Study Questions

Apply Your Knowledge
24.8
Which of the following is an appropriate safety control design objective?

Control objectives

1. Safety is one of the seven categories of control objectives.




Design materials that do not corrode


 

 Prevent high pressure in closed vessel (above vessel rating)
 


 Design vessel walls that are strong enough
 
Do not allow excessive reflux to return to the tower

 

 

 

 

 

 

 

 

 

 

 

 

24.9
Which of the following is an appropriate environmental protection control design objective?

Control objectives

2. Environmental protection is one of the seven categories of control objectives.




Do not process hazardous materials

 

 Do not allow liquid to overflow to the environment from the reflux drum
 


 Do not release hazardous vapors to the environment
 
Do not allow leaks from the pumps

 

 

 

 

 

 

 

 

 

 

 

 

 

24.10
Which of the following is an appropriate equipment protection control design objective?

Control objectives

3. Equipment protection is one of the seven categories of control objectives.

 

Do not blow up the tower
 
 Keep some liquid on each tray
 
 Keep the steam pressure in the reboiler below the maximum
 

Provide at least a minimum flow to the pumps by maintaining liquid in the levels

 

 

 

 

 

 

 

 

 

 

 

 

 

 

24.11
Which of the following is an appropriate smooth operation control design objective?

Control objectives

4. Smooth operation is one of the seven categories of control objectives.

 

Moderate flows from the tower to downstream units

 
 Do not change the reboiler duty
 
 Maintain the levels within max/min limits
 
 Maintain the pressure drop across the stripping section constant

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

24.12
Which of the following is an appropriate product quality control design objective?

Control objectives

5. Product quality is one of the seven categories of control objectives.

 

Maintain the light key in the bottom product at its specification

 
 Maintain the lighter than light key in the distillate below is maximum
 
 Control the pressure required for the product quality
 
 Maintain the heavy key in the distillate product at its specification

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

24.13
How can we affect profit when most valves are already used for higher priority objectives?

Control objectives

Profit is important; we have to pay the engineers ($$$$)!

We must have valves that are not used for other objectives.

 
 We cannot effect profit through process control
 
 We can adjust the set points of controllers implemented for other objectives
 

None of the above

 

 

 

 

 

 

 

 

 

 

 

 

24.14
Which of the following is an appropriate profit control design objective?

Control objectives

6. Profit is one of the seven categories of control objectives.

 

Maintain the light key in the bottom product at a value that maximizes profit

 
 Keep the tray temperatures at the highest profit values
 
 Control the pressure to the value that yields the highest profit
 
 I still don't believe that control affects profit!

 

 

 

 

 

 

 

 

 

 

 

 

 

 

24.15
Which of the following is an appropriate monitoring and diagnosis control design objective?

Control objectives

7. Monitoring and diagnosis is one of the seven categories of control objectives.

Here, we will focus on short-term monitoring - alarms.

 

Provide an alarm for every measured variable

 
 Provide an alarm on the pressure difference across the stripping section.
 
 Provide a high alarm on the pressure in the tower.
 
 Provide high and low alarms for the bottoms level

 

 

 

 

 

 

 

 

 

 

 

 

24.16
Which of the following is an appropriate monitoring and diagnosis control design objective?

Control objectives

7. Monitoring and diagnosis is one of the seven categories of control objectives.

Here, we will focus on longer-term monitoring and process diagnosis.

 

Provide a trend plot and historical data storage for every variable

 
 Measure the pressure with two sensors
 
 Measure several tray temperatures
 
 Measure the flow rate of the feed, although it is controlled by an upstream unit

 

 

 

 

 

 

 

 

 

 

 

 

24.17
Which of the following sensor ranges is correct?

Instrumentation

We need to specify the range for every sensor.

 

Reflux flow: 10-120% of design

 
 Tray temperature: design=300 ºC, range = 250-350 ºC
 
 Level: from drain to very top
 

Pressure: design = 5 atm,

range = 0 to 10 atm.

 

 

 

 

 

 

 

 

 

 

 

 

24.18
For controlling pressure in the tower, which valve would you manipulate?

Loop Pairing

 

 

There is no need to manipulate a valve, the pressure would be essentially constant.

 
 The distillate flow valve
 
 The liquid flow from the condenser
 
 The steam flow to the reboiler.

 

 

 

 

 

 

 

 

 

 

 

24.19
If you want to control the bottom product purity, but you do not have an on-stream analyzer, what dominant variable would you select to control?

Loop Pairing

Dominant variable - maintains important unmeasured variables near their desired values.

 

Use a Fenske-Gilliland model to estimate the bottoms purity.

  Maintain the reboiler duty as constant as possible.
  Control a tray temperature in the lower half of the column
  There is nothing that we can do.  We must install an on-stream analyzer.