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Drying of an organic powder
Journal of Loss Prevention in the Process Industries 12 (1999) 321–323
Short Communication
Drying of an organic powder I. Chilton
*
Hickson & Welch Ltd, Wheldon Road, Castleford, West Yorkshire, WF10 2JT, UK
Abstract A number of small-scale methods have been used to determine the safe drying temperature, on a 2 tonne scale, of an organic powder. Predictions of the temperature for which the Time to Maximum Rate (TMR) is 100 hours are used to compare the results. The predicted temperatures range from 50°C to 120°C raising the issue of taking the result from a single test method in isolation. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Process safety; Drying; Thermal stability; Time to Maximum Rate (TMR), DTA, DSC, ARC, PHI-TEC
1. Introduction Many products resulting from fine chemical manufacture are isolated as solids from aqueous or organic slurries. Subsequently these solids need to be dried. Process Safety Specialists will be asked to recommend safe drying conditions based on small-scale testing. Just what constitutes small scale will vary between test methods, and between companies. By definition, it will be smaller than the plant scale. This communication offers a comparison of some of the techniques. It arose not out of a desire to produce an academic paper but was a response to a production requirement to dry an organic intermediate, under vacuum, in a rotary cone drier. The scale was around 2 tonne.
drying temperature of 50°C was recommended. Fig. 1 shows the trace from the first of the test series. Ramped differential scanning calorimetry (DSC) (5°C min−1) showed an exotherm at 150°C which was considered to confirm the DTA result. No further testing was planned. However, the Production Department suggested that drying at 50°C may make this operation unacceptably slow. Furthermore, this was a toll-manufacturing project and it was known that the client company had previously dried the product at 80°C. Thus a series of isothermal DSC tests were undertaken to probe the kinetics of the decomposition. Exothermal activity was detected down to 120°C on a 5 mg scale. The set of isothermal results was used to generate TMR (time
2. Results Initial studies were conducted using simple differential thermal analysis (DTA). These tests, on a 2 g scale, indicated that the material exhibited thermal instability at 130°C. When tested at 120°C the material was shown to be stable for at least 100 h. It is usual in such testing to apply a scale rule. In this instance, a 60°C scale factor plus a 10°C safety margin was applied. On this basis a * Tel.: ⫹ 44-1977-556565; fax: ⫹ 44-1977-603664/515932. 0950–4230/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 0 - 4 2 3 0 ( 9 9 ) 0 0 0 0 3 - 0
Fig. 1.
Ramped DTA run.
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I. Chilton / Journal of Loss Prevention in the Process Industries 12 (1999) 321–323
to maximum rate) data (Fig. 2). At 67.5°C the TMR is calculated as 100 h which was deemed to be ample time to perform the drying operation with a large safety margin. On this basis the recommended drying temperature was accordingly raised. Because the client had already dried this material at 80°C, further test work was commissioned on both the ARC and PHI-TEC II calorimeters. Both these instruments indicated onset temperatures of around 120°C. Because these instruments are intended to behave adiabatically the data are often used without the application of a scale factor. However, two things led us to approach these results with caution. First of all, these sensitive adiabatic instruments were detecting thermal onset at temperatures very little lower than the non-adiabatic DTA and DSC. Secondly, and probably connected, high phi-factors were quoted for both the ARC and PHI-TEC II tests. The reader will, of course, be aware that the phi-factor is a measure of the thermal mass of the container compared with the thermal mass of the sample. For completeness it is calculated as follows:
⫽1⫹
mCPtestcell . mCPsample
For the ARC test the phi-factor was 1.74, for the PHI-
Fig. 2.
TMR from isothermal DSC.
TEC II test the phi-factor was around 2. The phi-factor for plant equipment is usually less than 1.1. As with DSC data, ARC and PHI-TEC II data can be used to probe the kinetics of the event in question. Simple thermal analysis of adiabatic reactions can be shown to lead to the following equation: ln k⬘ ⫽ ln(Cn0 ⫺ 1A) ⫺
E , RT
where C0 initial reactant concentration, n reaction order, A Arrhenius frequency factor, E activation energy, R gas constant, T absolute temperature, k⬘ pseudo rate constant as defined below
k⬘ ⫽
(dT/dt) , (Tf ⫺ T0)[(Tf ⫺ T)/(Tf ⫺ T0)]n
Tf T0
final temperature and initial temperature.
Figs. 3 and 4 show the raw data and a kinetics plot with n set at one. Once a value for E, the activation energy, has been
Fig. 3.
Raw ARC test data.
I. Chilton / Journal of Loss Prevention in the Process Industries 12 (1999) 321–323
Fig. 4.
ARC test kinetics.
extracted from the kinetics plot, the TMR can be calculated from the following equation: TMR ⫽
RT 2 . (dT/dt)E
At 103°C the TMR is 100 h. Alternatively, TMR information in the region of interest can be extrapolated more directly from the adiabatic time/temperature data. A plot of ln TMR versus ⫺ 1/T should give a straight line. These data are shown in Fig. 5 along with a phi-corrected extrapolation. A similar plot is obtained for PHI-TEC II data.
3. Summary The data in Table 1 were collected with regard to obtaining a safe drying temperature for this organic intermediate on a production scale.
4. Further discussion points The purpose of this communication was not to pass judgement on the relative merits and de-merits of these various items of test equipment. Nor was it intended to indicate which gave the “right” answer. It was merely intended to provoke thought. Given the results of one
Fig. 5.
323
Arc data: TMR plot.
Table 1 Data collected on safe drying temperature for the organic intermediate on a production scale Test method DTA DSC ARC (from kinetics) ARC (extrapolation) ARC (extrapolation, phi corrected) PHI-TEC II (extrapolation) PHI-TEC II (extrapolation, phi corrected)
Temperature TMR (h) (°C) 50 67.5 103 113
> 100 (not a true TMR) 100 100 100
111
100
102
100
101
100
test, in isolation, what would be the response? If the DSC data are correct then drying at 110°C could present a significant hazard. The TMR at this temperature can be calculated as 3 h, which could easily be exceeded in normal processing. Conversely, if the ARC data are correct, then a limit temperature of 50°C would place an unnecessary burden on production. It may cost money in terms of both time and equipment costs. There is one comment, which might fuel the debate. The organic intermediate in question can undergo an intramolecular rearrangement during drying. For this reason the client imposed an upper specification, on the dried material, of the impurity thus formed. Is this the event that all these tests are detecting?
Short Communication
Drying of an organic powder I. Chilton
*
Hickson & Welch Ltd, Wheldon Road, Castleford, West Yorkshire, WF10 2JT, UK
Abstract A number of small-scale methods have been used to determine the safe drying temperature, on a 2 tonne scale, of an organic powder. Predictions of the temperature for which the Time to Maximum Rate (TMR) is 100 hours are used to compare the results. The predicted temperatures range from 50°C to 120°C raising the issue of taking the result from a single test method in isolation. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Process safety; Drying; Thermal stability; Time to Maximum Rate (TMR), DTA, DSC, ARC, PHI-TEC
1. Introduction Many products resulting from fine chemical manufacture are isolated as solids from aqueous or organic slurries. Subsequently these solids need to be dried. Process Safety Specialists will be asked to recommend safe drying conditions based on small-scale testing. Just what constitutes small scale will vary between test methods, and between companies. By definition, it will be smaller than the plant scale. This communication offers a comparison of some of the techniques. It arose not out of a desire to produce an academic paper but was a response to a production requirement to dry an organic intermediate, under vacuum, in a rotary cone drier. The scale was around 2 tonne.
drying temperature of 50°C was recommended. Fig. 1 shows the trace from the first of the test series. Ramped differential scanning calorimetry (DSC) (5°C min−1) showed an exotherm at 150°C which was considered to confirm the DTA result. No further testing was planned. However, the Production Department suggested that drying at 50°C may make this operation unacceptably slow. Furthermore, this was a toll-manufacturing project and it was known that the client company had previously dried the product at 80°C. Thus a series of isothermal DSC tests were undertaken to probe the kinetics of the decomposition. Exothermal activity was detected down to 120°C on a 5 mg scale. The set of isothermal results was used to generate TMR (time
2. Results Initial studies were conducted using simple differential thermal analysis (DTA). These tests, on a 2 g scale, indicated that the material exhibited thermal instability at 130°C. When tested at 120°C the material was shown to be stable for at least 100 h. It is usual in such testing to apply a scale rule. In this instance, a 60°C scale factor plus a 10°C safety margin was applied. On this basis a * Tel.: ⫹ 44-1977-556565; fax: ⫹ 44-1977-603664/515932. 0950–4230/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 0 - 4 2 3 0 ( 9 9 ) 0 0 0 0 3 - 0
Fig. 1.
Ramped DTA run.
322
I. Chilton / Journal of Loss Prevention in the Process Industries 12 (1999) 321–323
to maximum rate) data (Fig. 2). At 67.5°C the TMR is calculated as 100 h which was deemed to be ample time to perform the drying operation with a large safety margin. On this basis the recommended drying temperature was accordingly raised. Because the client had already dried this material at 80°C, further test work was commissioned on both the ARC and PHI-TEC II calorimeters. Both these instruments indicated onset temperatures of around 120°C. Because these instruments are intended to behave adiabatically the data are often used without the application of a scale factor. However, two things led us to approach these results with caution. First of all, these sensitive adiabatic instruments were detecting thermal onset at temperatures very little lower than the non-adiabatic DTA and DSC. Secondly, and probably connected, high phi-factors were quoted for both the ARC and PHI-TEC II tests. The reader will, of course, be aware that the phi-factor is a measure of the thermal mass of the container compared with the thermal mass of the sample. For completeness it is calculated as follows:
⫽1⫹
mCPtestcell . mCPsample
For the ARC test the phi-factor was 1.74, for the PHI-
Fig. 2.
TMR from isothermal DSC.
TEC II test the phi-factor was around 2. The phi-factor for plant equipment is usually less than 1.1. As with DSC data, ARC and PHI-TEC II data can be used to probe the kinetics of the event in question. Simple thermal analysis of adiabatic reactions can be shown to lead to the following equation: ln k⬘ ⫽ ln(Cn0 ⫺ 1A) ⫺
E , RT
where C0 initial reactant concentration, n reaction order, A Arrhenius frequency factor, E activation energy, R gas constant, T absolute temperature, k⬘ pseudo rate constant as defined below
k⬘ ⫽
(dT/dt) , (Tf ⫺ T0)[(Tf ⫺ T)/(Tf ⫺ T0)]n
Tf T0
final temperature and initial temperature.
Figs. 3 and 4 show the raw data and a kinetics plot with n set at one. Once a value for E, the activation energy, has been
Fig. 3.
Raw ARC test data.
I. Chilton / Journal of Loss Prevention in the Process Industries 12 (1999) 321–323
Fig. 4.
ARC test kinetics.
extracted from the kinetics plot, the TMR can be calculated from the following equation: TMR ⫽
RT 2 . (dT/dt)E
At 103°C the TMR is 100 h. Alternatively, TMR information in the region of interest can be extrapolated more directly from the adiabatic time/temperature data. A plot of ln TMR versus ⫺ 1/T should give a straight line. These data are shown in Fig. 5 along with a phi-corrected extrapolation. A similar plot is obtained for PHI-TEC II data.
3. Summary The data in Table 1 were collected with regard to obtaining a safe drying temperature for this organic intermediate on a production scale.
4. Further discussion points The purpose of this communication was not to pass judgement on the relative merits and de-merits of these various items of test equipment. Nor was it intended to indicate which gave the “right” answer. It was merely intended to provoke thought. Given the results of one
Fig. 5.
323
Arc data: TMR plot.
Table 1 Data collected on safe drying temperature for the organic intermediate on a production scale Test method DTA DSC ARC (from kinetics) ARC (extrapolation) ARC (extrapolation, phi corrected) PHI-TEC II (extrapolation) PHI-TEC II (extrapolation, phi corrected)
Temperature TMR (h) (°C) 50 67.5 103 113
> 100 (not a true TMR) 100 100 100
111
100
102
100
101
100
test, in isolation, what would be the response? If the DSC data are correct then drying at 110°C could present a significant hazard. The TMR at this temperature can be calculated as 3 h, which could easily be exceeded in normal processing. Conversely, if the ARC data are correct, then a limit temperature of 50°C would place an unnecessary burden on production. It may cost money in terms of both time and equipment costs. There is one comment, which might fuel the debate. The organic intermediate in question can undergo an intramolecular rearrangement during drying. For this reason the client imposed an upper specification, on the dried material, of the impurity thus formed. Is this the event that all these tests are detecting?