The inner parts of the packed tower mainly include packing support device, packing compaction device, liquid distribution device, liquid collection and redistribution device, etc. It is very important to choose and design the inner parts of the packed column to ensure the normal operation and excellent mass transfer performance of the packed column.

Packing support

1. Introduction of supporting device

The function of the packing support device is to support the packing and the weight of the liquid in the packing layer, while ensuring the smooth passage of gas-liquid two phases. The commonly used packing support devices are grid plate type, hole pipe type, hump type, etc.

The choice of supporting device is mainly based on the diameter of the tower, the type and type of the packing, the material of the tower body and the packing, the gas-liquid flow rate, etc.

2. Influence of supporting design on distillation operation

If the support is not designed properly, the liquid flooding of the packed column may first occur on the support plate. In order to make the gas can pass through smoothly, for the ordinary packed column, the free cross-sectional area of the fluid on the supporting part is more than 50% of the packing surface, and should be greater than the void ratio of the packing.

In addition, it should be taken into account that the section on the supporting plate should be blocked after loading the filler, so the design should be taken as large as possible free section. The free section is too small, and the blockage will occur during operation. Increase pressure drop, reduce efficiency, and even form a flood.

3, as a packing support device needs to meet the situation

Since the packing support device itself will also have an impact on the flow state of gas and liquid in the column, as a packing support device, in addition to its influence on fluid flow, the packing support device should generally meet the following requirements:

(1) Sufficient strength and stiffness to support the weight of the packing and the liquid held (liquid holdup), taking into account the liquid holdup in the packing gap, as well as pressure fluctuations, mechanical vibration, temperature fluctuations and other factors that may be applied to the system.

(2) enough porosity (generally greater than the void ratio of the filler) to prevent the first liquid flooding at the support place; In order to make the gas can pass through smoothly, for the ordinary packed column, the free cross-sectional area of the fluid on the supporting part is more than 50% of the packing surface, and should be greater than the void ratio of the packing. In addition, it should be taken into account that the section on the supporting plate should be blocked after loading the filler, so the design should be taken as large as possible free section. The free section is too small, and the blockage will occur during operation. Increase pressure drop, reduce efficiency, and even form a flood.

(3) the structure should be conducive to the uniform distribution of gas and liquid phase, at the same time will not produce a large resistance (general resistance is not greater than 20Pa);

(4) Simple structure, easy to process, manufacture, installation and maintenance.

(5) There must be a certain amount of corrosion resistance.

Packing compaction device

1. Packing compaction device and its function

The packing compaction device is a device that the packing plate is placed freely on the upper end of the packing layer and the packing is compressed by its own weight.

In order to prevent the packing bed layer from loosening or bouncing under the action of upflow, and to keep the packing bed layer as a constant fixed bed in operation, the uniform gap structure must be maintained to make the operation normal and stable, so it is necessary to set the packing compaction device above the packing layer.

In order to facilitate installation and maintenance, the packing compaction device cannot be continuously fixed with the tower wall. For small towers, screws can be used to fix the tower wall, while large towers are fixed with lugs.

2. Classification of packing compaction device

Packing compaction device is divided into two categories: packing plate and bed plate, each of which has different types.

Packing plate is suitable for bulk packing made of ceramics, graphite, etc. which is easy to be broken.

Bed restraint plate is used for metal, plastic, etc., not easily broken bulk packing and all structured packing. Its function is to prevent high gas velocity and high pressure drop or the operation of the tower sudden fluctuation of packing upward movement resulting in empty packing layer, so that the mass transfer efficiency decreases.

Note: Because the metal and plastic fillers are not broken and elastic and will not sink when correctly loaded, the bed restrictor plate should be fixed to the tower wall. In order not to affect the installation and use of the liquid distributor, it is not possible to use continuous tower ring fixation. For small towers, screws can be used to fix the tower wall, while for large towers, lugs can be used to fix.

3. Why should the free cross-sectional area of the packing compaction device be greater than 70%?

In the design, in order to prevent the pressure drop at the packing compaction device is too large or even liquid flooding, it is required that the free cross-sectional area of the packing compaction device should be greater than 70%.

Its role is in high gas velocity (high pressure drop) and load sudden fluctuations, prevent the packing to produce relative movement, so as to avoid packing loose, damage. Because the packing is fragile, when the debris is deposited in the gap between the bed packing, so that the void ratio of the packing layer decreases, the packing plate can fall with the packing layer, tightly pressing the packing without forming the packing loose, reducing the production capacity and separation efficiency of the packing column.

Liquid distribution unit

1. About the liquid distributor

Liquid distributor can be divided into the initial distribution and redistribution, initial distributor is set in the packed tower, used to uniform liquid distribution in packing tower top surface, the stand or fall of the initial distributor had a great influence on the efficiency of packed tower, the design of the distributor, liquid distribution, effective wet packing layer drift phenomenon and channel flow increases with the decrease of the area, It is difficult to get satisfactory separation effect even if the packing performance is good.

Therefore, the design of liquid distributor is very important. Especially for the packed column with large diameter and low packing layer, the liquid distributor with good performance is especially needed.

2, the performance of the liquid distributor

The performance of liquid distributor is mainly determined by the density of distributing liquid points (i.e. the number of distributing liquid points on a unit area), the uniformity of distributing liquid points and the uniformity of liquid composition on each distributing liquid point. The design of liquid distributor is mainly to determine the structural size of these parameters.

For the selection and design of liquid distributor, general requirements:

Liquid distribution should be uniform;

The free cross section rate should be large;

Operation flexibility;

It is not easy to block, not easy to cause the entrain and foaming;

Can be made of a variety of materials, and easy to operate and install, easy to adjust the level.

3, the classification of liquid distributor

There are many kinds of liquid distributers, and there are many different classification methods, which are generally divided by the driving force of liquid flow or according to the structure form.

According to the flow driving force can be divided into gravity and pressure type, if according to the structure can be divided into multi-pass and overflow type.

Among them, the multi-pass liquid distributor can be divided into: lotus sprinkler, straight pipe porous distributor, row pipe multi-pass distributor and double row pipe multi-pass distributor, etc. Overflow type liquid distributor can be divided into: overflow plate type liquid distributor and overflow tank type liquid distributor.

There are various kinds of liquid distribution devices, including nozzle type, disc type, pipe type, groove type and groove type.

① Nozzle type distributor

The nozzle type distributor is shown in the figure below. The liquid is eject from the hole of the hemispheric nozzle, the diameter of the hole is 3~ 10mm, and the concentric circle is arranged. The spraying Angle is less than 80°, and the diameter is (1/3~1/5)D. The structure of the distributor is simple, and it is only suitable for the tower with diameter less than 600mm. Because the small hole is easy to plug, the general application is less

(2) Disk distributor

The tray type distributor has the form of the tray type sieve type distributor, the tray type overflow pipe distributor and so on. The liquid is added to the tray and runs down through the sieve or the overflow pipe. The diameter of the distribution plate is 0.6 ~ 0.8 times of the diameter of the tower, and the distributor is used in D<>

③ Tubular distributor

The tubular distributor is composed of perforated pipes of different structural forms. It is characterized by simple structure, large free cross section for gas flow and small resistance. But the small hole is easy to plug, the elasticity is generally small. The tubular liquid distributor is widely used in packed towers with medium and lower liquid loads. In vacuum distillation and screen corrugated packing tower, because of the small liquid load is often used. The tubular distributor has different shapes such as row pipe type and ring pipe type, as shown in the figure below. Single row or double row can be made according to the liquid load.

④ Tank type liquid distributor

The trough type liquid distributor is usually composed of a distributary trough (also known as the main trough or the first trough) and a distribution trough (also known as the secondary trough or the second trough). The first stage of the groove through the bottom of the hole will initially divide the liquid into a number of streams, respectively into the liquid distribution tank below. The bottom of the distribution groove (or the wall of the groove) is provided with a hole (or a conduit) to evenly distribute the liquid on the packing layer.

The tank type liquid distributor has large operating elasticity and excellent anti-fouling and plugging, especially suitable for the separation of atmospheric liquid load and liquid containing solid suspended solids and large viscosity. Because of its excellent distribution performance and anti-fouling performance, the trough distributor is widely used.

Trough plate distributor is a new type of liquid distributor developed in recent years. It organically combines the advantages of trough type and plate type distributor, and has three functions of liquid collection, liquid separation and gas separation. The structure is compact and the operating elasticity is as high as 10:1. The distribution of gas and liquid is uniform and the resistance is small. It is especially suitable for the occasions where entrain and clogging are easy to occur.

Liquid collection and redistribution device

1. Liquid collection and redistribution device

When the liquid flows downward in the packing layer, there is a tendency to gradually flow toward the tower wall, which is called wall flow phenomenon. In order to improve the mass transfer effect of the column, when the ratio of the height of the packing layer to the diameter of the column exceeds a certain value, the packing layer should be divided into sections. In order to improve the uneven liquid distribution caused by wall flow, a liquid redistributer is installed between each packing layer to collect the liquid from the upper packing layer and provide a uniform liquid distribution for the next packing layer. A liquid distributor shall be arranged in the packing layer at intervals of a certain height.

In general, the liquid collector and liquid distributor are used together to form a liquid collection and redistribution device. The function of the liquid collector is to collect the liquid flowing from the upper packing and then send it to the liquid distributor for liquid redistribution.

2, the type of liquid collection redistributer

There are many types of liquid collection redistributers, which can be broadly divided into two categories:

The first one is the liquid collector and the liquid redistributer are independent respectively, respectively assume the task of liquid collection and redistribution;

The other is a liquid collector and redistributer which integrates liquid collecting and redistributing functions.

Liquid redistribution devices include tube type or trough type liquid redistribution device, porous type liquid redistribution device and truncated cone type liquid redistribution device used in combination with louver type liquid collector.

The simplest liquid redistribution device is the truncated cone type redistribution device, which is simple in structure and easy to install. It is generally used in the packed tower with diameter less than 0.6m to overcome the effect of wall flow on mass transfer efficiency. As the height of the packing layer in this design is 8m, it needs to be segmented, so the porous disc liquid redistributer is selected according to the actual situation. In order to prevent the liquid from the upper packing layer from flowing directly into the riser, a cap should be set on the riser.

3. Wall flow phenomenon

When the liquid flows down the packing layer, it tends to flow toward the column wall, which is called wall flow. The wall flow will lead to the uneven distribution of gas and liquid in the packing layer, which will decrease the mass transfer efficiency. In order to reduce the wall flow phenomenon, liquid redistribution devices can be set up in the packing layer at certain height intervals.

4, the classification of liquid collector

Liquid collector mainly has two kinds of inclined plate liquid collector and disc liquid collector:

Inclined plate liquid collector is characterized by large free area, small gas resistance, generally less than 2.5mm liquid column, so it is very suitable for vacuum operation;

The disc liquid collector has a slightly higher gas resistance and can be used as a gas distributor.

Inclined plate liquid collector for large tower diameter

Gas inlet and outlet device

1. Gas inlet and outlet device

The gas inlet device of the packed tower tries to disperse the gas evenly and prevent the liquid flowing from the tower from flowing into the gas pipeline. The common method is to extend the intake pipe to the center line of the tower, and the end of the pipe is a downward 45O cut or a downward notch. In this way the gas is diverted from the cut or gap.

Because the intake pipe can not make the gas distribution evenly, so can only be used in the diameter of the tower below 500㎜. For a tower with a larger diameter, the end of the intake pipe is a downward bell. For a tower with a larger diameter, gas distribution measures should be taken.

2. Characteristics of gas inlet and outlet device

The outlet device of the gas should not only ensure that the airflow is smooth, but also remove the liquid fog entrained by the gas. At present, the commonly used defogging devices are folding plate defogging device and screen defogging device. Folding plate defogger, this device is simple, the effect of defogging is better. Screen mist eliminator. This device is highly efficient and can remove droplets larger than 5um in diameter.

An outlet device for liquids

The outlet device of the packed column can not only make the liquid unobstructed (discharged) out, but also ensure the formation of liquid seal to the gas in the column, and can prevent the liquid entrain gas. The commonly used liquid outlet device can adopt water seal. When the pressure difference between inside and outside the tower is large, the inverted U-shaped tube sealing device can be used.

Defoaming device

1. Defroth device

Because the gas leaves the packed column at the top with a large number of liquid droplets and fog drops, in order to recover this part of the liquid phase, often need to set the defroth at the top.

2, the type of defroth device

Commonly used defrothing devices are as follows:

Baffle defromer, a device that uses inertia to separate droplets, is usually used in a small tower.

The cyclone plate defrosting device is composed of several fixed cyclone plates. When the gas passes through, the rotating motion is generated, resulting in a centrifugal force field. Under the action of centrifugal force, the liquid droplets move to the tower wall to realize the separation of gas and liquid. Suitable for large tower diameter and high purification requirements.

Wire mesh defromer, which is used by rolling wire into a 100-150 height disk. There are various installation methods. The pressure drop of the gas through the defrosting device is about 120-250KP. The diameter of the screen defrosting device is determined by the maximum speed of the gas through the screen.