Application of the hottest graphite heat exchange

2022-08-08
  • Detail

Discussion on the application of graphite heat exchange equipment in the production of PS plate

I: preface

ps plate is the abbreviation of precoated offset plates, which is developed to adapt to the rapid development of lithography. Because PS version has the characteristics of high resolution, full dot reproduction, rich layers, easy balance of ink and water, and high printing resistance, it has been rapidly popularized in the printing industry. According to the statistics available at present, PS plate is still the most important lithographic plate material with the largest consumption at present. PS plate manufacturing industry has become one of the extremely important pillar industries in the current printing equipment manufacturing industry

chemical raw materials such as hydrochloric acid, sulfuric acid, sodium hydroxide, sodium fluoride and sodium dihydrogen carbonate are used in the production process of PS version, and they need to be cooled and heated. Most of these raw materials are highly corrosive media, so the equipment must be seriously corroded. It is necessary to require that the materials of many production equipment not only be corrosion resistant, but also have good heat transfer performance

at present, PS plate manufacturers generally use plate heat exchangers made of titanium alloy materials with a cost four to five times higher than that of graphite, such as 2% hydrochloric acid solution cooler in the electrolysis process of PS plate, 25% sulfuric acid solution cooler in the oxidation process, sodium fluoride in the sealing process, sodium dihydrogen carbonate and other solution coolers. Although the heat transfer efficiency is good, it is easy to block, inconvenient to maintain, and frequent cleaning, causing great inconvenience to the intense production; The cost per unit heat exchange area of graphite heat exchanger is also much lower than that of plate heat exchanger, and the thermal conductivity is quite high. It is convenient to install, clean, repair and maintain, which not only shortens the shutdown time, but also does not reduce the heat exchange area of the equipment, so it can well replace titanium alloy plate heat exchanger

the following is a brief introduction to the chemical equipment made of graphite

II: graphite chemical equipment

(I) impermeable graphite is the same host

impermeable graphite is a new structural material made of artificial graphite and synthetic resin through impregnation, molding, casting and other methods. The difference between it and all other non-metallic corrosion-resistant materials is that it has not only high chemical stability, but also high thermal conductivity, which greatly expands its application range

As a corrosion-resistant non-metallic inorganic material, impermeable graphite is widely used in chemical corrosion prevention. At present, impermeable graphite has been successfully and widely used in the manufacture of various types of heaters, cooling condensers, synthetic furnaces, membrane absorbers, pipelines, pipe fittings, valves, towers and accessories, pumps and equipment lining corrosion prevention. Its structural performance is as follows:

1 Phenolic impregnated graphite mechanical properties

unit weight 1.8 ~ 1.9 g/m3

compressive strength 58.8 ~ 68.6 MPa

tensile strength 7.8 ~ 9.8 MPa

flexural strength 23.5 ~ 27.4 MPa

partial impact strength 2.7 ~ 3.1 kj/㎡

hardness 25 ~ 35 Brinell

thermal conductivity, thermal conductivity 105 ~ 116 w/m · K

heat resistance 170℃

coefficient of thermal expansion 4.4 1/℃ × 106

2. Mechanical strength impermeable graphite can be used for various machining, such as turning, planing, sawing, milling, etc. these properties provide good conditions for processing and manufacturing various equipment

3. chemical stability impermeable graphite is resistant to most chemicals except strong oxidizing acids and bases, especially hydrochloric acid, which is resistant to any concentration below the boiling point

4. The thermal conductivity of impregnated graphite is 100 ~ 110 kcal/m · h ·℃. The thermal conductivity of impregnated graphite is similar to that of copper, aluminum, etc. it is usually used as a heat exchanger or other heat transfer equipment according to this characteristic of impermeable graphite

5. Thermal stability the coefficient of thermal expansion of graphite itself is very small, which is 0.24 × 10-5 1/℃。 Although the linear expansion coefficient of the resin is much larger, after impregnation and curing, it is widely confined in the gap and cannot expand freely, which makes the total linear expansion coefficient of impregnated graphite increase little, ranging from 0.4 to 0.45 × 10-5 1/℃。

(II) introduction to various equipment

heat exchange equipment and absorption equipment made of impermeable graphite have excellent corrosion resistance and heat transfer performance. They are applied to the heat transfer process of corrosive media, and are widely used to deal with corrosive media such as hydrochloric acid and sulfuric acid, as well as chemical unit operations such as heating, cooling, condensation, evaporation and absorption. Graphite heat exchange equipment and absorption equipment can be divided into tube type, block hole type (round block hole type and rectangular block hole type), spray type, immersion type, sleeve type and other structural forms according to their structural forms. Among them, tube type and block hole type have series designs, and technical standards for graphite equipment have been formulated. The following focuses on the characteristics, classification, scope of application and individual applications of the following tubular, block hole and tubular falling film absorbers

1. The connection between the tube and the tube sheet of the combined tubular graphite heat exchanger is an elastic sealing structure. The graphite tube can expand and contract freely under the action of temperature difference stress, which avoids the stress damage caused by the bonding between the graphite tube and the tube sheet under the action of temperature difference stress. Graphite material has high utilization rate and can be made into equipment with large heat transfer area. The cost per unit heat exchange area is lower than that of other structural forms, which is used in the heat exchange process with large processing capacity; Small fluid resistance, convenient maintenance, overhaul and cleaning; The allowable pressure of the equipment is 0.5MPa and the temperature is 150 ℃ Due to the good interchangeability of various equipment parts, they can be replaced separately, which greatly extends the service life of the equipment. It has been widely used in sulfuric acid, hydrochloric acid industry and epichlorohydrin production

2. The heat exchanger (round block hole, rectangular block hole) is composed of several graphite heat exchangers with material holes, upper and lower graphite heads and their metal cover plates, cylindrical steel shell (round block hole) or side covers at both ends (rectangular block hole type), and other main parts. The parts are sealed with gaskets and fastened with long bolts. The block hole heat exchanger has the following characteristics:

① the structure is solid, and the graphite block mainly bears compressive stress, which can make full use of the high compressive strength of graphite materials. It can increase the operating pressure and is suitable for occasions with thermal shock or vibration

② compact structure and small floor area

③ it has strong adaptability and can be used in many chemical processes such as heating, cooling, condensation, evaporation, reboiling, absorption and desorption

④ good interchangeability of parts, using the "building block" removable combined structure, requiring a small number of standard components, which can be assembled into equipment with different heat exchange areas. Its disassembly, installation, cleaning, maintenance and transportation are convenient, which has great advantages for manufacturing and maintenance

⑤ higher heat transfer coefficient can be obtained

for example, yxsq round hole sulfuric acid dilution cooler, the upper part is the mixing, dilution and distribution part of concentrated acid and desalted water, and the middle and lower parts are the cooling part, which can dilute 98% of concentrated sulfuric acid to 50~60%. As a more advanced equipment in the sulfuric acid dilution process, it can dilute the concentration of concentrated sulfuric acid to less than 65%, and can be directly cooled to the required temperature after dilution

3. Falling film graphite absorber is actually a vertically installed tubular or round block hole heat exchanger. The rows of tubes (or longitudinal holes on the block) of the heat exchanger are equivalent to many parallel water-cooled wet wall towers. An overflow pipe for distributing the absorption liquid is arranged above it. In the absorption process, the falling film absorber continuously removes the dissolution heat, and its heat and mass transfer effect is good. Compared with the adiabatic absorption of packed tower, it has significant advantages

the by-product hydrogen chloride used in epichlorohydrin production is now hydrochloric acid after water absorption. It has strong corrosivity and greatly shortens the service life of metal heat exchangers. Therefore, heat exchangers related to hydrogen chloride medium in epichlorohydrin production adopt very corrosion-resistant Zui heat exchangers after the broken graphite sample with a hemispherical impactor

III. application prospects

with the development of society and the progress of technology, the product quality is also gradually improving. Carbon steel and cast iron can no longer meet the requirements of the growing development of modern industrial production and the needs of large-scale production. With it, some new materials, such as alloy cast iron, stainless steel, titanium, engineering plastics and various anti-corrosion linings, have been widely used in production

most of the anti-corrosion treatment of carbon steel equipment adopts phenolic and epoxy resin coatings, especially the heat exchange equipment. After anti-corrosion, the heat transfer effect is greatly reduced, thus increasing the heat exchange area, so graphite has become a very suitable material in equipment manufacturing. For example, in the process related to HC1 gas or hydrochloric acid in PVC production by calcium carbide method, the main equipment includes: hydrochloric acid cooler, round graphite three in one hydrochloric acid synthesis furnace, graphite hydrogen chloride synthesis furnace, round graphite sulfuric acid diluter, mixed gas cooler, preheater and converter vinyl chloride cooler in polymerization section, EDC cooler in rectification section in ethylene oxychlorination production, etc., which have received good use results

the thermal conductivity of graphite is 480 7 J/(m · h ·℃), while the thermal conductivity of general alloys is much lower, such as 58 for 304 stainless steel 69 J/(m · h ·℃), so the thermal resistance of graphite with the same thickness is smaller than that of stainless steel, which also has stronger heat transfer ability. Although the thermal resistance of the pipe wall in the process of heat transfer is much smaller than that of the fluid and dirt, sometimes this effect can not be ignored

even the most corrosion-resistant alloy, the corrosion rate in strong corrosive media such as hydrochloric acid is 0 1 mm/a, while the corrosion rate of graphite is almost 0. Therefore, in highly corrosive environment, the service life of graphite heat exchanger is obviously longer than that of metal heat exchanger

graphite heat exchanger can not only effectively exchange heat and reduce energy consumption, but also improve the yield of relevant materials, so as to reduce material consumption. Its use in the production of epichlorohydrin not only saves the steam consumption of the cyclization reactor, but also improves the reaction yield of dichloropropane

although impermeable graphite equipment has been widely used in other fields, its application in PS version production is only in its infancy. As for actual production, impermeable graphite equipment can be used as long as the working pressure does not exceed 0.6MPa and the working temperature is lower than 160 ℃, and the medium contains corrosive substances

metal heat exchangers often need plugging even under other medium conditions, and they need to be replaced frequently when operating under acidic medium conditions, which will cause frequent shutdown. The direct loss of replacing tubes or heat exchangers and the indirect loss caused by shutdown cannot be counted. Therefore, it can be seen that graphite heat exchangers not only have good heat exchange effect, but also have high corrosion resistance and long service life, To prevent the device shutdown caused by frequent plugging or replacement of heat exchanger, the chemical plant can truly realize long-term, safe and stable operation, so it is worthy of further promotion

through the above introduction, we believe that with the passage of time and the deeper understanding and understanding of graphite equipment, its application prospect will be quite good

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