Thesis title: Process design of continuous distillation column in benzene-chlorobenzene separation process.
Literature review and investigation report: (explain the research status and development trend, the significance and value of this study, references)
1. Project background
Design is the soul of engineering construction, which plays a leading and decisive role in engineering construction and determines the level of industrial modernization. Engineering design is the bridge and link between scientific research achievements and practical productivity. Only through design can industrial scientific research achievements be transformed into real industrialized productive forces. Chemical design is a highly policy-oriented work, involving politics, economy, technology, environmental protection, laws and regulations. , and it also involves multidisciplinary, multidisciplinary intersection, synthesis and coordination, is a collective labor. Advanced design ideas, scientific design methods and excellent design works are the design directions and goals that engineering designers should adhere to. In chemical design, the design of chemical unit equipment is the core and foundation of the whole chemical process and device design, which runs through the design process. Therefore, as an undergraduate majoring in chemical engineering, it is very important to master the design method of chemical unit equipment.
Distillation is the most commonly used unit operation to separate liquid mixture (including liquefied gas mixture), which is widely used in chemical industry, oil refining, petrochemical industry and other industries. In the process of rectification, the gas-liquid phase is directly contact out for many times driven by energy agent (sometimes adding mass agent), and the volatile components are transferred from liquid phase to gas phase and the nonvolatile components are transferred from gas phase to liquid phase by using the different volatilities of the components in the liquid phase mixture, thus realizing the separation of the components in the raw material mixture. This process is a simultaneous process of mass transfer and heat transfer.
The task of this design is to design a distillation column with a certain capacity to realize the separation of benzene and chlorobenzene. In view of the small capacity of this design task, the benzene-chlorobenzene system is easy to separate, and the feed liquid to be treated is clean, the sieve plate tower is chosen for the design. The main contents of this course design are material balance, heat balance, process calculation, structure design and check. Limited to the author's level, there are inevitably shortcomings and fallacies in the design. Please criticize and correct me.
Sieve plate tower is one of the most commonly used plate towers in production. Plate tower has the advantages of simple structure, convenient manufacture and maintenance, large production capacity, small pressure drop of tray and high tray efficiency. It came out as early as 1832. For a long time, it has been mistakenly thought that the working range is narrow and the sieve holes are easy to be blocked, so it has been given a cold reception. The sieve plate has simple structure, low cost and great economic attraction. Therefore, since the 1950s, many researchers have re-studied the sieve tray tower. The results show that the narrow operating range of sieve tray tower is due to poor design (mainly low design point and easy leakage), while the well-designed sieve tray tower has a wide operating range. As for the problem that sieve holes are easily blocked, it can be satisfactorily solved with large-aperture sieve plate ⅰ.
In the early 1960s, American Distillation Research Corporation (FRI) made a systematic study on sieve tray tower on an industrial scale, using different materials and under different operating pressures, and widely changed the structural parameters such as sieve hole diameter, opening rate and weir height. These research results make the design of sieve plate tower more perfect, and the design method of large aperture sieve plate belongs to patent. Some research on large aperture sieve plate has also been done in China.
FRI research shows that the well-designed sieve plate is a kind of tray with high efficiency and large production capacity, which greatly promotes the popularization and application of sieve plate. At present, sieve plate has developed into the most widely used general tray. In China, the application of sieve plate is more and more common.
It can be said that sieve tray distillation column is a traditional distillation column. In the early days, it was neglected by industrial production for design reasons. However, due to the development of computing technology and the improvement of design level, sieve tray tower has been paid more and more attention and used by manufacturers. Its advantages are simple equipment, simple operation, convenient maintenance and low manufacturing cost.
2. Research status and development trend.
Gas-liquid mass transfer equipment is mainly divided into two categories: plate tower and packed tower. The distillation operation can adopt a plate column or a packed column. Plate tower is a kind of gas-liquid mass transfer equipment with step-by-step contact, and there are many types. According to the different gas-liquid contact elements on the tower plate, it can be divided into bubble tower, floating valve tower, sieve plate tower, perforated plate tower, tongue tower, floating tongue tower and buoyant jet tower. Plate column was first used as bubble column (18 13) and sieve plate column (1832) in industry. Later, especially after 1950s, with the rapid development of petroleum and chemical industry, a large number of new trays appeared, such as S tray, float valve tray, multi-downcomer sieve tray and tongue tray. At present, from the actual use situation at home and abroad, tray types mainly include sieve plate tower, float valve tower and bubble cap tower, of which the former is particularly widely used.
Sieve plate tower is a kind of plate tower. Its design intention is to make the vapor-liquid two phases fully contact on the tray to reduce the mass transfer resistance. On the other hand, it is to keep the two phases in countercurrent as a whole, so that the two phases can uniformly cross-flow contact on the tray to obtain greater mass transfer driving force. There are several layers of horizontal plates inside, and there are many small holes on the plate, which are shaped like screens; With or without overflow pipes. In operation, the liquid enters from the top of the tower, descends plate by plate through the overflow pipe (partially through the sieve hole), and accumulates a layer of liquid on the plate. Gas (or steam) enters from the bottom of the tower, rises through the liquid layer through the sieve holes, and bubbles out, so that the two phases fully contact and interact. A form of bubble contact gas-liquid mass transfer process, its performance is better than that of bubble column. In order to overcome the difficulty of high installation height of sieve plate, a circulating sieve plate was developed. In order to overcome the leakage phenomenon of sieve plate under low load, a sieve plate with disc under the plate is designed. Reduce the entrainment of fog on the sieve plate, shorten the spacing between plates, manufacture sieve plates with baffles and convex holes on the plates, and replace the inlet weirs with inclined foam-increasing platforms. Linde sieve plate has an air guide groove on the tray. Sieve plate tower is a hot and cold tower widely used in the process of H2S-H2O two-temperature exchange, which is used for rectification, absorption and dust removal.
Sieve plate tower is a common tower equipment in mass transfer process, and its main advantages are:
The structure of (1) is simpler and easier to process than that of floating valve tower, and its cost is about 60% of that of bubble-cap tower and 80% of that of floating valve tower.
(2) The treatment capacity is large, which can be increased by 10 ~ 15% compared with the bubble column with the same column diameter.
(3) The tray efficiency is 65438 05% higher than that of bubble column.
(4) The pressure is reduced, and the pressure per plate is about 30% lower than that of the bubble column.
The disadvantages of sieve plate tower are:
(1) The levelness of tray installation should be high, otherwise the gas-liquid contact will be uneven.
(2) The operating flexibility is small (about 2 ~ 3).
(3) The sieve plate with small holes is easy to be blocked.
Linde sieve plate, which is widely used at present, was developed by Linde subsidiary of United Carbide Company. It was first used in distillation column of air separation unit requiring low pressure drop, and began to be used in ethylbenzene-styrene distillation unit after 1963. In 1970s, Linde sieve plates were used in many companies' 120 vacuum distillation columns, among which 45 were over 5.0 m in diameter, and the maximum diameter was11.5 m. Linde sieve plates had two important improvements on ordinary sieve plates: one was to raise the tray upward at the outlet of downcomer, and the other was to add louver guide on the tray. This improvement increased the effective bubbling area, changed the tray operation from bubbling to spraying, and made the gas distribution uniform while reducing the liquid level gradient, thus reducing the dry plate pressure drop, reducing the entrainment and improving the mass transfer efficiency. At present, there are 10 sets of devices in China that have used the middle-running Linde sieve plate.
Distillation is the most widely used mass transfer and separation operation, and its wide application has made its technology quite mature, but the maturity of technology does not mean that there is no need for development and stagnation in the future. The development of mature technology often needs more energy, but because of its wide application, every progress, even a small progress, will bring huge economic benefits. Because of this, the research of distillation is still widely concerned and has made continuous progress.
Improving the thermodynamic efficiency and energy saving of distillation process has always been a research field of concern. The synthesis of separation sequence, the development of energy-saving separation process and the optimization of heat exchange network by using the concepts of thermal integration and pinch analysis, as well as the rational application of heat pump, multi-effect distillation, intermediate reboiler and intermediate condenser in the specific separation process to achieve energy saving, have always been active research fields that have been widely concerned.
For the materials that are difficult (or impossible) to separate by ordinary distillation, it is also a research field worthy of attention to develop the separation process of extractive distillation and azeotropic distillation, and to combine distillation with reaction to develop reactive distillation, which is of great significance to broaden the application scope of distillation and improve economic benefits.
With the development of fine chemical industry, the application of batch distillation is more and more extensive, and its research has also received due attention. Developing various new operation modes has obvious effects on saving energy consumption and shortening operation time. The simulation calculation of liquid holdup of batch distillation membrane has made some progress, which is of great significance to design and guide operation.
In order to develop a more reliable efficiency and pressure drop model, we should pay attention to the measured data, especially the industrial scale test data, which is the basis for establishing and verifying the model. In 1960s and 1970s, American Distillation Research Company and others carried out a series of industrial-scale experiments and obtained very valuable measured data, which laid an important foundation for the establishment of various models and the deepening of phenomenon understanding.
The research work of distillation has been very active and has made continuous achievements. With the continuous development and industrial application of various new separation methods, its important role in petroleum, natural gas, petrochemical industry, medicine and agricultural products chemistry will not change, and its position as the main separation method will not waver. As Phil pointed out at the 1987 International Distillation Conference, "If the mixture can be separated by distillation, the economically attractive method may be distillation." With the improvement of science and technology and industrial production level, distillation is widely used. It is important to further improve its technical level and make it perfect day by day through continuous efforts.
3 Significance and value of research
In this design, the binary mixture of benzene and chlorobenzene is separated by continuous distillation. The continuous distillation column is operated at normal pressure, and the separated benzene-chlorobenzene binary mixture enters the column from the middle of the continuous distillation column, and benzene with qualified content is extracted from the top of the continuous distillation column at a certain reflux ratio, and chlorobenzene is extracted from the bottom of the column, wherein the purity of chlorobenzene is not lower than 99.5%.
Equipment with large height-diameter ratio is called tower. Tower equipment is one of the most important equipment in chemical industry and refining production. It can make gas (or vapor) liquid or liquid-liquid two-phase close contact, and achieve the purpose of mass and heat transfer between phases. Common unit operations that can be completed in tower equipment include rectification, absorption, desorption and extraction. In addition, cooling and recovery of industrial gas, wet purification and drying of gas, and humidification and dehumidification with mass and heat transfer of gas and liquid.
In chemical industry or oil refinery, the performance of tower equipment has great influence on the product output, quality, production capacity and consumption quota, as well as the treatment of three wastes and environmental protection. According to relevant data, the investment cost of tower equipment accounts for a large proportion of the investment cost of the whole process equipment. Therefore, the design and research of tower equipment has been highly valued by chemical and oil refining industries.
As a tower equipment mainly used in mass transfer process, gas (vapor) and liquid must be fully contacted to obtain high mass transfer efficiency. In addition, in order to meet the needs of industrial production, tower equipment must also consider the following mass transfer efficiency. In addition, in order to meet the needs of industrial production, tower equipment must also consider the following requirements:
(1) Large capacity. Under the condition of large gas (vapor) liquid flow, there will be no large amount of entrainment, liquid blockage or flooding, which will damage normal operation.
(2) Stable operation and high elasticity. When the gas (vapor) and liquid load of tower equipment fluctuates greatly, it can still operate stably and has high mass transfer efficiency. And the tower equipment should ensure long-term continuous operation.
(3) The resistance of fluid flow is small. That is, the pressure drop of the fluid passing through the tower equipment is very small. This will greatly save power consumption in production and reduce daily operating expenses. For vacuum distillation operation, the large pressure drop also makes the system unable to maintain the necessary vacuum degree.
(4) simple structure, less material consumption and easy manufacture and installation. This can reduce the investment cost in the process of capital construction.
(5) corrosion resistance, not easy to block, easy to operate, adjust and maintain.
In fact, for any existing tower type, it is impossible to completely meet all the above requirements, but it is unique in some aspects.
According to the design task, the designed tower type is sieve plate tower. Sieve plate tower is a kind of plate tower that appeared very early. Since 1950s, a lot of industrial research has been done on sieve tray tower, and the performance of sieve tray tower has been gradually mastered, and a relatively perfect design method has been formed. Compared with bubble column, sieve tray column has the following advantages: the production capacity is increased by 20-40%, the tray efficiency is increased by 10- 15%, the pressure is reduced by 30-50%, the structure is simple, the tray cost is reduced by about 40%, and the installation and maintenance are easy. Therefore, it is widely used to oppose long-term neglect. In recent years, the research of sieve plate is still developing, including large-aperture sieve plate (the aperture can reach 20-25mm) and guide sieve plate.
The sieve plate is divided into sieve hole area, non-hole area, overflow weir and downcomer. The commonly used sieve holes in industrial towers are 3-8mm in regular triangle arrangement, and the ratio of spacing to aperture is 2.5-5. In recent years, a sieve plate with large aperture (10-25mm) has appeared, which has the advantages of easy manufacturing and not easy to block, but it has low night leakage point and low operating flexibility.
This topic combines theoretical teaching with practical application, which is helpful to improve the ability to deal with practical problems. Through the study of this topic, we can deepen our understanding of the basic principle of distillation process, be familiar with the process design method of sieve plate distillation column and cultivate our design ability.
The process has simple structure, low cost and flexible operation, and has strong engineering practical value. The popularization of this technology will accelerate the progress of energy-saving technology in China's industrial production process and promote the development of a large number of related technologies and industries.
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