Column control equipment

The essential plant items used in an ion-exchange colunm process are relatively simple, but a number of complications can arise in their operation which demand an element of skill on the part of operating personnel or alternatively the installation of elaborate column control equipment. 

As many emissions involve chlorinated compounds, corrosion is a major problem in many control methods. The corrosion of columns and surface condensers can be prevented or reduced by the correct material selection. However, corrosion remains a constant threat to the interior of incinerators. Additional pollution control equipment such as scrubbers may also be required to remove acidic compounds from treated gases before discharging into the atmosphere. 

Instrumentation. A Pharmacia BioPilot Column Chromatography system was used to perform large-scale size exclusion chromatography (SEC) with an 11.3 x 90 cm BioProcess column packed with Sephacryl S-200 HR gel. High performance size exclusion (HPSEC) and ion exchange chromatography (HPIEC) were conducted with Pharmacia Superose 6 and 12 (HR 10/30) and Mono-Q (HR 5/5) columns respectively, equipped with Beckman model 520 system controller and Beckman model HOB HPLC pumps. 

A batch distillation plant consists of a still or reboiler, a column with several trays, and provisions for reflux and for product collection. Figure 13.10(c) is a typical equipment arrangement with controls. The process is applied most often to the separation of mixtures of several components at production rates that are too small for a continuous plant of several columns equipped with individual reboilers, condensers, pumps, and control equipment. 

Barolo et al. (1998) developed a mathematical model of a pilot-plant MVC column. The model was validated using experimental data on a highly non-ideal mixture (ethanol-water). The pilot plant and some of the operating constraints are described in Table 4.13. The column is equipped with a steam-heated thermosiphon reboiler, and a water-cooled total condenser (with subcooling of the condensate). Electropneumatic valves are installed in the process and steam lines. All flows are measured on a volumetric basis the steam flow measurement is pressure- and temperature-compensated, so that a mass flow measurement is available indirectly. Temperature measurements from several trays along the column are also available. The plant is interfaced to a personal computer, which performs data acquisition and logging, control routine calculation, and direct valve manipulation. 

Loss of electric power. This is another failure that frequently sets the column relief requirements. A common practice (9) is to study the installation to determine the effect of power failures and to set the required relief capacity for the worst condition that can occur. All electrically driven equipment, such as pumps, compressors, and fans (including those in the site cooling water system or steam supply system), may fail, and so will electronic controllers and computer control equipment. 

Consideration must be given to the possibility of column controls and auxiliary equipment failing during a fire. This may induce a coolant, power, or controller failure related to the fire. 

It is important to properly design and operate the condensate pot. In one case history (351a), a column preheater equipped with a steam inlet control scheme and with a condensate pot (no pump) experienced condensate removal problems upon turndown. It was not stated whether the Fig. 17.1e or / arrangement was used. Arrangement 17.1c needs the pump for avoiding this type of problem. Arrangement 17.1/ needs a sufQdently tell condensate pot (Sec. 17.1.3) and adequate operation of the level controller at tumed-down rates in order to avoid this problem. The author suspects that in this case (351a), one of these needs was not fulfilled. 

The method of Meng et al. " can be used for the quality control, stabUity study, and validity term of nateglinide tablets. In the linear range 0.2—2 p,g/ml, the precision is 0.98%. The method of Ono, Matsuda, and Kanno was elaborated for determining nateglinide and its seven metabolites in plasma and urine. However, this method requires an expensive column switching equipment. 

The flow of solvent through the column should not be too rapid or the solutes will not have time to equilibrate with the adsorbent as they pass down the column. If the rate of flow is too low or stopped for a period, diffusion can become a problem—the solute band will diffuse, or spread out, in all directions. In either of these cases, separation will be poor. As a general rule (and only an approximate one), most macroscale columns are run with flow rates ranging from 5 to 50 drops of effluent per minute a steady flow of solvent is usually avoided. Microscale columns made from Pasteur pipettes do not have a means of controlling the solvent flow rate, but commercial microscale columns are equipped with stopcocks. The solvent flow rate in this type of column can be adjusted in a marmer similar to that used with larger columns. To avoid diffusion of the bands, do not stop the column and do not set it aside overnight. 

This desirable result is mainly due to the combined pieconcentrator/recovery column dampening the large disturbance from fresh feed, thus making the feed variations to the more sensitive Cl colimin small and easier to handle. The three-column system should also have this desirable feature. However, it requires more process, instrumentation, and control equipment, and the TAC is also higher than the proposed design. 

The Aspen Plus steady-state simulation in the last section is exported to the dynamic simulation of Aspen Dynamics. The tray sizing option in Aspen Plus is utilized to calculate the column diameter to be 0.3259 m and the tray spacing is 0.6096 m. Other equipment sizing recommended by Luyben is used here. The volume of the reboiler is sized to give 10 min holdup with 50% liquid level. The decanter is sized to be bigger to allow for two liquid phases to separate. The holdup time of 20 min is used in the dynamic simulation. Pressure-driven simulation in Aspen Dynamics is used with the top pressure of the azeotropic column controlled at 1.1 atm to allow for some pressure drop in the condenser and decanter to give the decanter at atmospheric pressure. The pressure drop inside the colunm is automatically calculated in Aspen Dynamics. Since the tray pressures in the colunms are different than the constant atmospheric pressure assumption used in steady-state simulation, the established base-case condition in Aspen Dynamics will be slightly different than Table 9.11. The final base-case steady-state condition used for control study can be seen in Table 9.15. 

The variety of expert systems in the field of equipment selection is very large. Examples are pumps, compressors, control equipment, column internals, reactors, valves, absorbers and extractors. The structure of these selection systems is often very similar. The knowledge base contains a set of objects (similar to an equipment list). For each of these equipment items a list of criteria is available which allows the user to decide whether the object can be used or not. Moreover, a ranking list can be derived from a set of evaluation criteria. 

Extraction, a unit operation, is a complex and rapidly developing subject area (1,2). The chemistry of extraction and extractants has been comprehensively described (3,4). The main advantage of solvent extraction as an industrial process Hes in its versatiHty because of the enormous potential choice of solvents and extractants. The industrial appHcation of solvent extraction, including equipment design and operation, is a subject in itself (5). The fundamentals and technology of metal extraction processes have been described (6,7), as has the role of solvent extraction in relation to the overall development and feasibiHty of processes (8). The control of extraction columns has also been discussed (9). 

An extraction plant should operate at steady state in accordance with the flow-sheet design for the process. However, fluctuation in feed streams can cause changes in product quaUty unless a sophisticated system of feed-forward control is used (103). Upsets of operation caused by flooding in the column always force shutdowns. Therefore, interface control could be of utmost importance. The plant design should be based on (/) process control (qv) decisions made by trained technical personnel, (2) off-line analysis or limited on-line automatic analysis, and (J) control panels equipped with manual and automatic control for motor speed, flow, interface level, pressure, temperature, etc. 

Special safety constraints apply to equipment selection, design, and operation in nuclear reprocessing (269). Equipment should be reHable and capable of remote control and operation for long periods with minimal maintenance. Pulsed columns and remotely operated mixer—settlers are commonly used (270). The control of criticaHty and extensive monitoring of contamination levels must be included in the process design. 

For the scaled-up column, suitable baffle plates are required to control axial mixing. For the final column layout the equipment vendor (Ghtsch Process Systems Inc.) should be consulted. 

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