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is clear that the gas efficiency of the process is increased as pressure is increased; more of the nitrogen that is fed to the system is recovered as product. Although the associated material balances must cover pressurization, production, and purging, students observe that the removal of waste oxygen from the packing during blowdown is greatly reduced at lower working pressures. This requires a great deal more nitrogen use in purging, and reduces the fraction of nitrogen recovered accordingly. The experimental data may be analyzed in additional ways to provide information related to adsorber design and scale up. The adsorption isotherms for zeolites or other porous media used in PSA systems generally exhibit adsorption isotherms that are classified as “favorable,” and the moving concentra- tion profile does not change shape throughout the adsorption bed.[13] This allows for straightforward scaling calculations to be carried out based on the bed length and representative time frames observed during breakthrough trials. The end time for the production step in a PSA cycle, based on overall product purity design requirements, is known at the break-point time (tb). The ratio of the break-point time and the amount of effec- tive bed length saturated at the break-point time (Lu) will be in proportion to the ideal time and the total length of the bed: tb=Lu (2) t∗ L automatically logged by the computerized data acquisition system, the manual style of operation keeps an entire working group of three students actively involved in the experiment without overwhelming them. In most chemical engineering departments, the junior- and senior-level laboratories are primarily regarded as the places in which the principles delivered in lecture courses are put into practice; students get hands-on experience with process equipment, and see things with their own eyes. At CSM, the Unit Operations Laboratory is additionally used as a means for driving the overall curriculum. A stand-alone, intensive summer lab course with extended laboratory working hours and a special emphasis on experimental design has allowed our department to make it the centerpiece of our B.S. degree program. As an example, the introduction of a PSA experiment in the laboratory spurs the inclusion of fixed-bed adsorption material in the junior-level Mass Transfer (Separations) and senior-level Transport Phenomena courses. As a result, all students performing the PSA experiment for the first time have received some instruction related to the theory and practice of industrial gas purification by adsorption. To assist in course learning outcome assessment, students are required to complete two concepts quizzes during the Unit Operations Laboratory—one at the course orientation, and another on the final day of the session. To examine the students’ progress in mastering the fundamental concepts behind PSA systems and their design, specific questions related to the experiment were created. Table 2 shows that students in recent lab sessions have shown improvement in both theory and applied system-related questions; generally, a much greater improvement is observed in the practical aspect. Conversations with students over the course of the session indicate that an improvement of practical system understanding is a result of the in-depth literature reviews required for report introductions. Additionally, students ap- pear to make better connections between theory and experi- ment after performing detailed data analysis and evaluation in these reports. Student feedback regarding the experiment itself has been generally positive. In end-of-session course reviews, the PSA system is often referred to in terms such as “the experiment I learned the most from, but which was the most difficult to conceptually understand.” Students also indicate an appre- ciation of the provided data-acquisition system, which gives freedom to the entire team to manipulate the experiment throughout the course of the day. There is always a special focus on explaining deviations in observed data trends from theory or pre-lab expectations. The availability of extensive automatic data sets has often provided students with additional material for analysis and explanations of such deviations. More often than not, this leads to a deeper understanding of the physical/chemical phenomena around which PSA systems are designed. This relation is easily applied to breakthrough curve data. For example, should the students choose a break-time con- centration of c c -1 = 0.10 for the 75 psig working pressure, a 0 net product purity of 99.9% N2 would result. The data show that this break-point concentration ratio occurs at a time of 318 seconds (5.3 minutes), which is 74.5% of the ideal time. Therefore, an effective 114 cm of the column length will have been saturated. Similarly, if a production step time of 1300 seconds is required for proper cycle timing, a larger column with the same diameter must provide an ideal time of close to half an hour—and through the proportionality stipulated by Eq. (1), it will require a length of about 6 meters. STUDENT EXPERIENCES AND LABORATORY ASSESSMENT The PSA system in our Unit Operations Laboratory is an excellent example of the type of experimental system that works very well in a laboratory setting in which students are given a great deal of freedom in the creation of an ex- perimental design and also in the selection of experimental objectives. Since its construction and commissioning in the summer of 2013, the PSA system has proven to be extremely flexible in its application. Students have chosen to study N2 or O2 production, overall cycle design or cycle step evaluation, the study of system scale up or the investigation of overall mass transfer coefficients, and so on. Although most of the routine measurements are 50 Chemical Engineering EducationPDF Image | PRESSURE SWING ADSORPTION IN THE UNIT OPERATIONS
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