Atmospheric columns control

Gearing PJ, Gearing IN. 1982b. Transport of no. 2 fuel oil between water column, surface microlayer and atmosphere in controlled ecosystems. Marine Environmental Research 6(2) 133-143. 

For column pressure control there are Ihrea genaial approaches vent bleed (to arraosphere or to vacuum system), hot vapor bypass, and flooded condenser. These approaches are illustrated in Fig. 5.11-2.3 For an atmospheric column, the vent approach is quite simple. The vapor bypass represents a temperature bleading method. Partial flooding of the condenser suiface adjusts the bent transfer capability or the condenser. The schemes are generally self-explanatory. 

FIGURE 5.11-2 Schemes Tor control of columa pressure (a) pressure control for na atmospheric column (vent bleed to atmosphere) (b) split ranga valves in a block and bleed arrangement (vent bleed to vacuum) . (c) hot vapor bypass pressure control and (d) flooded condeaser pressure control. 

Atmospheric columns. In atmospheric columns, pressure is controlled by having the column open to atmosphere, i.e., by variation of air flow in and vent gas out of the column through the column vent ("breathing ). If air ingress into the system is undesirable, an inerts purge is added at the vent, or the column is pressured to about 3 to 5 psig and operated as a pressure column. 

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. 

An additional problem with condensate temperature control, via cooling-water manipulation, relates to column safety. In an instance with which one of the authors is painfully familiar, an atmospheric column with such a control system was running at a very low feed rate. Condensate temperature became too low, so the controller closed the cooling-water valve located in the exit line from a vertical condenser. The water in the shell b an to boil, the valve could not pass the required volume of steam, the cooling-water pump stalled, and product vapor issued in great quantities fix>m the vent. Fortunately, an alert operator shut the column down before any damage occurred. 

Figure 8.4 shows a typical pressure control scheme for sub-atmospheric column operation used for total condensing service. The eductor is not controlled by regulating the motivating steam, because the turndown on the jets is very limited. Rather, the capacity is controlled by regulating the addition of non-condensable gas. This method provides a smooth and rapidly responding control system. 

Bottom pressure control of an atmospheric column via reboiler heat input will, in effect, control the column differential pressure and, thereby, the column vapour flow... 

Trichloroethene mass emissions rank just above mid-column, but the ranking falls when emissions are placed on a molecular basis, falls further when scaled by relative rates of attack by OH, climbs somewhat on the basis of ozone comparison, but falls again when comparing CO2 formation. The reporting of emission inventories has been expressed historically as mass emissions that are an important part of atmospheric pollution control and modeling. However, it is clear that the relative magnitude of these is not intended to reflect the relative impact of the emissions of a given compound on the chemistry of the atmosphere. 

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. 

The gas chromatograph (GC) resembles the MS in providing both qualitative and quantitative EGA but is significantly slower in operation. The interval between analyses is normally controlled by the retention time of the last component to be eluted from the column such delay may permit the occurrence of secondary reactions between primary products [162]. Several systems and their applications have been described [144,163— 167] sample withdrawal can be achieved [164] without the necessity for performing the reaction in an atmosphere of carrier gas. By suitable choice of separation column or combination of columns [162], it is possible to resolve species which are difficult to measure in a small low-resolution MS, e.g. H20, NH3, CH4, N2 and CO. Wiedemann [168] has made a critical comparison of results obtained by MS and GC techniques and adjudged the quality of data as being about equal. 

The schematic diagram of the experimental setup is shown in Fig. 2 and the experimental conditions are shown in Table 2. Each gas was controlled its flow rate by a mass flow controller and supplied to the module at a pressure sli tly higher than the atmospheric pressure. Absorbent solution was suppUed to the module by a circulation pump. A small amount of absorbent solution, which did not permeate the membrane, overflowed and then it was introduced to the upper part of the permeate side. Permeation and returning liquid fell down to the reservoir and it was recycled to the feed side. The dry gas through condenser was discharged from the vacuum pump, and its flow rate was measured by a digital soap-film flow meter. The gas composition was determined by a gas chromatograph (Yanaco, GC-2800, column Porapak Q for CO2 and (N2+O2) analysis, and molecular sieve 5A for N2 and O2 analysis). The performance of the module was calculated by the same procedure reported in our previous paper [1]. 

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