Thursday 10 March 2011

DEMETHANIZER COLUMN



Types Of Rectifying Columns

Devices in which vapour from a still pass counter currently to a portion of a condenser returned as reflux are turned as rectifying column, these are also defined as devices for bringing a stream of vapour and a stream of liquid into intimate counter current contact. In fractionating column there is also a still that generates vapours, still is also provided with a source of heat, the interchange of material between a liquid and a vapour phase which takes place in rectification is inherently a diffusional process and a process in which it takes place should provide for counter current flow of reflux and vapour and intimate mixing of the streams through the counter current patch.
There are two main types of rectifying column which are mainly used.
·        Plate column
·        Packed column.

Relative Merits Of Plate And Packed Towers

                   The choice between the use of packed tower and plate tower for a given mass transfer equation should be based on a detailed cost analysis for the two types of contactors. Thus the optimum economic design for each type would be developed in detail and the final choice would be based on considerations of cost and profit at the optimum conditions, in many cases however the decision can be made on the basis of qualitative analysis of the relative advantages and disadvantages of plate and packed column.
·        For a column dia of less than 3ft it is more usual to employ packed towers because of high fabrication cost o small trays, for a very large column big distribution problems may arise and this fact coupled with the weight considerations of a large volume of packing may lead to the choice of plate towers. Random packed towers are seldom designed with a dia larger than 4 ft and dia of commercial plate towers are seldom less than 2 ft.
·        Froth is produced by the bubbling action of vapors through liquids and diameters of commercial; plates are seldom less than 2 ft
·        Froth is produced by the bubbling action of vapors through liquids and use of a packed tower minimize the effect as contact is then between the vapors and liquid film flow over the packing.
·        If a system contains suspended solids or sludge, a plate column is to be preferred, since solids can accumulate in the voids coating of the packing material and render it ineffective.
·        Because ceramics are used for many packing it is usually more economical to used packed tower than to employ corrosion resistance material for a tray column, with perhaps associated problems in fabrication.
·        Because of the liquid on each plate there may be large quantity of liquid in a plate column where as in packed tower liquid flows as a thin film. Hold up of the packed tower is an advantage.
·        Stage efficiencies for packed towers must be on experimental tests with each type of packing the efficiency varies not only with the type and size of packing, but also with the fluid rates, the fluid properties, the column diameter, the operating pressure and in general, the extent of liquid dispersion over the available packing surface.
·        Because of liquid dispersion difficulties in packed towers, the design of plate towers is considerably more reliable and requires less safety factor when the ratio of liquid mass velocity to gas mass velocity is low.
·        Plate towers can be designed to handle wide ranges of liquid rates without flooding.
·        If the operation involves liquids that contain dispersed solid, use of plate tower is preferred because the plates are more accessible for cleaning.
·        Plate towers are preferred if interstage cooling is required to remove heats of reaction or solution, because cooling coils can be installed on the plates or the liquid delivery line from plate to plate can be passed through an external cooler.
·        The total weight of a dry plate tower is usually less than that of a packed tower designed for same duty. However, if liquid hold-up during operation is taken into account both types of towers have about the same weight.
·        When large temperature changes are involved as in distillation operation, plate towers are often preferred because thermal expansion or contraction of the equipment components may crush Packing.
·        Design information for plate tower is generally more easily available and more reliable than that for packed towers.
·        Packed towers have proved to be cheaper and easier to construct than plate towers if highly corrosive fluids are to be handled.
·        Packed towers are usually preferred if the liquids have a large tendency to foam.
·        The amount of liquid hold-up is considerably less in packed towers.
·        The pressure drop through packed towers may be less than the pressure drop through plate towers designed for the same duty. The advantages, plus the fact that the packing serves to lessen the possibility of tower wall collapse making packed towers particularly desirable for vacuum operation.

Types Of Plate Columns


There arc three types of plate columns widely used
·        Bubble-cap plate columns.
·        Sieve plate columns.
·        Valve trays.

Bubble-Cap Plate Columns


Bubble tray towers are the most widely used type of fractionating device in the large scale installations and is the best known vapour liquid contacting device. Through the years it has been a standard for the chemical and petroleum industry and a majority of the existing commercial vapour liquid contacting devices such as fractionators and absorbers contain bubble and cap tray.
There are two unique advantages of bubble cap trays which should be considered
·        The fixed seal arrangement of bubble cap trays enables them to be operated over a wider range of conditions while maintaining a relatively constant efficiency.     
·        The greater wealth of published performance data on and experience with bubble cap trays supports their choice when the process risk is likely to be high. However it should be recognized that these considerations do not always lead to the bubble cap tray.

Sieve Plate Columns

Sieve plate columns were used by Coffey as early as 1832 and have been extensively employed ever since. In these columns a series of horizontal plates are placed at a distance of 6 inch or more in the column shell. The plates are perforated with a number of holes distributed over the plate, a convenient size being 3/16" diameter holes spaced on 1/2" centers. The pressure and velocity of the vapour passing up through these holes must be sufficient to hold up a body of liquid reflux on each plate. The sieve plates of this kind cannot be- operated at less than 50% vapour rate or the liquid will drain through the perforations and the bubbling contact cases which result in great loss of efficiency.
Disadvantages of this type have limited their utility
·        Because the pressure drop through the openings of the
perforated plate is proportional to the square of the vapour
velocity, it varies more rapidly with changes in vapour rate than
a bubble cap plate, and this reduces its flexibility.
·        Flooding the liquid on the plate can dump if the vapour rate is momentarily stopped.
·        The plates operate properly only over limited range of vapour and liquid rates of How.
·        If the plates are not perfectly leveled all   the liquid will run through the low side and vapour will How through the high side.
·        If the plates corrode the holes become larger and efficiency falls
Although the cost of sieve plate is less than the bubble cap plate but on account of the disadvantages of sieve plate and advantages of bubble cap plate, we select bubble cap plate for the purpose of our design.

Valve Trays

Now-a-days valve trays are widely in use. But due to non-availability of relevant data about valve trays, this type was not used.
The major advantage of valve trays over sieve trays is that high efficiencies can be maintained over a wider range of operating liquid. Turnover ratio, defined as the ratio of the maximum allowable throughput rate to the minimum allowable throughput rate, as high as 10 can be obtained with valve trays, while the ratio is much less with sieve trays.
Valve-tray perforations are larger than those for sieve trays being in the order of 1.5 in. for circular holes and 6 in. long in the case of rectangular-slot holes. There are man)' methods to control the movement of the valve lids, but the lids generally allowed a vertical movement in the range of 1/4 to 1/2 in., and the weight of the lid varies depending on the intended use of the valves.
Basically, the overall model for valve trays is similar to that for sieve trays with the exceptions that the vapor flow area in the contractor can vary for valve units depending on the extent of the valve opening and the number of kinetic heads for calculation of the gas flow pressure drop through the contactor (h=liquid head through perforation) is usually greater for valve trays than for sieve trays. Thus, many of the basic principles of fluid mechanics as used for sieve-tray design also apply for valve-tray columns & can often be used by engineers for considering alternatives before contacting a specific manufacturer. Because valve units are more complex mechanically than sieve trays, valve trays are more expensive to fabricate, and this extra cost must be consider in pricing the units. On the other hand, proprietary know how will become available for the valve units if they are purchased, and this advantage to the user plus the advantage of wider range of turnover ratios may swing the balance in favor of the more expensive value trays. By the comparative studies of the cap we reach at the result there are many styles and dimensions of cap in use, the round bell shaped bubble cap is quite practical and efficient. It is recommended as a good basis for contacting requirements.

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