CG5052 - Agitation and mixing - 2 - 2015.pdf

1

The surface area for cooling to reactor volume ratio decreases upon scale-up

Hazard!!

2

53

SSDNP3

SSDNQ

23

SST DN ST

NV

Q

Torque:

52

2SS

S

q DNN

PT

222 tipspeedDNV

TSS

T

q

Tip speed:

SSSS DNDN

Average impeller

shear:

SN SSDN

Maximum impeller

shear:

Agitated tank hydrodynamics

For turbulent conditions and geometric similarity

3

53

SSDNP3

SSDNQ

23

SST DNS

T

NV

Q

Torque:

52

2SS

S

q DNN

PT

222 tipspeedDNV

TSS

T

q

Tip speed:

SSSS DNDN

Average impeller

shear:

SN SSDN

Maximum impeller

shear:

Agitated tank hydrodynamics

For turbulent conditions and geometric similarity

Basics of fluid flow and shear

conditions scale differently!

4

Scaling-up of heat transfer 5

1,

2,

1,

2,

S

S

T

T

D

D

Scaling-up of blending

2

1,

2,

1,

2,

S

S

T

T

D

D

Scaling-up of liquid-

liquid dispersion 1,2, TT

Scaling-up of solid-

liquid suspending

55,0

1,

2,

1,

2,

S

S

T

T

D

D

5


1,

2,

1,

2,

S

S

T

T

D

D


2

1,

2,

1,

2,

S

S

T

T

D

D




liquid suspending

55,0

1,

2,

1,

2,

S

S

T

T

D

D

Different phenomena scale

differently!

7

Unclear how to scale a single

phenomena!

8

The overall scaling-up problem

chemical reaction

yield selectivity safety

liquid-liquid extraction

yield efficiency

crystallization

purity size and shape polymorph

blending and homogenisation heat transfer to or from wall mass and heat transfer between phases

generation of contact surface area between phases

FLOW SHEAR

1. Agitation creates:

2. Which leads to:

3. On which the process goal

depends :

phenomena that scales

differently often in a complex manner, in

combination and changing

over time

Scaling-up - the overall problem

Difficult to characterise how the process goal depends on what we directly can achieve by the agitation - processes are

complex

Difficult to characterise in detail and scale-up how agitation influences on surface area generation, homogenisation, and

heat and mass transfer - hydrodynamics of agitated tanks are

complex

Process experiments have to be performed and

semi-empirical scaling-up procedures used

10

SCALING-UP PROCEDURE

1) define the process need

2) carefully identify all mixing parameters

that may have an influence on the process

3) select the most critical mixing parameters

4) scale on the most critical parameters

5) carefully review the behavior on each scale

11

Change the agitation rate to examine the role of power input -plot your process result versus N3 in a log-log plot

Use the small scale experiments to try to identify

the role of mixing

12

Use the small scale experiments to try to identify the role of mixing

Change the agitation rate to examine the role of power input -plot your process result versus N3

Change the impeller diameter to examine whether flow or shear is governing

Change impeller blade width to examine whether macroscale or microscale mixing is important

3

SSDNQ

SSDNMaximum shear:

Flow:

Use the small scale experiments to try to identify

the role of mixing

13


1,

2,

1,

2,

S

S

T

T

D

D


2

1,

2,

1,

2,

S

S

T

T

D

D




liquid suspending

55,0

1,

2,

1,

2,

S

S

T

T

D

D

Assumes turbulent conditions and geometric similarity

14

Non-geometric scale-up

Why geometric similarity?

Geometry do not control no mixing in itself!

Geometric similarity occupies one degree of freedom that can be used to control mixing!

Allow different geometry to enable an additional mixing related scale-up criterion

Ex liquid-liquid contacting:

Droplet generation occurs in the impeller region

Droplet coalescence occurs in the bulk region

1,2, TT

2

1,

2,

1,

2,

S

S

T

T

D

D

15

Mixing requirements

Require rapid attainment of uniformity:

fast chemical reactions

competitive chemical reaction

reaction crystallizations

liquid-liquid and gas-liquid processes influenced by mass transfer

Uniformity less important:

Heat transfer

Blending of miscible liquids

Slow chemical reactions

Suspending of solids

More easy performed in small scale and are more difficult to scale up

Can usually be scaled with few difficulties

16

Some final remarks

It is impossible to have all conditions equal in different scales Preoccupation with scale-up rules truncate thinking Geometric similarity do not capture an important mixing

property

Use intelligent experiments to identify what aspects of the mixing that is governing your process goal

Be aware of that the governing mechanism can depend on the scale

A reactor in the pilot plant has a volume of 1000 litres and a diameter of

1 m. It can be equipped with a propeller with a diameter of 0.3 m, or a

45o pitched blade turbine with a diameter of 0.5 m. It has been decided

to purchase reactors to the process lab to simulate 1000 litre in a

smaller scale.

Determine the dimensions of the reactors if the volumes are 20 or 50

litres, respectively, and the reactors are geometrically equivalent with

the pilot plant reactor.

A 1000 L baffled tank has the diameter 1 m and can be equipped with a

Rushton turbine, a propeller or a 45 pitched blade turbine. Determine the flow condition, power input and pumping capacity for 50, 100, 200 and

400 rpm and agitation diameters: 0,33 and 0,45.

Data:

Density 1000 kg/m3

Viscosity: 0.001 Ns/m2

Rushton turbine Ne=5.2 Nq=0.72

Propeller Ne=0.5 Nq=0.8

45 pitched blade turbine Ne=1.3 Nq=0.79

2Re SS

DN 53

SS

pDN

PN

3

SS

qDN

QN

Re P Q

You have done experiments with water in laboratory experiments and

are now considering what will happen with the power consumption

when you the same experiments with the process liquor:

Water Process liquor

=1000 kg/m3 =800 kg/m3

0.001 Ns/m2 0.005 Ns/m2

How will the power consumption in the process liquor compare

to that in the aqueous phase at

a.equal agitation rate and turbulent conditions?

b.equal Reynolds number and turbulent conditions

1

A catalytic powder is charged into a solution in an agitated baffled tank.

In 1000 litre scale this process is carried out with rotation speed 100 rpm

and stirrer diameter 0.3 m. To study the process you would like to scale

down to 6 litre scale. Determine appropriate stirrer speed and impeller

diameter in the 6 litre scale. Assume geometric similarity and that the

liquid is water.

22

1) Criterion: keep particles suspended from the bottom:

2) Suspending charactersitic: 85.0 SJS DN

3/1VDS mD labS 055.0,

85.0

arg,

,

arg,

,

elS

labS

elJS

labJS

D

D

N

N rpmN labS 425,

3/1

argarg,

,

el

lab

elS

labS

V

V

D

D

85.0 SJS DN

2

A catalytic powder is charged into a solution in an agitated baffled

tank. In 1000 litre scale this process is carried out with rotation

speed 100 rpm and stirrer diameter 0.3 m. To study the process

you would like to scale down to 6 litre scale. Determine

appropriate stirrer speed and impeller diameter in the 6 litre scale.

Assume geometric similarity and that the liquid is water.


2) Suspending characteristic: 85.0 SJS DN

mD labS 055.0,

rpmN labS 425,

Turbulent?

2Re SS

DN

150000Re arg el

21100Re lab

Turbulent?

2

A catalytic powder is charged into a solution in an agitated baffled

tank. In 1000 litre scale this process is carried out with rotation speed

100 rpm and stirrer diameter 0.3 m. To study the process you would

like to scale down to 6 litre scale. Determine appropriate stirrer speed

and impeller diameter in the 6 litre scale. Assume geometric similarity

and that the liquid is water.

Determine also the specific power input required assuming that the

power number is 4

24


2) Suspending charactersitic: 85.0 SJS DN

3) Geometric similarity and turbulent conditions: 23

ssT DN

55.0

arg,

,

arg,

,

elS

labS

elT

labT

D

D

53

ssp DNNP

V

DNN sspT

53

3/45arg, mWelT

3/115, mWlabT

2

A solid-liquid and liquid-liquid reaction was scaled-up to 3500 gal. The

scaling-up was performed based on the just suspended state of the

solid particles. However, upon analyzing the yield of the full scale

production plant, it soon became clear that the productivity had

decreased, giving only about 10% of the expected productivity. What

had gone wrong?

3

26


1,

2,

1,

2,

S

S

T

T

D

D


2

1,

2,

1,

2,

S

S

T

T

D

D




liquid suspending

55,0

1,

2,

1,

2,

S

S

T

T

D

D

3

A solid-liquid and liquid-liquid reaction was scaled-up to 3500 gal. The

scaling-up was performed based on the just suspended state of the

solid particles. However, upon analyzing the yield of the full scale

production plant, it soon became clear that the productivity had

decreased, giving only about 10% of the expected productivity. What

had gone wrong?

Would need more agitation to generate the same droplet

surface area per unit volume

There is more coalescence in the larger tank because of

longer circulation times

The liquid-liquid contacting forgotten

3

28

You have investigated a chemical reaction by labexperiements. In the

process two liquid solutions are mixed in the presence of a solid

catalyst. The catalyst particles have a diameter of 2 mm and a density

of 1900 kg/m3. The liquid phase has a density of 900 kg/m3 and the

viscosity is 0.002 Ns/m2. The experiments have been performed in a 1

L tank with four baffles and 0.11 m diameter, agitated by a pitched blade

turbine having a diameter 40 % of the tank diameter. In the experiments

it has been observed that the yield increases with increasing agitation

rate upto 700 rpm beyond which there is no improvement.

Measurements show that the reaction has gone to completion within 2

hours and that the temperature increases at most by 10 . The process is now to be transferred to the plant and be performed in a 4 m3 tank.

What should be the agitation in the plant and are there any particular scaleup issues to be aware of

4

29

catalyst particles: 2 mm and 1900 kg/m3.

liquid density: 900 kg/m3.

Viscosity: 0.002 Ns/m2

1 L tank, four baffles, pitched blade turbine, diameter 40 % of the

tank diameter.

700 rpm is the optimum

reaction time: 2 hours

temperature increases at most by 10 Plant: 4 m3 tank.

Scaling-up is normally based on geometric similarity do we have that?

Do we have turbulent conditions? The chemical reaction is obviously influenced by the agitation Liquid-liquid blending Solids suspending Evolution of heat

4

30

catalyst particles: 2 mm and 1900 kg/m3.

liquid density: 900 kg/m3.

1 L tank, four baffles, pitched blade turbine, diameter 1/3 of the tank

diameter.

600 rpm is the optimum

reaction time: 2 hours

temperature increases at most by 10 Plant: 4 m3 tank.

Scaling-up is normally based on geometric similarity do we have that?

Do we have turbulent conditions The chemical reaction is obviously influence by the agitation Liquid-liquid blending Solids suspending Evolution of heat 10 is not insignificant since the plant will have 15 times less

area/unit volume Normally the agitation rate cannot be increased sufficiently to

compensate Longer time is required for the process

4

31


1,

2,

1,

2,

S

S

T

T

D

D


2

1,

2,

1,

2,

S

S

T

T

D

D




liquid suspending

55,0

1,

2,

1,

2,

S

S

T

T

D

D

4


1,

2,

1,

2,

S

S

T

T

D

D


2

1,

2,

1,

2,

S

S

T

T

D

D




liquid suspending

55,0

1,

2,

1,

2,

S

S

T

T

D

D

3/2

1

2

V

V

3/5

1

2

V

V

3/55.0

1

2

V

V

23

SST DN 1,2, SS NN

SS DN

85.0 SJS DN

3/2 SS DN

4

Documents

CG5052 - Agitation and mixing - 2 - 2015.pdf