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What is cryogenic reactor

By GCM October 21st, 2023 128 views

I, What’s the scope of cryogenic temperature (deep cold )?

When the temperature is below -153°C (120K), it is called cryogenic temperature.

II,What is a cryogenic reactor?

As you can see from the range of deep cooling, any reactor that can work stably at temperatures below -153°C can be named a deep-cooled reactor.

However, a temperature of -153°C is very low. In practice, as long as the reactor can support the working temperature below -80°C, we generally call it a cryogenic reactor.
cryogenic reactor temperature range

III, How to tell a cryogenic reactor from a regular reactor at a glance?

Just look at the size of their vessel. Cryogenic reactors have a much larger belly than ordinary reactors.
Profile difference of cryogenic reactor and ordinary reactor

Of course, it does not mean that all reactors with a small height-to-diameter ratio are deep-cooled reactors.
The main reason is that most cryogenic reactors are required to evaporate at low temperatures, and a low aspect ratio has the advantage of enhancing the evaporation effect.

IV, Structure

The structure of the cryogenic reactor is similar to that of a conventional reactor, including reactors, agitators, coolers, and other components.
However, it is also equipped with special components such as a refrigeration system, temperature control system, and pressure control system to ensure that it can operate stably at extremely low temperatures.

In fact, together with the jacket, insulation layer, inner coils, refrigeration system, and temperature control system, these five components constitute the cryogenic system, which is the main difference between the deep-cooled reactor and the ordinary reactor.


A cryogenic system is more than just cooling. Instead, it contains heaters. The heater works in conjunction with the refrigeration system. The operator can regulate the temperature of the cryogenic reactor vessel through a temperature control system.

The vast majority of cryogenic systems, the refrigeration system, and the heating system are separated.

WHGCM adopts an integrated high and low temperature circulator to combine the refrigeration system and heating system into one. Furthermore, this equipment also integrates an intelligent temperature control system. So, it can realize the free rise and fall of temperature, the temperature range reaches -196°C to 350°C, and the temperature control precision can be +/-0.1°C.

High and low temperature circulation equipment

Advantages and functions of refrigeration heating temperature control system:

Temperature range from -196 to 350,

Superior performance, high precision, and intelligent Temperature control.

Multi-functional alarm system and safety features, 7-inch, 10-inch color TFT touch screen graphic display, the use of magnetic drive pump, no shaft seal leakage problems.

High temperature cooling technology, can be from 300 direct refrigeration cooling

The heat-conducting medium won’t be oxidized or absorbs water in the air.

Heating Power KW Cooling capacity kW  AT Optional Power Supplies
2.5 to 200

0.45~ 38  at  -95 ~ 100

110V220V230V380V440V460V

The cryogenic reactor has the characteristics of rapid heating, high temperature resistance, corrosion resistance, sanitation, and no environmental pollution.


V, Application:

The reactor is widely used in chemical, pharmaceutical, semiconductor, and other fields.

5.1 In the field of chemistry

it can be used to synthesize high-purity organic compounds, polymers, metal-organic complexes, etc.

5.2 In the field of medicine.

it can be used to prepare high-purity pharmaceutical raw materials, biological products, etc.

5.3 In the semiconductor field.

it can be used to prepare high-purity semiconductor materials, coating materials, etc.
Note:

During the use of the reactor, some safety issues need to be paid attention to.
Due to its extremely low working temperature, dangerous situations such as the evaporation of liquid nitrogen may occur, so operating procedures and safe operating procedures must be strictly followed.

VI, Developing history of Cryogenic Pressure Vessel Technology

6.1 Causes

Cryogenic technology was developed in the liquid air industry. The key equipment for cryogenic industrial processes is the cryogenic pressure vessel.

6.2 Low material selection

Cryogenic pressure vessels are used at low temperatures. The toughness and plasticity of steel at low temperatures are lower than them at room temperature, while brittleness is higher.

There would be a suddenly happened brittle fracture under low stress at the position of sharp notch or defect, when a low-temperature pressure vessel is using below a certain temperature.

The brittle fracture occurs in the in production equipment such as pressure vessels, chemical equipment, pipelines, etc. can cause huge losses.

6.3 Design difficulty

The design of low temperature pressure vessels is more complex than the design of normal temperature vessels.

Need to comprehensively consider many aspects including design temperature, the choice of materials, structural design, welding material selection, manufacturing inspection, post-weld stress relief heat treatment, and so on.

VII, Points for the design of low-temperature pressure vessels

7.1 Determination of design temperature

Design temperature below -20 ℃ is the key parameter to know whether carbon steel, low alloy steel, duplex stainless steel and ferritic stainless steel vessel is a low-temperature vessel.

Design temperature below -196 ℃ is to determine whether the austenitic stainless steel vessel is the low-temperature vessel.

In the design,  the relevant factors could affect the temperature of the container are need to be fully examed and analyzed. The factors including the container's employing location, the installation location is indoor or outdoor, the effect to  the container shell metal temperature by normal operating temperature, and the effect to metal by temperature of the medium inside the vessel.

7.2 Key points of material selection

The main failure mode of cryogenic pressure vessels is brittle fracture. The brittleness of steel at low temperatures is increased , toughness of steel is reduced. So, the chosen of vessel materials and at low temperature is hard than it at non-low temperature.

The steel for low temperature application should be very strict in the smelting method, chemical composition, heat treatment state and other aspects, and the factor of impact at low temperature to steel is also required.

For the steel with the lower limit of tensile strength of less than 540MP at design temperature below -20 ℃, its chemical composition is  P ≤ 0.025%, S ≤ 0.012%;

For the steel with  the lower limit of tensile strength greater than 540MP at design temperature below -20 ℃, it should be with P ≤ 0.02%, S ≤ 0.01%.

The welding material used between pressure components, or between non-pressurized component and pressure component should be steel with good welding performance.

7.2.1 Selection of steel according to service temperature

7.2.1.1 Design temperature -40℃ ~ -20℃

This type of container is represented by 16MnDR steel, which is mainly used for air storage tanks and nitrogen storage tanks installed outdoors subject to ambient temperature.

7.2.1.2 Design temperature -196℃~-40℃

Ni series of low-temperature steel can be selected for the manufacture of low-temperature vessels.

For example: -70℃ use 09MnNiDR, -100℃ use 08Ni3DR, -196℃ use 06Ni9DR.

7.2.1.3 Design temperature -273℃ ~ -196℃.

Austenitic stainless steel is usually selected for vessel fabrication. Such as S30408 and other austenitic stainless steel.

7.3 Structural design points

7.3.1 Points to be observed in the structural design of the vessel

7.3.1.1
The structure should be as simple as possible to minimize constraints, avoid excess additional stresses, avoid excessive temperature changes, and select materials consistently.
7.3.1.2
Sudden changes in the shape of the structure should be avoided as much as possible to reduce local stresses, which are a key cause of cracking.
7.3.1.3
The angle weld at the connection between the receiver and the shell should be concave and smooth transition, and the inner wall at the end of the receiver should be rounded.
7.3.1.4
Reinforcement of the receiver should adopt the whole reinforcement or thick-walled pipe reinforcement as far as possible. If the reinforcement ring structure is adopted, it should be full weld-through structure with smooth transition of weld seam. 
7.3.1.5
Welding of the support with the shell should be set up with pad.

7.3.2 Design of sealing fasteners

The stud fasteners for low temperature pressure vessel flange shall not be general ferritic commodity fasteners.
photo of Studs  -40℃~-70℃ Stud material is 35CrMoA.
 -70 ℃ ~ -100 ℃ stud material for 30CrMoA.

Proposed sulfur and phosphorus chemical composition limit content, and do low-temperature impact test. Design temperature below the above temperature should be used austenitic steel stud.

Stud should be used in the center of the unthreaded part of the core rod diameter is not greater than 0.95 times the root diameter of the thread or the full thread of the elastic stud.

7.3.3 Design of sealing gasket

Non-metallic materials with good elasticity and plasticity at low temperatures should be used, e.g., asbestos rubber sheet, flexible graphite, etc.
photo of non metallic material sealing gasket

At temperatures below -40℃, the metal material of the sealing gasket (such as metal shell of metal clad gasket, metal tape of winding gasket and solid metal pad) should use the metal without significant transformation in low temperature. For example: austenitic stainless steel, copper, aluminum, and so on.

7.3.4 Design of welded joints and selection of welding consumables

7.3.4.1
Welded joints are divided into five categories, A/B/C/D/E, each with different design requirements.

Classification criteria:

According to the size and nature of the force beared by the various welding joints of vessel when it is working at the low-temperature. "Pressure Vessel" GB150-2011 Standard.

The connecting joints between the base, supporting lugs, brackets, gaskets and other non-pressurized accessories and the inner and outer shell walls of the vessel are specified as Class E welded joints.

Category A welded joints shall be double-welded butt joint, or the double-sided welding guarantee the entire penetration, which has the same quality as double-sided welding.

If one-sided welding with a pad is used, the pad must be removed after welding.

Category B welded joints, although subjected to axial stresses, it needs to have the same structural design as category A welded joints.

Category C welded joints are full penetration welding structure regardless of which form is applied. Flange and cylinder, receiver, such as the use of downhand welding structure when the full cross-section welded structure, applicable to the design temperature is not less than -40 ℃, and the design pressure is not greater than 4.0MPa conditions.

Category D welded joints receiver and container wall angle joints should be complete penetration structure.

7.3.4.2
Welding between pressurized elements or pressurized and non-pressurized elements of low-temperature pressure vessels.

Use manual arc welding electrodes, choose low hydrogen alkaline welding electrodes.

Adopt submerged arc automatic welding, use alkaline or neutral flux.

7.3.4.3
Performance requirements for welded joints of dissimilar steels in low-temperature pressure vessels.

Ferrite steel and austenitic steel direct welding. 

Due to the line expansion coefficient difference between the ferrite steel and austenitic steel is large, it will cause temperature difference stress in the low temperature occasions. And the carbon of ferrite side will move to the weld metal  in the fusion welding, the strength at ferrite side and  plasticity weld metal are all reduced.

So, the following requirements should be followed when welding.

7.3.4.3.1 Generally, Cr23Ni13 or Cr26Ni21-type high-chromium-nickel or nickel-based welding consumables shall be selected, and no further stress-relief heat treatment shall be carried out in principle after welding. 

7.3.4.3.2 Welding process evaluation of this type of dissimilar steel and product welding test plate shall meet the following requirements.

- Welded joint tensile strength is not less than the smaller value of the lowest tensile strength in the both sides base material.

- The impact work at fusion line and  heat-affected zone at ferritic side should be determined by the V-notch impact work value KV2 corresponding to the ferritic steel tensile strength.

- The joint shall be subjected to a side bending test, with the diameter of the bend center equal to 4 times the thickness of the specimen, 180°. There shall be no cracking defects exceeding 1.5mm measured in any direction on the tensile surface after the bending test, and there shall be no cracking defects exceeding 3mm at the fusion line.

7.3.5 Manufacturing inspection

Low-temperature container welding should be strictly controlled line energy, usually using a thin welding rod. For small welding line energy it shall be multi-channel welding.

When low-temperature pressure vessel butt welds are allowed to use local flaw detection, inspection length shall not be less than 50% of the length of the welded joints. This is for safety based on the characteristics of low-temperature pressure vessels,  whose testing length is different with ambient temperature vessels.

It is not allowed to carry out in accordance with the normal temperature container detection length standard, that is, the use of not less than 20% of the length of each welded joint standard is wrong.

Low-temperature vessel with class A longitudinal welded joints, the  welded specimens should be prepared one-by-one. In the production, it shall be welded by the same welder, with the same material, the same welding process and conditions, and shall not be produced by other welders or made up after producing.

7.3.6 Post-welding stress relief heat treatment

Post-welding heat treatment can eliminate welding residual stresses generated during the welding, improve the mechanical properties of welded joints, and reduce the tendency of low-temperature brittle fracture of steel.

According to the design and manufacture standard heat treatment regulations, if there are requirements on the material of low temperature container and the thickness of welded joints, the container must do stress relief heat treatment.

 

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