Large-scale practical lecture: Advanced lectures on the fluid dynamics design of towers – presented by Wiseboy
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This post was last edited by wiseboy on 2020-12-24 at 12:04. Copyright notice: The copyright of this lecture belongs to Haichuan Chemical Forum and the author Wiseboy. The Haichuan Chemical Forum possesses all rights granted by copyright law, including the rights to reproduce, cite, and publish. There are things you can’t learn in college. ———————————————————————————————————————— Lesson 1: Introduction. Dear sea friends, chemical unit operation equipment is the most important component of chemical plants. The three main unit devices: heat exchangers, distillation towers, and reactors are the most important units. Regarding heat exchangers, I have devoted considerable space to providing a systematic overview of them in my university textbook on heat exchanger process design, through years of writing and updating. Today, at the request of many marine enthusiasts through calls and emails, I finally made up my mind to start explaining the design of tower equipment to everyone. A tower is a broad concept; here I will only discuss towers related to absorption, desorption, stripping, and distillation, with a focus on fluid dynamics design. We focus first on classic and fundamental theories, and I will correct those parts in textbooks and manuals that are missing, vague, or even inaccurate. Try to include some manual calculations so that everyone can understand the basic principles. However, for more accurate calculations that require fast processing speeds, relevant software is used. You can choose to do the calculations manually or using software, depending on your personal circumstances, especially your level of education and knowledge. The software used includes: 1. Aspen Hysys (USA), 2. PROII (USA), 3. WPTR (PRC). For easier reference, I have listed the courses as follows: Part 1: Introduction; Lesson 1: Introduction; Lesson 2: Theoretical foundations of column separation; Lesson 3: The origin of columns; Lesson 4: Gas-liquid equilibrium – an important topic in thermodynamics; Lesson 5: Thermodynamic fundamentals of columns; Lesson 6: Calculation of theoretical separation stages in columns – simplified algorithms; Lesson 7: Calculation of theoretical separation stages in columns – general algorithms; Lesson 8: Summary of calculation methods for theoretical separation stages in columns. Part 2: Fluid mechanics; Lesson 9: Column types covered in this course; Lesson 10: Key parameter – critical gas velocity; Lesson 11: Eckert correlation chart method for calculating critical gas velocity in packed columns (Part 1); Lesson 12: Eckert correlation chart method for calculating critical gas velocity in packed columns (Part 2); Lesson 13: Software algorithms for calculating critical gas velocity in packed columns (Part 3); Lesson 14: Calculation of empty column velocity and column diameter; Lesson 15: Eckert correlation chart method for calculating column pressure drop; Lesson 16: Software algorithms for calculating column pressure drop (Part 1); Lesson 17: Software algorithms for calculating column pressure drop (Part 2); Lesson 18: Differences between liquid flooding in packed columns and tray columns (Q&A session); Lesson 19: Formula-based methods for calculating critical gas velocity in packed columns (Part 1); Lesson 20: Formula-based methods for calculating critical gas velocity in packed columns (Part 2)
This post was last edited by wiseboy on 2020-11-30 at 16:22. Lesson 2: The theoretical basis for tower separation. The towers we are discussing here are used for the separation of components in fluid mixtures. The fluids here generally refer to gas and liquid mixtures, occasionally including solids. The basic principle of separation is that the composition (content) of components differs after reaching equilibrium between the gas and liquid phases. It is often said that for a mixture of two components with different boiling points, they can be separated by distillation. In fact: different boiling points result in different compositions (contents) at equilibrium between the gas and liquid phases. For example: at a normal temperature of 20°C and normal pressure of 101.325 kPa, with an ethanol mass composition of 20%, it is a liquid mixture. When we heat it to 95.61°C, part of it vaporizes to form a gas-liquid mixture. Pour this mixture into a container to allow the gas and liquid phases to separate; the ethanol content in each phase is different: the gas phase contains 67.13% ethanol, while the liquid phase contains 4.34% ethanol. This separation process results in the 20% ethanol present in the gas phase being purified to 67.13%. And the cost of this process is the energy consumption of the heater. And if you want to turn the gas phase into a liquid, you also need cooling.
This post was last edited by wiseboy on 2020-12-1 08:50. As can be seen from Lesson 2, heating plus gas-liquid separation – these two operations enable the purification of ethanol in one step; we refer to this process as one cycle of gas-liquid equilibrium. Then, the resulting vapor is condensed into a liquid, and by performing heating plus gas-liquid separation once more, ethanol can be purified again. Such multiple stages of balancing allow for the thorough separation of ethanol, resulting in a higher concentration. So the question arises: 20 levels of balance require 20 heating and gas-liquid separation units – is that not an overwhelming burden? Thus, the tower was created. Taking a plate tower as an example: one tray functions as a heating and gas-liquid separation device. You can have hundreds of trays, which is equivalent to hundreds of heating and gas-liquid separation units.
This post was last edited by wiseboy on 2020-12-1 09:17. In Lesson 2, we provided the balanced compositions of the gas and liquid phases, but how do you know these compositions? Of course, you can determine it through experiments. But it’s not possible to conduct experiments on everything. An important area of study in thermodynamics is the investigation of its laws: gas-liquid equilibrium. This relationship is used to describe the mathematical connection between the molar fraction of a component in the gas phase, y, and its molar fraction in the liquid phase, x: y = f(x). This relationship involves the equation of state, which is a quite complex theoretical framework for which many people have devoted their entire lives to studying. This lecture is unable to cover in-depth content; it only introduces some of its applications. Taking a liquid mixture of two components, A and B, as an example, assume that A is more volatile than B. In special cases: An ideal solution is an idealized, simple liquid, whose gas-liquid equilibrium is as described above and is relatively simple. In the formula, a is referred to as the relative volatility. In fact, the vast majority of systems are not ideal solutions; the above formula is perhaps most suitable for use in university teaching, as it can hardly be applied in practical engineering applications. The problem is that a large number of undergraduate graduates treat this balance as an accurate, universally applicable formula. This misconception leads some people to oversimplify the gas-liquid equilibrium, and as a result, they also view distillation technology in a simplistic manner.
Lesson 5: Fundamentals of Column Thermodynamics. In Lesson 4, we briefly discussed gas-liquid equilibrium, which is an important part of column thermodynamics. But it’s not limited to this; tower thermodynamics encompasses many aspects, some of which are fundamental concepts of thermodynamics. For example, for gas-liquid two-phase systems: specific heat, density, viscosity, thermal conductivity, surface tension, and so on. Because the tower separates a mixture, and the concentration, temperature, and pressure all vary. We like to, *are used to looking up information in manuals, but: what is the density of an ethanol solution with a concentration of 21.34%, at 183.45 kPa and 34.54°C? Could you check that for me? I guarantee you won’t find it in any manual on Earth. So the question arises: how do we obtain this data? Perhaps you thought of experimental measurement—this is possible, but it is not economical. Thermodynamic methods are the approaches used to estimate these physical properties. During the industrial revolution that lasted for over several hundred years, thermodynamics underwent tremendous changes. Today, thermodynamic methods can be used to estimate the thermodynamic properties of most mixtures with fairly high accuracy. However, this is not sufficient; various formulas and methods have their own ranges of applicability, which indicates that there is still a significant lack in research on this field. At this point, we won’t discuss specific thermodynamic methods and calculations, as those fall outside the knowledge scope of most people here. What we need to understand right now is: 1. What is the role of thermodynamic methods? ； 2. Each method has its own range of applicability, and we need to choose it properly.
This post was last edited by wiseboy on 2020-12-2 09:46. Lesson 6: Calculation of theoretical separation stages in column reactors – simplified algorithms. As mentioned earlier, separation in column reactors is achieved through multiple stages of gas-liquid equilibrium, with each stage of gas-liquid equilibrium representing one theoretical stage. We* usually call it the theoretical plate number. In the course of Chemical Engineering Principles, after making certain assumptions, the problem can be solved using graphical methods. These assumptions are mainly: 1. The mixture consists of two components; 2. The molar flow rate remains constant throughout the column. These approaches used in the course on chemical engineering principles are essential for understanding the basic concepts. But they are all simplified, rough algorithms, so no further explanation is needed here.
This post was last edited by wiseboy on 2020-12-7 10:01. Lesson 7: Separation Stage Calculation in Tower Theory – General Algorithm. In Lesson 6, Separation Stage Calculation in Tower Theory – Simplified Algorithm, we introduced the introductory algorithm from textbooks; its usefulness is very limited, as it can hardly be applied to most practical engineering projects. The reasons are as follows: 1. The assumption of constant molar flow. This is an approximation. 2. Suitable for two-component mixtures. Although we can identify two key components in a multi-component mixture to simplify the problem to a two-component system, this is clearly an approximation. To calculate the theoretical number of separation stages more accurately, the rigorous approach is to establish mass balance and energy balance equations for each tray in the tower and then solve them. We call them the 4 major equations of MESH. Where: M —— mass balance ； E — Energy balance ； S — Phase equilibrium ； S —— sum of components. Each tray has four sets of equations, and we need to solve for each of the N theoretical trays. Due to the high non-linearity of the S-phase equilibrium equation, the convergence of the solution process is very poor. Before the advent of computer technology, the amount of work required for such calculations was unimaginable, and very few people tried to solve them manually; yet there were pioneers who braved those challenges. After the 1970s, with the advancement and widespread use of computers, various algorithms for solving MESH equation systems emerged in large numbers, and many attempts failed; ultimately, however, numerous effective algorithms were developed. Among them, the three-diagonal matrix method, the relaxation method, and the over-relaxation method are the mathematical methods that were initially successful. Here, we would like to thank the thermologists and computational mathematicians for their hard work in this field, as well as for the remarkable achievements they have made in advancing distillation technology. We will not discuss this in detail, as it goes beyond the knowledge level of most readers who did not receive formal education in this field. But we need to keep in mind especially that: 1. The binary diagram method used in \"Principles of Chemical Engineering\" is of very limited use; it is only suitable for theoretical teaching. Don’t assume that just by studying \"Principles of Chemical Engineering\" you have learned how to perform distillation calculations, as this method has little practical value in engineering today. 2. Strict MESH equation set algorithms are the mainstream today, but you should not try to calculate them manually – don’t even think about it. 3. There’s no need to worry; computer technology has paved the way for us. Aspen, PROII, ChemCAD... are all powerful tools at our disposal, and all we need to do is fully understand their principles.
This post was last edited by wiseboy on 2020-12-8 15:16. Lesson 8: Tower Theory – Separation Stage Calculation – Summary. The theme of this lecture is fluid mechanics design, but we spent some class time outlining theoretical separation, that is, thermodynamic calculations, which are a prerequisite for fluid mechanics calculations. In my previous training on enterprise-level column fluid dynamics design (note: not for university teaching), participants were required to first understand this part. It is fundamental, but very complex. For most people, it’s sufficient to understand the principles and the calculation tools. Calculation tools are thermodynamic calculation solutions. It has been emphasized on many occasions that performing rigorous theoretical separation stage calculations by hand is now impossible. The main computational tools currently in use are: Aspen, Aspen Hysys, and PROII. We use an ethanol separation column to conclude this part of the calculation of the theoretical separation stages of the column. Example: Ethanol concentration tower: Feed: ethanol concentration (wt) 20%, at normal temperature and pressure. Separation requirements: Product: ethanol concentration (wt) ≥90%, ethanol recovery rate: ≥90%. Software used: Hysys. Tower settings: Theoretical plates (separation stages): 10, feed position: 6th plate (numbered from top to bottom). Results are as follows: Fun fact: Algorithm for calculating product recovery rate:
Lesson 9: Tower Types (Packings, Tray Sheets) Covered in This Lecture. As a comprehensive lecture, we will cover a wide range of tower types (packings, tray sheets), rather than just a few individual types: Bulk packing towers: 44 types of packings； Packed tower: 27 types of packing ； Plate towers: 8 types of trays, including sieve trays, guide sieve trays, inclined-orifice valves, F1 floating valves, trapezoidal guide floating valves, rounded-trapezoidal floating valves, ADV floating valves, and solid valves. These tower types cover almost all the commonly used tower types. The data provided is also the most comprehensive, scientifically verified, and corrected. In terms of the calculation of packed towers, we provide more accurate equations than those used by foreigners, such as the wet packing factor equation, to demonstrate the achievements of China’s science and technology.