Tyre Asia – May-June 2018

94 Tyre Asia April/May 2018

MIXInG

In this second and concluding part of the article on tandem process, the

focus is on machinery optimisation with regard to dispersion, distribution,

temperature control and process consistency

Tandem process:

Advantages

all the way

By Prof. Dr. Andreas Limper,

Dr. Harald Keuter, HF Mixing Group

Cooling properties and

equilibrium temperature

As the overall importance of good

cooling increases, the difference between

the tangential and intermeshing rotor

systems should again be clarified through

picture 4.

Image 4 shows the mixer empty volume

as a function of the temperature-

controlled surface for tangential and

intermeshing machines. Using the

example of 250-litre mixer it is clear that

the available cooling surface is 20 per

cent higher for the intermesh compared

to the tangential mixer. In principle, the

gap widens in larger machines, i.e. the

difference is greater in favour of the

intermeshing mixer.

If we consider the difference between

the cooling surfaces of a size IM320e

(upper mixer) and IM550eT (lower

mixer) tandem line, there is another side

effect to be considered. The IM320e

has a cooling surface of 8.5 m2 and the

IM550eT 11.3 m2. Based on a batch

weight of 250 kg, we have a specific

cooling surface of 340 cm2/kg for the

IM320e, and 450 cm2/kg in the IM550eT,

which corresponds to an increase of more

than 30 per cent in cooling capacity.

Therefore in the larger tandem lower

mixer the compound “sees” a larger

cooling surface. This results in lower

equilibrium temperatures in the tandem

lower mixer than in an “optimally” filled

machine.

Image 5 schematically shows the

equilibrium temperatures as a function of

the rotational speed for an internal mixer

and a corresponding tandem lower mixer

without a ram. Due to the difference

in the size of the two machines, lower

equilibrium temperatures should be

set in the tandem lower mixer over the

entire speed range. This means that

the rotor speed can be set higher in the

tandem lower mixer at the same target

temperature than in a standard internal

mixer. This enhances the distribution

performance due to greater material

movement. The lower mixer takes over

the distribution.

As explained above the lower mixer

has excellent cooling properties and so

chemical reactions such as silanisation

for example can also take place here due

to the intermeshing rotor geometry and

the size of the lower mixer.

The missing ram has a beneficial effect in

the tandem lower mixer as the resulting

reaction products are easily extracted

from the open system.

It can be estimated that a rotor speed of

up to 20 per cent higher can be selected,

depending on the mixture, when the

IM320e and IM550eT machine sizes are

compared.

Process division in dispersion

and distribution

The original idea of being able to produce

as many as possible, or ideally all

compounds in one step has not yet been

fully realized. however, there certainly

is a trend in this direction. The tandem

process opens up the possibility to

divide the process into the sub-steps of

distribution and dispersion.

In this case, due to the existing ram, the

upper mixer is responsible for dispersion.

The upper mixer must draw the recipe

ingredients into the mixing chamber and

complete the dispersive mixing process

before the compound can be conveyed to

the lower mixer.

The process division can be clearly

represented by the preparation of a

reactive base compound. Image 6 shows

Image 4: Empty volume as function of

temperature-controlled surface for tangential and

intermeshing mixers

Image 5: Equilibrium temperatures as function

of rotational speed for an internal mixer and

corresponding tandem lower mixer without ram.