column plate

Question 1

What is the main difference between the gas and liquid flows found in the plate and packed columns?

Answer 1

In plate columns, liquid is the continuous phase and gas is the dispersed phase because it is dispersed in the form of bubbles in the liquid. In packed columns, gas is the continuous phase and the liquid is the dispersed phase because it is dispersed in the form of film on packings.

Question 2

What are three types of plates used in plate columns?

Answer 2

Bubble cap, valve, and sieve plates

Question 3

Write one advantage and one disadvantage of sieve plates used in plate columns compared to other plate types?

Answer 3

Sieve plates are cheap because they are easy to be constructed. However, they are not good to handle fluctuating vapour and liquid flow rates. Especially when vapour flow rates fall they can easily lead to weeping.

Question 4

Define the ‘weeping point’ for plate columns. What design parameter would you vary to prevent weeping in sieve plate columns

Answer 4

It is an operating condition at which liquid leaks through the plate holes. It occurs when the vapour flow rate and its pressure under a plate are not sufficient to hold the liquid flowing above the plate. Total hole area and/or hole diameter can be decreased to prevent weeping

Question 5

Write the equation for calculating plate pressure drop (ht) and name the components contributing to ht.

Answer 5

Write the equation for calculating plate pressure drop (ht) and name the components contributing to ht. Answer: ht= hd+hw+how+hr
where hd=dry plate pressure drop, mm of liquid hw = weir length, mm how = liquid crest over weir, mm hr= residual liquid head, mm

Question 6

Name three types of common liquid flow arrangements that are used in plate column design?

Answer 6

Double pass, crossflow single pass, reverse pass

Question 7

What is the purpose of calming zones in plates used in plate columns?

Answer 7

Calming zones allow the gas bubbles carried over by the liquid to escape before the liquid enters the downcomer. If calming zones are not large enough, the liquid entering the downcomer will entrain the gas bubbles to the bottom plate which will decrease the efficiency of the plate.

Question 8

What is the consequence of the downcomer liquid level reaching the plate above in a plate column?

Answer 8

Liquid flow from one plate to the other will stagnate which will soon lead to flooding condition

Question 9

Choose any two of the following design parameters that can be changed to decrease the downcomer liquid level in a plate column? Why? 1) tray spacing 2) hole diameter 3) hole number 4) downcomer area 5) weir height

Answer 9

Plate pressure drop needs to be reduced to prevent downcomer liquid backup reaching the plate above.
i. Increase the downcomer area which will decrease how and therefore decrease the plate pressure drop ii. Increase hole diameter and therefore increase total hole area which will decrease vapour velocity through holes and therefore will decrease dry plate pressure drop.

Question 10

The percent flooding in a plate column is found to be very high and therefore needs to be decreased. The flow rates and the properties of the liquid and vapour phases are fixed. What design parameter needs to be changed to decrease the percent flooding? Why?

Answer 10

Percentage (%) flooding in a plate column = uv(design)*100/Uflooding. To decrease % flooding, you have to decrease uv(design). To decrease uv(design) for a given volumetric gas flow rate, you have to increase the net area (An) which can be increased by decreasing the downcomer area Ad. Another way to decrease the uv(design) is to increase the column cross-sectional area (Ac) or column diameter (Dc) while keeping Ad/Ac constant.

Question 11

Criterion that is used to check verifiable downcomer back-up

Answer 11

hb < 0.5 (lt + hw)

Question 12

Acceptable downcomer back-up hb

Answer 12

This criterion takes into account that there is froth over the liquid surface in the
downcomer
It ensures that the plate spacing is greater than the combined height of liquid and froth in
the downcomer

Question 13

Weep-point

Answer 13

– Operating condition at which liquid leaks through the plate
holes
– Vapour velocity at weep-point is the minimum required for
the stable operation of plate
– Operating vapour velocity must be greater than that at
weeping point
– Hole area must be chosen such that operating vapour
velocity is well above that at weep-point

Question 14

Determination of fractional entrainment of liquid by vapour leaving the plate

Answer 14

Acceptable trident value is about 0.1

Question 15

Relation between hole area and pitch

Answer 15

Hole pitch lp is the distance between the hole
centres
• Hole pitch should not be less than 2.0 dh
• Hole pitch normally varies between 2.5 to 4.0dh
• Holes are arranged in square or triangular
patterns; triangular is preferred

Question 16

Plate columns (Finite stage contactors)

Answer 16

• A finite-stage contactor consists of series of interconnected stages
• Each tray represents a stage
• In a theoretical stage, equilibrium is achieved between the contacting phases
• Plate columns are preferred
- when fluctuations in the vapour or liquid rate may occur
- where major changes in the overall capacity of the column are anticipated
- when long contact time should be provided between the gas and liquid
phases
• Plate columns
- are easy to clean
- have few distribution (gas or liquid) problems
- have relatively large pressure drop
- are relatively expensive

Question 17

Packed columns (Continuous contactors):

Answer 17

• Contacting between the phases occurs continuously throughout the equipment in a
packed column
• The overall analysis of a differential stage contactor is based on a differential
height of the contactor
• In continuous contactors, the analysis is not based on a tray or a single unit but
based on HETP ( height equivalent of a theoretical plate)
• HETP is defined as the height of the packed column necessary to give a separation
equivalent to that achieved in one theoretical plate.
• HETP is similar to a theoretical plate used in finite-stage contactors but it is based
on a differential change in equilibrium conditions and concentrations (- theoretical
plate is based on finite change in concentrations)

Question 18

Flooding

Answer 18

Flooding is determined by the pressure drop across the plate and the plate spacing. At
the flooding point the entrainment of liquid by vapour becomes excessive and the
downcomer liquid backs up to the next higher plate. Flooding point determines the
upper limit of the vapour velocity.

Question 19

Weeping

Answer 19

Weeping occurs if there is a slight gradient in liquid head across the plate at low
vapour velocities. It decreases the plate efficiency. Weeping point determines the
lower limit of the vapour velocity.