UNIT 7

Columns

This unit describes control strategies and operational considerations for distillation columns.

Learning Objectives When you have completed this unit you should:

1. Understand how operation close to the weep or flood point will disable control systems.
   
   
2. Know how to compensate for large diameter distillate receivers.
   
   
3. Determine the best plate for temperature control.
   
   
4. Be able to construct the best control strategy.

7-1. Process and Equipment Design Considerations

Column design that causes operation at the weep point or flood point will prevent stable operation. The operating point can be moved further away from the weep and flood points by a decrease and an increase in column pressure, respectively. It is desirable to reduce the pressure set point if possible, because the relative volatility, which is a measure of the ability of the column to separate components, improves as the pressure is lowered. Most columns can run at full vacuum if the operating point is far enough away from the flood point and the condenser has sufficient heat removal capability (duty) for the higher vapor flow.

When a column goes into weep, the vapor flow is insufficient to keep the liquid suspended on the trays. Liquid falls through the holes in the trays and can eventually end up mostly in the column sump, along with some trays. Temperature sensors in the upper portion of the column run dry. The pressure drop plummets and the sump level rises from the dumping.

When a column goes into flood, the vapor flow is so high it causes the froth from one tray to intermittently choke and break through to the next tray. The result is a percolation, much like in a coffee pot, that can cause spillover into the distillate receiver. The pressure rapidly fluctuates and the receiver inventory may surge. For large horizontal drums, the level change might not be noticeable due to the large cross-sectional area. The column temperature profile can become inverted as high boilers are burped upward and low boilers are swept downward.

Since the use of the best tray location is critical and can change with operating conditions, it is extremely important that the column have temperature connections at many trays. Sensors should be installed at the best tray and the ones immediately above and below it, as well as at enough other trays to get a good column temperature profile for diagnostics and validation of process models. The sensor should extend into the liquid froth above the tray to maximize the heat transfer coefficient but should not interfere with column traffic. Intrusion of the thermowell into a downcomer area might cause local flooding. In packed columns, sensor location is more critical due to the possibility that channeling will cause liquid to bypass the sensor.

The condenser and reboiler duty should permit operation at the lowest and highest column pressure, respectively Since the coolant pressure and temperature are less well regulated than steam pressure and temperature, the coolant to the condenser is often manipulated to control column pressure, and the steam to the reboiler is set to fix the energy per unit feed.

The distillate receiver diameter should be small enough so that the process integrator gain (ft./lb) is large enough to ensure typical level controller gain, and measurement error and noise do not translate into excessive error in the enforcement of the material balance. Thus, horizontal drums cause problems due to a small and nonlinear process gain. For these applications, either an exceptionally high controller gain (e.g., K_c > 20) or a translation of the control variable from level (%) to inventory (1b) should be used to facilitate tight inventory control The use of reset action and low gain will create slow, nearly equal amplitude-sustained oscillations. The amplitude increases with reset setting (repeats/minute) and control valve dead band (%) (Ref. 1).