Control bottom flow

Amplitude of controlled variable Output amplitude limits Cross sectional area of valve Cross sectional area of tank Controller output bias Bottoms flow rate Limit on control Controlled variable Concentration of A Discharge coefficient Inlet concentration Limit on control move Specific heat of liquid Integration constant Heat capacity of reactants Valve flow coefficient Distillate flow rate Limit on output Decoupler transfer function Error... 

Example 1.5. For a binary distillation column (see Fig. 1.6), load disturbance variables might include feed flow rate and feed composition. Reflux, steam, cooling water, distillate, and bottoms flow rates might be the manipulated variables. Controlled variables might be distillate product composition, bottoms product composition, column pressure, base liquid level, and reflux drum liquid level. The uncontrolled variables would include the compositions and temperatures on aU the trays. Note that one physical stream may be considered to contain many variables ... 

As a final example, suppose we are controlling the base level in a distillation column with the bottoms product flow rate. The valve would be AO because we want it to fail shut (we don t want to lose base level in an emergency). The level transmitter signal increases if the level increases. If the level goes up, we want the bottoms flow rate to increase. Therefore the base level controller should be increase-increase (direct acting). 

The reactor effluent is separated in a distillation column. The overhead is mostly excess reactant A which is recycled back to the reactor. The bottoms from the column is mostly product C. The reaction occurs in the liquid phase so the reactor feed streams are liquid. Reactant B is added directly to the reactor on flow control. The flow rate of the recycle stream is ratioed to the flow rate of the B feed stream. The composition of A in the column base sets heat input. The composition of C in the column overhead sets reflux. 

As a minimum, a distillation assembly consists of a tower, reboiler, condenser, and overhead accumulator. The bottom of the tower serves as accumulator for the bottoms product. The assembly must be controlled as a whole. Almost invariably, the pressure at either the top or bottom is maintained constant at the top at such a value that the necessary reflux can be condensed with the available coolant at the bottom in order to keep the boiling temperature low enough to prevent product degradation or low enough for the available HTM, and definitely well below the critical pressure of the bottom composition. There still remain a relatively large number of variables so that care must be taken to avoid overspecifying the number and kinds of controls. For instance, it is not possihle to control the flow rates of the feed and the top and bottom products under perturbed conditions without upsetting holdup in the system. 

Controlling Quality of Two Products Where the two products have similar values, or where heating and cooling costs are comparable to product losses, the compositions of both products should be controlled. This introduces the possibility of strong interaction between the two composition loops, as they tend to have similar speeds of response. Interaction in most columns can be minimized by controlling distillate composition with reflux ratio and bottom composition with boil-up, or preferably boil-up/bottom flow ratio. These loops are insensitive to variations in feed rate, eliminating the need for feedforward control, and they also reject heat balance upsets quite effectively. 

Figure 1.17 Schematic of a micro channel equipped with many electrodes at the upper (U ) and lower (Lf) walls for control of the C, potential. The arrows in the channel denote the directions of the electroosmotic velocities creating one type of flow pattern, here a counter-current arrangement of top and bottom flows (top) alternating-flow arrangement, demonstrating another type of control over the potential (bottom) [28] (by courtesy of ACS).

In this example, the five manipulated variables are so assigned to the five controlled variables that the heat input at the reboiler and the distillate composition are fixed and therefore the bottoms flow and composition are allowed to change with the variations in feed flow or composition. 

Once the large internal flow rates have been set via appropriate control laws, the index of the DAE system (7.21) is well defined, and a state-space realization (ODE representation) of the slow subsystem can be derived. This representation of the slow dynamics of the column can be used for the derivation of a model-based nonlinear controller to govern the input-output behavior of the column, namely to address the control of the product purity and of the overall material balance. To this end, the small distillate and bottoms flow rates as well as the setpoints of the level controllers are available as manipulated inputs. 

When dispensing fluid from a measuring pipette, let the tip touch the side of the receiving container and let the fluid flow. If you are dispensing the liquid by hand, you need to control the flow rate by placing your thumb on the end of the pipette. However, never let your thumb wander away from the end of the pipette because you will need it to stop the fluid flow. If you completely drain the pipette, count to two (to allow for any remaining fluid to flow to the bottom), and remove the tip sideways away from the receiving wall. Do not remove the tip with an upward or downward motion. 

Let s look again at the simple reaetor/column process in Fig. 2.5. In Sec. 2.4.2 we proposed two control structures where both the bottoms composition xB it he plant product) and the distillate composition xD (the recycle stream) are controlled, i.e., dual composition control. Bottoms composition must be controlled because it is the product stream leaving the plant and sold to our customers. However, there is a priori no reason to control the composition of the recycle stream since this is an internal flow within the plant. 

The scheme for the reactor/stripper process uses a PI controller to hold product composition (xB) by manipulating vapor boilup in the stripper. The same analyzer deadtime is used. Proportional level controllers are used for the stripper base (manipulating bottoms flow ), the overhead receiver (manipulating recycle flow), and the reactor (manipulating reactor effluent flow) with gains of 2. 

The magnitudes of various flowrates also come into consideration. For example, temperature (or bottoms product purity) in a distillation column is typically controlled by manipulating steam flow to the reboiler (column boilup) and base level is controlled with bottoms product flowrate. However, in columns with a large boilup ratio and small bottoms flowrate, these loops should be reversed because boilup has a larger effect on base level than bottoms flow (Richardson rule). However, inverse response problems in some columns may occur when base level is controlled by heat input. High reflux ratios at the top of a column require similar analysis in selecting reflux or distillate to control overhead product purity. 

R-V Reflux flow controls distillate composition. Heat input controls bottoms composition. By default, the inventory controls use distillate flowrate to hold reflux drum level and bottoms flowrate to control base level. This control structure (in its single-end control version) is probably the most widely used. The liquid and vapor flowrates in the column are what really affect product compositions, so direct manipulation of these variables makes sense. One of the strengths of this system is that it usually handles feed composition changes quite well. It also permits the two products to be sent to downstream processes on proportional-only level control so that plantwide flow smoothing can be achieved. 

R-B When the boilup ratio is high bottoms flow should be used to control bottoms composition and heat input should control base level. However, in some columns potential inverse response may create problems in controlling base level with boilup. 

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