heated to 95°C before being circulated through the cane bed. It can be seen

from Figure 12 that even without lime or flocculant addition, the juice turbidity was brought

to below the average mill CJ value within 40 minutes of filtering. The percolation rate, even

after 40 minutes, was above 0.4 m3/m

2/min, which was significantly higher than the runs

where lime and flocculant were added. To test the effect of gradual lime addition to the juice,

it was dosed in five batches of 100 mL. The pH increased by about half a unit, and

percolation rate continued to decrease. There was little evidence of a turbidity reduction as a

result of lime addition in this way. Percolation rate continued to decrease throughout the test.

Flocculant (1 ppm) was dosed after 81 minutes, and a step reduction of 20% in absorbance

was seen shortly afterwards.

Figure 12: Turbidity and percolation (run 24/08b) with heating only, then

lime addition, then flocculant addition. Starting material was scalding juice.

A similar test was performed using water instead of scalding juice as the starting material,

and the results are shown in Figure 13. Once again it is evident that there is a significant

turbidity reduction with just heating and filtration of juice through the cane, without a large

percolation reduction. The addition of lime both decreased percolation rate and turbidity. The

addition of floc further decreased the percolation rate. The absorbance reading was already

off the scale before floc was added.

Figure 13. Turbidity, percolation and pH (run 24/08a) with heating only, then

lime addition, then flocculant addition. Starting material was water.


359

The above results suggest that it may be possible to produce DCJ using no, or minimal lime.

The juice pH could be adjusted after the diffuser using calcium-free bases such as sodium

bicarbonate or carbonate (Lionnet, 2000). This would be of particular benefit where ion

exclusion chromatography, which relies on a low calcium feed, was being considered as a

downstream process.

5.7 Optimal temperature for DCJ production

Lamusse (1980) suggested that operating temperatures below 70°C would not be enough to

precipitate all the protein in the juice. Heating the juice above 85°C in a diffuser is generally

not recommended due to increased colour formation, inversion losses and heat losses. The

turbidity curve for run 22/06 in Figure 14 suggests that insufficient heating increases the time

required to reduce the turbidity of DCJ. Operating temperatures between 75 and 80°C are

adequate. Higher temperatures were not tested. The scalding juice temperature is, however,

required to be high in order to heat the cane quickly from ~25°C to the operating temperature.

Figure 14. Turbidity and temperature curves suggesting

an effect of temperature on turbidity removal.

Figure 15 shows the percolation of run 22/06 to begin lower than the base curves, but to

approach the same value after 20 minutes.


Figure 15. Percolation rate curves comparing run 22/06

(lower temperature) to the base level runs.


360

5.8 Colour change during DCJ production

A colour comparison is only possible for the runs using MJ as the starting material, as the

ratio of scalding juice (which is high in colour) to cane is far more in the pilot plant than in a

full scale diffuser. During each of the MJ piloting runs, samples were taken at different times

for laboratory analysis. The ICUMSA colour of the samples was measured, and the results

are shown in Figure 16.

Figure 16: Colour change during direct clear juice (DCJ) production.

The results in Figure 16 showed the mill MJ colours and CJ colours to be quite different. This

is likely due to the catch sampling employed and the fact that the mill CJ and MJ samples

probably did not originate from the same batch of cane. In run 25/10, for example, the CJ

colour is higher than the MJ colour, while the reverse was measured for the other two runs. It

is, however, useful to note that the DCJ colour did not appear to increase with filtration time.

5.9 Effect of type of lime

Run 19/07 was conducted using saccharate, whereas all other runs used MOL. The results of

run 19/07 are shown in Figure 17, along with the base level results. There is no discernable

difference between the use of saccharate and MOL.


Figure 17: Turbidity and percolation results using saccharate (run 19/07).


361

5.10 Visual comparison of DCJ and mill CJ

As part of the DCJ research project at the SMRI, a new analytical method was developed as a

quick visual comparison of the suspended solids between juice samples (Jensen, 2011).

Two samples of DCJ (collected after ~13 minutes) were centrifuged simultaneously in small

centrifuge tubes in order to compare the settled suspended solids in the juice with a sample of

centrifuged mill CJ. The DCJ samples shown in Figure 18 contained noticeably less settled

matter than the CJ sample. The white markers on the centrifuge tubes indicated the level of

the settled solids after centrifugation.

CJ = clear juice DCJ = direct clear juice MJ = mixed juice

Figure 18. Centrifuged juice from run 30/09/2011(left) and 25/10/2011 (right).

Conclusions

It was observed that DCJ of better quality than mill CJ could consistently be produced on a

piloting scale. The filtration time required for DCJ to reach average mill CJ quality was

~9 minutes. There was a definite reduction in percolation rates during DCJ production. The

percolation rate could however be increased back to normal levels after agitation of the bed

with a ‘lifting screw’. Furthermore, the following were tentatively observed during DCJ

piloting:

No observable difference between using saccharate or milk of lime.

The bed has capacity to filter out more solids than just those originating from the bed

itself.

A minimum percolating temperature of 75°C is required for quick reduction in DCJ

turbidity.

No increase in juice colour with filtration time.

Based on historical results, developments in diffuser technology over the last 50 years, and

results from piloting at the SMRI in 2011, it appears that the replacement of conventional

clarification with DCJ production is worth looking at on a full scale once again.


362

Recommendations

It is recommended that further aspects of DCJ production be investigated to get a better

understanding of the process. The investigations should include more detailed studies on:

The formation of acetic acid during DCJ production.

A comparison between milk of lime and lime saccharate.

A comparison in colour between DCJ and mill CJ.

A comparison between the fouling characteristics of DCJ and conventional CJ.

pH correction with alternative bases after the diffuser.

Optimal temperature for DCJ production.

Design of lifting screws.

Possible impact on cane payment.

Furthermore, DCJ production could open the door to advancements in a cane sugar factory

which were previously unattractive or not considered. These include:

Colour removal (or other) additives which could be filtered out in the diffuser.

A relook at energy efficiency due to a reconfigured front end.

New technologies which were previously unattractive due to the variability of juice

quality from conventional clarification.

New diffuser designs.

Deep bed filtration of DCJ.

Acknowledgements

The author is grateful for the help received in preparing this paper, and would like to

acknowledge inputs from Ramesh Ramsumer, Seppy Ramsuraj and Rendani Ramaru who

assisted during with piloting; Dr Dave Love for his advice with setting up the piloting rig;

Tongaat Hulett Sugar for the loan of their piloting column; Maidstone for their assistance

with sample collections; Brian Bailey from Sugarequip (Pty) Ltd for the loan of their

turbidity meter; Dr Richard Loubser for his help with instrumentation and data capture; Steve

Davis for his valuable input and support of the work.

Abbreviations used

abs absorbance

BT bubble disengagement tank

CJ conventional defecation clear juice

DCJ direct clear juice

DJ draft juice

floc flocculant

MJH mixed juice heater

MJ mixed juice

HTC heat transfer coefficient

SJH scalding juice heater

SMRI Sugar Milling Research Institute NPC

tcph tons cane per hour

WSM white strap molasses


363

REFERENCES

Anon (1985). Laboratory Manual for South African Sugar Factories. 3rd Edition, South African

Sugar Technologists’ Association, Mount Edgecombe, South Africa. p 262.

Chen JCP (1972). Questions frequently asked about sugar cane diffusion. Zuckerindustrie 22(5): 261-

265.

Ivin PC, Clarke ML and Blake JD (1987). Comparison of extractives from milling and diffusion. Proc

Aust Soc Sug Cane Technol 9: 191-200.

Jensen CRC (2001). The elimination of filtercake in a cane sugar factory by recycling defecation

muds to the extraction plant. Proc Int Soc Sug Cane Technol 24: 231-236.

Jensen CRC and Kitching SM (2007). Options for retrofitting white sugar milling (WSM)

technologies into existing raw sugar factories. Proc Int Soc Sug Cane Technol 26: 1504-1512.

Jensen PS (2011). Suspended solids by volume % - A quick, comparitive method for mixed juice

suspended solids analysis. Technical Note No. 13/11, Sugar Milling Research Institute, Durban,

South Africa.

Lamusse JP (1980). Practical Aspects of Cane Diffusion. Elsevier: pp 197-253.

Lamusse JP (1981). Recommendations for improving the performance of the cane diffuser at central

Tocuyo. Technical Report No. 1278 RCP, Sugar Milling Research Institute, Durban, South Africa.

p 2.

Lionnet GRE (2000). An old technologist’s perspectives of new separation technologies. Proc S Afr

Sug Technol Ass 74: 257-260.

Lionnet GRE (2005) The effect of selected factors on percolation in pilot diffusion columns. Proc S

Afr Sug Technol Ass 79: 249-257.

Love DJ and Rein PW (1980). Percolation behaviour of a cane diffuser. Proc Int Soc Sug Cane

Technol 17: 1900-1924.

Meadows DM, Schumann GT and Soji C (1998). Farewell to filters: the recycle of clarifier muds to

the diffuser. Proc S Afr Sug Technol Ass 72: 198-203.

Mkhize SC (2003). Clear juice turbidity monitoring for sugar quality. Proc S Afr Sug Technol Ass 77:

414-422.

Payne JH (1965). Cane diffusion – The displacement process in principle and practice. American

Factors Associates, Honolulu, Hawaii, USA. pp 110-117.

Payne JH (1968). Cane diffusion – Ring diffuser at Pioneer Mill Company. American Factors

Associates, Honolulu, Hawaii, USA. pp 1517-1518.

Peacock SD and Love DJ (2003). Clear juice heaters – Do we need them? Proc S Afr Sug Technol Ass

77: 452-462.

Rein PW (1995). A comparison of cane diffusion and milling. Proc S Afr Sug Technol Ass 69: 196-

200.

Rein PW (2007). Cane Sugar Engineering. Verlag Dr Albert Bartens. pp 210, 173, 226.

Sahadeo P, Lionnet GRE and Davis SB (1998). The effect of clarifier mud carryover on sugar quality.

A preliminary pilot plant investigation. Technical Report No. 1785, Sugar Milling Research

Institute, Durban, South Africa. p 1.

Schäffler KJ (1988). Acetic acid production in cane diffusers and the resultant effect on vapour pipe

corrosion in evaporators. Proc Conf Sug Process Res pp 62-74.

Van Der Riet CB and Renton RH (1971). The Empangeni diffuser installation 1967-1970. Proc S Afr

Sug Technol Ass 69: 49-60.

Vukov K (1965). Kinetic aspects of sucrose hydrolysis. Int Sug J 67: 172-175.

Wright PG (2006). Notes on the consistency of clarifier underflow mud. Proc Aust Soc Sugar Cane

Technol 28: (see index)


364

APPENDIX 1

Calculations:

Sugars™ model parameters and basis:

Cane throughput = 300 t/h

Brix % Cane = 16%

Sucrose % Cane = 13.6%

Fibre % Cane = 15%

Imbibition % Fibre = 350%

Juice throughput = 380 t/h (including mud recycle)

Sucrose % Juice = 11.1%

Clarifier configuration = Mud recycled to diffuser

Evaporator configuration = 5th effect with condensate flash to following effect

Heating vapour = V1

Crushing hours per year = 4200

Sugar price (R/t) = R4100/ton (24.71 c/lb. as of 02/04/2012)

Steam/bagasse ratio = 2

Bagasse price T/t (coal equivalent) = R208

Exchange rate = ±R8/1US$ (02/04/2012)

Steam saved if no flashing required

Flash temperature = 104°C

Heating vapour = V1

Exhaust steam saved = 1.61 t/h

Annual Rand savings = 1.61 x 4200 x 208/2

Annual Rand savings = R703 248

Steam saved if clarifier heat losses removed

Clarifier temperature drop = 2°C (100 to 98°C)




Sucrose saved if clarifier inversion losses removed (Vukov model)

Brix % Juice = 13.04

Average temperature = 99°C

pH = 7.1

Residence time (mins) = 60

Inversion % = 0.18

Sucrose inverted per year (t) = 0.0018 x 380 x 0.111 x 4200 = 319

Annual Rand savings = 319 x 4100

Annual Rand savings = R1 307 411

Steam saved if V2 used instead of V1 in final juice heaters

Temperature rise in heater = 9°C (89 to 98°C)





365

APPENDIX 2

Results and discussion of a single run

Figure 19 shows an example of the logged data from one of the runs. For this run, MJ, diluted

with water to 8 brix, was used as the starting material. After the initial batch clarification,

~5 kg of mud was removed from the bottom of the heated tank before the juice was circulated

through the column. This was to reduce the solids load imposed on the bed. The 5 kg of mud

was later added to the column to observe its effect on turbidity and percolation rate.

Figure 19. Turbidity and percolation chart of piloting run 30/09/2011.

A detailed commentary to the chart in Figure 19 is given in Table 7.

Table 7. Commentary to turbidity and percolation chart of piloting run 30/09/2011.

Time

(mins) Commentary

0 The tank valve is opened and juice flows from the tank to the diffusion column.

1 The first juice from the column flows through the turbidity meter.

3 Juice turbidity is dropping but bubbles into the meter give a false reading.

3 All the juice has passed once through the cane bed.

5 The DCJ absorbance is equal to that of mill CJ (measured after the DCJ run). Percolation

is ~0.2 m3/m

2/min.

7 The first sample of juice (DCJ 1) is taken for lab analysis.

11 All the juice has passed twice through the cane bed.

13 The second sample of juice (DCJ 2) is taken for lab analysis.

19 The ‘lifting screw’ is removed from the bed.

20 Percolation rate increases from 0.1 to 0.8 m3/m

2/min.


366

20 All the juice has passed three times through the cane bed.

20 Turbidity increases to 5 abs units as mud trapped in the bed is released through bed

agitation with the lifting screw.

26 Turbidity is once again decreased to below the mill CJ reading however percolation rate is

now ~0.75 m3/m

2/min.

34 5 kg of mud removed earlier is added into the tank.

34-38

Bubbles entrained in the juice corrupted the absorbance readings. The juice turbidity trend

can however be seen to have been negligibly influenced by the addition of mud into the

tank.

38 The fourth sample of juice (DCJ 4) is taken for lab analysis.

39 Percolation rate dropped from 0.5 m

3/m

2/min to 0.3 m

3/m

2/min as a result of extra mud

addition.

46 Poured 5 kg of mill CJ into the tank. A spike in the turbidity shows the mill clear juice to

have higher turbidity than the circulating juice.

52-64 Bed manipulation attempted from the top of the column. No increase in percolation rate

resulted, bud interruptions in the turbidity were witnessed.

64 The fifth sample of juice (DCJ 5) is taken for lab analysis.

64 The tank valve is shut and pump stopped.

72 Mill CJ (collected at the same time as the cane) is poured through the turbidity meter in

order to measure its absorbance.

The samples DCJ1 to DCJ5, which were removed during the trial as indicated in Figure 19,

were kept for laboratory analysis. The results from the analysis are shown in Table 8.

Table 8: Laboratory results of piloting run 30/09/2011.

Mill MJ DCJ1 DCJ2 DCJ3 DCJ4 DCJ5 Mill CJ

Time into run (mins) 0 7 13 19 38 64 −

ICUMSA turbidity 15 406 13 085 9 553 4 138 7 520 7 534 11 307

% of CJ turbidity 136% 116% 84% 37% 67% 67% 100%

Online turbidity (abs) 3 1.6 0.85 0.5 0.7 0.25 2.20


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