Reflux Control

Post- Oil Energy Technology After the Age of Fossil Fuels 

Feed flow optimization (a) maximizing throughput by fully loading the condenser (b) maximizing throughput against a reboiler constraint (c) maximum recovery of the heat content of bottom product by economizer. 

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Resoles. Like the novolak processes, a typical resole process consists of reaction, dehydration, and finishing. Phenol and formaldehyde solution are added all at once to the reactor at a molar ratio of formaldehyde to phenol of 1.2—3.0 1. Catalyst is added and the pH is checked and adjusted if necessary. The catalyst concentration can range from 1—5% for NaOH, 3—6% for Ba(OH)2, and 6—12% for hexa. A reaction temperature of 80—95°C is used with vacuum-reflux control. The high concentration of water and lower enthalpy compared to novolaks allows better exotherm control. In the reaction phase, the temperature is held at 80—90°C and vacuum-refluxing lasts from 1—3 h as determined in the development phase. SoHd resins and certain hquid resins are dehydrated as quickly as possible to prevent overreacting or gelation. The end point is found by manual determination of a specific hot-plate gel time, which decreases as the polymerization advances. Automation includes on-line viscosity measurement, gc, and gpc. 

Temperature Limit Ramped change of set point Reflux Control (1)... 

Figure 3.60. Model representation of a continuous binary distillation column. PC is the cooling water controller, LC the reflux controller.

Feed enters T-2 at tray 5. There is a pump-through reboiler. Another pump withdraws material from the bottom and sends it to tower T-3. Liquid is pumped from tray 18 through a cooler and returned in part to the top tray 20 for temperature and reflux control. A portion of this pumparound is withdrawn after cooling as unsaps product. Steam leaves the top of the tower and is condensed in the barometric. 

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. 

External instead of internal reflux control can be used in some cases when the external reflux flow (L) is controlled under the cascade control of accumulator level. To overcome the accumulator lag, the reflux rate, L, is manipulated in direct proportion to the distillate rate (D), rather than by waiting for the response of a level controller (Figure 2.90b). 

Kerkhof and Vissers showed that for difficult separations an optimal reflux control policy yields up to 5% more distillate, corresponding to 20-40% higher profit, than either constant distillate composition or constant reflux ratio policies. 

The startup time for optimal reflux control is 120 min. and for side-stream flowrate control is 130 min. These results (for each case obtained after 44805 numerical integrations of the system (l)-(3) with 200 differential equations) are remarkable in comparison with operating at constant control parameters where the startup time is 380 min. For all these regimes the final stationary state corresponds to an average value of the normalized derivatives less than 10 (maximum integration step is 1 s.). 

In figures 1 and 2 are presented the optimal reflux control and the optimal side-stream flowrate control (subscript s indicates final, stationary value). 

Figure 3. Evolution of benzene concentration in the top product for startup at constant control parameters and at optimal side-flowrate control (—), and at optimal reflux control (—).

This equation can be rearranged to calculate the external reflux that maintains a specified internal reflux control (FJ ), i.e.. 

FIGURE 15.65 Schematic of an internal reflux controller applied for composition control of the overhead of a column. 

This approach, called internal reflux control, is shown schematically in Figure 15.65. Note that the composition controller outputs the internal reflux flow rate, and the internal reflux controller calculates the external reflux flow rate, which is used as the setpoint for the flow controller on the reflux. 

The condenser is usually an Allihn type, because of its large surface area, and usually it is placed vertically so all of the condensed vapors will drip into the reflux control. 

Figure 3-29 shows a simpler, stopcock-type reflux controller. The reflux ratio is controlled by how much the stopcock is opened. This type does not work well with viscous materials, and you cannot measure the reflux ratio. 

Control system. electronic control for oil bath according to temperature or differential column pressure — electronic control or follow-up control for heating jacket — reflux control with vapour division — vacuum control — fraction collector control — recording of all data — protection against superheating and interruption of cooling water supply... 

Automatic reflux control by mechanical means or by electronic time switches. 

Automatic reflux control by a magneticallj operated swinging funnel and timer (Fig. 312). 

The fraction collector 23 normally contains sixty 20 ml receiving tubes and is buUt into a vacuum desiccator with a wire-gauze safety cover. It can be employed at atmospheric pressure and at reduced pressures down to 1mm. After 30 tubes have been filled a signal sounds alternatively, the apparatus may be made to switch over automatically to the next circle of tubes. When all the tubes are full there is another signal and the reflux controller is switched off so as to stop the distillate take-off. 

The accuracy of reflux control by means of electronic timers has been thoroughly studied by Rock et al. [13]. Geinmecker and Stage [67] have found that constant, reproducible and load-independent values for reflux can be obtained only with elec-tromagneticaUy controlled components. Deviations may be due to the following effects. 

In smaller columns, the overhead condenser is often mounted above the column, and reflux flows to the column by gravity. The reflux drum and/or reflux control valves are frequently omitted sometimes, the bottom section of the condenser is used as a surge compartment. 

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). 

Internal reflux control. It was stated earlier (Sec. 16.6, guideline 7) that a major strength of scheme 16.4d is its ability to minimize the impact of disturbances in the cooling medium on the column. A similar capability can be incorporated into the alternative schemes (16.4o-c, e) by adding simple instrumentation to set up an internal reflux controller (Fig. 19.2). This controller automatically adjusts the reflux rate for changes in reflux subcooling. 

In scheme 16.4d, the action of the level controller (which controls the reflux) eliminates these fluctuations. The internal reflux controller (IRC) achieves the same function by computing the column internal reflux and controlling it at a desired value. A simple, approximate correlation often used is (68)... 

Gravity reflux control cycling. Figure 19.3 shows an application of scheme 16.4d to a gravity reflux system. Level in the condenser liquid compartment (which is equivalent to the reflux drum) is controlled by... 

Controlling the internal reflux to the section below the side draw Subtracting the measured side-product flow from the measured reflux flow (the latter may need correction for subcooling see Sec. 19.2) gives the internal reflux to the section below the side draw. An internal reflux controller (IRC) uses this computed internal reflux to manipulate side-product flow (Fig. 19.7a). A limitation of this technique is that the internal reflux is calculated as a small difference between two large numbers, and can therefore be in error. The error escalates as the internal reflux becomes a smaller fraction of the total liquid traffic above the side draw. 

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