Food Process Design I an der Universität Hohenheim

Stefan-flux is a compensating volume flow. The difference in the volume flux from one side to the other is leading to a pressure difference which leads to a flow which is interfering with the diffusion process.

It can occur through:

Single sided diffusion:

One sided exchange of one component through a semi-permeable wall or away from a source (e.g. dry ice sublimating, water evaporating)

Non equimolar diffusion:

Exchange of any non equal amount (volume) of substances in inverse directions of two components (e.g. coke made out of coal)

CSTR (continuous stirred tank reactor)

not sure

When the fluid enters the tube, the flow profile is a "block" (no difference between wall and centre velocity). Therefore the transfer coefficient is higher (?) at that point (beginning of tube). Later on, the flow profile changes to a parabolic (curve like) shape with different velocities at the wall and the centre. This then leads to a different coefficient than from the beginning.

Does that sound right?

The shift is to a higher aw value with the same product moisture (with higher temperature). This is because warmer air can hold more water than cold air, due to a higher binding energy.

1) Freezing --> freezing rate influences crystal size

2) Drawing vacuum

3) Apply heat for drying by heating plates (--> sublimation ice)

4) Discharge

Vacuum pump doesn't need to be running during the entire process, only when pressure is getting higher again (but just to be sure: let it run all the time)

0.63 in general

(0.7 molds, 0.75 yeasts, 0.8 bacteria)

For most food powders a aw value of 0.3 is desired, because here all the reactions are at the lowest point

separation of the variables; first order system; initial and boundary conditions

Convection:

-tray/ cabinet dryer

-fluid bed dryer

-spray dryer

-tunnle dryer

Contact/conduction:

-drum dryer

-freeze dryer

-mixer dryer

Radiation:

-freeze dryer

CSTR (continuous stirred tank reactor)

complex structure (muliphasees, gels, ...)

- intermolecular interactions

- variation of the chemical composition

- parallel chemical reactions

-> matrices can change during storage and processing

=> not ideal condition => instative processes ?

target: high heat transfer, low pressure drops? -> depends on the geometry

Corrugated structure leads to a resistance to flow (with high friction loss) due to the narrow path between the plates. Therefore, there is a large pressure capacity and the flow becomes turbulent along the path. This requires more pumping power than the other types of heat exchangers. A higher pressure leads to a higher heat transfer, both depend on a the corrugated angle

pros: homogeneous flow profil (turbulent flow), higher heat transfer, lower pressure drop?

cons: high friction loss, requires high pumping power