Reboiler Circuits

Definition & Types of reboilers –

A. Thermosiphon – Flow through thermo siphon circuit is sustained by difference in density of leg of liquid flowing to the reboiler & leg of mixed phase material flowing to tower from reboiler.There are three types of thermo siphon reboiler circuits.

1. Once through This type of arrangement in which liquid from bottom tray of fractionator flows

towards reboiler & mixed phase leaving reboiler consists of liquid phase which is nothing but bottom product & vapor passes through bottom tray.

Reboiler feed rate is set by tower heat balance & bottom product rate.

2. Circulating This type of circuit is an arrangement in which liquid from bottom tray flows to

surge section at bottom of tower. Part of this liquid taken off as bottom product & remaining flows to reboiler.

Liquid circulating at rate higher or lower than sum of rate of vapor flowing to bottom tray & bottom product.

3. Preferential This type of system is similar to circulating except that by means of suitable

baffling in bottom of tower, degree of segregation between bottom product & reboiler feed is attained.

It is usual practice to design the reboiler circuits at rate equal to or greater than sum of rate of vapor to bottom tray of tower & bottom product.

B. Kettle Type It is special case of once through reboiler. Separation between bottoms product & vapor is effected in reboiler itself rather

than in tower.C. Pumped

Flow through pumped circuit is maintained by head of pump drawing reboiler feed from tower.

SELECTION OF TYPE OF CIRCUIT

A. Thermo siphon Vs Pumped Circuits Pumped circuits are only necessary under following conditions – 1. When utilizing fire heaters.2. When utilizing an exchanger reboiler in preferential or circulating systems

To limit temp. rise for materials which may decompose or polymerize. To attain higher velocities in reboiler tubes to minimize deposition of suspended

solids. To prevent high percentage of vaporization to limit deposition of salts on tubes.

Where high heat flux is possible due to high LMTD

B. Once-through Thermo siphon circuits It has three principle advantages over other circuits –

It approaches theoretical stage in distillation. Greater temp. Spread between bubble point of bottom product & bottom tray

liquid, greater LMTD less surface area required. Residence time is relatively low.

Disadvantages of once-through circuit –

Hydraulics is very sensitive to variations in through put rate or % vaporization. Once through reboiler should not be used if tower bottom product rate should

be vary over wide range Also not used if wt % of vaporization of reboiler feed may expected to increase

above 70% It is clearer that at high % vaporization problem of vapor binding is more acute hence once through reboiler is used for range 30 – 50% of vaporization.Only pertinent reason for selecting once through reboiler is improvement in MTD.

C. Circulating Thermo siphon circuits Advantages over other circuits –

There are no tower internals for this system, while careful design of tower internals required for once-through reboiler system.

Overcapacity of circulation may be selected independently of bottom product feed rate & tower heat balance.

Disadvantages of circulation circuit –

Feed temp. is bubble point of bottom product and reboiler outlet temp. is still higher as it is function of % vaporization and flash curve of bottom product.If bottom tray temp. is close to bottoms temp. this disadvantage is reduced.Hence, for given heating medium temp more area is required than for once through reboiler.