Internal reflux control

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). 

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. 

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)... 

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. 

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.

In summary, it is best to use configuration 19.66 with the desired MB control scheme as per Sec. 16.6. If the desired control scheme is 16.4a, 6, or e, internal reflux control (Fig. 19.7a) should be incorporated. This internal reflux control cannot be used with scheme 16.4d. 

It is possible to install an internal reflux controller. If ATis the temperature drop across the condenser, Cp the specific heat of the reflux material and X its latent heat of vaporisation then... 

The temperature difference can vary for reasons other than subcooling. For example an increase in the heavy key component in the overhead vapour will cause an increase in vapour temperature. The internal reflux controller will then reduce the reflux flow -the opposite of what is required to deal with the composition change. Using, instead of the overhead temperature, a constant set at a typical value can resolve this. But then the correction for subcooling, although directionally correct, will not be of the correct magnitude. 

The alternative approach of selecting the energy balance scheme and checking for its limitations might show that it does not handle well disturbances to condenser and reboiler duty. We have seen (Figure 12.36) how the use of internal reflux control might be applied to the hrst of these problems. If the disturbances to the reboiler duty arise from heat... 

Internal reflux can he controlled without affecting product yield. The maximum internal liquid reflux is fixed by the thermodynamic state of the feed relative to the product stream. Excessive reflux will diminish product yield. 

In an operating column the effective reflux ratio will be increased by vapour condensed within the column due to heat leakage through the walls. With a well-lagged column the heat loss will be small and no allowance is normally made for this increased flow in design calculations. If a column is poorly insulated, changes in the internal reflux due to sudden changes in the external conditions, such as a sudden rain storm, can have a noticeable effect on the column operation and control. 

A great many assumptions have been made in this qualitative approach. Little account has been taken of the dynamics of the plant at this stage and the stability of xD in the face of changes (perturbations) in xF. The plant may be affected by ambient conditions, e.g. a sudden shower of rain will have a cooling effect which will cause vapour rising in the column to condense and increase the column internal reflux a long reflux line can lead to considerable degradation of the control (see Section 7.6). However, even with this simplistic approach, it is possible to examine a number of the control problems associated with the plant. 

The controls required to keep the internal reflux flow of the column constant (a) and the controls needed to eliminate the effect of accumulator lag in controlling the external reflux to a column (b). 

The bypassed vapor heats up the liquid there, thereby causing the pressure to rise. WTien the bypass is closed, the pressure falls. Sufficient heat transfer surface is provided to subcool the condensate, (f) Vapor bypass between the condenser and the accumulator, with the condenser near ground level for the ease of maintenance When the pressure in the tower falls, the bypass valve opens, and the subcooled liquid in the drum heats up and is forced by its vapor pressure back into the condenser. Because of the smaller surface now exposed to the vapor, the rate of condensation is decreased and consequently the tower pressure increases to the preset value. With normal subcooling, obtained with some excess surface, a difference of 10-15 ft in levels of drum and condenser is sufficient for good control, (g) Cascade control The same system as case (a), but with addition of a TC (or composition controller) that resets the reflux flow rate, (h) Reflux rate on a differential temperature controller. Ensures constant internal reflux rate even when the performance of the condenser fluctuates, (i) Reflux is provided by a separate partial condenser on TC. It may be mounted on top of the column as shown or inside the column or installed with its own accumulator and reflux pump in the usual way. The overhead product is handled by an alter condenser which can be operated with refrigerant if required to handle low boiling components. 

If the draw is markedly less in rate than the internal liquid flow, it can be drawn under flow control from an internal or external sump, and arranged so that the tower internal reflux flows through the sump. It is a good idea, nonetheless, to have a seal baffle or a seal pan set up so that if the side draw rate is set higher than the internal reflux flow, the downcomer will not become unsealed. 

If the draw stream rate is to be at or near the total internal liquid flow rate, it is best to provide an internal or external sump, with the draw rate controlled by the liquid level. The residual internal reflux can be a controlled flow to the tray below the draw. 

Two or three trays just above the flash zone are wash trays to prevent entrainment of dark color bodies into the heavy gas oil. The internal reflux flow on these trays should be limited to a few percent of the molal vapor rate, for good energy conservation and minimum residue handling. These trays are often equipped with anti-blowing baffles. The wash rate is sometimes controlled by using a total draw for the heaviest gas oil, then metering the wash oil rate. 

Regarding reflux, large-diameter columns carmot be operated with the internal reflux caused by a temperature gradient along the column that is operative only for diameters below 15 cm. Thus, an external reflux through a pump reinjected part of the extract must be implemented, which also permits easy control of this parameter. 

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). 

Upon entry into a column, a subcooled reflux quickly heats up to its boiling point by condensing column vapor. The liquid downflow (or "internal reflux ) in the top section therefore equals the external reflux plus that condensate. Frequently, the condensate makes up about 10 to 30 percent of the internal reflux flow, and it is not uncommon to find situations where it exceeds 40 percent of the internal reflux. Subcooling variations alter the quantity of condensate formed and are therefore reflected as changes in internal reflux. Inside the column, only internal reflux matters, and this will fluctuate even when the external reflux stream is flow-controlled. 

---