EXPERIMENT: Pressure drop over a bubble cap plate.
AIM : This experiment is conducted in an experimental test ring in which the effect of the variation in vapor and liquid flow rates on the pressure drop across bubble cap plates is simulated using air and water to represent the vapor and liquid respectively.
THEORY: In a pilot tube the relation between the gas velocity and pressure drop is
Where, is the differential pressure as expressed in the head of the fluid flowing.
= density of air= 1.21 kg/m3
’=density of liquid (water) =1000 kg/m3
Volumetric gas flow rate V (m3/s) is calculated by assuming a flat velocity profile inside the tube. This is justifiable as the flow is turbulent V= v * s’
Where s’= pipe cross sectional area
Vapor velocity inside the distillation column (v) =
• Allow water to pass through the equipment to ensure the plates are loaded with liquid and then stop the water flow. Wait till the excess water is drained from the plates.
• Set the inclined manometer to a suitable inclination to measure the differential pressure.
• Switch on the blower and measure across the pilot tube, keeping the air flow rate constant. Select the appropriate valves only (one “high” valve and one “low” valve) and measure the pressure drop across the three plates.
• Reuse the air flow rate by partially closing the inlet of the blower and repeat the above procedure to obtain pressure drop at various air i.e. vapor velocities.
• Repeat the above with two different water (i.e. liquid) flow rates which are kept constant.
• Measure the water flow rate by measuring the amount of water collected in a known time interval.
A distillation column can use either trays or packing. Their mechanisms of mass transfer differ, but the key for both is a good approach to equilibrium through the generation of large amounts of interfacial area. In a trayed column, liquid flows down the column through down comers and then across the tray deck, while vapor flows upward through the liquid inventory on the tray. The most common gas disperser for cross-flow plates has been the bubble-cap. This device has a built-in seal which prevents liquid drainage at low gas flow rates.
Gas flows through a center riser, reverses the flow under the cap, passes downward through the annulus between riser and cap and finally passes into the liquid through a series of openings or “slots” in the lower side of the cap. Trays and packing materials are widely used in distillation. Normally packed columns are used for gas-liquid and liquid-liquid contacting operations. That Means packed columns are used for distillation, gas absorption and liquid-liquid extraction. However, these Columns are used extensively for absorption and, to a limited extent, for distillation. But some time the packing materials are not suitable for the distillation process. Those times we can use trays to achieve the requirement.
The types of trays used in distillation
Bubble cap tray
High Capacity Trays
A bubble cap tray has riser or chimney fitted over each hole, and a cap that covers the riser. The cap is mounted so that there is a space between riser and cap to allow the passage of vapour. Vapour rises through the chimney and is directed downward by the cap, finally discharging through slots in the cap, and finally bubbling through the liquid on the tray. Advantages of Bubble cap plate
• Bubble cap trays are used primarily where large turndown ratios are required. • Their construction allows very low liquid rates to be handled with little or no leakage. • Due to their ability to operate at low vapor and liquid rates, bubble cap trays are used in a significant portion of fractionation tray installations.
Disadvantages of bubble cap trays
• Capacity of perforated trays is often plotted as a function of percent hole area. Actually, the capacity of a perforated tray is not much affected by hole area unless the lack of hole area increases pressure drop and down comer backup to unacceptable values. For example, if a perforated tray has sufficient hole area to limit dry tray pressure drop to a reasonable value (about 2″ to 3″ liquid at 80% flood) the perforated tray will have the same capacity as a valve tray. A bubble cap tray cannot be designed to have as much hole area as a valve tray and will, therefore, have less capacity. • Bubble cap trays cannot be used to achieve high flow rates. • Not like the other trays, for bubble cap trays it has low efficiency.
Distillation is the dominant process for separating large multi component streams into high purity products. So, the chemical process industries’ ongoing quest to improve energy utilization, reduce capital costs, and boost operating flexibility is spurring increasing attention to distillation column optimization during design. Designers often approach column optimization in an iterative manner, heavily relying on vendor experience and information. A good understanding of mass-transfer and pressure-drop fundamentals, as they relate to optimization, will enable the column designer to independently judge vendor offerings and effectively determine the optimal equipment design. This article will address the following optimization goals: (1) maximizing theoretical stages per height of section or column, (2) minimizing pressure drop per theoretical stage,
(3) maximizing the operational range, turn-down, or turn-up. Tray pressure drop
Typical tray pressure drops lie in the range of 250 – 1500 N/m*m (or 2.5 mbar – 15 mbar or 25 – 150 mm Water Column, in whatever units one prefers). Usually, the drop in pressure caused by gas flowing through a tray is small in comparison to the system pressure. Except for vacuum columns, where it can become quite substantial and the gas velocity in the perforations may become comparable to the velocity of sound. The tray pressure drop plays an important part in filling up the down comers. To compensate for the pressure drop, a liquid head builds up in the down comers, to enable the liquid to flow down against it. When the tray pressure drop becomes excessive with respect to the height of the down comers, flooding will be the result.
The tray pressure drop is composed of (at least) two (major) contributions:a pressure drop caused by the gas flowing through the perforations in the tray floor. This contribution depends on gas flow rate, fraction free area and the pressure drop coefficient of the particular perforations (or valves) being used. This pressure drop coefficient depends on relative hole thickness (i.e. the ratio of tray thickness over hole diameter), hole shape and nearness of other holes (ratio of hole pitch to hole diameter).a pressure drop caused by the liquid present on the tray. This liquid hold up effect primarily increases with an increase in outlet weir height, decreases with an increase in gas flowrate and increases with an increase in liquid flowrate. To a lesser extent, it depends on physical properties of the gas/liquid system. Other kinds of trays that are used in industry
We use many kinds of trays to achieve some goals
The optimization goals are:
(1) Maximizing theoretical stages per section or column height, (2) Minimizing pressure drop per theoretical stage, and
(3) Maximizing the operational range, turn-down, or turn-up. Industries that used Bubble cap trays
• Glycol Dehydration
• Caustic Scrubber (Wash Section)
• Amine Columns (Wash Section)
– H2S or CO2 Removal
Errors involved in practical
• In the apparatus there wasn’t any meter to calculate the inclination of the manometer. So we faced great difficult to find the angle of manometer. By using an appropriate method to calculate the angle we can minimize the error. • In the apparatus there wasn’t any method to change the water flow rate quickly. We have to collect water in to a bucket in a one minute of time period. That was not a good method, as we cannot change the water flow rate as we wish.