Distillation

oPtiMiZation FoR MaXiMisation PResenteD by : nabanita Deka lPG DePaRtMent

oil inDia liMiteD DateD-04.03.2011

Basics of mass transfer

Mass transfer : Transfer of material from one homogeneous phase to another. The driving force for mass transfer is concentration difference or difference in activity coefficient.

Mass transfer operation involves changes in

composition of solution & mixtures. Transfer of substance through another on a molecular scale.

Mass transfer coefficient : Rate of mass transfer per unit area per unit conc. difference. It depends on diffusivity,viscosity,density,velocity

& linear dimension D. Why Mass transfer operation: Any chemical process requires : 1. Purification of raw material. 2. Separation of product from byproduct

Types of Distillation

Distillation Types 1. Flash distillation. 2. Batch Distillation. 3. Steam Distillation. 4. Azeotropic & Extractive Distillation Flash Distillation : Vaporising a definite fraction of

the liquid in such a way that the evolved vapor is in equilibrium with residual Liquid.

Batch Distillation : Used for Feed composition may change from batch to

batch.Negligible hold up in the column & condenser relative to that in receiver & kettle.

Types of Distillation

Steam Distillation : 1. Possible to distill an organic compound at much lower

temp. 2. At constant system pr.PT ,steam lowers the partial &

vapor pressure of organic compound & its corresponding boiling pt. Due to immiscibility of water ,it can be separated from

product by simple condensation & followed by decanting.

Application : Purification of heat sensitive material as an alternative to vacuum distillation.

Azeotropic & Extractive Distillation. Very close boiling mix can be separated economically by

this technique.

Solvent for Distillation

Solvent when added will increase the difference between

volatilities of light & heavy component. The attraction of solvent to one of the component reduces the volatility of solvent & the component to which it is attracted.

Solvent for Distillation should be Non-corrosive. Should not react with feed to form undesirable product. Non-toxic. Azeotropic solvent should have volatility near the major

component desired in overhead product & in Extractive distillation its volatility should be lower than major component to be withdrawn at bottom.

Points of discussion

Distillation is the dominant process for separating large multi-component streams into high purity products.

Through today’s presentation we would try to understand the following optimization goals for sieve trayed columns:

1. Maximizing theoretical stages per height of section or

column. 2. Minimizing pressure drop per theoretical stage. 3. Maximizing the operational range, turn down or turn

up.

Trayed columns

In a trayed column liquid flows down the column through the dowcomers and then across the tray deck, while vapour flow upward through the liquid inventory on the tray.

Types of trays based on the flow pattern: 1. Cross flow (most common & least expensive) 2. Parallel flow (if designed properly would give 10%

higher efficiency) As column diameter increases, the ratio of weir length

to throughput decreases. So, for larger dia columns multiple pass (no. of downcomer per tray) trays are used to increase the weir length.

Factors affecting tray efficiency

Flow pattern: As column diameter increases, the ratio of weir

length to throughput decreases. So, for larger dia columns multiple pass (no. of downcomer per tray) trays are used to increase the weir length.

Tray deck type: Sieve trays are the most commonly used whereas

bubble cap trays are used less frequently.

Factors affecting tray efficiency

Pressure drop Throughput (weeping and flooding)

Steps towards higher efficiency

To maximize the number of theoretical stagesfor a given section height (For sieve trays)

1. Keep the fraction open area low, in the range of 5% 2. Use the smallest practical perforation diameter. 3. Consider parallel flow trays To minimize the pressure drop per theoretical stage

(For sieve trays) 1. Use the largest fraction open area that will not result in

weeping 2. Use the smallest practical perforation diameter. 3. Specify a low outlet weir (<50mm) 4. Consider parallel flow trays

Steps towards higher efficiency

To maximize the operating range (For sieve trays): 1. Lower open area 2. Use the smallest practical perforation diameter. 3. Maximize the tray spacing 4. Decreasing the liquid inventory by increasing the

number of tray passes or lowering the weir height.

Thank you