Prevention and cleaning of coking of the hottest h

Prevention and cleaning of coking of heat transfer oil

heat transfer oil heating instead of steam heating has a wide promotion and application prospect in all walks of life. At present, it has been used in wood-based panel and petrochemical industry. Therefore, ternary materials will be well used in textile, chemical fiber, printing and dyeing, light industry, crude oil transportation, food, highway engineering, building materials and other industries in the future. Heat transfer oil is an organic heat carrier. As a medium, it circulates in the liquid phase in the heat exchange equipment system to transfer heat. Heat transfer oil is also an excellent heat transfer medium. It has the heat transfer performance of high temperature and low pressure, has significant energy-saving effect, and has the advantages of high stability, good heat transfer, high thermal efficiency, high boiling point under low pressure, good safety under normal pressure at 350 ℃, large heat capacity, and low operation and maintenance costs

the coking process of heat transfer oil under high temperature includes induction, adsorption, hardening, falling off and other steps. Coking reduces the heat transfer rate of the furnace tube, reduces the thermal efficiency of the heat transfer oil furnace, causes energy waste, and increases the temperature difference inside and outside the furnace tube. When the outer wall temperature of the furnace tube reaches 600~700 ℃, it is easy to burn through the furnace tube, causing fire accidents, equipment damage, and even casualties. Coking is the enemy of heat transfer oil furnace. How to prevent coking of heat transfer oil furnace and cleaning after coking is a problem worth studying

I. composition of heat transfer oil

heat transfer oil is generally refined by adding various additives to the base oil. The base oil accounts for more than 90% of the total heat transfer oil and plays a decisive role in the performance of heat transfer oil. The proportion of additives is small, but it can strengthen or give some special properties to heat transfer oil

the base oil of heat transfer oil mostly adopts the refined narrow distillation range crude oil fraction with high aromatic hydrocarbon content, such as the high boiling point residual oil of high-temperature cracking of crude oil and catalytic cracking diesel oil. After acid-base clay refining or hydrofining, the base oil with low acid value, low chromaticity and low boiling point can be obtained. The sealing ring or combination pad should be replaced in time, and the base oil with low component content. The crude oil fraction with high aromatic hydrocarbon content is selected because this oil has good thermal stability, and the order of thermal stability of various hydrocarbons is aromatic cycloalkanes alkanes. With the increase of flash point and initial boiling point of crude oil fraction, the viscosity and freezing point of base oil also increase correspondingly. Selecting narrow distillation range fraction has good low-temperature fluidity and freezing point while ensuring high flash point and initial boiling point of base oil

when producing heat transfer oil, it is also necessary to add appropriate high-temperature antioxidant with good compatibility and composite coke inhibitor cleaner to better improve and improve the thermal stability and anti-oxidation stability of heat transfer oil. In order to ensure the normal use of heat transfer oil in the northern winter, an appropriate amount of pour point depressant can be added as needed to reduce the freezing point of heat transfer oil to a certain extent. In addition, the alkaline component in the additive has a certain neutralization effect on the acids generated during the use of heat transfer oil

II. Index analysis of heat transfer oil

the indexes of heat transfer oil mainly include idle point, acid value, viscosity and carbon residue

    1. Flash point

flash point is the lowest temperature that guides the flash of the mixture formed by oil vapor and surrounding air when the hot oil is close to the flame under certain heating conditions. Its size identifies the evaporation tendency and safety of heat transfer oil. When the flash point is low, the fraction in the oil is light, the evaporation is large, and the safety is small; When the flash point is high, the distillate in the oil is heavy, the evaporation is small, and the safety is high. The flash point of different brands varies greatly, the lowest is only 1400 ℃, and the highest is more than 2000 ℃

    2. Acid value

acid value is the total amount of organic and inorganic acids in the heat transfer oil (the total amount of potassium hydroxide consumed per gram of heat transfer oil). Organic acids are divided into low molecular organic acids and high molecular organic acids. Low molecular organic acids and inorganic acids are corrosive to metals, especially in the presence of water molecules. Most of the heat transfer oil is polymer organic acid, which has little corrosion to the equipment

    3. Viscosity

viscosity refers to the thinness and fluidity of heat transfer oil under specified conditions. When the mechanical load and speed are the same, the greater the viscosity of the heat transfer oil used, the greater the power consumption. Most domestic heat transfer oils are used in the high-temperature heat transfer stage, while almost all brands of heat transfer oils have similar viscosity at high temperature. When the viscosity of the heat transfer oil increases, the flow point of the heat transfer oil also increases. When the heat transfer oil is cooled, paste or solid incomplete substances will appear in the furnace tube, which will block the furnace tube, resulting in the failure of the hot oil pump to rotate and the failure of the oil furnace to heat up. Therefore, the flow point of heat transfer oil is also a very important indicator

    4. Carbon residue

carbon residue is a mixture of polycyclic aromatic hydrocarbons, cavitates and asphaltenes. Heat transfer oil is easy to decompose, dehydrogenate and condense into carbon residue under the condition of insufficient air. The amount of carbon residue can roughly determine the coking tendency of heat transfer oil in high temperature use. The heat transfer coefficient of carbon residue is very different from that of metal. 5. transmission system: there are two main types of transmission part of electronic universal experimental machine. The more serious coking is, the greater energy consumption is. Therefore, carbon residue is the main factor affecting the normal use of heat transfer oil furnace. See Table 1 for the relationship between carbon residue and parameters of heat transfer oil furnace

III. cause of coking of heat transfer oil

high temperature heat transfer oil circulates and transmits heat energy in the heat transfer oil furnace, and colloids are produced at the same time. The colloid is sticky, and the heat transfer oil with good quality can suspend the colloid in the oil. During the circulation process, part of the colloid can be filtered out through the filter. However, if a small part of colloid is attached to the inner wall of the furnace tube, it is easy to form coking. In addition, during the circulation of heat transfer oil, if there is air in series, it is easy to degrade and polymerize, forming low boiling matter and high boiling matter. The low boiling matter can be discharged into the atmosphere through the high-level tank, while a small amount of high boiling matter can be dissolved in the heat transfer oil. If the solubility of the heat transfer oil reaches a supersaturated state, the high boiling matter will adhere to the inner wall of the pipe, which is another reason for coking. Moreover, when the operating temperature exceeds its design temperature, autocatalytic thermal decomposition is often caused, resulting in coking in the tube. The leakage of the process material department is brought into the heat transfer oil system, forming corrosion products and rust, and the impurities and pollutants brought in during the overhaul will promote the coking of the inner wall of the pipe. The main components of heat-conducting oil scale are wax colloid, coke, asphalt, carbide, organic polymer, etc