Continuous Crystallization

Transcription

Traxxys__________________________________
Consultancy for sustainability
Technoproject
CS0108
Oscillating Baffled Flow Crystallizer
Objective
Parties
Proof of Principle Oscillating Baffled Flow Crystallizer
Contacts
Phone
Email
Edwin Poiesz
(Industry Partner)
076 – 5303222
[email protected]
Jan van Krieken (Industry 0183 – 695695
Partner)
[email protected]
Hans Vreeswijk
(Industry Partner)
Ian Laird
(Technology Provider)
0182-542 729
[email protected]
0044
1355245993
- [email protected]
Henk
Akse
(Project
initiator/coordinator/
reporting editor)
Frans van den Akker
(coaching from DSTI) / Jan
Koning
T 0348 – 410907 [email protected]
M
06
–
41237695
033 - 4676497
Frans.vandenAkker@dstioffice.
nl
Date
Project code
Status
January 11th 2011
2010-005
Final
Revision
2.0
Filename
Project 2010 005 1 11-1-11
©
Traxxys 2011
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CONTENTS
page
Executive summary
3
1. The Feasibility Study On The Process Of Preparation Of Carboxyline 25 for Cosun
5
2. The Feasibility Study On Lactic Acid Crystallization Process for Purac
12
3. The Batch Feasibility Studies On Dewax Process of Squalane by Cooling
Crystallization for Croda
21
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EXECUTIVE SUMMARY
INTRODUCTION
This Technoproject has been executed within the framework of the NL GUTS knowledge network in cooperation
with DSTI (ISPT). An Oscillating Baffled Flow Reactor / Crystallizer (OBFR or OBFC) is essentially a
continuous plug flow pipe device that can be operated as a crystallizer and as a reactor. Innovations are the
oscillation that can be superimposed on the ingoing flow as well as the baffles with orifices. These innovations
add new independent process parameters like oscillation frequency and amplitude, orifice design diameter and
design distance between the baffles. PURAC and CRODA have tested the concept in its application as a
crystallizer. COSUN has tested the concept in its application as a reactor.
COSUN
The reaction of inulin and SMCA can be performed in a OBFC with excellent temperature and pH control. For a
satisfactory Degree of Substution and reaction efficiency more trials are recommended to identify the best
experimental conditions for this reaction.
PURAC
Continuous crystallization of lactic acid in an OBFC is possible if the correct experimental set-up is used. The
important parameters studied show that the following are important for lactic acid continuous crystallization:
Good mixing, by applying strong oscillation,
such as 2 Hz of frequency and 50 mm of amplitude
o
A slow cooling rate of 0.06 ~0.08 C/min is preferred for generating big particles
A solid content: <15% is preferred
A 25 mm ID OBFR with Teflon or similar baffles
Further studies using a DN25 OBFR would further validate the last point.
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CRODA
o
o
The obtainable
filtration rate was affected by the cooling rate in both temperature ranges – from 65 C to 20 C
o
and from 20 C to the final temperature - the slower the cooling rate, the faster the filtration. Oscillation rate plays
an important role in the obtainable filtration rate - the less oscillation, the faster the filtration. The filtration time in
the dewax process of squalane in an OBFC is similar to that in a Stirred Tank Crystallizer when a slow cooling
rate was applied. The process time in an OBFC was reduced to 5 hrs from 16 hrs with similar filtration rate. More
study is required to enable this process to be carry out in a continuous crystalliser as the current process time (5
hrs) is still too long for COBC.
IN CONCLUSION
For the applications of COSUN and PURAC, the proof of principle of OBFR/OBFC has been demonstrated
effectively. For the application of CRODA, results of OBFC are similar to those of Stirred Tank Crystallizers so
there is no competitive advantage for OBFC over STC in the CRODA case.
Woerden, January 11th 2011
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Part A. The Feasibility Study On The Process Of Preparation Of Carboxyline 25
For Cosun
By
NiTech Solutions Ltd
Project No: Ni035
Notebook Ref: KO023 & VR011
Operators: Kayleigh O’Neil
Vashal Raval and Lihua Zhao
Author: Lihua Zhao
Reviewers: Xiong-Wei Ni, Ian Laird
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In this project, Cosun wants to assess the efficiency of COBC on the synthesis of CMI with SMCA.
The critical success factor for this study is the reaction efficiency:
• the DS (degree of substitution) should be around 2.5 for 15% CMI solution
• SMCA must be completely converted. Otherwise the reaction is deemed as failure
The main challenges are:
• Solid additions during process
 Both inulin and SMCA are solids. They were added in solid forms for better reaction efficiency
• Mixing
 Efficient mixing is critical as the initial reaction involves solid-liquid mixing
 Two reagents were added simultaneously to the reaction mixture - good mixing is required
• Temperature control
 The reaction is exothermic, higher than the required temperature would introduce side reaction and
unwanted products
2. Phase I – batch work
Based on the proposal, the work was carried out in two phases – batch and continuous work:
• Phase I - Batch work: 3 days a week for 3 weeks;
• Phase II - Continuous work: 5 days work in 1~2 weeks
In Cosun’s procedure, inulin and SMCA were added as solids during reaction. Solid addition in continuous operation
was problematic, so the procedure was modified after discussion with Cosun in a teleconference:
st
•
Initial reaction between the 1 portion of SMCA slurry and inulin was carried out in a stirred vessel (Figure
1, left photo). This slurry was then fed into a continuous reactor during the Phase II study.
•
For the 2 part of the addition, the SMCA solution or slurry was used to replace the solid SMCA in the
original Cosun’s procedure. In this way, the method would be transferable to the continuous operation.
nd
2.1 Starting materials
Inulin was supplied by Cosun; SMCA and sodium hydroxide were purchased from Sigma-Aldrich (Catalogue No:
291773 and 71691 respectively).
2.2 Experimental set-up
Figure 1 shows the batch experimental set-up. The initial SMCA/inulin and water slurry were made in a stirred vessel
shown on the left. This slurry was then transferred to a 500 mL jacketed batch OBR. The reaction temperature was
controlled by circulating fluid through the jacket via a bath circulator; and monitored by a thermocouple inside the
reaction mixture. Two Watson-Marlow peristaltic pumps were used for simultaneously addition of SMCA slurry and
50% of NaOH solution. Oscillation was provided by a linear motor connected on the top of the baffle string. The pH
was not monitored for the first three experiments including the benchmark run. After the meeting with Cosun in
September, a probe of 6mm ID was inserted through a side port of OBR for pH monitoring during the additions.
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Figure 1 Photos of batch experiment set-up
2.3 Modified experimental procedure
•
•
•
•
•
•
•
•
•
Set-up OBR and a STR as shown in Figure 1
Mix 98.5 g of SMCA (98%) in 76 g water in a 500 mL vessel as shown in Figure 1 (left) with a
overhead stirrer (SMCA was not completely dissolved)
Add gradually 102.2 g of inulin under strong stirring until a slurry has been obtained
Transfer this slurry to a 500 mL OBR
Heat the slurry to 75 ºC (most solid dissolved)
Mix 195.53 g of SMCA in 152.25 g of water in a beaker, then homogenize it for 1 minute for better
pump ability or dissolve the SMCA in 237g of water
Add small amount of NaOH to OBR to adjust pH between 10.5~11.5
Add the above SMCA solution/slurry and 210.64 g of NaOH (50% sol.) simultaneously over a
certain amount of time maintaining temperature at 75 ºC and pH between 10.5 and 11.5 (using the
pH probe inside the OBR)
Maintain the temperature for a certain period of timeafter addition has finished in order to gain a
full reaction of all reagents
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•
•
•
•
Cool the content down to room temperature and drain the content to a 2 L beaker
Dilute with 304.4 g of water (adjust water quantity based on how much water used for dissolving)
to dissolve NaCl (by-product) in solution
Check pH again. If necessary, adjust pH around 7with 50% NaOH or 6N HCl
Take 250 mL diluted solution and send it to Cosun for analysis
In total 10 experiments were carried out. The experimental conditions were shown in Table 1.
2.4 Batch experimental results and discussion
The following experimental conditions were studied and the results were shown in Table 2:
o
• Reaction temperature: 75 and 85 C
nd
• Water quantity in 2 portion of SMCA for dropwise addition
o SMCA solution for addition (more water)
o SMCA slurry for addition
• Oscillation conditions
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In the first three experiments,
SMCA solution and 50% NaOH were added to the initial inulin/SMCA slurry in around
o
Compared to the Cosun’s current procedure,
40 minutes at 75 C without monitoring the pH change during addition.
o
the reaction exotherm observed was smaller (temperature raised 7 C at the beginning of the addition), which may be
due to more water used for the reaction. The reaction efficiency was low, as the DS was only 1.8, 1.91 and 1.99
respectively; and the levels of SMCA left were higher too.
After discussion with Cosun, a pH probe was fitted inside the OBR to monitor the pH during the addition. It was found
that the change of pH was much slower compared to the Cosun’s process due to the additional water used in OBR
reaction. Subsequently the water quantity in SMCA solution for dropwise addition was reduced to minimum by
homogenizing the SMCA into a slurry. In this way, combined with increased temperature, the pH change was much
faster during the dropwise addition. The addition was finished in 2 hours.
Comparing to the targeto DS of 2.5, the lower DS in OBR may be caused by insufficient temperature control (the
temperature rose to 98 C during addition). It is expected that DS and reaction efficiency would be improved in
continuous operation as both temperature control and additions are much easier in a COBR. The experimental
condition of KO023079 was chosen for continuous trials.
3. Phase II – Continuous operation
Based on batch experiments, a COBC was designed:
 A jacketed glass COBC with 15 mm ID is used. The whole COBC system consists of a feeding vessel,
oscillator, 14 meters of baffled tubes with two sets of addition ports
 Oscillator consists of a linear motor coupled with a piston; and the oscillation frequency and amplitude are
controlled by a control box
 The
initial inulin/SMCA/water slurry is prepared in a feeding vessel with an overhead stirrer, and heated to 85
o
C before pumping into COBC as the main flow
 Two sets of NaOH and SMCA addition ports are used
o The ratio of NaOH and SMCA slurry is fixed
o 50% sodium hydroxide is pumped into COBC before the SMCA slurry
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o micronised SMCA/water slurry is pumped into COBC shortly after NaOH is added
o
 The temperatures of the COBC are measured along the length of COBC, and are controlled between 83~86 C
 pH is measured a few minutes after NaOH and SMCA addition. The addition rates of NaOH and SMCA are
controlled to maintain the pH between 10.5 ~11.5
Figure 2 shows some photos of the COBC set-up.
Figure 2 Photos of COBC set-up
The experiments were focused on the investigation of the suitable addition rates of SMCA slurry and 50% sodium
hydroxide to control the pH at 10.5 ~ 11.5 and control the reaction exotherm. After several trials, one condition was
selected for the continuous experiment:
•
The flow rate for inulin/SMCA/water solution was 53 mL/min
•
The 1 portion of 50% NaOH addition was 14.5 mL/min, and the 1 SMCA was 26 mL/min
•
The 2 portion of 50% NaOH addition was 12.5 mL/min, and the 2 SMCA was 26 mL/min
•
Reaction temperature was 85 C
•
pH was 10.5~11.5
st
st
nd
nd
o
o
It is found that COBC can effectively control the reaction exotherm - the temperature was stable at 83~86 C. pH can
also be controlled within the desired range of 10.5~11.5.
The samples from initial experiments were sent to Cosun for analysis. The results shows that the DS was very low –
1.46 only with the reaction efficiency of 46% (VR011102B). This is probably caused by insufficient reaction time as
the residence time was only 17 minutes in COBC compared with a batch time of 2 hrs. The lower SMCA ratio and
lower sodium hydroxide used in continuous trials (Inulin: SMCA:NaOH = 1:3.19:2.76) compared to batch trials were
another reason for the lower DS.
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Based on these result, two more trials were carried out: one with more NaOH (molar equivalent of SMCA) and slightly
longer reaction time (VR011105); and another one with increased NaOH quantity and SMCA (Inulin: SMCA: NaOH =
1: 3.27: 3.35) with longer reaction time (VR011106).
Surprisingly, the results were not improved. Compared to VR011102, there were more SMCA, monochloroacetic acid;
and higher glycolic acid. These indicated that increasing sodium hydroxide quantity might have caused too high pH,
and resulted more by-products between SMCA and the base. The results also suggest that the optimisation should be
towards the reduction of the SMCA quantity from experiment VR011102 to match molar number sodium hydroxide
solution added; and have longer reaction time after each addition.
4. Conclusion
From the above study, we can conclude that the reaction of inulin and SMCA can be performed in a COBC with
excellent temperature and pH control. But for satisfactory DS and reaction efficiency, more trials should be
recommended and carried out to identify the best experimental conditions for this reaction.
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Part B. The Feasibility Study On
Lactic Acid Crystallization Process
For Purac
Project No: Ni033
Notebook Ref: BT024
Operators: Ben Taylor, Vishal Raval and Lihua Zhao
Author: lihua Zhao
Reviewers: Xiong-Wei Ni, Ian Laird
Distribution list: Frans van den Akker(DTSI), Henk.N.Akse (Traxxys), Jan van Krieken (Purac)
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Purac is a leading supplier of sustainable biobased building blocks and of a wide range of products produced from
renewable resources, and lactic acid is one of their products. The crystallization of lactic acid was chosen to evaluate
the technical feasibility of the NiTech crystallization technology.
The process challenges are:
• Lactic acid starts polymerizing at a temperature higher than 45 °C
• The starting solution is very viscous
• Large particle sizes are required for better separation from mother liquor
• Crystal sizes are normally 500 ~1000 μm in batch
Critical success factors are:
1. Increase the capacity – current crystallization time is 5 hours.
a. Reduce residence time is the key
b. 3 x current volumetric production is desirable
2. No encrustation
3. Particle sizes and PSD – should be large enough for separation from viscous mother liquor. (The larger, the
purer the crystals by centrifuge. So large ones are desirable.)
4. Yield – high is good but not important, as mother liquor can be recycled
2. Current process description and objectives
The current crystallization of lactic acid at Purac was performed in batch over 5 ~ 6 hours:
• Dissolving lactic acid by adding about 6% water
• Heating the solution carefully to 45 °C (avoid temperature above 45 °C)
• Cooling the solution slowly to 38 °C, and adding about 0.5% seeds slurry
• Keeping the temperature at 38 °C for 30 minutes to allow the seed crystals to grow (in case the crystals dissolve,
repeat the seeding procedure at 36 or 37 °C)
• Cooling the above slurry to 20 °C at a cooling rate of 0.05°C/min or slower
• Separating the solids with mother liquor in a centrifuge ( ~ 50% solid)
• Checking the product purity by color comparison
Preparation of seed crystals (slurry):
The ball mill method: Charge a ball mill pot with X g of lactic acid crystals and X/10 g of demineralized water. Grind
the crystals for a certain period of time - grind 15 minutes at maximum speed. This yields a suspension of small lactic
acid crystals with about 30wt% of solids.
This can be translated to a mortar and pestle method. Just use the 10:1 ratio of crystals: water and grind a small
amount. The resulting suspension is the seed slurry.
3. Phase I – batch experiments
Based on the proposal, the work will be carried out in two phases – batch work and continuous work:
• Phase I: 9 batch experiments
• Phase II: continuous work: 5 days work in 1~2 weeks
Pure lactic acid was provided by Purac. Water content was determined by Karl-Fisher in NiTech. Particle sizes were
measured by an optical microscope.
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3.1 Batch experimental set-up and program
Figure 1 is a photo of the batch experimental set-up. The crystallization experiments were carried out in a jacketed
batch oscillatory baffled crystallizer (OBC). Temperature was controlled through jacket fluid by a bath circulator. A
thermocouple was placed inside the crystallizer for monitoring the internal temperature. Oscillation was provided by a
linear motor connected to the top of a baffle string.
Filtration was carried out using a Buchner funnel under vacuum (Grade 1 Whatman filter paper). The filtration was
very faster, finishing in several seconds.
Figure 1 A photo of the batch OBC™
In phase I, 9 batch experiments were carried out in an OBC, and the following effects on the outcomes were
investigated:
• concentration of lactic acid;
• cooling profile;
• oscillation conditions
• seeding quantities
The experimental conditions were shown in Table 1.
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From these experiments, we found that (Table 1 and Figure 3):
• Slow cooling rate gave bigger particle sizes
• Crystal grew very fast at the nucleation temperature
• Longer holding time at the nucleation temperature gave bigger particle sizes
• Lower final temperature gave bigger particles
• No significant difference for water contents in filter cakes at above conditions. Stronger vacuum pump afforded
less water content
Figure 3 Microscope images of the crystallization products from 95% lactic acid
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In order to further improve the flowability of the product slurry, Purac suggested using 98% lactic acid (BT024046)
and 100% lactic acid (BT024047~049) for crystallization, as the nucleation temperature are much higher at these
concentrations;
and viscosity owill be lower
at the higher temperatures. For 100% lactic acid, the nucleation temperature
o
o
was 49 C, cooling from 50 C to 46 C gave solid content similar to above experiments with much improved flow
ability and viscosity.
o
It was found that cooling rate of 0.06 C/min
gave the particle sizes similar
to the benchmark experiment BT024041.
o
o
With faster cooling rate, such as 0.21 C/min (BT024048) and 0.23 C/min (BT024049), a strong oscillation was
required to suspend the solids crystallized out over a short period. Particle sizes were smaller with faster cooling rates
(Figure 4). No seeding was required, as the nucleation happened readily at this concentration.
Figure 4 Microscope images of crystals from 98% (BT024046) and 100% lactic acid
Purac was satisfied with the batch results and ready for the Phase II continuous trials. 100% lactic acid concentration
was chosen for continuous trials, as this condition gave better flow ability and smaller viscosity.
4 Phase II - Continuous experiments
4.1 COBC set-up
Based on the data from batch experiments, a 15 mm ID continuous jacketed glass COBC was designed and built. In
this COBC, the oscillation was achieved by a linear motor connected to a piston via a control box; temperature was
controlled by bath circulators using jacket water and monitored by internal thermocouples at different locations of
COBC.
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4.2 Continuous experiments
The first practical challenge for continuous experiments was to choose a suitable temperature to melt the solid lactic
acid and transfer the melt to the COBC without crystallization happening inside the tubing. This problem was solved
by putting a hot water jacket around the tubing. Another practical issue was how
to keep the minimum degradation of
o
lactic acid during melting, because the nucleation
temperature
was
about
49
C
for
100% lactic acid, and lactic acid
o
starts degradation at the temperature above 45 C. If the degree of degradation is different, then nucleation temperature
would change. A good approach to this was to choose a suitable temperature to melt the lactic acid and keep the same
melting time for each experiment. In this way, the nucleation temperature was almost the same each time.
For the first two experiments, a twenty-four meter long COBC with glass baffles was used for crystallisation. Several
experimental conditions were investigated: flow rates (85, 100 and 200 mL/min), cooling rates (0.05 and 0.12 ºC/min)
and oscillation conditions; but the mixing was poor, some big crystals were hardly moved. So the length of COBC was
reduced to 12 meters. With the shorter COBC, the mixing was improved, but still not satisfactory due to the
combination of the glass baffles and the high viscosity. We incorporated PFA baffles into the middle of the COBC
(Figure 5) to further improve mixing.
Figure 5 Photos of the COBC set-up
The experimental procedure is described below:
1. Melt lactic acid in two 2 L conical flasks in a water bath and stirred with overhead stirrers
2. Flush the COBC with water to get rid of all the air bubbles
3. Start oscillation with a frequency of 2 Hz and amplitude of 50 mm
4. Switch on all the bath circulators connected to COBC with desired temperatures
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5. Pump the recycled lactic acid solution to COBC (~10 L) to replace the water
6. Pump freshly prepared lactic acid solution into COBC
7. After 30 minutes, pump a small amount of seeds slurry from the first bend via a valve into COBC (~30 seconds
at a flow rate of 60 mL/min)
8. Continue pumping freshly prepared lactic acid into the COBC and observe the nucleation
9. Collect some slurry from the outlet of COBC into the preheated centrifuge bowl and centrifuge for 5 minutes
after one residence time from the nucleation happened
10. Analyse the crystals by an optical microscope and calculate the solid content of the slurry
A total of 8 continuous experiments were carried out, and the experimental conditions were shown in Table 2.
4.3 Continuous experiment results and discussion
Due to the time constraint, the following effects on mixing were briefly investigated and discussed below:
1. COBC length
2. COBC baffles
3. Cooling rate
4. final temperature
4.3.1 The effect of COBC length
The length of COBC had a considerable effect on oscillation due to the viscous nature of the materials: when the
length of COBC was reduced from 24 to 12 meters, the oscillation was improved.
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4.3.2 The effect of baffles
The batch experiments in phase I revealed that the mixing was good in a 50 mm ID batch OBC with adequate
oscillation (2 Hz, 20 mm). It was surprising to see that only the fluid at the middle of COBC moved, but those at the
side of COBC cells didn’t when a all glass baffled COBC was used. These could be attributed by the baffle formation
(glass forged vs. PFA baffles) as well as the mixing mechanisms (moving baffles vs moving fluid).
When a section tube containing PFA baffles (similar to the batch OBC) was inserted in the middle of COBC, the
mixing was much improved. This confirms the effect of the formation of baffles on mixing, in particular, dealing with
viscous liquids. On this note, some big particles were stuck in baffle holes of small diameter (~6 mm) and blocked
flow. A 25 mm ID COBC is probably better for lactic acid crystallization.
4.3.3 The effect of cooling rate
Cooling rate had a significant effect on the particle sizes. With fast cooling rates, the particle sizes were smaller. This
is in accordance with the findings from other projects. The microscope images from several experiments were shown
in Figure 6 (they were taken with the same magnification).
o
VR024168 (0.14 C/min)
o
VR024171 (0.14 C/min)
o
o
VR024169 (0.09 C/min)
VR024170 (0.09 C/min)
o
o
VR024172-1 (0.065 C/min) VR024172-2 (0.065 C/min)
Figure 6 Microscope images of COBC experiments
4.3.4 The effect of final temperature
The final temperature affects the solid contents at the outlet. The lower the final temperature, the higher the solid
content of the slurry. Higher solid content in a viscous liquid is likely to cause blockage of the COBC, so the solid
content should be kept less than 20%, preferable <15% to ensuring a smooth continuous run.
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4.3.5 The effect of oscillation
The effect of oscillation was briefly studied: the oscillation of 2 Hz and 50 mm seemed to give better mixing than 1 Hz
and 50 mm.
5. Conclusion and Suggestion of future work
Because of the limited time, more continuous runs were not possible.. From we have done so far, we can conclude:
• Continuous crystallization of lactic acid in COBC is possible if the correct experimental set-up is used
The important parameters studied show that the followings are important for lactic acid continuous crystallization:
Good mixing:
•
Strong oscillation, such as 2 Hz of frequency and 50 mm of amplitude
•
Slow cooling rate: 0.06 ~0.08 C/min is preferred for generating big particles
•
Solid content: <15% is preferred
o
A 25 mm ID COBC with PTFD or similar baffles
Further studies using a DN25 COBC would further validate the last point.
20
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Part C. The Batch Feasibility Studies
On Dewax Process of Squalane
by Cooling Crystallisation
For Croda
Project No: Ni034
Notebook Ref: KO023
Operator: Kayleigh O’Neil, Lihua Zhao
Author: Lihua Zhao
Distribution list: Frans van den Akker(DTSI), Henk.N.Akse (Traxxys), Hans Vreeswijk (Croda), Krzysztof
Rakowski (Croda)
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The current Croda’s dewax process of squalane was performed by cooling crystallisation in a batch crystalliser with a
crystallisation time of around 14 ~ 16 hours. The process was described in a patent (WO WO9716400) with following
procedure:
• Heating the crude Squalane to 65 °C with 65 rpm stirring rate (if any solids at this temperature, filter the hot oil to
remove the solid)
• Holding the solution at 65 °C for 30 minutes
• Cooling the solution to 20 °C at a cooling rate of 1.5°C/min
• Cooling the solution from 20 °C to the end temperature (0°C to –10°C) at a cooling rate of 0.04°C/min
• Holding the slurry at the end temperature for 3 hours
• Filtering the solids in a thermostated testfilter using a 40 μm filtercloth (Propex) with a filteraid (Perlite). The
filter is pressurized with nitrogen at 0.5 bar and 2.0 obar.
• Check the clarity of the filtered oil after placing at -10 C for 2 hrs
The critical success factor is to improve specific cake resistance or filtration rate:
• Check the clarity of the filtered oil after dewax process
• Filtration rate at the end temperature of 0°C and –5°C
The challenges are:
• The dewax process needs to be performed slowly (14~16 hrs), otherwise the filtration will be very slow due to the
formation of small crystals
• The vacuum filtration has to be used in NiTech as we do not have a pressure filter. Vacuum filtration is very
slow, and the filter should be kept at -5 or 0°C during filtration
2. Experimental
Starting material: Crude squalane, purified squalane and filter aider perlite were provided by Croda.
Experimental set-up: The crystallisation was carried out in a 250 mL jacketed glass OBC. Two different final
temperature were investigated in this study. For each of the conditions, a benchmark experiment in a stirred jacketed
flask was carried out to compare the filtration rate. Figure 1 is photos of crystallizers used for benchmark experiments
(left) and in OBC experiments (right).
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Figure 1 Photos of the crystallizers (left, STR and right OBC)
In all the experiments, temperature was controlled by a bath circulator through the jacket fluid in crystallizers, and
monitored by a thermocouple inside the crystallizers. For benchmark experiments, an overhead stirrer with 65 rpm
stirring rate was used for mixing. In OBC experiments, the mixing was achieved by oscillation through a linear motor
connected to the baffle string at the top of the OBC (Figure 1, right).
Filtration was performed in either a pre-chilled unjacketed Buchner funnel with grade 1 Whatman paper (pore size 11
μm) or a jacketed sintered funnel (pore size 20 ~50 μm, thickness 4~5 mm) under vacuum. When unjacketed funnel
was used, ice bags were used to wrap the funnel to keep it cold during filtration (Figure 2).
Unjacketed Buchner funnel Jacketed sintered funnel
Figure 2 Buchner funnel used for filtration
Initially, crystallization was carried out at a final temperature of -5 ºC. With this final temperature, it was found that
the filtration was slow in a pre-chilled ice-wrapped funnel. In order to keep the temperature at -5 ºC during the long
filtration time, filtration was changed to a jacketed sintered funnel. Surprisingly the filtration in this sintered funnel
was extremely slow; the filtration was unable to finish in a whole day for all the experiments including the benchmark
experiment. But clean squalane from Croda can be filtered off easily in this sintered funnel (12 minutes) (Table 1).
23
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After consulting with Croda, the final temperature of crystallization was adjusted to 0 ºC, and filtration was performed
in a pre-chilled unjacketed funnel and wrapped with ice-bags during filtration. At this condition, the filtration rate was
much improved. The experimental conditions were listed in Table 2.
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3. Results and discussion
An optical microscope was used for observation of the wax particle shapes. Figure 2 shows the photos from various
experiments during crystallization. In all the experiments, the needle-shaped wax crystals were observed.
Figure 2 Microscope images of wax crystals
3.1 Comparison of OBC experiments with benchmark experiments
The effect of experimental conditions on filtration rate was briefly studied. It was found that crystallization in a batch
OBC gave small improvement compared to stirred vessel on filtration rate at both final temperature of -5ºC and 0ºC
(Table 3) with the same cooling profile (Figure 3).
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The cooling profile during filtration in the last four experiments (KO023078, 080, 082 &085) were recorded (Figure
3). During filtration the temperature rose from 0 ºC to 7 ºC for all four experiments (Figure 4).
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3.2 Effect of oscillation on filtration rate
It was also found that oscillation condition plays an important role in filtration rate. Stronger oscillation resulted in
long filtration time and poor filtration rate (Table 4). This can be due to stronger oscillation breaking down the needle
crystals to smaller particles which would slow down the filtration.
3.3 The effect of cooling rate on filtration rate
Cooling rate also plays significant role in filtration rate. The faster the cooling rate, the slower the filtration (Table 5).
Cooling rates at both temperature zones (65 ºC to 20 ºC) and 20 ºC to final temperature affect the filtration rate:
 By changing cooling rate from 1.5 ºC/min in original Croda’s procedure to 0.1 ºC/min from 65 ºC to 20 ºC, the
filtration rate was almost improved 50% in OBC.
o
o
o
o
 By changing cooling rate from 20 C to 0 C from 0.25 C/min to 0.1 C/min, the filtration rate improved 50%
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It is encouraging to see that the filtration rates were almost the same when the process
time was reduced from 980 minutes (KO023080) to 305 minutes in KO023085, although
it is not clear if the wax contents in filtrates were same or not.
3.4 The effect of final temperature on filtration rate
The final temperature also has significant impact on filtration rate. At a final temperature
of 0ºC, the filtration was faster and filter cake was much dried (Figure 5). This is probably
contributing to both improved viscosity and less solid content at 0 ºC.
Figure 5 A photo of the filter cake from KO023078, 080, 082 & 085
3.5 The effect of the pore size of filter paper
The pore size of the filter paper affects the purity of the product. When a grade 54
Whatman filter paper (pore size 20~25 μm) was used in experiment KO023078, the
filtrate from cold test at 0 ºC for 2 hrs was much cloudier than the one used Whatman
grade 1 filter paper (pore size 11 μm) in KO023080 at the same experimental conditions
(Figure 6). Some needle crystals must have sizes less than 25 μm.
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Figure 6 Photos of KO023078 (left) and KO023080 (right) after 2 hrs at 0ºC
4. Conclusion
From the above discussion, we can conclude:
•
Fitration rate was affected by cooling rate in both temperature ranges – from 65
o
o
o
C to 20 C and from 20 C to the final temperature - the slower the cooling rate,
the faster the filtration
•
Oscillation rate plays important role in filtration rate - the less oscillation, the
faster the filtration
•
The filtration time in the dewax process of squalane in OBC is similar to that in
STC when slow cooling rate was applied.
•
The process time in OBC was reduced to 5 hrs from 16 hrs with similar
filtration rate.
But more study is required to enable this process to be carry out in a continuous
crystalliser as the current process time (5 hrs) is still too long for COBC.
29
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Continuous Crystallization

Transcription

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