Distillation columns accumulator level control

Four alternative control schemes are commonly used for distillation column control, as shown in Figure 3.15 through Figure 3.18, respectively. Scheme 1 directly adjusts the material balance by manipulation of the distillate flow. If the distillate flow is increased, then the reflux accumulator level controller decreases the reflux flow. As less liquid proceeds to flow down to the sump, the sump level controller decreases the bottoms flow a like amount. The separation is held constant by manually setting the reboiler steam flow to maintain a constant energy per unit feed. 

Problem Reflux flow from a vertical accumulator was erratic. Reflux flowed by gravity to the distillation column on flow control with no accumulator level controller. 

For example, assume that you want to perform tests on the plant, represented by Figure 15.74. The plant is a simple distillation column with overhead accumulator pressure controlled by moving the hot vapor bypass, bottoms level maintained by bottoms product draw rate, and the overhead accumulator level controlled by adjusting the overhead product draw rate. Reflux is on flow control, and the reboiler is on temperature control. Typical move sizes for this plant are shown in Table 15.12. 

Direct MB control. Here the composition (or temperature) controller directly regulates a material balance (i.e., product) stream. The other product is regulated by a level or by pressure. The stepwise action of the Fig. 16.4d scheme is as follows. Suppose the concentration of lights rises in the column feed. This will be sensed by a drop in column temperature, and the temperature controller will increase distillate flow. Accumulator level will fall, and the level controller will reduce reflux. This will lower the bottom level, and the level controller will reduce bottom flow. 

If the main source of disturbance in the column is the heating medium, the disturbances may interact with the accumulator level control, and scheme 16.4d may lead to reflux flow fluctuations. The unconventional Fig. 16.66 scheme may be better for smoothing these disturbances. Favorable experiences have been reported with this scheme, and its use has been recommended when distillate flows are small (258). Like its unconventional counterpart (Fig. 16.6a), scheme 16.66 may not be suitable if feed flow is large compared to the column boilup. It may also suffer from interaction between the level and pressure controls (301). 

Column acted like a strq>per, reflux-to-distillate ratio was 0.43. When reflux flow was on accumulator level control, a small change in heat input led to large changes in reflux flow. Reflux flow at times fell below the minimum required for tray wetting. 

The feed flow is often not controlled but is rather on level control from another column or vessel. The liquid product flow s (distillate and bottoms) are often on level rather than flow control. Top vapor product is, however, usually on pressure control. The reflu.x is frequently on FRC, but also may be on column TRC or accumulator level. 

A proportional plus integral controller is used to control the level in the reflux accumulator of a distillation column by regulating the top product flowrate. At time t = 0, the desired value of the flow controller which is controlling the reflux is increased by 3 x 10-4 m3/s. If the integral action time of the level controller is half the value which would give a critically damped response and the proportional band is 50 per cent, obtain an expression for the resulting change in level. 

A proportional plus integral controller is used to control the level in the reflux accumulator of a distillation column by regulating the top product flowrate. At time t = 0 the desired value of the flow... 

In the multiloop controller strategy each manipulated variable controls one variable in a feedback proportional integral derivative (PID) control loop. Taking a single-feed, two-product distillation column with a total condenser and a reboiler as an example, a basic list of possible controlled variables includes the distillate and bottoms compositions, the liquid levels in the reflux accumulator and the column bottom, and the column pressure. The main manipulated variables are the reflux, distillate, and bottoms flow rates and the condenser and reboiler heat duties. 

Eollowing are two examples (16.1 and 16.2) of a distillation column that demonstrate the effect of applying different pairing strategies. In both examples the control loops for the column pressure and the liquid levels in the condenser accumulator and the column bottom are determined independently based on practical considerations. Thus, the column pressure is controlled by various techniques that may involve the condenser coolant rate, and the liquid levels are controlled by the product flow rates. What remains to be decided is how to pair the distillate and bottoms compositions with the reflux rate and the reboiler heat duty. The same distillation column is used in both examples, having a total condenser and a reboiler, one feed and two products. The column is designed to separate a benzene-toluene mixture into benzene and toluene products with specified purities. 

Liquid-level control Figure 13.2d and e show two feedback systems used for the control of the liquid levels at the bottom of a distillation column and its condenser accumulation tank. 

If the inventory of main components can be handled by local control loops, the inventory of impurities has essentially a plantwide character. The rates of generation, mainly in chemical reactors, and of depletion (exit streams and chemical conversion), as well as the accumulation (liquid phase reactors, distillation columns and reservoirs) can be balanced by the effect of recycles in order to achieve an acceptable level. Because... 

In contrast, consider the response of schemes 16.4a,6, or e to a similar change. The drop in accumulator level will reduce distillate flow, while reflux flow rate will remain unaltered. The same quantity of reflux will enter the column, but at a lower temperature. It will be reheated upon entry by vapor condensation, ind this will increase liquid flow down the column. This is not the desired response. Eventually, the appropriate response will be established, but not until the control tray temperature drops and the temperature controller takes corrective action. In scheme 16.46, a subcooling disturbance disturbs at least the top part of the column. With the Fig. 16.4a and e schemes, it will distimb the entire column. 

Control of the column material balemce may be difficult when the distillate stream is small relative to reflux flow. Unless large swings in distillate flow are acceptable, changes in the small distillate flow will have little impact on accumulator level, and schemes 16.4a, b, and e will not maintain a steady accumulator level. In most cases, the level in the accumulator will be allowed to drift and will be periodically adjusted manually by changing reflux, boilup, or condensation rate. However, this mode of control does little to maintain the column material balance, and over a period of time may cause accumulation or depletion of the light components. Scheme 16.4d does not suffer from the above problem and is often favored with small distillate flows (300). 

An energy balance scheme as in Fig. 16.7 usually requires continuous adjustment of a product rate (in Fig. 16.7, the distillate rate) by the operator, and a very slow level control action (in Fig. 16.7, accumulator level). Consider a rise in concentration of the light component in the feed. The bottom section temperature will drop, and the temperature controller will raise boilup. Column pressure will rise, and the pressure controller will increase the condensation rate. The accumulator level will rise, and the level controller will pour more reflux into the column. This in turn will reduce control tray temperature, and the temperature controller will raise boilup again. This will continue until reflux and boilup sufiiciently rise to keep the bottom section temperature up. In the meantime, the light component accumulates in the system, and this will cause further increase of reflux and boilup. 

Due to the above limitations, an energy balance control scheme such as scheme 16.7 is not recommended, but some situations exist where this scheme can offer better product composition control than many other alternatives. Superfractionators with a reflux to distillate ratio of 10 to 1 or more are one example. Here, distillate flow may be too small to satisfactorily control either accumulator level or column temperature. The author has experienced a satisfactory operation of scheme 16.7 in a propylene-propane splitter, with intermittent operator intervention to ac ust the material balance. The cycles in reflux and reboil (see above) could be tolerated, as the column was not operating close to its limits. In this column, scheme 16.7 gave tighter composition control than scheme 16.4d. 

Distillate and bottoms were controlled by accumulator and sump levels, respectively, feed and reflux on flow control and boilup was temperature-controUed Tower pall rings were replaced by higher-capadfy rings (bottom) uid wire-mesh structured packing (top) to increase c )acity and reduce reflux. The column was sensitive to ambient disturbances (e.g., rainstorms). The reflux reductions escalated this sensitivity to an extent that annulled the revamp benefits. The temperature control was ineffective due to its narrow range of variation. Problems were solved by controlling boilup on sump level and bottom product on flow control. 

Once the basic concept of material-balance control has been selected for a process, one must apply the same concept to all process steps. It is for this reason that the first step in designing column controls is to determine the material-balance control arrangement. Control in the direction of flow is the most commonly used concept (although the least desirable), and a frequently encountered arrangement is shown on Figure 1.5. Here level in the condensate receiver (also commonly called reflux drum or accumulator) sets the top product, or distillate flow, while the level in the base of the column sets the bottom product flow in other columns base level sets steam or other heat-transfer media to the reboiler, in which case the condensate receiver level sets top product flow. 

When excess water is added to the system, the level of the reactor increases. The level controller increases the liquid flow rate, leaving the reactor to compensate. Assuming the distillation column separates the ternary mixture almost perfectly, the unreacted ethylene oxide and the excess water are driven overhead into the distillate stream. This stream feeds the recycle tank, and thus increases the level. The flow rate of the stream to the recycle pump is increased to compensate. This increased flow is recycled to the reactor and increases the level. The cycle begins again, which results in accumulation of water in the system. The recycle stream snowballs . 

---