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Optimized Wedge Wire Screen for Continuous Catalytic Reforming Unit

Abstract: A lot of continuous catalytic reforming (CCR) units have been established and commissioned along with the more high requirements of environmental protection and the development of aromatics industry. The internals applied in the reactor and re-generator which is one of the core equipment of CCR unit,such as center—pipes,inner and outer screens were mainly made up of axial screens,which is very important to keep the reaction and regenerating process.How ever many center—pipes failures of reactor and inner and outer screens for re-generator have been happened in the running unit,which was made of traditional axial screens,the normal continuous production cannot be processed.The optimized axial screens which could over come defects of traditional axial screens were described,and it could be subjected to the conditions of CCR unit

Key words:continuous catalytic reforming(CCR) unit;center pipes;inner and outer screens;Wedge wire screens

 

Catalytic reforming is one of the main process methods for producing high quality gasoline and aromatics. With the increasing environmental protection requirements in our country and the needs of the development of the aromatic hydrocarbon industry, the processing capacity of catalytic reforming has been rapidly improved, and the scale has become larger and larger.

 

According to the different ways of catalyst regeneration, catalytic reforming technology can be divided into three types: semi-regenerative reforming, cyclic regeneration and continuous regeneration reforming (referred to as continuous reforming).The liquid yield, hydrogen yield, aromatics yield and octane number of the reformed oil of the continuous reforming technology have been improved to varying degrees, with better economic benefits and market competitiveness. In recent years,the catalytic reforming units newly built in China with a scale of 400,000 t/a and above all use continuous reforming technology.Currently , the continuous reforming process technology is mainly provided by UOP , Axens and Luoyang Petrochemical Engineering Company ( Lpec ) .

 

1 Introduction to continuous reformer

In the continuous reforming process, the catalyst moves continuously between the moving bed reactor and the regenerator. The catalyst that loses activity after the reaction in the reactor will be continuously regenerated in the regenerator and then returned to the reactor, thereby ensuring the reforming reaction. continuous. The continuous reforming reaction process is carried out in a radial moving bed, the catalyst passes through the annular space formed by the fan-shaped cylinder and the central tube from top to bottom, the material radially passes through the catalyst bed from the fan-shaped cylinder (or outer mesh), and the product after the reaction The catalyst is separated from the catalyst through the central tube, and the schematic diagram is shown in Figure 1.

reactor stracture

The coke produced in the reforming reaction is deposited on the surface of the catalyst, which reduces the activity of the catalyst and requires regeneration to restore the activity of the catalyst. The catalyst regeneration process generally includes coking, chlorination renewal, roasting, reduction and other processes. The coking process is to use a regeneration gas (containing oxygen and nitrogen) to burn off the coke deposited on the catalyst, while taking away the heat released by combustion. The scorching process takes place in a re-generator. The coking reaction of the re-generator is also carried out in the radial moving bed. The catalyst to be regenerated moves slowly downward by gravity in the annular space formed by the outer mesh and the inner mesh on the upper part of the re-generator, and the regenerated gas passes through the outer mesh and reaches the The catalyst bed is charred, and the gas after charring is separated from the catalyst through the inner mesh. The schematic diagram of the charring part of the re-generator is shown in Figure 2.

wedge wire stracture

2 Reactor central tube and regenerator inner and outer nets

Reactor central tube and regenerator inner and outer nets are the core components to realize the continuous reforming process.

The central tube of the reactor is mainly composed of a punched inner cylinder, an axial strip screen (also known as an axial Johnson screen or wedge wire screen) and the like.

The regeneration process is different, and the internal network structure of the regenerator is not exactly the same. The inner mesh of UOP technology is made of inverted conical axial strip screen. The inner mesh of Axens and Lpec technology is mainly composed of punched inner cylinder and axial strip mesh, and its main components are axial strip mesh. .

The outer screen of the re-generator is also mainly composed of axial strip screen. The central tube of the reactor and the inner and outer nets of the re-generator not only play a role in ensuring the smooth progress of the reaction and regeneration, but also meet the reliability of operation under high temperature and temperature changes.

The reforming reaction is carried out at 450 ~ 550 ℃, and the temperature of regeneration scorch is as high as 450 ~ 550 ℃. Due to the high temperature and harsh working conditions, the design of the reactor central pipe and the inner and outer nets of the re-generator is very important, which is directly related to the smooth progress of the reaction and coking process, the service life of the catalyst and the performance of the catalytic reformer. No long-term safe operation. Correspondingly, as an important component of the central tube of the reactor and the inner and outer meshes of the re-generator, the reliability of the axial strip screen in the harsh conditions of the reforming reaction and the regeneration and scorching process is particularly important.

 

3 Conventional axial bar screen

3.1 Basic bar screen

The conventional axial bar screen is formed by the basic bar screen (commonly called Johnson screen or wedge wire screen). Basic strip screen processing method: Arrange the support rods (or reinforcing ribs) evenly on the circular tire, and the V-shaped cross-section wire moves in a circular motion along the circular tire, while the circular tire moves axially. During the movement, when the V-shaped cross-section wire is in contact with the support rod (or reinforcing rib), it can be connected by resistance welding to form a basic strip screen (circular strip screen). image 3. The length direction of the V-shaped cross-section wire is the hoop direction, and the support rods are equally spaced around the support rod with a certain helix angle. The support rod is axial and evenly distributed along the circumferential direction. The basic bar screen can be used directly as a filter element, an internal part of a fixed bed, etc.

wedge wire screen stracture

3.2 Structure of conventional axial strip screen The axial strip screen is formed by secondary processing of the basic strip screen, and the process is as follows. (1) Cut the basic strip screen along a certain axial section, and flatten it into a flat plate, see Figure 4.

(2) Take the length direction of the V-shaped section wire in the flat plate as the axis and roll it into a circle, the length direction of the V-shaped section wire changes from the circumferential direction to the axial direction, and the support rod changes from the axial direction to the circumferential direction, as shown in Figure 5.

(3) Weld i connecting plates on both sides of the interface, and then weld the connecting plates to form a conventional axial strip screen

wedge wire flat panel

wedge wire screen stracture2

At present, the central tube of the reactor and the inner and outer meshes of the regenerator, which have been put into industrial application, are all made of conventional axial strip screen meshes. The conventional axial bar screen is composed of V-shaped section wire, support rod and connecting plate. The length direction of the V-shaped section wire is axial and evenly distributed along the circumference. The support rod is annular and evenly distributed along the axial direction. The flow direction of the catalyst is consistent with the length direction of the V-shaped cross-section wire, reducing the wear of the catalyst

3.3 Problems with conventional axial bar screen

(1) During the process of flattening and re-rolling the basic bar screen, the welding point between the V-section wire and the support rod is damaged. Its strength is reduced, and these parts are prone to damage or even failure during operation.

(2) The structure of the longitudinally welded joint of the axial strip screen has a large mutation. Compared with other parts, the temperature difference stress formed under the condition of high temperature and large temperature gradient is easy to cause cracking and damage in this part.

(3) When starting, stopping and the temperature of the material changes, the temperature of the longitudinal welded joint of the axial strip screen is difficult to synchronize with the temperature of the surrounding V-section wire, and the resulting temperature difference stress makes its ability to resist temperature changes changes. weak.

(4) The V-section wire of the basic strip screen is wound on the support rod in the helical direction. When unfolding and re-forming the axial strip screen, the V-section wire is not completely vertical, but has a certain slope. (helix angle in circumferential motion). During the operation, the solder joints between the V-section wire and the connecting plate will be subjected to tension, pressure and bending moments caused by various loads, and it is very easy to cause excessive deformation and failure. (5) The diameter and length of the central tube and the inner and outer nets are getting larger and larger with the need for larger-scale devices. The length of the conventional axial strip screen produced by this process is limited by the basic strip screen, which will cause the central pipe and the inner and outer meshes to be segmented in the length direction, which increases the number of circumferential welded joints and thus increases the possibility of failure.

  1. The new wedge wire screen

The conventional axial strip screen has defects. During the use process, the reactor center tube and the inner and outer meshes of the re-generator in the domestic continuous reforming device are often damaged to varying degrees, which not only leads to high prices The loss of reforming catalysts has increased, and the number of abnormal plant shutdowns is also increasing.

The disadvantages of high cost of these internal parts, long manufacturing cycle and inconvenient maintenance will directly affect and restrict the safe, stable and long-term operation of the reformer. According to the above analysis, combined with the new domestic manufacturing technology, the author proposes a new type of axial strip screen, the schematic diagram of the structure is shown in Figure 6_8]. The schematic diagram of the conventional axial strip screen structure is shown in Figure 7.

wedge wire screen stracture 3

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