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Structure Part2 | Reactor design

By WHGCM September 28th, 2023 149 views

Continuing on from the previous section - the heat transfer device


(2) Jacket

A jacket is the most commonly used heat transfer structure in stirred reactor design. The reactor jacket is composed of a cylindrical shell and a bottom head. There are two ways to connect the jacket with the inner cylinder: a detachable connection and a non-detachable (welding) connection. The detachable connection structure is used in poor operating conditions, or the requirement of regular inspection of the outer surface of the inner cylinder and the need for frequent cleaning of the jacket occasions. The detachable connection is the inner cylinder and jacket through the flange to connect.
The non-removable connection is mainly used for carbon steel reactors. The jacket is connected to the inner cylinder by welding. A non-detachable connection is reliable in sealing and simple in manufacturing and processing. The jacket is equipped with an inlet and outlet of steam, cooling water or other heating and cooling medium. When the heating medium is steam, the inlet pipe should be close to the upper end of the jacket, and the condensate is discharged from the bottom; when the heating (cooling) medium is liquid, the inlet pipe should be located at the bottom, so that the liquid is fed in from the bottom and out from the top, which is conducive to discharging the gas and filling the liquid.
The material of the Jacket could be carbon steel or glass. They are not required for corrosion resistance, cause the jacket is usually not contact the reactant directly. But, because of the jacket need to stand with certain pressure, the material needs to resist pressure, so a certain anti-force feature is needed. 
In design practice,  engineers usually count out the force from the pressure, material and reactor vessel's size. Then use specialty tools software to get the thickness of the jacket, which has described in a previous article already.
WHGCM usually applies a non-removable connection jacket because of its advantages of easy operating and non-maintains requirements. 
From the profile of the reactor, we could identify if it does have a Jacket. 
The reactor that has no jacket is usually smaller in diameter, its height/dia ratio is bigger than the reactor with a jacket. So, it looks very slim.  
Conversely, it is difficult to tell if a reactor is jacketed or not. This is because some reactors are not jacketed but have an insulation layer. The insulation layer is also attached to the outside of the reactor cylinder like the jacket does. Simply from the appearance, we can't tell whether its external structure is an insulation layer or jacket.

(3) Reactor Coil

The reactor coil in this article is not the one in Iron Man. It is a component used on the reactor vessel for heating and transforming. 
Iron Man's coil is for generating electromagnetic force and immobilizing radioactive reactive materials. And the coils here are for regulating temperature, they're not the same thing at all.
In the chemical reactor design, if it is required a large heat transfer area, while the jacket heat transfer can not meet the requirements or it is not suitable to use the jacket heat transfer, we can apply the coil as heat transfer for the reactor during reactor design. 
The coils are either placed inside the reactor or will be welded to the outside of the reactor.
When the coil is placed in the vessel/kettle, immersed in the reactant medium, the heat can be fully utilized, and the heat transfer effect is better than the jacket structure. 
Because it is in direct contact with the reactants, if the reactants are corrosive, then the material of the coil must be corrosion-resistant.
Also, in large kettles, the coils are usually large and care should be taken while designing by avoiding the agitator. At the same time, in order to prevent deformation of the coil, it is necessary to install a bracket for the coil and fix the bracket to the kettle.
It can be seen that locating the coil inside the kettle will increase thermal efficiency but will cause difficulties in manufacturing as well as future cleaning and maintenance. In industrial production, generally, the reaction is used in the easier-to-clean materials, such as soluble or non-adhesive liquid media, or gas-liquid two-phase media using the built-in coil.
The coil in the cylinder body needs to be fixed, fixed in a variety of forms. When the central diameter of the coiler is small, the number of circles is less, the coiler can use the import and export pipe fixed in the kettle cover or kettle bottom.
However, it is difficult to overhaul the coil, and the heat transfer effect may be reduced due to the accumulation of condensate. The coil and jacket can be used at the same time to increase the heat transfer effect. Coil generally by the nominal diameter of 25 ~ 70mm seamless steel tube winding. Commonly used structural shapes are circular spiral, plane ring, U-shaped vertical coiler, spring concentric circle group parallel form coiler.
The import and export of the coiler should be located at the same end, generally at the upper head, so as to make the structure simple, easy to install, and dismantle.
There is also a form where the coil is welded to the outside of the kettle. The coil basically wraps around about two-thirds of the cylinder. Although this form of coil is not as high as the built-in coil in thermally efficient, its material requirements are limited to the needs of the heat-conducting medium only. So the materials are generally relatively inexpensive, and it is usually used in very large quantities accordingly.
Also, the externally welded coils are in the form of half-pipes, so it is required to be welded to the kettle body throughout out the lines. Compared to the built-in coil, it needs obviously more work, but because of the material saving and low the cost of labor,  the external coiled kettle is cheaper than the kettle with built-in coils in many cases.

(4)Reactor Kettle lid 

The lid ( cover head ) of the reactor is often made detachable to meet the needs of assembly and disassembly. In WHGCM, we usually called this kind of lid style an Open Style Reactor. Accordingly, another type of top cover is welded to the cylinder and cannot be opened, which we call a Closed Style Reactor.
The Open Style Reactor, its lid is connected to the cylinder through a flange. With a jacketed reactor, most of the receiver opens in the top cover. In addition, most of the kettle transmission device is also directly supported on the top cover. Therefore, the top cover must have enough strength and rigidity. The structural forms of the top cover are flat cover, disc-shaped cover, conical cover, and the most used is oval cover.
For Closed Style Reactors, in order to minimize the stress on the top cover. Generally, an oval lid is used, and sometimes also use a semi-circular lid.

(5)Nozzle ports on the reactor

There are many purposes of using the Nozzle ports  
The ports for the material inlet and outlet,  are called as feeding ports and discharging ports.
For installation and maintenance, there is a manhole or handhole. 
To observe the material mixing and reaction state, the sight glass port is applied. 
For the measurement of the reaction temperature, we could make the thermometer port on the reactor.
In order to ensure safety, some ports could also be opened to assemble with safety devices, and so on.


Other Parts

1、 Feeding pipe 

The feeding pipe is connected to the feeding port of the reactor.
The feed pipe is generally located at the top side. The lower end of the feed pipe is generally 45 ° cut to prevent material flow along the wall.

2、Discharging pipe 

The discharging pipes connect with the discharging port. They have two types, the upper discharging pipe, and the lower discharging pipe. 
The lower discharge is suitable for viscous or contains solid particles of the medium. When the material needs to be transported to clean the higher position or needs to be closed transport, must be equipped with a pressure tube, so that the material could be discharged from the upper part. 
The upper discharge often uses compressed air or other noble gas, the material from the kettle through the pressure tube pressure to the next process equipment. In order to make the material excluded cleanly, should make the lower end of the pressure out of the tube position as low as possible, and the bottom is made and the bottom of the kettle similar shape.

3. Stirring device of reactor/reaction kettle

The stirring device is the key part of the reaction kettle. The reactants in the reaction kettle are stirred with the help of a stirrer to achieve the purpose of fully mixing the materials, enhancing the collision of material molecules, accelerating the reaction rate, strengthening the effect of mass transfer and heat transfer, and promoting the chemical reaction. 
Therefore, the design and selection of a reasonable stirring device is an important means to improve the production capacity of the reactor. A stirring device usually includes a stirrer, stirring shaft, supporting structure, as well as baffle, guide cylinder, and other components. China has standardized the production of the main parts of the stirring device for use.

(1) Type of agitators

① Propulsive agitator/Propeller agitator/Propeller Stirrer 

The shape of the propulsive agitator is similar to that of a ship propeller, Figure 1-4.

The propulsive agitator is generally made by integral casting method, commonly used materials are cast iron or stainless steel, or welded molding can also be used. The upper surface of the paddle is a spiral surface, and the number of blades is generally three. The diameter of the paddle is small, generally about 1/3 of the inner diameter of the cylinder, the width is larger and gradually widens from the root to the outside. The propulsive stirrer is simple in structure, convenient in manufacturing and processing, and it produces axial movement of the liquid when it is working, with a small shearing effect of the liquid and a good effect of up and down tumbling. Mainly suitable for low viscosity, large flow rate occasions. 
Figure 1-4 Propulsive Agitator
Figure 1-4 Propulsive Agitator

② Paddle Stirrer/Paddle Agitator 

Paddle Agitators (Figure 1-5) have a simple structure and are generally made of flat steel or angle steel, alloy steel, non-ferrous metals, etc. The paddle agitators are divided into two types: flat blade and folded blade, as shown in Figure 1-5. According to the installation method of the paddle, the paddle agitator is divided into two types: flat blade and folded blade, as shown in Figure 1-5.
Figure 1-5 paddle types
A flat blade and the direction of rotation of the blade perpendicular to the main tangential direction of the flow of materials, if added with a baffle can also produce a certain degree of axial mixing effect. Folded blade type and the direction of rotation into a tilt angle, resulting in more axial shunt than the flat blade. A small paddle and shaft connection is often used to weld, that is, the paddle is directly welded to the hub, and then the key, and stop screws will be connected to the hub on the mixing shaft. The larger diameter paddle and mixing shaft connection is mostly detachable connection. One end of the paddle to make half of the collar, and two paddles against the bolt to clamp the collar on the mixing shaft.

③ Turbine agitator 

The structure of the turbine agitator is shown in Figure 1-6. The turbine structure is like the impeller of the centrifugal pump, and the blades on the blades have the shapes of flat and curved. Turbine agitators form more, and can be divided into two categories of open type and disc. The diameter of the paddle of the turbine agitator is generally 0.25 to 0.5 times of the inner diameter of the cylinder, and generally below 700mm. The ratio of the outer diameter, width, and height of the paddles is generally 20:5:4, and the circumferential speed is generally 3-8 m/s. The turbine agitator is suitable for the mixing operation of various viscous materials.
Figure 1-6 Turbine Agitator
Figure 1-6 Turbine Agitator

④ Anchor, Frame, and Ribbon Agitators 

Anchor-type agitator consists of vertical paddles and horizontal paddles shaped in the same way as the bottom head (Figure 1-7). The agitator can be keyed to the shaft and then the shaft end cap can be screwed on from the lower end of the shaft. If the paddles of an anchor agitator are reinforced with crossbeams, it becomes a frame agitator, see Figure 1-7(b) and Figure 1-7(c).
Figure 1-7(b) for the single-stage type, and Figure 1-7(c) for the multi-stage type. Anchor and frame stirrer's common feature is the rotating part of the larger diameter, up to 0.9 times the diameter of the cylinder body, generally taking D / B = 10 ~ 14. Due to the larger diameter, can make the kettle the whole liquid layer to form turbulence, and reduce sedimentation or agglomeration, so in the reaction kettle the application of more.
Figure 1-7 Anchor Agitator
Figure 1-7 Anchor Agitator
The Spiral Belt Agitator is made of spiral belt, shaft sleeve, and support rod, as shown in Figure 1-8.
Its paddle is a spiral belt with a certain width and a certain pitch, which is connected to the mixing shaft through the transverse tie rod. The outer diameter of the spiral belt is close to the inner diameter of the cylinder, when stirring the liquid shows complex movement, mixing and mass transfer effect is better.
Figure 1-8 Spiral belt Agitator
Figure 1-8 Spiral belt Agitator

(2) Standard and selection of Agitator

①Standard of Agitator 

Since there are many kinds of stirring processes, operating conditions, and media, the stirrers are of various types. 
In order to ensure the production quality of Agitator/Stirrers, reduce manufacturing costs and increase the interchangeability of parts. There are a lot of corresponding standards for the structural forms of several commonly used stirrers and technical conditions for standard stirrers. 
In China, the current agitator/stirrer standards are agitator/stirrer form and basic parameters (HG/T 3796.1-2006), stirring shaft diameter series (HG/T 3796.2-2006), paddle stirrer (HG/T 3796.3-2006), open turbine agitator/stirrer (HG/T 3796.4-2006), disc turbine agitator (HG/T 3796.5-2006), propeller-type agitator (HG/T 3796.8-2006), anchor frame agitator (HG/T 3796.12), and the series of mixing shaft diameter (HG/T 3796.2-2006). (HG/T 3796.12-2006).
The content of the standard agitator includes three parts: structural form, basic parameters, and dimensions, technical requirements, drawing catalog, and so on. Standard agitators can be selected according to the production requirements when needed.

② Agitator type selection 

As the factors affecting the mixing process and effect are extremely complex, involving fluid flow, mass transfer, heat transfer, and many other aspects. A variety of selection data are based on their own experimental focus on the basis of the conclusion is not the same, mostly with empirical. The actual selection, according to the flow state, stirring purpose, stirring capacity, speed range, and the highest viscosity of the liquid, etc., consult the relevant tool manual to determine.

③ Stirring shaft 

The stirring shaft is a component that connects the reducer and the stirrer, to transmit the power. The agitator shaft is a non-standard part and needs to be customized. As a custom reactor manufacturer in China, WHGCM is also one of the main reactor agitator suppliers and fabricators. We are able to customize all kinds of agitators as per the reaction required.
The material of the agitator shaft is basically No.45 steel. If strength is low, using Q235 steel is also fine. When the medium is corrosive or iron ion pollution is not allowed, we should use acid-resistant stainless steel or anti-corrosion measures for the normal steel. 
The structure of the mixing shaft is the same as the general mechanical drive shaft. The mixing shaft generally adopts a round cross-section solid shaft or hollow shaft. Its structure would be determined by the type of agitator, the shaft support form, and the coupling requirements. 
The mixing shaft is usually supported by a pair of bearings in the gearbox, and the supporting style is a cantilever beam.
The mixing shaft is often thin and long, and it is for the driving of the stirrer. 
The stirring shaft works under the joint efforts of bending and torsion. 
When deformation is serious, it will produce a large centrifugal force and the shaft would not rotate normally, and even be damaged. 
If the tolerance of shaft diameter is big, the agitator has passed the balance test, and low speed is allowed, then we could choose the uplift of the tolerance.
If the above conditions are not permitted, then we should install the middle bearing or bottom bearing. 
The mixing shaft diameter needs to be double-checked by strength calculations, stiffness calculations, and critical speed calculations. And we should consider the medium corrosion conditions to the shaft too.
a. Double-check the mixing shaft diameter with strength conditions 
The mixing shaft is under the joint effective of torsion and bending. The shaft will be damaged if the shaft cross-section of the shear stress is too large. So the maximum shear stress should be less than the material's allowable shear stress.
b. Double-check the mixing shaft diameter with the stiffness condition. 
The mixing shaft is subject to the joint action of torque and bending moments. The oversize of torsional deformation will make the shaft to vibrate, and distort. And the shaft will fail to seal well.
So the maximum torsion angle of the unit length needs to remain inside the allowable range. 
In order to install the components, or to meet manufacturing needs, the stirring shaft usually has a keyway, shoulder, threaded holes, chamfering, retreat groove, and other structures. 
The structures would weaken the cross-section of the load-bearing capacity. 
So the diameter of the shaft should be appropriately enlarged in accordance with the calculated diameter. 
And at the same time, double-checks on the diameter with critical speed and the radial displacement are also needed.

(3) Baffle and deflector

① Baffle plate 

The installation of the baffle can change the tangential flow into axial and radial flow of the fluid. It could increase the turbulence of the liquid as well. Then the mixing effect is improved.

② Guide cylinder

 
The deflector is a cylinder installed outside the agitator. 
It is commonly used in propeller-type and turbine-type agitators. The role of the deflector is to make the liquid discharged by the agitator form an up-and-down cycle flow inside and outside the deflector.
By this means, fluid turbulence is increased, while the chance of short-cut is reduced. So it could increase the circulation flow rate and control the flow pattern.

4. Reactor transmission device

The reactor's transmission device is usually on the reactor's lid.
It is usually using vertical deployment.  The transmission device includes the motor, reducer, bracket, coupling, stirring shaft, etc., as shown in Figure 1-9. 
Figure 1-9 Transmission device of the reaction kettle
Figure 1-9 Transmission device of the reaction kettle
1-motor; 2-reducer; 3-coupling; 4-support; 5-stirring shaft; 6-axis sealing device; 7-flange; 8 -lid
The function of the transmission device is to transfer the rotation from the motor to the stirrer.
The ratio speed is adjusted to the mixing speed required by the reducer. Then the coupling will take the stirring shaft to rotate. And make the agitator rotate at last.

(1) Motor selecting

The motor of the reactor is usually used with the support of reducer. So the selections of the motor and reducer are done at the same time.  As a reactor fabricator, we supply the motor and reducer together during the reactor fabrication.
During the reactor design, the engineer usually selects the motor as per reducer. And the motor type should be selected according to the motor power, working environment, and other factors. 
The working environment includes explosion-proof, protection level, corrosion grade, and so on. 
So, the motor selection is mainly to confirm the series, power, speed, installation method, etc.
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