A pilot-scale laboratory experience for an inductive learning of hydrodynamics in a sieve-tray tower

Depósito de Investigación de la Universidad de Sevilla https://idus.us.es/ This is an Accepted Manuscript of an article published by Elsevier in Education for Chemical Engineers, Vol. 29, on October 2019, available at: https://doi.org/10.1016/j.ece.2019.08.001 © 2019 Institution of Chemical Engineers. Published by Elsevier B.V. En idUS Licencia Creative Commons CC BY-NC-ND 1 A pilot-scale laboratory experience for an inductive learning of hydrodynamics in a sieve-tray tower Francisco Javier Gutiérrez Ortiz Department of Chemical and Environmental Engineering School of Engineering (E.T.S.I.) University of Seville, Spain Abstract A specific inductive teaching approach for undergraduate students of Chemical Engineering bachelor’s degree based on a laboratory experience is presented. Thus, the simple experiment performed to show an aspect of theory previously taught in a classroom is overcome. The specific laboratory experience involves a sieve-tray tower whereby a binary distillation is carried out. However, the focus is not on this separation operation but in the hydrodynamics associated with the start-up of the process, which needs a boiler as an ancillary system to supply the required energy. All these process units are usually taught operating under stationary conditions, which are achieved quickly if small laboratory units are used. In this paper, the hands-on experience is performed in a pilot plant, which makes students feel closer to industry and allows them to visualize different physical phenomena. Students work with their teammates, and they can acquire technical knowledge and develop transferable skills that prepare them for their future profession in a more engaged and motivational way. From their feedback, students feel that their mood increase, because they understand the phenomena shown in the experience better when it is performed first and then explained. Therefore, the approach is very well received by the students, so this new teaching scaffolding should be more and more developed and implemented in the chemical engineering curriculum. It is rewarding for the teaching staff because the laboratory experience helps students construct meaning by themselves in a more active and efficient way. This approach makes it possible to find and correct misconceptions in the students, to demonstrate procedures and methods used in the chemical engineering and to explain particular concepts within the knowledge area. But what is more important, it makes students protagonists of their own learning and gives them more confidence. Keywords Inductive learning, hands-on, laboratory, practice, experience, curriculum 2 1. Introduction A common teaching approach is to introduce a new topic to students, give some lectures, readings, and assignments on it, include it on an examination, and then move on to another topic (Felder and Brent, 2003). This is a traditional engineering instruction, namely deductive, which focuses teaching heavily on the theoretical knowledge, based on the narration of the textbook or power point slides conducted by lecturers. More recently the examination based on worked examples and videos of more or less realistic examples has been included in this didactic model of teaching and learning (Pott et al, 2017). This approach may be adequate to prepare students for an examination on the topic, but it has a limited impact on conceptual understanding (Abulencia et al., 2012). Actual engineering practice does not match with the real emphasis in many chemical engineering curricula around the world. However, the only way to develop a skill is practice. Engineering is a practical discipline; it is a hands-on profession where doing is key (Feisel and Rosa, 2005). An inductive approach pursues this, and it starts with specific observations and challenges, guides the participants to explanations and strategies, and then generalizes the results by providing the theoretical and research backing (Felder et al., 2011). Thus, an inductive teaching approach may be an alternative to the deductive/classical engineering instruction. The inductive learning approach is supported by social constructivism, especially by Vygotsky and Biggs who, among other proposals, proposed that instruction should involve students working together in small groups (Biggs, 1996). Inductive teaching and learning include different instructional methods, such as problem-based learning, project-based learning, case-based teaching, and just-in-time teaching (Prince and Felder, 2006). But, it also embraces experiment-based learning through laboratory experience related to discovery learning, as promoted in this work and explained below. In engineering curricula, many of the laboratory sessions do still follow tight recipes consisting of rigid instructions leading to a determined result (closed-ended problems). So, students must follow a sequence of tasks whereby they observe events that are unavoidable, in such a way that there is no space for any other thing. This type of practice is far from stimulating any creativeness and engagement in students, because it involves still passive activities. As pointed out by Lamancusa et al. (2008), recipes require little processing, emotional involvement or imagination on the part of students. Moreover, in the desire to prepare students for all contingencies that they may encounter in their careers, teachers take the risk of overwhelming them with facts on a superficial level, at the expense of a deep understanding and transference. 3 Therefore, this paper distinguishes the traditional laboratory-practice from the modern laboratory-experience, as we could call it, whereby students are active participants of their learning. In the hands-on experience proposed in the paper, teamwork is promoted, and students interact with the plant in an autonomous way. Thus, they must be able to start-up, operate and shut-down a facility, manipulating a number of actuators, measuring several variables and supervising the safety elements. Different conditions are requested for each group, and all students can “observe” and “see” the effect of their own actions, learning by doing but with more degrees of freedom than those found in a traditional laboratory. Indeed, learning by doing means that students actively engage in practicing real-world problems, and it can help students link each competency or skill to a broader framework. This new approach makes students become familiarized themselves with elements, devices, machines and process units as well as many of the followed procedures that are very similar to those used in the industry. This approach was initiated in the Department of Chemical and Environmental Engineering at School of Engineering (ETSI) of University of Seville more than fifteen years ago, and it has been adopted recently by others (Chen et al, 2019). The underlying pedagogical design of learning modules is based on experiential learning theory by Kolb (1984), especially when teaching implies to ask and reflect about questions such as why? what? how? and what if? This laboratory experience is designed for students in the 3rd academic year of the Chemical Engineering Bachelor’s Degree at the School of Engineering (ETSI, in Spanish) in Seville (Spain). The subject is completely practical and consists of a series of hands-on experiences carried out in the laboratory. One of them is described in this paper based on a pilot-scale sievetray tower, which is a main process unit widely used in the Chemical Industry. Tray towers are cylindrical vessels arranged in a vertical position containing trays to provide a large and efficient contact surface between two phases (vapour and liquid), thus improving the mass transfer between them. The liquid flows across each tray and downwards through the segmented/circular tube downcomer to the tray below. The liquid enters at the top of the column from a condenser and flows downward by gravity. The vapour rises through the orifices of the trays, and then bubbles through the liquid on the tray. For a given liquid flowrate, as the vapour flow-rate increases, the pressure-drop increases. For each liquid flow-rate, a range of vapour flow-rates allows the full liquid-loading of the trays, thus setting up a stable fluid-dynamic regime in which the drag force of the vapour does not prevent the free drainage of the liquid. Tray towers are largely applied to distillation. This is a very important separation operation that is usually taught only based on the number of ideal equilibrium stages (theoretical 4 trays) under steady-state conditions. However, the real number of trays also depends on the fluid-dynamic and mechanical design of the tower whereby dimensions and arrangements are decided. The problem is not trivial since such a decision represents a compromise between several opposing tendencies, because the achievement of high tray efficiencies, which reduces the number of real trays, involves conditions leading to operational difficulties. Thanks to the laboratory experience, students grapple with the handling of a sieve-tray tower and end up understanding: - the way in which it is started-up and how it becomes progressively liquid-loaded, - how the transient regime takes place in the loading process, - how the pressure balance is established through all the trays, - why one tray becomes flooded, - what happens if no correction is done, - how to correct this situation and the way in which pressure is balanced again in the tower. In this way, students can understand in a better way how to design a sieve-tray tower, learning how the steady-state conditions are achieved and when the tower can be put into operation to perform the distillation; all this is overlooked on many occasions. As far as the author knows, there are few papers about active practical teaching in laboratory with the educational focus described in this paper, despite it can provide practical material to chemical engineering instructors. Similar works to this may be those by Farrell and Cavanagh (2014) and by Silva et al (2017). However, most papers focus on showing some aspects of the theory previously dealt with in a traditional classroom (Orbey et al, 2017; Baz- Rodríguez et al, 2016; Fink and Luyben, 2015; Nassar et al., 2011; Mehrotra et al., 2012). Therefore, there should be more papers of the type described here, so an increasing number of engineering educators can share valuable approaches, strategies, and techniques on practical teaching and learning. Indeed, although other universities also use glass columns to illustrate distillation and other unit operations, most papers regarding this are: (1) related to web-based laboratories (Klein and Wozny, 2006; Alliet-Gaubert et al, 2016), which are useful, but students cannot tread on a real plant, (2) focused on a computer simulation (Fraser and Linder, 2009) rather than on the interaction with a real plant, 5 (3) concentrated on the separation operation, such as distillation operation itself and process control (Kazameas et al, 2015), or (4) aimed to optimize the operation of a distillation column (Naranj et al., 2012). In this work, the hands-on experience is performed in a pilot plant, which makes students feel closer to industry and allows them to visualize different physical phenomena. Thus, it is neither a remote laboratory nor one where students follow recipes. In addition, the focus is not on distillation itself or on other aspects as a controlled operation or a simulated process. Students must work with several systems simultaneously, so they must be very well coordinated, and they think about some phenomena during the experience and after that, when they work on the report and edit videos recorded in the practical session. 2. Pedagogical objectives The main objective is to make students become familiarized themselves with real process units, in a pilot-scale plant, so they can understand better the theoretical material explained after the laboratory experience. Although the focus is mainly on the hydrodynamics of a sieve-tray tower, there are many other aspects that are also dealt with during the class. This allows the acquisition of procedural knowledge (knowledge of how to do something, or “know-how”), which is very important for an engineer. Moreover, many phenomena occur during the handson experience, so students also revisit critical concepts, reinforcing and improving their cognitive background. Finally, this practical work offers an opportunity to emphasize how different operating variables are coupled, so they must be considered all together when interpreting and designing a real process. Therefore, the final aim is to achieve a more effective way of learning based on the inductive approach, trying to obtain a greater balance with the classical engineering instruction that is still mainly deductive. The practical learning described in this paper helps students acquire additional transferable skills and strengthen technical aspects, which is necessary for the chemical engineering graduate of today, who will work in a world that is increasingly complex and uncertain than at any time before. 3. Experimental During the class, students work collaboratively in groups and discuss among them, manipulating all the actuators of the plants, recording the measurements, learning from their own actions and observe what happen when making decisions in an autonomous way. This is 6 carried out under the instructor supervision, who interacts with them by asking questions and proposing different roles, so each student work in a part of the plant though they take turns around the plant. There is a short preparatory material that students must read before attending the laboratory class. In order to ensure this has been done beforehand, the instructor usually assesses it by a short quiz of ten minutes of duration, before moving the students to the laboratory, although oftentimes the teaching assistant asks students once they are working on the plant. They are aware that one way or another of performing this initial assessment can be done. 3.1. Experimental setup The facility consists of two systems: a boiler and a tray tower, which include measurement, control and safety elements. The main components of each system are the following: - Steam boiler (Figure 1), with five streams (fuel, air, water, flue gas and steam). - Tray tower (Figure 2): sieve-trays, a kettle (reboiler), and a condenser. The tower is initially fed into the bottom flask with an aqueous solution of ethanol. The tower (0.24 m OD, 4.5 m height) is made of borosilicate glass and equipped with 15 sieve-trays, made of stainless steel AISI 304, with a spacing of 0.22 m. The temperature profile is obtained by K-type thermocouples located at different trays (#1, 4, 6, 8, 10, 12, 14 and bottom flask). The bottom flask has a volume of 0.21 m3, the reboiler (0.22 OD, 0.8 m length) has a double coil, and the condenser (0.32 m OD, 1.0 m height) also has a double coil, and all of them are made of borosilicate glass. The energy input requires the use of saturated steam obtained in a water-tube type boiler that consumes diesel. The system usually operates in a batch mode, although it can also operate in a continuous mode. The aim is learning by doing, which includes procedural (start-up, operation and shutdown protocols) and conceptual aspects. The evolution of the fluid dynamics inside the tower over time is illustrated, from the start-up until reaching a stable hydrodynamic regime in which all the trays are fully liquid-loaded. There are companies that provide educational labs of different scales for universities, but in our case, we have designed and assembled most of the experimental units by ourselves, buying materials and adapting them to our facilities. Sometimes, students have participated in this design and arrangement through their capstone design projects. This is a cheaper way to have educational plants, but it implies that instructors need to spend more time on it. 7 Although the facility was assembled a number of years ago, and it is already amortized, the cost of the material for the tray tower and the boiler including all the tubing and accessories could be estimated about one hundred and thirty thousand euros. 3.2. Experimental procedure First, all students and instructors are equipped with personal protective equipment (PPE) that consists of protective clothing (laboratory coat), helmet, goggles and gloves, to protect them from injuries. Shoes must have rubber and non-slip soles. The hazards addressed by the PPE include physical, electrical and hot materials in this hands-on experience. Students work in teams, and the number of groups is between 8 and 11. The size of groups ranges from 4 to 6 students, and there are between 45 and 65 students enrolled per year in this practical subject. Each group works on this specific laboratory experience for a week; in the subject, there are more laboratory-experiences, so students work on other experimental units during the semester, and each experimental unit is tutored by an instructor. The laboratory work is supplemented with other activities to be performed outside, in the library or at home. Each team works in a different plant for 9-10 hours a week on average. The instructor shows all the parts of the boiler and tray tower, describing them and interacting with students by asking questions (why, what, how, what if) and making them take part actively from the first minute in laboratory. Students must read the start-up procedures given by the instructor to understand why the instructions must be sequentially followed. At this point, they are already dealing with questions such as what, why and what if. It has been argued that one negative aspect of laboratory sessions is that experimental procedures are specific for the units used in the laboratory. This may be true if those procedures are given to be merely followed, without any further explanation and without trying to extrapolate them to similar procedures used in industry. Thus, for instance, the water-tube boiler is different from any other boiler system, but all boilers have many shared aspects to be considered such as: (1) the water treatment (reduction of Ca2+ and Mg2+ concentrations, at least, by ionic exchange with Na+; there is a regeneration circuit that is also shown and discussed); (2) streams required to get the flame and flue gas (fuel and air); (3) the steam traps (why and how they work, and which their location is); (4) the measurement and supervision elements in all streams (instruments to measure flow-rate, pressure and temperature, mainly); 8 (5) safety elements to protect equipment and people (thermostats, pressure switches, electrical switches, redundancies, and so on), and some other technical devices. The same method is followed for the tray tower system, which consists of a tower, a bottom flask, a reboiler and a condenser, a feeding stream using a pump, and a distillate stream. These two latter are shut off due to different reasons that students must realize. The feeding to the tower by pumping would involve a second part of the experience more related to the distillation operation itself, which requires much more time. This second part was performed before the adaption of studies for higher education across the European Union (from 2001 to 2010), when the equivalent number of ECTS (European Credit Transfer and Accumulation System) was quite higher than currently. Likewise, the distillate stream does not work either because the tower is not fully liquid-loaded in all the trays. Students are asked to reflect on the usefulness of a column with several trays / separation stages, if not all of them are operational. The tray tower has similar components to those found in industry, including utility streams such as cooling water for the condenser and steam (coming from the boiler system) for the reboiler. Once the start-up is over, students are divided into two subgroups, so each one works with the tower or the boiler; however, after a while of about 30-40 minutes, the subgroups exchange their tasks, and all students work in all parts of the pilot plant. In this way, during the period in which the tray tower becomes fully liquid-loaded, students manipulate some operating parameters, thus verifying how they influence the overall process. These manipulated variables, such as the steam flow-rate for the reboiler or the cooling water flow-rate for the condenser, are changed by control valves. In addition, students also operate on/off variables that make it possible to put into operation or turn off devices and streams during the start-up and shut-down of the plant. Moreover, students track the measurements of instruments to see the evolution of variables, supervise the boiler cycles and so on. When there is enough condensate, the top tray (#1) begins to be loaded with liquid, and progressively the same occurs with the trays below. Students can follow the whole process looking at the tower directly because it is made of glass. In this way, students experience how the work is in a real installation in operation, which is managed and controlled by themselves, so they feel the real engineering very close to them. Thus, students get excited with engineering when learning by doing. Finally, when a stable hydrodynamic regime is achieved in the tray tower, shut-down procedures are applied both to the tower and the boiler, in a similar way to those previously explained for the start-up. 15 shown during the laboratory experience. Video V2b shows how the trays under the tray #7 are gradually loaded with liquid. - Fully liquid-loaded tower and shut-down Finally, once the tower becomes fully liquid-loaded and a stable hydrodynamic regime (Figure 9; video V3) is achieved, another procedure should be performed to carry out a distillation operation. However, as this is not the aim of the laboratory experience, a shut-down procedure is applied first to the tray tower system and then to the boiler system. Video 4 shows how the tray tower loses the liquid load when the steam entering the reboiler is progressively reduced to zero. Likewise, an intermittent strong weeping is observed during this final stage, because a remaining vapour flow-rate is still moving up to the condenser for a short time. 5.2. Data analysis and application to design There is some work to do outside the laboratory, in the library or at home, and students must write a report in which they explain the different procedures followed and physical phenomena observed. As well, students must draw some streams relative to the boiler system with all the components, mentioning the reason to dispose each one. Special relevance is focused on the devices regarding the measurement, supervision and safety. In addition, plant data must be represented in graphs with the trends of steam temperature and pressure over time as well as the flue gas temperature versus time. Likewise, the temperature profile along the sieve-tray tower must be also depicted over time, considering the steam flow-rate entering the reboiler and the cooling water used at any time. Finally, they must solve a design problem of a tray tower by the theory provided in the Appendix. 5.3. Pedagogical remarks From this hands-on experience, a number of learning aspects may be considered by any other instructor. First, students must manipulate the actuators in an autonomous way and actively interact with the plant, with our guidance. Then, they will learn how physical phenomena happen as a consequence of their action. In addition, to provide a better understanding of physical phenomena, continuous questions (such as what, how, why, and what if) should be asked, which makes them reflecting-in-action. Students use this reflection as a feedback to observe with renewed eyes again what they are looking at. Moreover, they will reflect-on-action when writing a report and editing their videos. In this way, by this meaning- 16 centred approach, students will be able to apply this practical knowledge to design problems, and they will be able to face real problems, many of which involve complexity and uncertainty. 6. Evaluation of the learning approach All students must pass this practical subject, so there is no way to compare groups that have followed this approach with others who have not. Therefore, the perspective of the instructors is the only one possible to compare this approach with the traditional one. Thus, after more than five years and in the instructors’ opinion collected by annual informal surveys, the described approach is clearly more transferable and provides more skills than a traditional lecture. In the end, students will have to solve many problems that arise in real units, so being familiarized themselves with these as much as possible will allow them to perform their job in a better way, by teamwork and team discussion. The learning is not faster, but this approach engages students in their own learning more actively. In addition, this method promotes a deeper learning in the students since it is a meaning-oriented approach and, hence, it encourages an active understanding through a continuous reflection. Using the methodology above described, the instructor can teach on the basis of direct observations during the laboratory experience, by asking students questions such as why, what, how and what if. This is carried out without using a straight (theoretical) explanation but making them think about the physical phenomena in a more active and interactive way. On the part of instructors, the interest of students is much higher than that felt in a traditional classroom and that observed when students do practice after the theory has been explained in the classroom. Although students can observe many physical phenomena almost simultaneously, they can understand them surprisingly well, especially as compared with the situation in which the teacher tries to explain first all those phenomena in a classroom. As aforementioned, both types of reflection, as distinguished by Schön (1983), take place in this approach. Thus, students reflect-in-action during the development of the hands-on experience, thinking about a number of issues raised by the instructor. In addition, students also reflect-on-action since they must carry out a work out of the laboratory and must bring a technical report that includes a personalized design problem. Moreover, they edit videos they record during the experience, whereby they must reflect on what they learned in the laboratory. 17 With respect to the feedback of students, no formal quantitative evaluation is performed beyond a simple survey, which they voluntarily answer. The most frequent questions are the following: - Do you perceive that this learning approach help you grasp the related theoretical concepts? - Do you think the resources and quality of the protocols are suitable to carry out the hands-on experience? - Was this experimental work useful for your learning of chemical engineering topics? - Was this experiential learning more suitable than the traditional lecture for you? - Are you pleased with the overall scope of the subject? In the last five years, the average results for all these questions have been ranged from 4 to 5, which is very positive, taking into account that the minimum value is 1 (strongly disagree) and the maximum one is 5 (strongly agree). So far, the participation of students in these surveys has been around 80-90% of the enrolled students on an average number of 55 students per year. In addition, they are asked by the instructor to provide general comments on this specific laboratory experience (verbal feedback). Most students give a positive feedback to this inductive learning approach as they enjoy working in the pilot plant, because of a number of reasons, where the most common are: - to be able to participate during the class, - to interact with their teammates and the instructor by discussing about the questions asked (especially, the question what if?), and - the perception of that doing things they feel more useful and become more selfconfident. Some different comments collected in the last two years are the following: “So far, I oftentimes felt that I was clumsy as I did not understand what the lecturer meant on many occasions, and I was ashamed to ask; now, after this experience, this has changed, and my mood has been boosted.” / “I never imagined enjoying myself in a class as today; wish there was more classes like this and less on the blackboard.” / “I believed that I would have to wait for finishing the degree to feel myself so close to the practical profession, but I’ve got it today.” / “Today, I have realized how the pressure balance is established in a real unit operating in a plant, beyond the equations presented in power point.” / “It is unbelievable how many things we have performed during this morning and how I have caught some issues I did not understand.” / “The theory given in the guideline is very 18 descriptive and a numerical example is provided, but I think I would have performed the problem of tower design as a routine, without understanding what I was doing.” / “The edition of videos and the drawing of streams helped me fix in my mind what I had learned in the lab.” / “The lab report is well addressed but, perhaps, the lab session could last longer, reducing the homework, so we [the work team] could seize the time in a better way.” Some students also mentioned that the laboratory experience gave them the chance to practice in an overall process that a priori “they thought impossible to operate” and for which they felt “a little scared”. These ideas expressed by students show the pedagogical aspects of the evaluation: 1. Generally, and in the case of chemical engineering, as a broad-based practical and practitioner degree and discipline, the link of this experiential learning to practicality and the profession is clearly stated. In fact, the contemporary chemical engineer requires competences not just in the knowledge domain, typically provided by traditional lectures, but also those related to transferable skills through experimentation, analysis, communication and teamwork, which is the basis of the described approach. 2. Specifically, the link between pressure balance equations and physical phenomena is shown in such a way that students can realize of it by reflecting in and on action and, hence, they will be able to perform design tasks with deeper knowledge of how to do something (“know-how”). 3. This approach promotes the teamwork during the practical class but also outside, when students must work on the report and the edition of videos. Moreover, they usually solve the design problem by working collaboratively as a team. In this way, this practical learning allows students to apply the calculation in a more correct way and with more confidence. On the other hand, not everything has been good regarding the practical learning and students' experience. One of these aspects is the workload to be done outside the laboratory. Initially, the structure of reports consisted of the following items (the maximum number of pages is included in brackets): 1. Cover page with title, group number and name of the students (1 page). 2. Index with pagination (1 page). 3. Objectives of the hands-on experience (1 page). 4. Theoretical introduction (2 pages). 19 5. Description of the equipment and materials, including a sketch of the experimental unit (3 pages, apart from the Figures). 6. Experimental procedure (2 pages). 7. Results and discussion (4 pages, apart from the Figures and Tables). 8. Conclusions (1 page). 9. References cited in the report: books, journals, databases, the Internet, etc. (2 pages). 10. Personal contribution such as incidents, comments, suggestions for improvements, possible supplementary work (not limit). A number of students found the workload was somewhat hard, and initially, many students dedicated 15 or more hours. We realized that the laboratory report and the design problem, as well as the edition of videos, had to have a smaller scope. Now, the structure is more reasonable, and some repetitive or less relevant aspects are not demanded, such as the points 4, 5, 6 and 9 of the initial structure. In the edition of videos, many of these aspects are covered by most of groups. The current workload is less than 10 h on average, including the 3.5 h in the laboratory. Therefore, the work schedule must be very well structured, and everything must be well organised to be able to teach all that previously mentioned without overwhelming the students. Another aspect to improve is the duration of the laboratory work, which should be longer, but it is limited by the own curriculum. This would involve a reduction of the theoretical material, and in particular the extent of fundamental sciences inherent in programmes which are already overcrowded. However, the discussion about what and how to reduce the programme is out of the scope of this work and will be addressed in another one. 7. Conclusions Students who participate in this laboratory experience are more motivated, demonstrate better teamwork skills, and have a greater understanding of issues related to specific professional practice and how to apply it to design problems. The laboratory experience provides students with hands-on skills that include elements of design, plant operation, problem solving and analytical thinking, apart from communication and teamwork. This inductive approach helps students acquire technical knowledge and develop transferable skills, but without forgetting them once the academic year or the degree is over. In this way, what they do will be better lodged in their memory. The idea is not to give hands-on experiences to students in order to supplement their theoretical studies, but to understand these latter when they are 20 explained after performing the laboratory experience. This methodology engages students in different activities in which instructors are facilitators rather than one-way information dispensers. This learning method also implies to cover less material and focus on the threshold concepts along with procedural and practical knowledge. It is necessary to achieve a better preparation for real-life problems, since technology (industry) strives to get what is effective and efficient for a particular purpose. Against the typical laboratory practice based on recipes, the laboratory experience shown in this paper by a specific case-study is not an uninspiring exercise to pass the course but helps develop the ability of students to define and solve openended (real) problems in a more confident and skilled way. Therefore, moving lectures from classroom to laboratory allows doing a better use of the class schedule and makes it possible for students and professors to interact with each other. Appendix Calculation of the column diameter, active area and number of holes The vapour velocity across a tray tower is limited by the entrainment of liquid droplets in the rising vapour stream and by the capacity of the downcomer for the liquid. The Souders and Brown equation gives the gas(vapour) superficial velocity of flooding: 𝑣𝐶𝜌𝜌 𝜌 (1) 𝜌 and 𝜌 are the liquid and vapour(gas) densities, respectively, and CF is the flooding constant that can be obtained by Eq. (2). 𝐶󰇟𝛽𝛼𝑙𝑜𝑔𝐹󰇠 󰇡𝜎 0.020󰇢. (2) 𝜎 is the liquid surface tension (N/m), and 𝐹 is the flow parameter given by Eq. (3). 𝐹𝐿 𝑉𝜌 𝜌. (3) 𝐿 and 𝑉 are the mass flow-rates of liquid and vapour, respectively. Note the fluid properties that are needed for the design. The values of parameters 𝛼 and 𝛽 are obtained by Eq. (4a) and (4b)). 𝛼0.0744 𝑡𝑠 0.01173 (4a) 𝛽0.0304 𝑡𝑠 0.0150 (4b) where 𝑡𝑠 is the tray spacing, which is one of the data of the design problem (Figure 3) or a parameter to be preliminary specified and after checked on, as explained later. 21 With the value of 𝐶, the flooding velocity (𝑣) is calculated, and an operating superficial velocity for the vapour phase (𝑣) is obtained by multiplying a factor that usually ranges from 0.75 to 0.85, in order to limit the liquid drag in the tower to 10% (safety design). The column diameter is obtained from the total area that is computed by dividing the vapour volumetric flow-rate by the operating velocity, once the area lost is discounted because of the downcomer. Likewise, the active area (𝐴) is calculated by subtracting twice this latter value and the loss of area due to mechanical supports from the total area. Then, the number of holes can be obtained by computing the total area occupied by the holes. The arrangement of holes is such that they are placed in the corners of equilateral triangles with a distance-to-diameter ratio of 2.0 to 5.0. The ratio of the total area of the holes (𝐴) to the active area (𝐴) is given by Eq. (5): 𝐴  𝐴 0.907𝑑 𝑝 (5) 𝑝 is the separation between holes and 𝑑 is the hole diameter. Pressure drop All the terms related to gas pressure drop are usually expressed as clear liquid heights with density 𝜌 on the tray. The pressure drop for the vapour 󰇛ℎ󰇜 is the sum of the pressure drop when the vapour goes through the dry holes (ℎ) and those terms regarding the effects caused by the presence of the liquid, which result from the depth of the liquid on the tray (ℎ), and the liquid surface tension when holes are wet (ℎ), which leads to a residual pressure drop (Eq. 6). ℎℎℎℎ (6) The pressure drop due to the flow of vapour through the dry tray is calculated by adding the losses in pressure due to the contraction and expansion undergone by the vapour at the inlet and outlet of the orifices, respectively, and the pressure drop resulting from friction within the short tubes of the holes: ℎ1 2𝑉𝜌 𝑔𝜌𝐶󰇩0.401.25 𝐴  𝐴 󰇛4 𝑓 󰇜𝐿 𝑑1 𝐴  𝐴 󰇪 (7) The Fanning friction factor (𝑓) is obtained by the Moody chart. 𝐶 is a coefficient that depends on the ratio of the tray thickness (𝐿) to the hole diameter (𝑑) and is given by Eq. (8). 𝐶1.09𝐿 𝑑/ (8) when 𝐿𝑑 ⁄ ranges from 0.2 to 2.0; 𝑉 is the vapour velocity passing through the set of holes: 𝑉𝑄 𝐴  (9) 𝑄 is the gas(vapour) volumetric flow-rate. 22 On the tray, the liquid is being bubbled by the vapour that is going through it and a foam is formed. The equivalent clear liquid column is an estimate of the value that would be obtained if the foam or vapour contained in the liquid coalesced (Eq. 10). ℎ6.1⋅100.725ℎ 0.238h𝑣𝜌1.225𝐿𝜌 𝑧 (10) ℎ is the weir height, and 𝑧 is the average flow width, which is calculated as the arithmetic mean of the tray diameter and the circle chord associated with the weir length. 𝑣 is the vapour velocity relative to the active area. 𝑣𝑄 𝐴  (11) The residual pressure drop of vapour is because of the resistance to the surface tension of the liquid that the vapour must overcome when leaving the holes: ℎ6𝜎 𝑑𝜌𝑔 (12) To calculate the height of liquid that must be in the downcomer, the pressure drop associated with the flow of liquid in the downcomer (ℎ) must be obtained, disregarding the pressure drop when crossing the tray since it is quite lower (Eq. 13). ℎ3 2𝑔𝑄 𝐴  (13) 𝑄 is the liquid volumetric flow-rate, and 𝐴 is the smaller of two areas: the cross section of the downcomer or the free area between the downcomer and the tray below, regarding the liquid entrance at this tray. Finally, the distance ℎ (difference in the liquid level inside and outside the downcomer) is the sum of the pressure losses due to the liquid and vapour(gas) flow-rates (Eq. 14). ℎℎℎ (14) Checks on design For a safety reason, during the operation, the total level of the equivalent clear liquid in the downcomer with respect to the tray below should be lower than half the tray spacing (Eq. 15). ℎℎℎ𝑠𝑝𝑎𝑐𝑖𝑛𝑔 2 (15) At this point, it is advisable to check if the initial hypotheses are met; otherwise, an iterative calculus must be performed, setting a tolerance in a chosen parameter. These limits are referred to the sum of the weir height, ℎ, and the crest that remains on the weir, ℎ, which can be calculated for a segmentedtype tray by the Francis formula (Eq. 16) along with Eq. (17): ℎ0.666󰇩𝑄 𝑊󰇪/ (16) 23 𝑊 𝑊𝐷 𝑊󰇩𝐷 𝑊1󰇪,2ℎ 𝐷𝐷 𝑊 (17) ℎ and 𝑊 must be obtained simultaneously and iteratively; 𝑊 and 𝑊 are the real length and the effective length of the weir, respectively. Likewise, it is also advisable to verify that the vapour velocity exceeds the minimum required to avoid weeping, but limiting the liquid drag (<10%). Weeping: The minimum vapour velocity through the holes is calculated by Eq. (18): 𝑉0.0229𝜎 𝜇󰇧𝜇 𝜎𝜌𝑑𝜌 𝜌󰇨.𝐿 𝑑.󰇧2 𝐴 𝑑 √3𝑝󰇨.󰇛/󰇜.  (18) 𝜇 is the vapour dynamic viscosity, and 𝑙 is the distance between two consecutive downcomers, which depends on the ratio 𝑊𝐷 ⁄. Liquid drag: The fractional liquid drag for sieve-trays according to the flow parameter (𝐹) and the approximation to the flooding (𝑣/𝑣󰇜 can be obtained by using Figure 10. 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Flooding in the tray #6 (a), correction (b), liquid loading of the tray #7 from the overloaded tray #6 (c), and balancing the liquid loading in the trays #6 and # 7 (d) (a) (b) (c) (d) 33 Figure 9. Fully liquid-loaded tray tower under a stable hydrodynamic regime: trays #1 - #5 (a), and trays #6 - #10 (b) Figure 10. Liquid drag (Treybal, 1981) 0.005 0.01 0.02 0.04 0.10 0.2 0.4 1.0 0.001 0.002 0.010 0.02 0.04 0.10 0.2 0.4 1.0 𝑣 𝑣  E- fractional drag 0.004 (a) (b)