Continuous distillation columns - Characteristics at different operating conditions

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Continuous Distillation Columns

Characteristics at different operating conditions

Group Members

• Group #7

Hafiz Annus Mahmood SP10-BEC-015

Mohammad Abubakar SP10-BEC-022

Noaman Ahmed SP10-BEC-037

Saim Khan SP10-BEC-044

What is Reflux ?

• In simple terms, it is the stream returning to the distillation column after being condensed from the condenser and after being re-boiled from the re-boiler

• It is employed to increase purity of required product

Total Reflux

• A condition at which no distillate and no bottom product leaves the distillate column

• The vapors leaving the column are condensed and the liquid leaving the column is vaporized and re-circulated back to the column

• There is sufficient vapor and liquid content for steady state operation

Material Balance on a Distillation Column

• General material balance on a distillation column

F = D + B– Where, F, D, and B are either mass or mass flow

rate of feed, distillate, and bottom product, respectively

Operating conditions based on Total Reflux

• Four types of ‘Total Reflux’ conditions exist

1. Type 1: Total Reflux in Continuous Columns: , F = D + B, when D and B are finite

2. Type 2: Total Reflux in both column sections:D = 0, B = 0, F = 0 (the conventional total reflux)

Operating conditions based on Total Reflux (contd.)

• Type 3: Total Reflux in Rectifying SectionD = 0, B = F

• Type 4: Total Reflux in Stripping SectionD = F, B = 0

Type 1

• Total reflux is achieved by approaching total reflux conditions in both rectifying and stripping sections

• Operating condition is reached when reflux ratio is increased indefinitely at fixed feed rate and non-zero product rates

Type 1 (contd.): The Θ method of convergence

• Following specifications are made on the column1. Feed rate and its composition (Xi)

2. Distillate rate, D3. Column Pressure4. Number of Stages, N5. Type of condenser (partial or total)

Type 1 (contd.): The Θ method of convergence

• Reflux ratio, Lj / Vj+1 = 1 (as Vj+1 approaches infinity)

• No need for energy balance on each stage (no latent heat and sensible heat calculations required to bring feed to boiling temperature)

Type 1 (contd.): The Θ method of convergence

• Supposition: Both liquid and vapor phases form Ideal Solutions throughout the column

• Method proposed by Fenske

The Θ method of convergence for Type 1

(Mathematics and Example)

34.68

The Newton Raphson Method for Type 1(Example)

What’s the difference then ?

• The Θ method of convergence is comparatively faster than Newton-Raphson method

• The calculated D values using the Θ method have more accurate values with increasing number of iterations compared to Newton-Raphson method

Operating Characteristics at wide ranges of distillate and reflux rates

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