Functions of Tray In Distillation Column

     One of the most prominent hardware used for mass transfer is tray. Tray columns are widely used in various types of mass transfer operations. All the simulation results, which predict a certain number of theoretical stages, can be converted to actual trays depending upon tray efficiency for a particular service.

     Basic functioning of a tray/plate is mass transfer. It actually brings about vapor-liquid contact. More the vapor-liquid contact, more would be the mass transfer. This is practically achieved when a liquid is held on a perforated tray and vapors pass through this liquid layer from below the liquid depth through the perforations. It should be noted here that, flow rate should be adjusted such as the liquid "may not" come down from the tray above "through perforations". If this happens, then vapor-liquid contact would be less which will result in low vapor liquid contact and hence it lowers the efficiency of tray. This condition is said to be weeping.

     Similarly, vapor velocity must not be so high, since it may take the liquid over the tray to the tray above it, in form of droplets. This will again reduces the efficiency of the tray in the similar manner. This process is called entrainment.

     In any conventional tray vapour rises through the liquid pool on the tray deck and then disengages from the liquid in the space above the deck. Liquid enters the tray from a downcomer above and leaves via a downcomer below.

Conventional Tray has three functional zones:

1. Active area for mixing vapour and liquid: This is the zone where mass transfer occurs.
2. Vapour space above the active area: This is the zone in which liquid is separated from vapour.
3. Downcomer between trays. This zone has two functions, first moving liquid from one contacting tray to another and second disengaging vapour from liquid.

What is Turndown Ratio And It's Significance In Distillation Column

Turndown Ratio:
                              The ratio between minimum vapor load to maximum vapor load, thus it indirectly defines both the operating range (low or high vapor flow rates) and also the minimum capacity before trays start to leak. A low turndown ratio depicts the tray to be less flexible in operation i.e, it cannot handle a range of vapor flow rates. 

Operational Problems In Distillation Column

Major operational problems in distillation column due to adverse vapour flow conditions can cause

• foaming
• entrainment
• weeping/dumping
• flooding

Foaming

                Foaming refers to the expansion of liquid due to passage of vapour or gas. Although it provides high interfacial liquid-vapour 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.

Entrainment

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

Weeping/Dumping

                                This phenomenon is caused by low vapour flow. The pressure exerted by the vapour 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 pressure drop in the column and reduced separation efficiency.

Flooding

               Flooding is brought about by excessive vapour flow, causing liquid to be entrained in the vapour up the column. The increased pressure from excessive vapour 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.

Plate Columns and Comparison of Tray Types

    For purpose of distillation, plate columns and packed columns can be used. In plate columns each plate constitutes a single stage, or in packed columns where mass transfer is between a vapor and liquid in continuous countercurrent flow.

     In order to design a plate type distillation column, following factors must be considered:

1) The type of plate or tray
2) The vapor velocity, which is the major factor that determines the diameter of the column.
3) The plate spacing, which is the major factor fixing the height of the column when the number of stages is known.

Types of Trays:
     The main requirement of a tray is that it should

a) provide an intimate contact of vapor and liquid phases, because more the contact of these two, more will be the mass transfer which brings about more enrichment.

b) it should be capable of handling more desired flow rates of vapor and liquid without excessive entrainment or flooding.

c) be stable in operation or have flexibility in operation.

d) be reasonably easy to erect and maintain.

Arrangement of Flow:
     The arrangements for the liquid flow over the tray depend largely on the ratio of liquid to vapor flow. Three layouts are shown here, of which the cross-flow array is much the most frequently used.

a) Cross-Flow: Normal, with a good length of liquid path giving a good opportunity for mass transfer.

b) Reverse: Downcomers are much reduced in area, and there is a very long liquid path. This design is suitable for low liquid -vapor ratios.

c) Double-pass: As the liquid flow splits into two directions, this system will handle high liquid-vapor ratios. 

     The liquid reflux flows across each tray and enters the downcomer by way of weir, the hight of which largely determines the amount of liquid on the tray. The downcomer extends beneath the liquid surface on the tray below, thus forming a vapor seal. The vapor flows upwards through risers into caps, or through simple perforations in the tray. Weir and downcomer is shown in second figure as follows:

Types Of Trays:
    Purpose of tray is to provide an intimate contact of liquid and vapor, and to make a low drop of pressure. So far in industry, following three types of trays are usually used:

a) Seive or Perforated Trays:
                                              These are much simpler in construction, with small holes in the tray. The liquid flows across the tray and down the segmental downcomer. This type of tray offers a very low pressure drop and is cheaper than the rest of the two, but it brings about less vapor-liquid contact as compared to the other two.

     The general form of the flow on a sieve tray is typical of a cross-flow system. With the sieve plate the vapor velocity through the perforations must be greater than a certain minimum value in order to prevent the weeping of liquid stream down through the holes. At the other extreme, a very high vapor velocity leads to excessive entrainment and loss of tray efficiency.

b) Bubble Cap Trays:
                                   This is the most widely used tray because of it's range of operations, but is now-a-days unable to compete with the third type which offers more flexible operation. The individual caps are mounted on risers and have rectangular or triangular slots cut around their sides. The caps are held in position by some form of spider, and the area of the riser and the annular space around the riser should be about equal. With small trays, the reflux passes to the tray below over two or three circular weirs, and with the larger trays through segmental downcomers.

     This type of tray provides good efficiency than seive tray, flexible in operation (i.e, can be used for a range of liquid-vapor flow rates) but  is most costly and offers great pressure drop as compared to the other two. Bubble cap trays are capable of dealing with very low liquid rates and are therefore useful for operation at low reflux ratios.

c) Valve Trays:
                        These may be regarded as a cross between a bubble-cap and a sieve tray. The construction is similar to that of cap types, although there are no risers and no slots. It may be noted that with most types of vlave tray the opening may be varied by the vapor flow, so that the trays can operate over a wide range of flowrates (i.e, it provides more flexibility in operation than a bubble-cap tray).
 
     It is low is cost than a bubble- cap tray since it is simple in construction. Because of their flexibility and low price, valve trays are tending to replace bubble-cap trays. It operates at the same capacity and efficiency as sieve trays. It has high turn-down ratio, i.e, it can be operated at a small fraction of design capacity.

Steam Distillation

     Steam distillation is used where a material to be distilled has a high boiling point, and particularly where decomposition might occur if direct distillation is employed. In this distillation type, steam is passed directly into the liquid in the still, but it should be kept in mind that the solubility of the steam/water must be very low otherwise it would contaminate the product and one will have to bear more cost to separate water from the product. Steam disillation is perhaps the most common example of differential distillaion.

Explanation:

     Two cases may be considered in distillation using steam. The steam may be superheated and so provide sufficient heat to vaporize the material concerned, without itself condensing. The second case will exist when some of the steam may condense, producing a liquid water phase.

     If there is no liquid phase present, then from the phase rule there will be two degrees of freedom,

                                             F = C - P + 2
                                    =>     F = 2 - 2 + 2
                                    =>     F = 2

F = degrees of freedom (tells about the number of parameters to be essentially specified to specify a system)
C = no. of components of the solution (binary solution considered here, hence 2 components)
P = no. of phases (2 in this case i.e, steam and liquid solution phase)

     Both the total pressure and the operating temperature can be fixed independently, and partial pressure of high volatile component, which must not exceed the vapor pressure of pure water, if no liquid phase is to appear (since at higher partial pressure than water will cause steam to condense, as at high pressure boiling point increases. See Effect of pressure on boiling point of liquid )

     When a liquid water phase is present (3 phases will exist , there will be only one degree of freedom),
                                             F = C - P + 2
                                       =>  F = 2 - 3 + 2
                                      =>   F = 1

      here, water will form a separate phase since being polar it is not miscible with the organic solution which is a non-polar one.

     Since, degree of freedom for this case is 1, therefore, selecting one variable (temperature or pressure) fixes the system, with the water and the other component each exerting a partial pressure equal to tis vapor pressure at the boiling point of the mixture. In this case, the distillation temperature will always be less than that of boiling water at the total pressure in question. Consequently, a high boiling organic material may be steam-distilled at temperature below 373K at atmospheric pressure. By using reduced operating pressures, the distillation temperature may be reduced still further, with a consequent economy of steam.

Comparison of the two cases:

     Where there is no liquid water phase present, the steam consumption will be high unless the steam is very highly superheated. With a water phase present, the boiling point of the mixture will be low, and consequently partial pressure of the component will have a low value. Thus, on a molar basis the steam consumption will again be high, although due to the relatively low molecular weight of the steam, the consumption may not be excessive. Steam economy may be effected by using indirect heating of the still, having no liquid water phase present, or by operating under reduced pressure.

Properties Of Solvent Used For Extractive Distillation

The solvent to be used is selected on the basis of :

1) Selectivity

2) Volatility

3) Ease of separation from the top and bottom products

4) Cost of separation

The selectivity is most easily assessed be determining the effect on the relative volatility of the two key components of addition of the solvent. The volatile the solvent, the greater the percentage of solvent in the vapor, and the poorer the separation for a given heat consumption in the boiler.

Extractive Distillation And It's Comparison With Azeotropic Distillation

Exractive Distillation:
   
     Exractive Distillation is carried out for such azeotropic solutions in which relative volatility is very low. In this case continuous distillation of the mixture to give nearly pure products will require high reflux ratios with correspondingly high heat requirements. In addition, it will necessitate a tower of large cross-section containing many rays.

     Basic principle for separation of this type of solutions is to add a substance that will alter the relative volatility of the original constituents, thus permitting separation. The added solvent should be of lower volatility as compared to the components and hence it does not appreciably vaporizes in the fractionation column. This solvent must be continuously fed near the top of the column and it runs down the column as reflux and is present in appreciable concentrations on all the plates.

     Actually, this extractive agent differentially affects the activities (activity coefficients) of the components, and hence alters the relative volatility of the mixture. It is important to note that the solvent must not form an azeotrope with any of the components.

Comparison With Azeotropic Distillation

     Extractive distillation is usually more desirable than azeotropic distillation since no large quantities of solvent have to be vaporized. In addition, a greater choice of added component is possible since the process is not dependent upon the accident of azeotrope formation. It cannot, however, be conveniently carried out in batch operation.