Structural characteristics and types of cracking furnace, the most important equipment for ethylene production

"Triene" and "triphenyl" are the basis of petrochemical industry, and all kinds of important organic chemical products should be produced from them. Therefore, the ethylene plant for producing "trienes" and "triphenyls" has also become the leader in the petrochemical industry, and its production scale, output and technology can reflect the development level of the national petrochemical industry.

Lesson: What are "trienes" and "triphenyl"?

Trienes: ethylene, propylene, butadiene

Triphenyl: benzene, toluene and xylene.

Do you know the production method of ethylene? Do you know the ethylene factory? Bian Xiao first introduced the common production methods, and introduced in detail the structural characteristics and types of the cracking furnace, the most important equipment in the production process.

Production method of ethylene

1 tube furnace cracking technology

It is the most mature technology to produce ethylene by pyrolysis of petroleum hydrocarbons in a tubular cracking furnace at high temperature.

2 catalytic cracking technology

Catalytic cracking, that is, hydrocarbon cracking reaction is carried out in the presence of catalyst, which can reduce the reaction temperature and improve the selectivity and product yield. The advantages of catalytic cracking technology make it one of the most promising technologies to improve the cracking process.

3 Synthesis gas to ethylene (MTO)

MTO synthesis route is a route that takes natural gas or coal as the main raw material, first produces synthesis gas, then converts synthesis gas into methanol, and then produces olefins from methanol, which is completely independent of petroleum. In the 2 1 century when oil is increasingly scarce, it is expected to become an important route to produce olefins.

At present, almost all ethylene plants in the world use tubular furnace steam cracking technology, and other process routes are still at the level of technology development or industrial test due to economic problems or technical bottlenecks, and no or few industrial production plants run all the year round.

Structure of cracking furnace

The cracking furnace in ethylene plant consists of convection section, radiation section (including radiation furnace tube and burner) and quench boiler system.

The cracking reaction is carried out in the furnace tube of the radiation section to produce ethylene and propylene. In the convection section, the waste heat of high-temperature flue gas is recovered, so that the raw materials are superheated to the cross-temperature required by the reaction, and boiler feed water and ultra-high pressure steam are preheated at the same time. The function of the quench boiler system is to terminate the secondary cracking reaction, recover the high-temperature heat of the cracking gas, and generate ultra-high pressure steam.

The basic process is as follows:

Classification of cracking furnaces

The types of ethylene cracking furnaces can be technically divided into double radiation chambers, single radiation chambers and millisecond furnaces.

According to the furnace type, it can be divided into CBL cracking furnace (independently developed), SRT cracking furnace, USC cracking furnace, KTIGK cracking furnace, millisecond cracking furnace and Pyrocrack cracking furnace.

CBL cracking furnace

CBL furnace is a highly selective cracking furnace jointly developed by Beijing Institute of Chemical Technology, China Petrochemical Engineering Construction Company and Lanzhou Chemical Machinery Research Institute in 1990s.

The convection section of the CBL cracking furnace is arranged at one side of the upper part of the radiation chamber, and the top of the convection section is provided with a flue and an induced draft fan. The convection section is provided with raw material, dilution steam, boiler feed water preheating, raw material overheating, dilution steam overheating and high pressure steam overheating sections. Dilution steam injection: Class I and II are secondary steam injection, and Class III is primary steam injection.

The main feature is that the traditional primary mixing of diluted steam and hydrocarbons in convection section is changed into a new secondary mixing process. The ratio of primary steam to secondary steam should be controlled within an appropriate range. After adopting the new technology of secondary mixing, the temperature of the material entering the radiation section can be increased by more than 50℃.

In this way, the cracking temperature can be reduced by 5℃-6℃, the flue gas temperature in the radiation section can be reduced by 20℃-25℃, the maximum tube wall temperature can be reduced by 14℃-20℃, and the whole furnace heating can be reduced by about 10%.

Heating adopts the joint arrangement scheme of side wall burner and bottom burner. The sidewall burner is a flameless burner, and the bottom burner is an oil-gas combined burner.

The furnace has the characteristics of high cracking selectivity, flexible adjustment and long operation period.

SRT cracking furnace

Structure of SRT tubular cracking furnace

SRT cracking furnace, that is, short residence time furnace, was developed by American Rhuems Company in 1963 and industrialized in 1965. Since then, the furnace type and furnace type structure have been continuously improved, and SRT-I ~ VI cracking furnaces have been introduced one after another.

The continuous improvement of furnace type is to further shorten residence time, improve cracking selectivity and increase ethylene yield, which has great flexibility for different cracking raw materials. SRT furnace is the most widely used furnace type in large ethylene plants in the world.

The convection section of the SRT cracking furnace is arranged at one side of the upper part of the radiation chamber, and the top of the convection section is provided with a flue and an induced draft fan. The convection section is equipped with preheating of feed, dilution steam and boiler feed water.

Starting from SRT-ⅵ furnace, high-pressure steam superheating was also set in the convection section, and the high-pressure steam superheating furnace was cancelled. In the process of preheating and steam dilution of raw materials in convection section, primary steam injection is generally used, and secondary steam injection is also used when heavy raw materials are cracked.

In the early SRT cracking furnace, the flameless burner with side wall was used to burn fuel gas. In order to meet the needs of oil burning in cracking furnace, the layout scheme of combining side wall burner with bottom burner is adopted at present. The maximum heat supply of the bottom burner can account for 70% of the total heat load. The thermal efficiency of SRTⅲ furnace reaches 93.5%.

Folding ultra-supercritical cracking furnace

Schematic diagram of ultra-supercritical cracking furnace structure

The USC cracking furnace (superselective cracking furnace) of Si Tong Webster (S.W.) Company is a single-row double-radiation vertical tubular cracking furnace, and the radiation coil is W-shaped or U-shaped. Due to the small pipe diameter, there are many coil groups in a single cracking furnace (16-48 groups).

Every two or four groups of radiation coils are equipped with a USX-type (sleeve-type) first-stage waste heat boiler, and the cracked gas at the outlet of several USX waste heat boilers is collected and sent to the second-stage waste heat boiler. In the later stage, the two-way double-tube waste heat boiler (SLE) was adopted, and the two-stage waste heat boiler was merged into one stage.

The convection section of the ultra-supercritical cracking furnace is arranged above the radiation chamber, and the top of the convection section is provided with a flue and an induced draft fan. The convection section is provided with heat recovery sections such as preheating of raw materials and dilution steam, preheating of boiler feed water and overheating of high-pressure steam. Most USC cracking furnaces have a convection section corresponding to the radiation chamber, and there are also cases where two radiation chambers share a convection section.

When the fuel of the device is all gas fuel, most of the ultra-supercritical cracking furnaces use side wall flameless burners; If the device needs to use part of liquid fuel, the scheme of joint arrangement of sidewall burner and bottom burner is adopted. The bottom burner can burn gas or oil, and its heat supply can account for 60%-70% of the total heat load.

Because the radiation coil of USC cracking furnace is a small diameter short tube and long tube, the single tube processing capacity is low, and there are more coils in each cracking furnace. In order to ensure that the feed in the convection section can be evenly distributed to each radiation coil, a Venturi nozzle is arranged at the entrance of the radiation coil.

USC cracking technology is named according to the choice of residence time, cracking temperature and hydrocarbon partial pressure conditions, which makes ethane and other by-products less and ethylene yield higher. Short residence time and low hydrocarbon partial pressure make the cracking reaction have good selectivity.

Folding KTIGK cracking furnace

GK no.1 cracking furnace

The early GKⅰ cracking furnace was a double-row vertical tube cracking furnace, and the GKⅱ cracking furnace developed in the 1970s was a mixed-row (double-row inlet and single-row outlet) branch reducing furnace.

On this basis, GK Ⅲ, GK Ⅳ and GK Ⅴ cracking furnaces were developed successively. GK-V cracking furnace is a two-way branch reducing pipe. Due to the reduction of pipe pass, the length of the pipe is shortened, and the residence time can be controlled within 0.2 seconds. GK cracking furnace generally adopts a first-class waste heat boiler.

The convection section is arranged on the upper side of the radiation chamber. Besides preheating raw materials, diluting steam and boiler feed water, the convection section also carries out superheating of high-pressure steam.

GK cracking furnace adopts the scheme of joint arrangement of sidewall burner and bottom burner. The bottom burner can burn oil or gas, and its maximum heat supply can account for 70% of the total heat load. The sidewall burner is a flameless burner that burns gas.

Different cracking raw materials adopt different furnace tube configurations, which has great flexibility for raw materials. The new radiant section furnace tube has short residence time and high thermal efficiency.

Folding millisecond cracking furnace

Schematic diagram of millisecond cracking furnace structure

Kellogg's millisecond furnace is a vertical tube cracking furnace, and its radiation coil is a unidirectional straight tube. The convection section is located on the upper side of the radiation chamber. After the raw materials and dilution steam are preheated to the crossover temperature in the convection section, they are sent to the radiation pipe from the bottom of the cracking furnace through the crossover pipe and pigtail pipe. The material flows from the bottom to the top, leaves the radiation tube from the top of the radiation chamber and enters the first waste heat boiler.

When cracking light hydrocarbons, install a permanent three-stage waste heat boiler; When cracking distillate oil, only two-stage waste heat boilers are set. The convection section also preheats boiler feed water and superheated high-pressure steam, and the thermal efficiency is 93%.

The millisecond furnace uses a large burner at the bottom, which can burn gas or oil. Due to the small diameter of millisecond furnace and the large number of single furnace tubes, in order to ensure the uniform flow of radiant tubes, pigtail tubes are set at the entrance of radiant tubes to control the flow distribution.

The millisecond furnace has a small diameter and a large number of furnace tubes, which makes the cracking furnace complex in structure and relatively high in investment. Because the cracking pipe is one pass, there is no elbow, the resistance drop is small, and the hydrocarbon partial pressure is low, so the ethylene yield is higher than other furnace types.

Folding pyrolysis type

Linde Company began to develop thermal cracking furnace in 1960s. This type of cracking furnace is usually of double radiation section and single convection section structure. In order to adapt to different raw materials, cracking furnaces 4-2, 2-2 and 1- 1 are adopted.

Among them, the cracking furnace 1- 1 has high selectivity, short residence time, and the single group of furnace tubes has the smallest throughput, but the olefin output is high. After 1990s, Pyrocrack 1- 1 furnace tubes were mainly used in cracking furnaces designed by Linde Company.

How to save energy?

As the core of the ethylene plant, the cracking system accounts for about 70-80% of the energy consumption of the whole plant. The energy consumption of the cracking system determines the energy consumption level of the device. The following Xiaoqi takes the ethylene plant of Zhongsha Petrochemical Co., Ltd. as an example to explain how to achieve the purpose of energy saving and consumption reduction by optimizing the operation of cracking furnace.

First, the optimal operation of cracking system

1 Optimize the operation of cracking furnace

Reduce the cracking depth of the cracking furnace, strengthen the optimal operation management of the cracking furnace, pay attention to the changes of cracking raw materials in real time, analyze and adjust them, and realize the optimal product distribution.

2. Optimize cracking raw materials

Strictly control the quality of raw materials, and timely adjust the sulfur injection amount according to the daily feed analysis. Light naphtha and heavy naphtha are sent to different cracking furnaces, stored and refined respectively, and put into use.

3 Optimize process operation and site management

Improve the thermal efficiency of the cracking furnace;

A. During the normal operation of the cracking furnace, check whether the burner combustion in the furnace is in good condition, and adjust the throttle opening according to the burner combustion. If the burner is burnt out or blocked, it should be replaced and cleaned in time to prevent coking and thermal insulation deformation of the furnace tube caused by local overheating.

B. control the oxygen content of flue gas.

C. pay attention to the rigor of the furnace to reduce heat loss. In daily operation, check whether the fire hole and ignition hole are closed or damaged, and deal with them in time to reduce cold air leakage.

4. Optimize the scorching scheme and shorten the scorching time.

Charcoal burning in cracking furnace is a completely energy-consuming working condition. By adjusting the ratio of dilution steam to air, analyzing the contents of CO and CO2 in the carbonization gas in time, and switching the side wall burner in the pure air carbonization stage, the furnace can be fully heated, which can effectively reduce the carbonization time.

In addition, it is not good to bake for too long or too short. When the furnace temperature is high during combustion, the temperature in convection section is higher than that during normal operation, and hot spots exceeding the maximum allowable value of the furnace tube are easy to appear in radiation section. If the combustion time is too long, it will inevitably cause damage to the furnace tube, shorten its service life and increase maintenance costs. If the scorching time is short and the scorching is not complete, the next coking period will be short and the production will be affected.

Second, implement new energy-saving technologies.

1. Adding an air preheater to the burner at the bottom of the cracking furnace can save fuel gas consumption, improve combustion conditions and reduce flue gas emission, thus improving the thermal efficiency of the cracking furnace.

2. Recover refinery dry gas to improve unit efficiency.

3. Modification of coke oven tank

When burning, coke powder is easily taken out of the top of the tank, which pollutes the environment and consumes a lot of cooling water. By reforming the coking tank, the distribution of small particles in coke powder is increased, which can effectively improve the problem of coke powder overflow.

4. Waste gas recovery project

Desalinated water is used as the working water source, and the exhausted air is condensed into water through the suction of the power head to recover steam.