Formation, influence and solution of heat transfer oil coking in asphalt mixing plant

13 Oct 2019 09:01
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I. Introduction
Compared with traditional heating methods such as direct heating and steam heating, heat-conducting oil heating has the advantages of energy saving, uniform heating, high temperature control precision, low operating pressure and safety.

Therefore, since the 1980s, the development and application of China's heat transfer oil has developed rapidly. It has been heated in various industries such as chemical and chemical, petroleum processing, petrochemical, chemical fiber, textile, light industry, building materials, metallurgy, grain and oil food processing. Widely used in the system.

This paper mainly discusses the formation, hazards, influencing factors and solutions of coking during the use of heat transfer oil.

Second formation of coking
There are three main chemical reactions in the heat transfer process: thermal oxidation, thermal cracking and thermal polymerization.

Coking occurs from thermal oxidation reactions and thermal polymerization reactions.

The thermal polymerization reaction occurs because the heat transfer oil is heated during the operation of the heating system, and the reaction generates macromolecular high-boiling substances such as condensed aromatic hydrocarbons, colloids and asphaltenes, which are gradually deposited on the surface of the heater and the pipeline to form coking.

The thermal oxidation reaction mainly occurs because the heat transfer oil in the expansion tank of the open heating system contacts the air or participates in the circulation, and the reaction generates low molecular or high molecular alcohol, aldehyde, ketone, acid and other acidic components, and further generates colloid. Viscous materials such as asphaltenes, and finally coking; thermal oxidation is caused by abnormal conditions. Once it occurs, thermal cracking and thermal polymerization will be accelerated, the viscosity will increase rapidly, heat transfer efficiency will decrease, and overheating and furnace tube coking will occur.

The resulting acid can also cause corrosion and leakage of equipment.

Three coking hazards
The coking caused by the heat transfer oil will form a heat insulation layer, and the air flow pulverizer will cause the heat transfer coefficient to decrease, the exhaust gas temperature to increase, and the fuel consumption to increase; on the other hand, since the temperature required for the production process remains unchanged, the heating furnace The temperature of the pipe wall will rise sharply, causing the furnace tube to bulge and rupture, and finally the furnace tube will be burned through, causing the furnace to ignite and explode, causing serious accidents such as equipment and operator personal injury.

In recent years, such accidents have not been common.

Four factors affecting coking
(1) Heat transfer oil quality
Through the analysis of the formation process of the above coking, it is found that the oxidation stability and thermal stability of the heat transfer oil are inseparable from the coking speed and quantity.

Many fire and explosion accidents are caused by poor thermal stability and oxidation stability of the heat transfer oil, causing severe coking during operation.

(2) Design and installation of heating system
Whether the various parameters and equipment provided by the heating system design are reasonable and directly affect the coking tendency of the heat transfer oil.

The installation of each device is different, which will also affect the life of the heat transfer oil.

Equipment installation must be reasonable, and timely rectification during commissioning is beneficial to extend the life of the heat transfer oil.

(3) Daily operation and maintenance of the heating system
Different operators have different objective conditions such as education level and technical level. Even if the same heating equipment and heat transfer oil are used, the control level of the heating system temperature and flow rate are not the same.

Temperature is an important parameter for thermal oxidation reaction and thermal polymerization of heat transfer oil.

As the temperature increases, the reaction rate of the two reactions increases sharply, and the tendency to coke increases.

According to the relevant theory of chemical engineering principles: as the Reynolds number increases, the coking rate slows down.

The Reynolds number is proportional to the flow rate of the heat transfer oil.

Therefore, the larger the flow rate of the heat transfer oil, the slower the coking.

Five coking solutions
In order to slow down the formation of coke and extend the mission of using heat transfer oil, measures should be taken from the following aspects:
(1) Select a suitable grade of heat transfer oil to monitor the trend of physical and chemical indicators
The heat-conducting oil is divided according to the highest operating temperature. The mineral-type heat-conducting oil mainly has three grades of L-QB280, L-QB300 and L-QC320, and the maximum use temperatures are 280 ° C, 300 ° C and 320 ° C, respectively.

The heat transfer oil of the appropriate grade and quality conforming to the SH/T 0677-1999 “heat transfer fluid” standard should be selected according to the maximum heating temperature of the heating system.

At present, the recommended maximum use temperature of some commercially available heat transfer oils is quite different from the actual measurement results, which is misleading to the user, and safety accidents occur when it occurs, which should attract the attention of the majority of users!
Heat transfer oils formulated with refined heat-stable refined base oils and high-temperature antioxidants and anti-scaling additives should be used.

The high temperature antioxidant can effectively delay the oxidation thickening during the operation of the heat transfer oil; the high temperature antiscalant can dissolve the coke in the furnace tube and the pipeline, disperse it in the heat transfer oil, and finally pass through the bypass filter of the system. Filter it to keep the tubes and tubing clean.

After three months or six months of use, the heat transfer oil should be traced and analyzed for its viscosity, flash point, acid value and carbon residue. When two of them exceed the specified limit (the residual carbon is not more than 1.
5%, acid value is not greater than 0.
When 5mgKOH/g, the flash point change rate is not more than 20%, and the viscosity change rate is not more than 15%), it is considered to add some new oil or all oil change.

(2) Reasonable design and installation of the heating system
The design and installation of the heat transfer oil heating system shall strictly implement the design rules of the hot oil furnace formulated by the relevant state departments to ensure the safe operation of the heating system.

(3) Standardize the daily operation of the heating system
The daily operation of the heat transfer oil heating system shall strictly implement the safety technical supervision procedures of the organic heat carrier furnace formulated by the relevant state departments, and monitor the changing trend of the temperature and flow rate of the heat transfer oil in the heating system at any time.

In actual use, the average temperature at the outlet of the furnace should be at least 20 ° C lower than the maximum temperature of the heat transfer oil.

The temperature of the heat transfer oil in the expansion tank of the open system should be lower than 60 °C, and the maximum temperature should not exceed 180 °C.

The flow rate of the heat transfer oil in the hot oil furnace should not be lower than 2.
5 m / s, to increase the degree of thermal oil turbulence, reduce the thickness of the stagnation bottom layer and convective heat transfer in the heat transfer boundary layer, improve the convective heat transfer coefficient, and achieve the purpose of enhancing fluid heat transfer.

(4) Cleaning of the heating system
The thermal oxidation and thermal polymerization products first form a polymeric high carbon viscous material that adheres to the tube wall and can be removed by chemical cleaning.

The high carbon viscous further forms incompletely graphitized deposits, and chemical cleaning is only effective for parts that have not been carbonized.

Fully formed graphitized coke.

Chemical cleaning of such substances has not solved the problem, and mechanical cleaning is often used abroad.

It should be checked frequently during use. When the formed high carbon viscous material has not been carbonized, the user can purchase a chemical cleaning agent for cleaning.

Six conclusions
1.
The coking caused by the heat transfer oil during heat transfer is derived from the reaction products of the thermal oxidation reaction and the thermal polymerization reaction.

2.
The coking of the heat transfer oil causes a problem of a decrease in the heat transfer coefficient of the heating system, an increase in the exhaust gas temperature, and an increase in fuel consumption, which may cause accidents such as fire, explosion, and personal injury of the operator.

3.
In order to slow the formation rate of coking, a heat-conducting oil formulated with a refined heat-stable refined base oil and a high-temperature anti-oxidation and anti-scaling additive should be selected.

For the user, the product whose maximum use temperature is determined by the authority's inspection should be selected.

4.
Reasonable design and installation of the heating system, the daily operation of the heating system should be standardized during the use, and the viscosity, flash point, acid value and carbon residue of the heat transfer oil during operation should be checked regularly to observe the change trend.

5.
A chemical cleaner can be used to clean the coking that has not been carbonized in the heating system.

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