EXERCISE 3 ANIS ATIKAH AHMAD

1. Consider the distillation column shown in Figure Q1.1: Figure Q1.1: Distillation column It would be reasonable to control the liquid level in the reflux drum, h D by manipulating either reflux flow rate, R, or distillate flow rate, D. Explain how would the nominal value of reflux ratio (R/D) influence your choice. As a specific example, assume that R/D = 4

It is required to select manipulated variable that have large effects on controlled variables, which is stated in Guideline 6. In this case, manipulating the large exit stream, R (reflux flowrate) will give large effects on liquid level rather than manipulating the small stream, D (distillate flow rate), since R/D> 1 (where R is greater than D).

2. Consider the liquid storage system shown in Figure Q2.1. Only volumetric flowrate q 1 and q 2, can be manipulated. Determine the model degrees of freedom, N F, and the control degrees of freedom N FC. Figure Q2.1: Liquid storage system

Variables: q 1, q 2, q 3, q 4, q 5, q 6, h 1, h 2 Equations: Total no of equation: 5 Total no of variables: 8

Thus, degrees of freedom, N F = N v - N F = 8 – 5 = 3 Degrees of freedom are also equal to the summation of control degrees of freedom, N FC and number of disturbance variables, N D : N F = N FC + N D Disturbance variable: q 6 [N D = 1] (Usually the input) Thus, control degrees of freedom, N FC = N F - N D =3-1 = 2

3. The loss of the coolant to a process vessel can produce an unacceptably high pressure in the vessel. As a result, a pressure relief valve is used to reduce the pressure by releasing the vapor mixture to the atmosphere. But if the mixture is toxic or flammable, the release can be hazardous. For the distillation column in Figure Q3.1, propose an alarm/SIS system that will reduce the number of releases to the environment, even though the occasional loss of coolant flow to the condenser is unavoidable. (The pressure relief valve at the top of the column is not shown in Figure Q3.1) Figure Q3.1: Distillation column

The proposed SIS system is as follows: