【Q&A Question 117】April 27, 2017
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【Q&A Question 117】April 27, 2017: What is the function of a fractional distillation tray? The reference answers will be visible after responding; points are awarded by identifying the key points. (Unless otherwise specified, all questions and answers are based on hydrogenation units.) The tray is the place where the vapor and liquid phases come into contact, enabling heat and mass transfer; it allows the two phases to separate quickly after contact, with the vapor phase containing droplets when there is little liquid, and the liquid phase containing bubbles when there is little vapor, thus adapting to changes in load. Scoring criteria: 2 Wealth points for incorrect attempts; 3–8 points for *incomplete* answers. 10 Wealth points for correct answers, provided with a detailed explanation; 15–20 Wealth points if the answer is correct but lacks sufficient details. Please score according to these criteria. 2017 Q&A Summary Thread (updates starting now) http://bbs.hcbbs.com/thread-1657793-1-1.html 2017 Daily Question Summary Thread (updates starting now) http://bbs.hcbbs.com/thread-1657794-1-1.html 2016 Q&A Summary Thread (updates completed) http://bbs.hcbbs.com/thread-1597792-1-1.html Petrochemical Sector – “Creative Ideas for Energy Savings” Campaign (Sponsors sought; highest returns ever) http://bbs.hcbbs.com/thread-1666758-1-1.html Sponsors are being sought for this campaign; those interested can contact the campaign coordinator. Petrochemical deep processing edition – the event to share pictures of chemical equipment has started! A vast amount of wealth, as well as HChuan coins, are waiting for you to claim them: http://bbs.hcbbs.com/thread-1760733-1-1.html. Notice regarding the collection of chemical engineering technical materials in the \"Petroleum and Chemicals Section\": http://bbs.hcbbs.com/thread-1614796-1-1.html (Source: HChuan Chemical Engineering Forum)
Provides a space for gas-liquid mass and heat transfer
The function of a fractional distillation tray is to provide an excellent contact area between gas and liquid during the separation process, thereby facilitating heat and mass transfer.
The gas phase rises, while the liquid phase overflows through the tray; thus, the gas and liquid load within the entire tower reaches an equilibrium state
Provide a site for vapor-liquid phase contact to increase the contact area
Component separation is achieved through heat and mass transfer on the tray, that is, the exchange of heat and substances between the gas/liquid phases
Distillation columns are generally used to separate all or most of the low-boiling-point impurities from the top of the column, so as to obtain a product at the bottom with a relatively low impurity content for further processing. Therefore, distillation serves as a pre-treatment step in the product purification process. In distillation operations, the fractions obtained from the top of the column are usually mixtures that also contain some of the components from the bottom of the column. Moreover, the separation requirements in distillation are generally not strict (otherwise it would be distillation); unless the distillation ratio is very high (in which case the yield of the product at the bottom decreases). The product at the bottom also typically contains impurity components from the top of the column. Distillation columns generally have a limited number of theoretical plates, and the reflux ratio used during operation is also low； Distillation columns are generally used to separate products of higher quality from either the top or bottom of the column; they typically have a relatively large number of theoretical plates, and there are strict requirements regarding the reflux ratio during operation. The main difference between distillation and fractionation is that distillation demands high separation efficiency in order to obtain the desired product at a high yield. In other words, distillation requires that the concentration of key components in the material at the top of the column be strictly below a specified threshold, and similarly, the concentration of key components in the material at the bottom of the column must also be strictly below a specified threshold. Simply put, distillation is an important method for separating products of high quality with high yields, while fractionation is usually used as a pre-treatment step before product purification. Fractionation can be employed as a means of obtaining a product only when the quality requirements for the product are not high or when there is a large difference in boiling points between the impurities and the product itself.
It primarily provides a good contact area between gas and liquid, facilitating heat and mass transfer. The liquid with a higher content of light components and a lower temperature, which flows upward through the tray, comes into contact with the steam at a higher temperature that flows downward. The temperature of the reflux liquid rises; the light components are evaporated into the gas phase, while the hot steam is condensed by the cooler liquid, causing the heavier components to cool down and return to the reflux liquid. As a result, the reflux liquid passes through a tray, causing its composition of heavier components to increase, while the vapor passing through each tray sees an increase in its composition of lighter components. This is the mass transfer process within the tray, also known as the concentration effect. The vaporization of the light components in the liquid phase requires heat – the latent heat of vaporization – which is supplied directly by the heat of condensation released when the heavier components in the gas phase condense. Therefore, heat transfer also occurs simultaneously with the mass transfer process on the distillation tray.
Provides a site for gas-liquid mass and heat transfer
In the process of stream separation, the fractionation tray plates, packing tray plates, or packing mainly provide a good contact area between gas and liquid, thereby facilitating heat and mass transfer. The liquid, which has a higher content of light components and a lower temperature, flowing from top to bottom on the tray or over the packing surface comes into contact with the steam, which has a higher temperature and flows from bottom to top. The temperature of the reflux liquid rises; the light components are evaporated into the gas phase, while the hot steam is condensed by the cooler liquid, causing the heavier components to cool down and return to the reflux liquid. As a result, the reflux liquid passes through a tray, causing an increase in the concentration of the heavier components, while the vapor passing through each tray sees an increase in the concentration of the lighter components. This is the mass transfer process occurring on the trays or packing, also known as the concentration effect. The vaporization of the light components in the liquid phase requires heat – the latent heat of vaporization – which is supplied directly by the heat of condensation released when the heavier components in the gas phase condense. Therefore, heat transfer also occurs simultaneously with the mass transfer process on the distillation tray.
During the shunting process, it mainly provides a good contact area between gas and liquid to facilitate heat and mass transfer.