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Department of Chemical Engineering and Biotechnology

University of Cambridge

CET IIA Exercise 4 Plant control and P&ID drafting

Submission: Thursday 21

st February, by 15:00 hrs. Submission via Camtools site ChemEng

BioT 2011 of P&ID in PDF format, A3 page size and 2-page commentary, A4 page size, in

PDF format.

The objectives of this exercise are:

1. To gain understanding of steady state and unsteady state plant behaviour using

UniSim dynamics;

2. To understand the function of various control loops placed on a section of plant using

UniSim dynamics;

3. To draft a P&ID for a section of plant using information gained from objectives (1)

and (2).

Process description

The plant section under study can be considered to be a candidate storage and volatiles

separation system that would precede the distillation columns that were designed in Exercise

2; the plant flowsheet is shown in Figure 1.

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Figure 1. Plant flowsheet

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Middle oil from a coal tar distillation plant that is contaminated with n-pentane is stored in a

heated tank, V-100, under a nitrogen blanket. This oil is pumped from the tank by P-100 and

flows through two pre-heaters, E-101 and E-100. E-101 recovers heat from the exiting

process stream to pre-heat the oil and E-100 uses high pressure steam to bring the oil to the

required temperature. A pipe, PIPE-100, having a defined volume joins these two heat

exchangers. The hot oil is then flashed in V-101, with the resultant vapour stream being rich

in n-pentane. The liquid stream from V-101 passes through PIPE-101 that has a defined

volume, through the feed pre-heater E-101 and onto the next section of the plant. The plant is

sited in a windy part of northern Canada, and operation is studied during early spring where

the maximum day-time temperature is 25 C and the minimum night-time temperature is

-20 C; all vessels and pipes are subject to heat loss.

Task 1 understanding start up, steady state and shutdown control

Download the file CETIIA-Exercise4.usc from Camtools and load it into UniSim. A

separate handout has been made available that describes the various features of this

simulation and how to use the simulation controls; please ensure that you have read and

understood this document before proceeding any further. Remember that you can stop the

integrator at any time during the course of a simulation and examine data for streams and unit

operations in exactly the same way that you would for a steady state simulation. It must be

remembered that only key control loops have been included in this simulation.

a. Plant start up.

Run the simulation for an elapsed time of 400 minutes and then halt it. Write brief

explanatory notes on the operation of the following control loops:

i. LIC-100. Observe the level in V-100 (red trace on levels graph) and the position of

valve VLV-100 (red trace on valve position graph);

ii. TIC-102. Observe the temperature of V-100 (red trace of temperature graph) and the

position of valve controlling the steam supply into V-100 (cyan trace on valve

position graph);

b. Steady state operation

Continue running the plant simulation until an elapsed time of 1600 minutes is reached. Halt

the simulation. Write brief explanatory notes on the behaviour of the control loops listed in

section (a) and of the n-pentane composition of the vapour exiting the flash drum, V-101

(blue trace on levels and composition graph). For reference, the pink trace on the flows graph

corresponds to the ambient air temperature.

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c. Plant shut down

Save your simulation at 1600 minutes before continuing into the shut down phase. Once

saved, run the simulation and implement the following shut-down strategy:

i. With the simulation stopped, put the control loop T-101 into manual mode and

change the set point of LIC-100 (level in V-100) to around 2 %.

ii. Run the simulation for an elapsed time of about 150 minutes and then halt it.

iii. Put all remaining control loops into manual mode and (i) remove the remaining liquid

from flash drum V-101 by fully opening the control valve VLV-104 and (ii) manually

close the HP steam control valve, VLV-102.

iv. Run the simulation until a negative pressure warning is flagged. Stop the simulation.

v. Examine the levels in vessels V-100 and V-101 and observe any key warnings

returned by the simulation.

Make brief explanatory notes on the additional control measures that would need to be

implemented on the plant to ensure safe shut down, based on your observation of the shut

down behaviour of the plant. Also detail the mechanical design considerations that you would

need to include for tank V-100 to ensure that the vessel will be safe for entry during a

maintenance period.

Task 2 P&ID drafting

Now that you have explored the dynamic behaviour of the plant and considered the additional

control aspects and mechanical design implications for V-100, draw a P&ID of the section of

the plant outlined in Figure 2. Use Visio to prepare your P&ID with the paper size set to A3.

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Figure 2. Highlight shows section of plant to be drafted as a P&ID

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Remember that your P&ID should clearly indicate every pipe, valve and control system that

is used to control the process. Standard symbols should be used to indicate unit operations

and instrumentation should be drawn and tagged according appropriate international

standards. You may assume that all piping has a nominal diameter of 80 mm. Where control

signals come from other parts of the plant, use flags in a similar way to process streams. In

addition to the illustrating the control loops in the simulation, the following aspects should be

addressed:

The tank should not overflow whilst being filled.

P-100 should not cavitate if the tank is accidentally emptied.

P-100 needs to be started and tested prior to starting up the rest of the plant.

The tank should not collapse when it is being emptied.

The tank should not over-pressurise on a hot day.

Provision needs to be made to take samples from the tank for laboratory analysis.

The total amount of product fed to the tank from the ship needs to be recorded.

The total amount of product taken from the tank by the process needs to be

recorded.

During shutdowns, people need to be able to enter the tank (with breathing

apparatus) to inspect the interior.

Submission

The explanatory notes prepared for Task 1 should be no longer than two sides of A4 with a

minimum font size of 11 point; note that this is not a full report, but it must convey your

observations and explanations concisely. Please submit this document as a PDF file. The

P&ID prepared for Task 2 should consist of one A3 worksheet. Please also submit this

document as a PDF file.

Your PDFs should be submitted to the Camtools site ChemEng BioT 2011 by Thursday

February 21st at 15:00. Late submissions will generate zero marks unless you have a genuine

reason, in which case a tutors note from a Tutor or DoS should be sent to Dr Bart Hallmark

within five calendar days of the deadline.

Advice

Accurate P&IDs take time to create. Clear P&IDs also take time. Bad P&IDs can be drawn

very quickly. You are advised to give yourself sufficient time to be able to not only draw, but

check and revise your P&ID up to three or four times. Do not leave this exercise to the day

before the deadline - unless you want to fail.