SEPARATION BY THERMAL DIFFUSION

BY-VIJAY BUNKAR

THERMAL SEPARATION PROCESSES

THERMAL SEPARATION PROCESSES ARE MASS TRANSFER OPERATIONS DRIVEN BY MOLECULER FORCES.

MASS AND OFTEN HEAT IS EXCHANGED BETWEEN ATLEAST TWO PHASES IN CONTACT.

THE PHASES ARE MIXTURE PHASES AND AUXILLARY PHASE. AUXILLARY PHASE IS GENERATED BY EITHER ADDING HEAT OR

BY MEANS OF AUXILLARY SUBSTANCES. REQUIRED DRIVING SUCH AS CONCENTRATION AND

TEMPERATURE GRADIENT ARE FORMED BY AUXILLARY SUBSTANCE.

DIVING FORCES CONCENTRATION AND TEMPERATURE GRADIENT

NATURE OF MIXTURE – HOMOGENEOUS.THERMAL DIFFUSION -GAS –GAS AND LIQUID LIQUID

OPERATION.PHASES IN CONTACT ARE MISCIBLE.

SEPERATIONS STEPS

AN ADDITIONAL PHASE IS GENERATED BY SUPPLYING ENERGY TO THE SYSTEM, OR BY ADDING A AUXILLARY COMPONENTS.

MASS AND OFTEN HEAT IS EXCHANGED BETWEEN PHASE.THIS IS ACHIVED BY ADDITION AND REMOVAL OF ENERGY.

AFTER COMPLETION OF THE INTERCHANGE PROCESS,THE PHASES ARE SEPERATED.TOGETHER WITH THE SEPERATION OF THE PHASES A PARTIAL SEPERATION OF MIXTURE OCCURS.

THERMAL DIFFUSION ?

DIFFUSION CAUSED BY NONUNIFORMITY OF TEMPERATURE OF GAS MIXTURE.

RESULTS IN NON ZERO VELOCITY OF SPECIES.

PRODUCE A CONCENTRATION GRADIENT INSIDE THE GAS MIXTURE WHICH COULD BE UTILIZED FOR GAS SEPRATION

THE TWO GASES WILL SEPRATE UNTILL A CONCENTRATION GRADIENT OCCURS RESULTING IN CONCENTRATION DIFFUSION OF EQUAL MAGNITUDE IN THE OPPOSITE DIRECTION.

UNDER EQUILLIBRIUM CONDITIONS THE RATE OF TRANSFER OF THE LIGHT MOLECULES TOWARDS THE HOT SURFACE RESULTIG FROM THERMAL DIFFUSION WILL BE EXACTLY COUTERBALANCED BY CONC. DIFFUSION.

HEAVY MOLECULES WHICH TEND TOWARD THE COLD SURFACE,ARE SWEPT TO THE BOTTOM OF THE COLOUM WHILE THE LIGHT MOLECULES AT THE HOT SURFACES ARE SWEPT TOWARDS THE TOP OF COLOUMN.

THE MAXIMUM SEPERATION IS LIMITED BY CONC. DIFFUSION SET UP IN AXIAL DIRECTION AS A RESULT OF SEPERATION ITSELF.

DISADVANTAGES

IRREVERSIBLE PROCESS REQUIRE LARGE QUANTITY OF ENERGY PER GRAM

OF MATERIAL SEPERATED. THERMAL DIFFUSION COLOUMN CANNOT BE

SCALED UP. GAP BETWEEN HOT AND COLD WALLS CANNOT BE

INCREASED. LIMITED TO MICRO AND NANO SCALE

APPLICATIONS.

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