Internal vapor controller

Similar problems can occur with vapor sidestreams, but the solution is not as easy because we cannot provide vapor holdup in the system. One approach is to use an internal vapor controller. The flowrate of the vapor sidestream and the flowrate of the steam to the reboiler are measured. The net flowrate of vapor up the column above the vapor sidestream drawoff tray is calculated. This flow is then controlled by manipulating the vapor sidestream drawoff rate. 

Figure 19.7 Internal reflux and vapor rate control in side-drawoff columns, (a) Internal reflux control, liquid side product (b) internal vapor control, vapor side product.

Controlling the internal vapor flow to the section above the side draw Reboiler heat duty is measured and divided by the latent heat of the boiling mixture the measured side product flow is subtracted from the quotient to give the internal vapor rate in the section above the side draw. In a steam (or condensing vapor) reboiler, the internal vapor rate is computed as a constant times the measured steam rate less the measured side product flow, with the constant equal to the ratio of the latent heat of steam to that of the boiling mixture. An internal vapor controller (IVC) uses this computed internal vapor to manipulate product flow (Fig. 19.76). A limitation of this technique is that internal vapor is computed as a small difference between two large numbers and can therefore be in error. The error escalates as the internal vapor rate becomes a smaller fraction of the total vapor traffic below the side draw. 

When the internal reflux in the section above the side draw is low compared to the side product flow, scheme 16.4e should be avoided. The internal vapor control in Fig. 19.76 may work well, but it will become increasingly troublesome as the internal vapor becomes a smaller fraction of the total liquid traffic below the side draw. 

A variety of test procedures and use guidehnes have been developed. In addition, companies or associations may develop internal standards. The Federal Register, 33 CFR, Part 154, contains the USCG requirements for detonation arresters in marine vapor control... 

Liquid distribution in a packed bed is a function of the internal vapoi/liquid traffic, the type of packing employed, and the quality of the liquid distributors mounted above the packed bed. Vapor distribution is controlled by the internal vapor/liquid traffic, by the type of packing employed, and by the quality of the vapor distributors located below the packed beds. 

Effectiveness of Internals in Controlling foam. Originally, the separators contained limited internals—an inlet disengaging device, surge baffles, an anti-foam baffle, and a demister at the outlet. After severe foaming occurred, however, a bank of parallel plates (Dixon plates) was filled in each separator on one of the Ninian and one of the Brent trains The plates provide a large surface area and are sloped so that the separated oil flows dow n to the surface of the liquid. The installation is illustrated m Ftg S. The parallel plates arc 4 in. apart and extend from near the top of the vessel in the vapor space down to the normal liquid level. 

Stable column operation is guaranteed by keeping the internal reflux of the distillation tower constant. Consequently, internal reflux controls are designed to compensate for changes in the temperature of the external reflux caused by ambient conditions. Figure 2.90a is controlled by a typical internal reflux control system (top) and the equations that need to be solved in calculating the required external reflux rate are shown at the bottom. This control system corrects for either an increase in overhead vapor temperature or a decrease in external reflux liquid temperature. 

The operating line for the rectifying section can, then, be expressed in terms of the external reflux ratio. It is chosen because of the ease of measuring and controlling external liquid flowrates compared to internal vapor ones ... 

Liquid-flow-modulated heat pipes have two separate wicking structures, one to transport liquid from the evaporator to the condenser and one that serves as a liquid trap. As the temperature gradient is reversed, the liquid moves into the trap and starves the evaporator of fluid. In addition to these liquid-vapor control schemes, the quantity and direction of heat transfer can also be controlled through internal or external pumps, or through actual physical contact with the heat sink. 

Failure of Reflux Controller. A common practice is to set the relief requirement equal to the column internal vapor rate to the top tray. In case of a side reflux or pumparoimd, the relief requirement is commonly set equal to the difference between vapor entering and leaving the section (9). 

The internal vapor rate controller can be substituted by a differential pressure controller measuring pressure drop in the section above the side draw (260, 263, 362). This option, however, is less satisfactory (260, 263) because of the shortcomings inherent in pressure-drop controllers (Section 19.4). 

The Fig. 19.86 system can be modified to become an improved version of the Fig. 19.8a system by cascading the composition controls onto the product ratio controls (68). If needed, an internal vapor flow control can be used to control the side-draw rate, and the bottom level controller cascaded onto the steam ratio control. 

A normal refrigerator has many sources of ignition within it— the thermostat, interior light, the hght switch on the door, the defrost heater, the defrost control switch, the compressor unit, and the air circulation fan. Most of these are located within the space being maintained cool, but self-defrosting units contain an internal drain that can permit the internal vapors to flow into the compressor space below the usable space. 

Note that the Zob(C) = 0.05 disturbance (Fig. 11.23), which crashed the two-temperature control scheme, causes no problem in the internal composition control structure and that the time scale has been increased to 300 min to make sure that there is long-term instability. The internal composition controller detects the increase in reactant A composition on tray 6 because there is less B coming into the column in the Fqa feed. The composition controller reduces the flowrate of Fqa. The temperature on tray 3 increases as less D is produced because less B is entering the column. The temperature controller cuts back on the vapor boilup, and the reflux ratio cuts back on the reflux. The final steady state produces somewhat lower purity bottoms and less of it. Distillate purity is higher with only a slightly lower flowrate because of the C coming in with the Fqb feed. 

Mercuric iodide crystals grown by physical vapor transport on Spacelab 3 exhibited sharp, weU-formed facets indicating good internal order (19). This was confirmed by y-ray rocking curves which were approximately one-third the width of the ground control sample. Both electron and hole mobiUty were significantly enhanced in the flight crystal. The experiment was repeated on IML-1 with similar results (20). 

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