Downcomer: A device to direct the liquid from a distillation column at a location above the bottom tray and below the top tray.
Definition of downcomer
: a pipe to conduct something downward: such as. a : a pipe for leading the hot gases from the top of a blast furnace downward to the dust collectors and flue system.
Downcomers. Downcomers are used to guide liquid flow from an upper tray to a lower tray. The most common is the segmental-type, which can be either straight or sloped (angled).
The Liquid – Vapour flow factor. FLV =
Downcomer backup is a function of the tray pressure drop, the head at the inlet side of the tray, and the frictional losses in the downcomer itself.
Downcomer Choke flooding is mainly function of downcomer geometry- DC top area / bubbling area. May occur when downcomer is not properly sized for vapor disengagement.
Downcomer Flooding:- In distillation column, liquid flows in downward direction by gravitational force but it flows against the pressure. When liquid flows from one tray to next below tray, it flows from lower pressure to higher pressure. Hence, to compensate that, it elevates certain level inside the downcomer.
Foaming in a distillation column is the expansion of liquid that provides high interfacial liquid-vapor contact. Although one of the least common causes of distillation column malfunctions, foaming can increase until the fluid in one tray mixes with the liquid on the above tray in a process called entrainment.
Weeping is when liquid flows downward through the holes in a distillation tray.Normally vapor rises up through the holes and contacts the liquid on the tray.If the vapor rate is too low the liquid may be able to drop to the next stage through the holes, resulting in less than optimal vapor/liquid contact (and therefore ...
Vapour Velocity. Vapour velocities are determined for both the rectifying and stripping sections of the column. They may be different. If too low, weeping occurs – liquid flows through the holes in the sieve tray, for example. If too high flooding will occur and liquid will back to the next plate.
downcomer in Chemical Engineering
A downcomer is a pipe which takes a substance from a distillation tray. The recycled flow up the catalyst bed, down the downcomer, and back into the catalyst bed is much more stable and predictable. The liquid overflows and drains through the downcomer to the tray below.
Why are downcomers placed outside a furnace? Natural circulation. enters the water wall creating Natural circulation. This is the reason , why downcomers are placed outside the furnace.
Arrangement b (often referred to as the false downcomer) is popular. It offers better liquid distribution than the others, does not suffer from hydraulic jump, and provides some flexibility in inlet nozzle orientation. The width of the false downcomer should be the same as the width at the bottom of a downcomer (307).
Downcomers are pipes leading from the top to the bottom of the boiler. Downcomers carry the water from steam drum to the bottom part of the boilers where it enters the distribution headers to be heated in the combustion zone. Risers are pipes leading from the bottom to the top of the boiler.
Reasons for having External Downcomers
It is known that as the pressure and temperature increases, the density of water decreases. Thus the density difference between water and the steam reduces & thus at higher pressures, the natural circulation of water-steam is impaired.
The downcomer outlet, or spreader, is located at the oil-water interface. Water, being heavier than oil, will separate and settle to the bottom of the vessel.
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.
Hydraulic loads in terms of F-factor (gas load factor) and liquid load are low to moderate in deep vacuum distillations and the physical properties (liquid to gas density ratio, surface tension and viscosities) are not considered to be close to critical.
Flooding is a common abnormal process condition wherein the distillation column stops generating a separation, thus causing the quality of the top and/or bottom products to go off specification.
The System Factor is sometimes called the Foaming Factor because there appears to be a relationship between the values of the system factor used in practice and a systems tendency to foam. Quite often one simply finds a suitable value of the system factor in an appropriate reference (see, for example, Section 6.2.
Foam is created when the surface tension of water (attraction of surface molecules toward the center, which gives a drop of water its round shape) is reduced and air is mixed in, causing bubble formulation. As one passes across a curved surface or interface, a jump in pressure occurs.
Reflux in industrial distillation
Inside the column, the downflowing reflux liquid provides cooling and condensation of the upflowing vapors thereby increasing the efficiency of the distillation column.
The higher pressure drop and liquid flowrate backs liquid higher into the downcomer until liquid spills onto the tray above. The liquid continues building, flooding in order the higher trays. This is called jet flooding because the flooding is initiated by the jetting of liquid from the tray transfer area (Figure 3).
What is Weeping of Distillation Column? If a very small fraction of the liquid flows from a tray to the lower one through perforation or openings of the tray deck, the phenomenon is called 'weeping'.
The differential pressure between two locations in a distillation column has two components. They are static head due to the mass of the vapor between them and dynamic pressure drop due to the resistance of internal to the flow.