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For the purposes of the present communication, the ex- perimental goals and data analysis for one student-designed experimental set are presented: the production of high-purity nitrogen from air using CMS. In this case, students chose to examine the effects of adjusting system working pressure on the production of nitrogen from a PSA air separation cycle using CMS. The selected cycle style has four steps, and would in practice be very similar to the cycle shown in Figure 1(a). However, as there is no product storage vessel, purging is accomplished by using nitrogen from a compressed gas cylinder. Monitoring of effluent gas from the column in use with the oxygen sensor is possible for every step in the cycle except blowdown, in which case the gas must be released very rapidly to the atmosphere from the column. The oxygen balance for the blowdown step will therefore be determined by first completing balances for the other three steps: how much oxygen enters the column during pressurization, how much oxygen builds up during product collection, and how much is released from the surface during purging. An important consideration for the students involves ar- gon (1% of the entering air) within the system. A literature review of CMS quickly reveals the nature of its operation in a kinetically controlled adsorption system when applied to air separation: It is expected that oxygen will be held up in the column owing to the microporous packing combined with oxygen’s smaller kinetic diameter versus nitrogen (3.64 Å vs. 3.46 Å). Considering that argon has a similar kinetic diameter to oxygen gas (3.40 Å),[12] it follows that the column will effectively screen out oxygen and argon—thus, a 0% O2 reading at the sensor would imply a 100% N2 product stream. The main process variable adjusted in this particular case was the system working pressure. While house compressed air is available at nearly 90 psig, compressor cycling and house air use by other laboratory experimental stations creates a practical upper regulated limit of about 80 psig. The students designed a series of three experiments with working pressures of 25, 50, and 75 psig. In each case, the column was purged with nitrogen gas at atmospheric pressure before beginning the experimental trial. Flow from the columns was always limited to 10 L min-1 using the exit flow controller. The process steps were carried out as follows: 1. Pressurization: The air feed pressure was adjusted using the regulator to match the selected working pressure, and the column inlet valve was opened with the exit valve closed. The rate of air entering the column was monitored, and when the value was equivalent to the chosen product flow rate (10 L min-1), the product valve was opened. 2. Production: The composition of the gas leaving the col- umn was monitored until its instantaneous oxygen com- position reached about half of the value for air (10.5%). The column feed was then closed, and the column exit was redirected to the exhaust line’s atmosphere vent. 3. Blowdown: The column is returned to atmospheric pressure as quickly as possible. When the pressure in the exhaust line reaches zero, the column exit is redirected to the product line, and low-pressure nitrogen is intro- duced as feed. 4. Purge: Pure nitrogen is introduced into the column at a rate of 10 L min-1 to rinse the packing of adsorbed oxygen. The oxygen sensor is monitored, and the nitro- gen feed is stopped when the exit oxygen concentration reaches zero. The system outlet flow is controlled and constant, while both the feed flow and the outlet oxygen concentration are directly measured and automatically recorded. Analysis of the column’s performance is typically expressed in terms of nitrogen recovery, net product purity, adsorption analysis, and cycle design through proposed step-by-step timing. Where applicable, the oxygen concentration, feed and exit flow rates, and elapsed time for each cycle steps may be used to track the oxygen and nitrogen in the system, allowing for simple evaluation of product purity and product recovery. For the case of nitrogen production, monitoring the oxygen concentration for a known total product flow over time during the production step permits the direct calculation of the net product purity. The calculation of nitrogen recovery requires only a determination of the amount of nitrogen in the product as a fraction of the sum of nitrogen sent to the column (as air) during the pressurization and production steps, as well as the amount of nitrogen used during the purge step. Adsorption analysis is a general examination of the col- umn’s performance during the pressurization and produc- tion steps. Generally, students find that the generation of breakthrough curves can provide a number of useful system insights; examples of these are how much capacity the adsor- bent has under various conditions, how much of the bed may be saturated after a given time, and how all of this information might apply to scaling up the PSA system to meet a specific production target. The assigned text for our Unit Operations Lab[13] provides some general background for the analysis of adsorption in packed beds. Some of the most useful material for students performing the PSA experiment is the discussion of concen- tration profiles in a column during adsorption, which may be inferred from breakthrough curves. An important general result relates the saturation capacity of the packing (Wsat) to the amount of time that would be required for the solute to break through in the absence of both axial dispersion and mass transfer resistance (t*): uct∗=Lρ(W −W) (1) 00 bsat0 Here, the superficial velocity of the fluid and its solute concen- tration (u0, co) are considered along with the physical proper- ties of the packed column: its length (L), the bulk density of the particles within (ρb), and finally the saturation and initial 48 Chemical Engineering EducationPDF Image | PRESSURE SWING ADSORPTION IN THE UNIT OPERATIONS
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