Department of process engineering U218

Int.CHISA 98

CONTINUOUS DIRECT OHMIC HEATING OF LIQUIDS

Rudolf Zitny, Jiri Sestak, Martin Dostal and Milan Zajicek

Summary:

Ohmic heating used in food processing for, e.g., sausage heating, pasteurisation/sterilisation of wine, milk, fruits, meat is either
BATCH or CONTINUOUS.
The continuous ohmic heating encouters the following problems:

RTD-residence time distribution (usually too broad, nonuniform processing)
Overheating and Fouling of electrodes

New design of a continuous direct ohmic heater with porous electrodes could solve these problems, by improving uniformity of heating.
This arcicle presents:

MATHEMATICAL MODEL OF LAMINAR FLOW IN THE HEATER THROUGH POROUS ELECTRODES

Assuming creeping flow it is possible to derive presure and flowrate profiles in an ANALYTICAL form.

Pressure drop is expressed in terms of dimensionless distance from the inlet

as well as the volumetric flowrate (cross-flow through electrodes)

STREAM FUNCTION AND RESIDENCE TIME DISTRIBUTION FOR A GIVEN PERMEABILITY

Variance of RTD reveals a minimum (optimum from point of view of the heater performance) at a certain permeability of electrodes. However, the arrangement giving smallest variance of the whole system (as can be observed e.g. by stimulus response technique applied to inlet-outlet) is not the best. Actuall optimum (RTD of the heating zone) corresponds to a slightly higher permeability of electrodes.

Numerical modelling of flow inside the heating vessel was performed by finite element method (using program COSMOS/M) and by control volume method, using FLUENT (3D model, approximately 500000 cells).

Results compare rather well with results obtained by visualisation and with measeurement of local velocities using ultrasonic velocity profile detector UVP Dantec.

TEMPERATURE PROFILES
Mean temperature of heated liquid:

This equation describes integral energy balance in a heating zone between electrodes. Using this mean temperature profiles can be calculated. More details can be obtained by numerical solution of two dimensional temperature field at parallel channels (side channel ->, and central channel <-). The following Fig. demonstrates the effect of perforation to the uniformity of temperature field in the heating zone:

STERILISATION EFFECTS - F VALUES
An integral effect of thermal treatment of a liquid food (e.g.milk) can be expressed by so called F-value, taking into account both the residence time and the kinetic of biochemical reaction. F-value is defined by the following equation

The F-value differs according to a selected streamline; the highest is usually at electrodes (long residence time and high temperatures), while insufficient thermal effect is at the center (shortest residence time and lowest temperature). Optimal design of a heater should by characterised by uniform distribution of F-values (the ratio F_max/F_min should be as small as possible. F-values as a function of distance from electrode for three different perforation are shown in Fig.

CONCLUSION:

Fmax/Fmin= 370 (all gaps closed)-the worst
Fmax/Fmin= 324 (4 gaps open)
Fmax/Fmin= 13.5 (all gaps open)-the best

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@TEC: 3. 3.2003 Change language to peoples mailto: Zitny

Department of process engineering U218

Int.CHISA 98

CONTINUOUS DIRECT OHMIC HEATING OF LIQUIDS

Rudolf Zitny, Jiri Sestak, Martin Dostal and Milan Zajicek

Summary:

MATHEMATICAL MODEL OF LAMINAR FLOW IN THE HEATER THROUGH POROUS ELECTRODES

STREAM FUNCTION AND RESIDENCE TIME DISTRIBUTION FOR A GIVEN PERMEABILITY

TEMPERATURE PROFILES

STERILISATION EFFECTS - F VALUES

CONCLUSION: