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Distillation & Controls blog content

Introduction
Where distillation is selected?
Distillation types
Flooding Point 
Entrainment
Foaming
Weeping
Tray stability
Turndown Ratio
Column Diameter
Factors to consider when selecting high performance trays. 
Flow Profile within the Column
Packings

 

Manufacturing steps in Chemical Process Industries are

  • Raw  Material   / Feed stock  preparation
  • Reactions
  • Separation 
  • Purification 
  • Product storage and dispatch 
  • Distillation technique : for Separation and Purification 
  • Main drivers : Relative Volatility, Boiling Points and vapor Pressure
  • Controls required to  :
  • Meet  Purity / Product  Specifications 
  • Minimize loss of valuable products 
  • Reduce  /Eliminate Quality give-away
  • Energy Conservation 

Introduction

Definition : Distillation is a procedure in which vaporization is followed by condensation to separate out phases of fluid mixture to separate out phases of fluid mixture with help of thermal energy.

Distillation is a method of separating mixtures based on differences in volatility of components in a boiling liquid mixture. 

Distillation is a unit operation, or a physical separation process, and not a chemical reaction. 

Chemical process in which solvent is required or side product which is recoverable. This solvent is recovered with Distillation unit operation.

Applied to the cases where all components are present in both phases.

Instead of introducing a new substance into the mixture in order to provide a second phase as done in Gas absorption or solvent extraction. The new phase is created from original solution by vaporization and condensation.

Where distillation is selected?

Mixtures where Specific gravity of components is close and not separate through sedimentary separation

Distillation types

  • Flash Distillation
  • Batch Distillation
  • Continuous Distillation 
  • Steam Distillation
  • Extractive Distillation
  • Azeotropic Distillation

Read in details Click here

It is also categorized by

Types based on nature of feed
Types based on Products 
Based on column Internals 


Distillation general diagram



Types based on nature of feed

Binary Distillation : feed contains only two components 

Multicomponent Distillation: Feed contains more than two components (Cracker plant quench tower )

 

Types based on Products 

Multiple product Column has more than two products (Crude column)

Based on use of additional component to help separation 

Extractive Distillation : 

Extra feed appears in bottom product stream ( Xylene separation Aromatics, Butanes/ butenes  separation in Butadiene recovery ) 

Azeortopic Distillation : 

Extra feed appears in top product stream  ( water methanol separation is TAME unit )

Based on column Internals 

Tray column 

Trays of various designs are used to hold the liquid and provide better contact between vapor and liquid and hence better separation 


Packed column

Instead if trays packings are used to enhance contact between vapor and liquid.

Selection of Contacting Device

Contacting device selection is based on Tray / Packing type. Also we should check the 
  • Vapor handling capacity
  • Liquid handling capacity
  • Mass transfer efficiency
  • Flexibility for wide range of operation
  • Pressure drop
  • Cost

Distillation general procedure

Generally distillation is carried out in following manner

1. Feed mixture which may be binary or multi component. Mixture temperature is decided depend on "It should be in liquid phase" to avoid vapour lock and hammering. Temperature may be cooled or pre heated

2. Start heating and vaporizing and establish temperature gradient through reflux. How to do this explained in this section. 
3. Maintain temperature gradient. Heated vapours going up side and cooled reflux travels bottom side. Temperature maintain through reboiler and reflux rate control. If variation occurred it impact on product purity.  

4. Recover product from top, bottom and intermediate sections. High volatile products recovers from top section and less volatile from bottom.

Flooding Point 

          Flooding is brought about by excessive vapor flow, causing liquid to be entrained in the vapor up the column. 

The increased pressure from excessive vapor also backs up the liquid in the downcomer, causing an increase in liquid holdup on the plate above.  

Depending on the degree of flooding, the maximum capacity of the column may be severely reduced. Flooding is detected by sharp increases in column differential pressure and significant decrease in separation efficiency.

 

Entrainment

         Entrainment refers to the liquid carried by vapor up to the tray above and is again caused by high vapor flow rates. 

It is detrimental because tray efficiency is reduced. lower volatile material is carried to a plate holding liquid of higher volatility.

It could also contaminate high purity distillate. Excessive entrainment can lead to flooding.

Foaming

Foaming refers to the expansion of liquid due to passage of vapour or gas. 

Although it provides high interfacial liquid-vapor contact, excessive foaming often leads to liquid buildup on trays. In some cases, foaming may be so bad that the foam mixes with liquid on the tray above. 

Whether foaming will occur depends primarily on physical properties of the liquid mixtures, but is sometimes due to tray designs and condition. Whatever the cause, separation efficiency is always reduced.

Weeping

This phenomenon is caused by low vapor flow. 

The pressure exerted by the vapor is insufficient to hold up the liquid on the tray. Therefore, liquid starts to leak through perforations. Excessive weeping will lead to dumping. 

That is the liquid on all trays will crash (dump) through to the base of the column (via a domino effect) and the column will have to be re-started. Weeping is indicated by a sharp pres. drop in the col.  and reduced separation efficiency.

 

Tray Pressure drop 

For normal operation pressure drop per tray
Pressure 2 ~ 4 inch water
Vacuum  2 ~ 4 mm Hg

Tray stability

A tray is stable when it can operate with acceptable efficiencies under condition that fluctuate, pulse or surge.

Turndown Ratio

Ratio of max. allowable vapor rate at or near flooding condition to the min. vapor rate when weeping or liquid leakage becomes significant.

Column Diameter

Determined by amount going through the column.

Factors to consider when selecting high performance trays. 

  • Capacity & Hydraulic limitations
  • Pressure Drop
  • Efficiency
  • Operating range
  • Resistance to Fouling
  • Existing column Configuration
  • Equipment Cost / Installation Cost.
A tray can be divided into two parts 
  1. Area occupied by downcomers and 
  2. Area occupied by valves.
  3. Free area – The total available area above the two phase froth where liquid droplets are separated from up going vapor
  4. Hole area (Escape area) – The sum of area of all openings (holes) on the active area. It is often expressed as percentage of active area.
Tray

Flow Profile within the Column

Flow Profile within the Column

Above figures show the direction of vapor and liquid flow across a tray, and across a column. Each tray has 2 conduits, one on each side, called ‘downcomers’. 

Liquid falls through the downcomers by gravity from one tray to the one below it. A ‘weir’ on the tray ensures that there is always some liquid (holdup) on the tray and is designed such that the holdup is at a suitable height, e.g. such that the bubble caps are covered by liquid.

General Characteristics of Tray 

Trays are usually made of sheet metals of special alloys. 

Thickness is governed by corrosion rate. 

Trays must be stiffened & supported & fastened to the shell to prevent movement owing to surge of gas, with allowance for thermal expansion. 

Large trays must be fitted with man – ways.

Tray spacing should be such that insurance against flooding & excessive entrainment where tower height is important consideration. 

For all except smallest dia tower, 20 inch spacing is considered. For small dia tower spacing of 6 inch is considered.

 

Packings

Random Packing

1st generation – Rasching ring, saddles
2nd generation – Pall ring
3rd generation – Proprietary packing

Features

More capacity or more efficiency than trays. Low delta P than trays, Excellent revamp tool. Easy installation, Sensitive to vapor liquid maldistribution. Price higher than tray.

Structured Packing

Proprietary – Gempack (Glitch), Flexipack (Koch), Mallapack (Sulzer), Intalox (Norton)

Features 

More capacity & more efficiency than trays. Very low pressure drop. Sensitive to vapor liquid maldistribution. Unreliable for high pressure distillation. High price.Mechanical check point
  • Associates Pumps
  • Fin fans & condenser
  • Safety Valves
  • Pipe supports
  • Expansion of overhead pipe line
  • Ladders & Platform
  • Corrosion & Vibration
  • During turnaround – Tray inspection, traying & de-traying 
Instrument/Elec. check point
  • Instrument mounting
  • Control valves
  • Level & Pressure controls
  • Interlock 
  • Analyzers
  • Weather proofing of instruments
  •  Variable speed drive
  • Earthing & bonding
  • Lighting & aviation lighting 

Trouble shooting

  • High differential pressure 
  • Loss of reboiler / Reflux
  • Flooding
  • Foaming
  • Tray/ Packing displacement
  • Tray and Reboiler fouling/coking
  • Overhead fouling  

Read more about "Distillation" from Industrial Guide - Click here

 

Thanks for reading -
Naitik Patel
Industrial Guide

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