Feed conditions in distillation column with respect to feed plate and reflux

Feed conditions in distillation column
with respect to feed plate and reflux
1
By:
IHSAN WASSAN
U.S - Pakistan Center for Advanced Studies in Water
Mehran University of Engineering and Technology, Jamshoro
Simultaneous Heat & Mass Transfer

• DISTILLATION – A BRIEF INTRODUCTION
• MAIN COMPONENTS OF DISTILLATION COLUM
• FEED CONDITIONS
• FEED PLATES
• REFLUX
• TYPES OF REFLUX
CONTENTS
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Distillation- a brief Introduction
• A process that separates two or more components into an overhead distillate and
bottoms.
• The bottoms product is almost exclusively liquid, while the distillate may be liquid
or a vapor or both.
• The separation process requires three things.
• First, a second phase must be formed so that both liquid and vapor phases are
present and can contact each other on each stage within a separation column.
• Secondly, the components have different volatilities so that they will partition
between the two phases to different extent.
• Lastly, the two phases can be separated by gravity or other mechanical means
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• The distillation column
contains:
• one feed stream and two
product streams
• The column is broken in
two sections.
• The top section is referred
to as the rectifying section.
• The bottom section is
known as the stripping
section.
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Distillation Column - main components

The feed contains a mole percent of
the light component, Zf.
The product stream exiting the top
has a composition of XD of the light
component.
The product stream leaving the
bottom contains a composition of XB
of the light component.
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• The top product stream passes through a total condenser.
• This effectively condenses all of the vapor distillate to liquid.
• The bottom product stream uses a partial reboiler. This allows for the
input of energy into our column.
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• Knowing the relative volatility for a system is also useful in
determining the amount of separation possible.
• A relative volatility of 1 indicates that both components are equally
volatile and no separation takes place, and when the relative volatility
is low, ( a < 1.05), separation becomes difficult and expansive because
a large number of trays are required
• The higher the relative volatility, or the lower the pressure, the more
separable are the two components
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Relative volatility

• The condition of the feed stream F determines the relation
between the flow rates in the stripping and enriching
sections.
• Let q be the ratio of the moles of saturated liquid in the
feed to the total amount of feed then
• The magnitude of q is related to the thermal condition of
the feed. Making enthalpy and material balance around
the feed plate in Figure
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Feed conditions
Fig. Relationships between flows
above and below the feed entrance..
Effect of feed conditions

• Assuming
• Substituting in above equation,
we get
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Feed conditions
Effect of feed conditions (continue ….)

Solving for q , gives
Where
• HV = specific enthalpy of the feed at the dew point (saturated vapor)
• HF = specific enthalpy of the feed at its entrance conditions
• hL = specific enthalpy of the feed at the bubble point (saturated liquid)
• HV - HF = heat needed to vaporize 1 mole of feed at its entrance conditions
• HV - hL = molar latent heat of vaporization of feed
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Feed conditions

• The feed-line or q-line equation, which is the locus of the intersection of the enriching and stripping
operating-line equations, can be derived as follow:
• At the intersection of the two operating lines
Subtracting Eq. (4.4-18) from (4.4-19) we obtain
After substituting and
rearranging we get :
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• Feed is a saturated liquid:
HF = hL ⇒q = 1, q-line is vertical
• Feed is a saturated vapor:
HF = HV ⇒q = 0, q-line is horizontal
• Feed is a mixture of saturated vapor and liquid:
hL < HF < HV ⇒ 0 < q < 1
• Feed is a superheated vapor:
HF > HV ⇒q < 0
• Feed is a sub-cooled liquid:
HF < hL ⇒q > 1
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Feed conditions
Fig. Slope of the q-line for various feed conditions.
There are five possible feed conditions listed below and illustrated in Figure.

Feed Plates
• Feed plates are fitted in the distillation column to separate different
mixtures
• They usually called trays
• Plates helps components to settle and to attain maximum separation
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Reflux
• Reflux is a technique involving the condensation of vapors and the
return of this condensate to the system from which it originated.
• It is used in industrial and laboratory distillations.
• In that context, reflux refers to the portion of the overhead liquid
product from a distillation column or fractionator that is returned to
the upper part of the column as shown in the schematic diagram
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Reflux and feed plates relationship
• The more reflux provided for a given number of theoretical plates,
the better is the column's separation of lower boiling materials from
higher boiling materials.
• Conversely, for a given desired separation, the more reflux is
provided, the fewer theoretical plates are required
• It means if we have few plates and more reflux we can achieve high
separation efficiency ( good conditions)
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Reflux types
Total reflux
• Total reflux is the operating condition where vapor and liquid are
passing each other in the column but no product is removed.
Minimum reflux ratio
• Minimum reflux ratio is the lowest value of reflux at which
separation can be achieved even with an infinite number of plates.
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Feed conditions in distillation column with respect to feed plate and reflux

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