A distillation column is a tower-type vapor-liquid contact device for distillation. As the main equipment of the distillation process, there are two main types of plate columns and packed columns. According to the operation mode can be divided into continuous distillation column and batch distillation column. Today we will take you to understand the structure and principle of plate column.
Plate column
Plate towers are usually composed of a cylindrical shell and a number of plates (or plates) set horizontally along the tower height at a certain spacing.
Plate tower plate
The plates of a plate tower can be divided into two categories: those with drop tubes and those without drop tubes. In general, the liquid with a drop tube is staggered flow, and the liquid without a drop tube is counterflow.
Plate tower can be divided into
Bubble Tower, floating valve tower, sieve plate tower, tongue and slant plate and so on. Among them, the bubble tower, floating valve tower and sieve plate tower are the most widely used in industrial production.
1 Blister tower
Blister tower plate is the earliest industrial application of the tower plate, it is composed of gas pipe and bubble. Blister is installed on the top of the ascending tube, divided into two kinds of round and strip, the former is more widely used. There are three sizes of blister, f80, f100 and f150mm, which can be selected according to the size of the tower. The lower periphery of the bubblers has a lot of teeth slits, which are generally triangular, rectangular or trapezoidal. Blisters are arranged in triangular shape on the tower plate.
The edge of the blister is equipped with longitudinal tooth slits, and the center is equipped with a gas lift tube. The rising gas pipe is directly connected to the tower plate. The gas phase under the tower plate enters into the rising tube, and then blows out from the teeth to contact with the liquid phase on the tower plate for mass transfer. Due to the rising tube, the liquid leakage phenomenon under low gas velocity is avoided.
Advantages: the tower plate operation flexibility, tower efficiency is also higher, more widely used.
Disadvantages: the structure is complex, the tower pressure is reduced, low production intensity, high cost.
2 sieve plate tower
Sieve plate tower plate referred to as the sieve plate, its structure is characterized by a number of uniform holes in the tower plate, aperture is generally 3 ~ 8mm. sieve holes in the tower plate for the positive triangular arrangement. Overflow weir is set on the tower plate, so that the plate can maintain a certain thickness of the liquid layer.
The advantages of the sieve plate tower are simple structure, low cost, large production capacity, small drop of liquid surface on the plate, the gas pressure is reduced, while the tower plate efficiency is higher.
The disadvantage is that the operating flexibility is small, the sieve holes are easy to clog, and it is not suitable to deal with easy coking, viscous materials.
3 Floating valve tower
Float Valve is the 20th century after World War II began to study the 50s began to enable a new type of tower plate, and then gradually appeared in a variety of types of float valve
Its type has a round, square, strip and umbrella, etc.. More use of circular float valve, and circular float valve is divided into a variety of types.
Characterized by the float valve canceled the bubble tower bubble and rising gas pipe, instead of openings in the tower, the valve installed on the limit of the three legs. However, the valve piece is easy to fall off or jammed during operation.
Float valve can float freely with the change of gas speed up and down, which improves the operating flexibility of the tower plate, reduces the pressure drop of the tower plate, and has high efficiency of the tower plate, which is widely used in the production.
Plate tower overflow device
The overflow device of plate tower refers to the overflow weir (outlet weir) and the descending liquid pipe.
The liquid is discharged to the bottom of the tower by gravity from the top plate by plate, and forms a flowing liquid layer on the plate surface of each layer of the tower plate; the gas is pushed by the pressure difference, and is discharged from the top of the tower through the openings evenly distributed on the tower plate and spread to each layer of the tower plate in turn.
Tower plate on the gas-liquid two-phase contact state is to determine the two-phase flow on the plate hydrodynamics and mass and heat transfer law of the important factors. When the liquid flow rate is certain, with the increase of gas velocity, the following contact states can occur:
1 bubble contact state When the gas velocity is low, the gas passes through the liquid layer in the form of a bubble. Due to the small number of bubbles, the formation of gas-liquid mixture is basically liquid-based, gas-liquid two-phase contact surface area is not large, the mass transfer efficiency is very low
2 honeycomb contact state With the increase in gas velocity, the number of bubbles is increasing. When the bubble formation rate is greater than the bubble floating rate when the bubble accumulation in the liquid layer. Bubbles collide with each other to form a variety of polyhedral bubbles. As the bubble is not easy to rupture, the surface is not renewed, so this state is not conducive to heat and mass transfer.
3 foam contact state When the gas velocity continues to increase, the number of bubbles increased dramatically, bubbles continue to collision and rupture, most of the liquid on the plate at this time in the form of liquid film exists between the bubbles, the formation of a number of small diameter, perturbation is very intense dynamic foam, due to the foam contact state has a large surface area, and constantly updated, is a better contact state.
4 jet contact state When the gas velocity continues to increase, the liquid on the plate upward sprayed into droplets of varying sizes, the larger diameter droplets fall back to the tower plate by gravity, the smaller diameter droplets are taken away by the gas, the formation of liquid foam entrainment. The droplets return to the tower plate and are dispersed, this droplet formation and aggregation repeatedly, so that the mass transfer area increases, the surface is constantly updated, is a better contact state.
Industrial production generally want to present foam state and spray state two states.
Because the gas velocity of the spray contact state is higher than the foam contact state, so the spray contact state has a larger production capacity, but the spray state liquid foam entrainment is more, if not well controlled, it will destroy the mass transfer process, so most of the tower are controlled in the foam contact state work.
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