Separation and Purification by Crystallization - American Chemical

Chapter 15

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Effects of Seeding on Start-up Operation of a Continuous Crystallizer H. Takiyama, H. Yamauchi, and M . Matsuoka Department of Chemical Engineering, Tokyo University of Agriculture and Technology, 24-16 Nakacho-2, Koganei, Tokyo 184, Japan

Design strategy of start-up operation was investigated to shorten the required time for start-up operation, by analyzing the changes in the crystal size distribution and highlighting the role of the size distribution of the seed crystals and the time of addition. The main results are as follows. (1) The crystal size distribution and time of addition of seed crystals influence the transient behavior of products crystal size distributions during start-up. (2) The required time to attain steady state can be shortened when previous products were used as the seed crystals.

Plant structures and operations are complicated in the chemical processes because of multiple productions, energy conservation, pollution and safety. Operation design is therefore essential to advanced operations. For the start-up operation with great changes in operating conditions, the following items should be evaluated(i). (1) Required time for start-up operation. (2) Switching time from the off-specification product line to the product line. In the industrial crystallization, operation design which satisfies required product specification is also important. However, methods of detennining the seed crystals specifications i.e., size distribution and mass of seed crystals, and the time of addition have not been considered systematically. The required time of start-up operation is known to be different by 10 residence times even in the laboratory scale depending on the seed crystals specifications^). In the present study relations between the procedure and conditions of start-up operations and the product crystal size distributions are discussed based on the analysis of product crystal size distributions. The purpose of this study is, therefore, to obtain design strategies for start-up operation by use of a typical MSMPR (Mixed-Suspension Mixed-Product Removal) crystallizer which has been used for the theoretical analysis. The ammonium-sulfate water system was used. Experimental Experimental apparatus. Details of experimental apparatus are shown in Figure 1. The crystallizer is a stirred vessel of 720 ml capacity water jacket. The crystallizer is 172

© 1997 American Chemical Society

Botsaris and Toyokura; Separation and Purification by Crystallization ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

15.

TAKIYAMA ET AL.

Effects of Seeding on Start-up Operation

173

equipped with a draft tube made of acrylic resin and four baffles. The slurry flows inside the draft tube downward. The crystallization temperature was controlled by a programmable thermostat ( E Y E L A NCB-3100) with a temperature sensor set inside the vessel The slurry was stirred with a marine type propeller made of stainless steel. Feed solution was supplied with a centrifugal magnetic pump, and its flow rate was adjusted with a needle valve. The slurry was withdrawn by using a syringe at one minute intervals. The syringe worked as a level controller as well as a sampler. The slurry collected during 6 minutes was filtered every 20 minutes, and the crystal size distribution was measured by sieving after drying. The solution concentration was measured by sampling that was carried out with a small syringe having a cotton filter every 20 minutes. Experimental conditions. The feed conditions were fixed as follows: The solution concentration was 44.5 wt% and the flow rate was 6.00 X10' m /s, hence, the residence time being 1.2 Χ 1 0 seconds. The agitation speed was decided as 11.7 s" from preliminary experiments to satisfy MSMPR conditions (3). The operating temperature was 25 \^ (2). 7

3

3

1

Start-up methods. In this study, methods of start-up of a continuous cooling crystallizer are considered. Although several start-up methods are possible two typical ones as shown in Figure 2 were chosen. This description is based on the SFC (4) (Sequential Function Chart). The SFC is composed of "step" and 'transition". "Step", which is denoted with a box, shows the state of apparatus. 'Transition" with a bold line indicates a condition that makes a move of the "step". The items on the side of "step" are actual operations. The feed solution was charged to the crystallizer from the feed line. The feed was stopped when the liquid level arrived at the set up value, i.e. the hold up was fulfilled. Then, the solution was cooled to the operating temperature. Seed crystals were added when the solution temperature became constant at the set up value. After that, the continuous feeding and discharge were started. In this study, the experimental duration was more than 10 residence times in order to ensure the attainment of steady state. This method of start-up is defined as SU Method 1. On the other hand, SU Method 2 is as follows. After the solution temperature of the initial charge in the crystallizer became constant, the feeding and discharge were started. Seed crystals were then added to the solution with a known level of supersaturation. Four kinds of seed crystals were used and their size distributions are shown in Figure 3. The specifications of seed crystals are as follows: (1) No seeds. (2) Monodisperse seed crystals at the number based average size, (Δ) (The size of seed crystals being equal to the number based average size of crystals which would be obtained under the same operating condition.) (3) Monodisperse seed crystals at the mass based average size (•), and (4) Crystals having the product crystal size distribution which would be obtained under the same operating condition, in other words, previous products. The amount of seed crystals was 20.0 g, equivalent to the slurry density of 27.8 kg/m . 3

Judgment of steady state. The judgment of the time needed for the response to reach the steady state is generally referred to settling time θ„ which is necessary in order to evaluate the required time for start-up operation. Attainment of the steady state was determined from the convergence of the crystal size distribution. This was done by the use of the performance index (ε) which is defined as the change in the mass based crystal size distribution in every 1 residence time.


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SEPARATION A N D PURIFICATION BY C R Y S T A L L I Z A T I O N

Fig. 1

Experimental apparatus


TAKIYAMA ET AL.


START Start

Ch arging of the

Feed line ON

πlother liquor ^

Level

Sel ting

Cooling ON

coperation temp,

Feed line OFF

Temp.

Set sding Temp. Fe

20

Q

o

__

a

o

o

0

H

0.5

I Ϊ ο ϋ

10%I

I I 10

I I I t I I I I

Time Fig. 4

I ι ι ι ι

I I i 1 I

20 XI τ

[-]

Changes in ε and C V value


15.

TAKIYAMA ET AL.

177


X

•\»& (.t ydL -i4> (t„)-dL \ t

t

t

t

i

1 0 0

Δί,./τ

dL.

(D

Mpj(ti) denotes the crystal mass on the j-th sieve at time f„ and / is the total number of sieves, which is 9 in this study. This function evaluates a change in the area of the mass based size distribution in 1 residence time. w is a weighting factor defined by Equation (2), and is a function of size so as to give a maximum weight to the class of the mass average size, i.e. the weighting factor of the class of the mass average size is 1.0. ;

( 2 )

Φ Ai

( M)' M

n

L

L

'

d L

M

The above equation expresses the ratio of the mass of crystals in the j-th sieve class to the mass of crystals in the mass average size class, where, n(L) is the population density function approximated with a straight line at the steady state (see Figure 3). A n experimental run with more than 20 residence times was carried out to determine the value of ε at the threshold of the attainment of steady state, and the results are shown in Figure 4. Values of the coefficient of variation (CV) are also plotted in the diagram as O , since the C V values are usually used to discuss the product crystal size distribution in crystallizers (5). The C V value was calculated by Equation (3). CV-[(m -m /ml)-if 3

m

k

s

(3)

-f\(L)L dL k

The C V value of the product crystals from an MSMPR crystallizer is theoretically given as 0.5. Since each values of ε and C V is significantly affected by errors in crystal size distribution arising from sieve analysis, the trends are approximated by smooth curves to eliminate abrupt changes and are shown as broken curves in Figure 4. The values of ε are sensitive to the changes in production quantity particularly in the begmning of the experiment, however, both the values of C V and ε converged except the begmning stage of the experiment. From the trend of these values the threshold of the attainment of the steady state in terms of the performance index ε was decided to be 10%. For this particular case, Θ, was therefore determined as 3 residence times. The calculated values of ε might have been affected by the relatively large crystals exfoliated from the scale on the crystallizer wall in the later stages of the experiment, and this causes fluctuations of the ε values. Experimental Results and Discussion No seeds. For the case of SU Method 1 without seeds, typical changes in the crystal size distribution are shown in Figure 5. The moment of the start of feeding was defined as time 0. The abscissa is the dimensionless time based on the average residence time and the ordinate is the population density. Each curve shows changes in the population density of each sieve class. This shows that the crystal size distribution shifts to bigger size with time, and smaller crystals attained steady state faster. Changes in ε are shown in Figure 6 for this case, and Θ, was found to be 7


SEPARATION A N D PURIFICATION B Y C R Y S T A L L I Z A T I O N

Crystal size Lx10 [m] 3

5 Time Fig. 5

10 t/r[-]

Changes in population density (No Seeds)

100

ι •« No seeds 1

1

c£ 80 co Q CO

ο

60

CO

-o-o-rHo.5 >

ϋ

S

4 0

υ c

I

ο ϋ

20 J

i

5 Time Fig. 6

I

I

S

I

I

10 t/r [-]

I

15

Changes in ε and C V value (No Seeds)


15.

TAKIYAMA ET AL.


179

residence times from the definition. The C V values also attained 0.5 after about 7 residence times. Changes in the average size and the production rate are shown in Figure 7. Each value was normalized by the value at the steady state. If on-line measurement of crystal size distribution was possible, the changes in the production rate for the class of the mass average size would be used to decide the switching time from off-specification product line to the product line. The average size was observed to increase continuously till about 7 residence times. The slurry density in this particular experiment was 24 kg/m . The total production rate also increased gradually up to 7 residence times. A decay oscillating change was observed in the production rate of the mass average size. 3

Monodisperse seeds. Changes in the population density when the monodisperse crystals at the number based average size were added are shown in Figure 8, where Ο shows the population density of the seed crystals. A peak arising from the seed crystals was found to shift to bigger size. It took long time for this peak to disappear. This tendency was pronounced when excess amounts of the seed crystals were added. Θ, was found to be 7 residence time after seeding. Figure 9 shows the case when the monodisperse crystals at the mass based average size were added. The movement of the peak of the seed crystals is also observed. Θ, evaluated from the ε values was 7 residence times. Previous product seeds. Figure 10 shows the case when the seed crystals with crystal size distribution that would be obtained at the steady state were added. It is shown that the population density of each size became very stable soon after seeding. Changes in the dimensionless average size, total production rate and production rate of the class of the mass average size are shown in Figure 11. The changes of the mass average size were negligible soon after seeding. The total production rate attained stability at about 1.0 in a few residence times. The oscillated changes of the production rate at the mass average size were small compared with those of the case of no seeds (Figure 6). Θ, was 2 residence times in this case. SU Method 2. Figure 12 illustrates the changes in the population density for the case of a different start-up method (SU Method 2). The previous product crystals were added after 1 residence time when the temperature had become constant. In this experiment, small amounts of crystals were found in the solution at about 0.5 residence time after feeding, suggesting that at the moment of seeding the solution supersaturation was large enough for nucleation. A small peak was observed in the small size region after seeding, however its movement to bigger size was not seen clearly. Changes in mass average size and the production rate are shown in Figure 13. The total production rate became stable after overshooting. The overshooting of the production rate of the mass average size, followed in about 3 residence times because of the time needed for growth of nuclei which were generated at the moment of seeding. The required time for start-up operation in this case is determined as 7 residence times as shown in Figure 14, and the changes in ε for SU Method 2 were similar to the case of no seed. It is, therefore, suggested that methods of addition of seed crystals without overshooting should be necessary in order to shorten the value of 8. S

Conclusions Relations between the start-up operating conditions and the specification of product crystals during start-up were experimentally examined, and the following conclusions were obtained. (1) Differences of the size distribution, amounts of seed crystals and time of addition influence the transient changes of products crystal size distributions during start-up. (2) The dynamic decay of oscillations of production rate of each crystal size after


180

SEPARATION AND PURIFICATION B Y C R Y S T A L L I Z A T I O N

1.5

ι

ι

ι

ι

1

1

1

1

1

1

'

'

'

1

No seeds

[I\ 8

S _J

}θ.5

:.

-A

I

.

I

A

1—1

1

1

1

1

1

1

1

Time t/r [-]

5

1 1

L_

•

I

0

I

I

15

"

Changes in mean crystal size T1.5

- τ — ι — ι — r

No seeds a.

1 Q.

j

1F Mp(Total)

0.5h fj£

I

I

I

I

I

I

I

I

L

Time t/r [-]

5

1

15

0

Changes in total production rate -i

1

1

1

5

1

1

1

ι

Time t/r H

ρ-

1

0

1

5

Changes in production rate (L ) M

Fig. 7

Changes in mean crystal size and production rate (No Seeds)


TAKIYAMA ET AL.


Time

Fig. 8

Effect of seed crystals on stability of population density (Monodisperse seed crystals at number based average size)

Time

Fig. 9

t/r[-]

t/r[-]

Effect of seed crystals on stability of population density (Monodisperse seed crystals at mass based average size)


SEPARATION AND PURIFICATION BY CRYSTALLIZATION

Time t/r[-] Fig. 10 Changes in population density (Previous products)


TAKIYAMA ET AL.


1.5

—ι

"A A A A A A

2

A

A

1

1

1

1 1 1 ι r Previous products

J

*

-.0.5 J

ι

ι

i

ι

L

Time t/r [-]

5

1

15

0

Changes in mean crystal size -. 1.5

-ι

1

1

1

1

1

1

1

1

1

1

Γ

1

Previous products

8

bI ο.

·

1

·

·

·

·

·

·

0.5

·

• Mpfjotal)

i Ξ

·

—I

0

I

I

I

Time t/r [-]

5

1

15

0

Changes in total production rate 1.5

-ι

1

1

1

ι

1

1

ι

1

1

•

Ίτ · 0

1

1

r — τ

Γ"

Previous products

Mp(L )

5

M

— I "0

5 Time t/r Η

1

0

L 1

5


Fig. 11 Changes in mean crystal size and production rate (Previous products)


184

SEPARATION AND PURIFICATION

BY

CRYSTALLIZATION

Time t/r[-] Fig. 12 Changes in population density (SU Method 2)


TAKIYAMA ET AL.


1.5

-τ

" I " "

A

1

. . *

.

A

1

1 Α

1

r — i

λ

Α

1 Ί

.

SU Method 2 j A

A

A

"sO.5

5

Time t/r [-]

1

0

1

5

Changes in mean crystal size „ 1.5

—ι

8

I

1

1

1

j

1

1

1

1—

1

SU Method 2

. I .

•·• · ·· · · ··

1 0.5

• Mpfjotal)

5

Time t/r [-]

1

15

0

Changes in total production rate - i — ι — ι — ι — ι — ι — « — ι

SU Method 2

M (L ) P

5

Time t/r H

1

M

15

0


Fig. 13 Changes in mean crystal size and production rate (SU Method 2)


186

SEPARATION A N D PURIFICATION B Y CRYSTALLIZATION

100 80

. . . . . .

ι

ν

ν

CO Q 60 CO

S

ι

I

·

ι

ι

ι

I

• Previous products " (SU Method 1) ν Previous products _ (SU Method 2)

?ο

ο

ι

ο No seeds

ο

40

ο c

I

Ζ '

20

·.··

3

· t

·

8 * *

·

*

Λ

v # v § 8 w v o

I I I 15Τ I . . ? 10 Y . . . y15 γ Time XI τ [-]

Fig. 14 Effect of start-up method on stability of crystal size distribution start-up could be small in the case where previous products were used as the seed crystals. (3) The time of addition did not influence the required time for start-up operation (θ ) under the conditions of SU Method 2 and no seeds. 5

Nomenclature CV dLj L

coefficient of variation sieve aperture width mass average crystal size LmILmoo dimensionless mass average size : number average size Mpj crystal mass on y-th class sieve Mp(u>tai)IMp(totai) «, : dimensionless total production rate Mp(L )/Mp(L ) oo : dimensionless production rate of the class oiL m &-th moment n(L) : function of population density U : time of i-th sampling Wj weighting factor ε performance index φ : volume shape factor 6 settling time needed for the response to reach the steady state τ : mean residence time : :

M

M

M

M

k

f

s" s

Literature Cited 1. Naka, Y.; Takiyama, H.; O'Shima, E. IFAC Workshop, London, 1992 2. Takiyama, H.; Yamauchi, H.; Matsuoka, M. 13th Industrial Crystallization, in press 3. Matsuoka,M.; Inoue, K.; Annual Research Report of Surface & Multiphase Eng. Res. Lab, TUAT, 1984, Vol.2 4. Baker, A.D.; Johnson, T.L.; Kerpelman, D.I.; Sutherland, H.A. Proceeding of the 1987 American Control Conference, 1987 5. Randolph, A.D.; Larson, M.A., Theory of particulate processes 2nd.ed., Academic Press, 1985


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