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students to work on a given experiment for a period of up to 8 hours. Longer lab periods provide significant freedom for team-by-team experimental design decisions, and addition- ally make possible the use of larger, more flexibly applied laboratory modules. An operational schematic of the PSA experiment in use for the Unit Operations Laboratory at CSM is shown in Figure 2. Six process measurements are continuously logged using a commercial data acquisition system (DataTaker model DT85): feed pressure (P1), purge pressure (P2), product temperature (T1), purge temperature (T2), input gas flow, and product oxygen concentration. Students are provided with a selection of feed gases for the system: compressed air, nitrogen, and a 20% oxygen mixture in argon. The compressed air is available from building utility lines (~ 80 psi), and the other gases are sourced from compressed gas cylinders (regulated to ~ 20 psi). The process feed gases are supplied to a four-way valve (V1) with check valves to prevent backflow during gas interchange. The selected feed is manually regulated, and the feed pressure is automatically logged with a pressure transducer (Omega model PX309- 100GV) The feed gases are dried sequentially by refrigera- tion (Parker model PRD15-A11516016TXU) and calcium sulfate desiccant packing. A bypass option is provided at V2 to allow the feed stream to go directly to the exit line, which is a useful method for verifying the feed composition. Feed flow to the columns is measured using a thermal mass flow meter (Aalborg model GFMS-011327). The flow pathways to and from the columns are controlled using the valves on the various control panels located around the PSA assembly. Three-way input valves (V3 – V6) allow each column to be provided with fresh feed gas or the product of its counterpart through lines equipped with manual threaded flow-adjustment valves (V7 – V10). This allows for a number of experimental options—a small amount of product gas may be used to purge a saturated column (backfilling), pressure may be equalized between columns as an independent cycle step, or two columns may be connected in series when flow- adjustment valves are fully open. The adsorption columns are four steel pipes (schedule 40, 10.2 cm ID), each 152 cm in length. The columns are verti- cally wall-mounted, and contain randomly packed pellets of molecular sieve. The leftmost columns, designated Column A and Column B, each contain a porous carbon molecular sieve (CMS, Hengye CMS 260). CMS has pores with microporous openings[10]; these pore mouths allow gases with smaller kinetic diameters (such as O2) easier access to the internal surface than molecules with a larger kinetic diameter (such as N2). The rightmost columns, designated Column C and Col- umn D, each contain zeolite 13X (13X, Hengye 13X812MS), which is a silica-alumina clay with a significant amount (up to 20%) of sodium oxide. Nitrogen molecules have a much greater surface equilibrium concentration than oxygen Vol. 52, No. 1, Winter 2018 molecules on 13X, resulting in an initially oxygen-rich gas phase when the zeolite is exposed to air.[11] The availability of both packing types enables students to examine the stepwise performance of a system controlled either kinetically (CMS, nitrogen production) or by surface/gas equilibrium (13X, oxygen production). The study of cycle steps is isolated to either nitrogen or oxygen production, so Columns A and B are not used in cycles with Columns C and D, and vice versa. Columns A and B contain 7.5 kg of CMS (packing density = 0.559 g cm-3, void volume = 9.35 L), and Columns C and D contain 8.6 kg of 13X (packing density = 0.637 g cm-3, void volume = 9.83 L). Exiting streams from the column may be routed to either an exhaust line or to a product line using another series of three-way valves (V11 – V14). The exhaust line may either vent to the atmosphere, or be connected to house vacuum (ca. -12 psig) using V16. On the product line, flow is controlled with a thermal mass flow controller (Tylan General model FC-261V-4S). The exit composition is measured with an electrochemical oxygen sensor (Vernier model O2-BTA), which has been provided with a higher excitation voltage (12 DC volts vs. the stock 5 DC volts) to increase its span from 0 – 27% O2 to 0 – 100% O2. STUDENT EXPERIMENTAL WORK AND DATA ANALYSIS The instruction style for the Unit Operations Lab at CSM requires students to thoroughly familiarize themselves with the overall system before creating a list of experimental objec- tives as well as a detailed plan for achieving those objectives. After a cursory analysis of the experimental system, students quickly realize the critical role of the oxygen sensor. Coupled with the feed and exit flow measurements, the oxygen sen- sor allows material balances to be carried out over time on oxygen—enabling students to determine, for example, the amount of oxygen held up in the column during an absorption trial, or the amount of oxygen released to the atmosphere dur- ing a blowdown step after solving an oxygen balance on all other steps. Stepwise oxygen balances (and the implication of these balances for inferred nitrogen or argon balances) are the common elements of a wide range of experimental designs. These designs may include: • Isolation of enriched nitrogen or enriched oxygen of a specified purity • Purging of one column with product gas from another (backfilling) • Application of vacuum for vacuum swing adsorption (VSA) study • Use of columns in series or parallel arrangement • Pressure equalization between columns, or traditional 4-step cycles 47PDF Image | PRESSURE SWING ADSORPTION IN THE UNIT OPERATIONS
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