Pinch analysis

Question 1

List the order, from first to last priority, of the process design hierarchy.

Answer 1

Reactor, separation and recycle system, heat recovery system, heating and cooling utilities, water and effluent treatment.

Question 2

What is the heat capacity flowrate a product of?

Answer 2

mass flowrate and specific heat capacity, provided specific heat capacity is constant

Question 3

What are the advantages and disadvantages of increasing Tmin?

Answer 3

A smaller HX area is needed, which means lower capital costs. However not as much heat will be transferred overall.

Question 4

What happens to the duty when you increase Tmin?

Answer 4

The remaining heating and cooling duties required would increase.

Question 5

What does the area where the duty of the hot and cold composite curves overlap on the graph represent?

Answer 5

The amount of energy recovered.

Question 6

How do you create a composite curve?

Answer 6

Find the total duty required by each of the streams (cold streams and hot streams must be handled individually) in each relevant temperature interval and plot each section starting from the lowest temperature.

Question 7

What is the pinch point?

Answer 7

The temperature at which any heat crossing that temperature will only have to be removed again through hot or cold utilities, which is very energy inefficient.

Question 8

In which direction does heat flow above the pinch?

Answer 8

Into the cold streams from the hot utility.

Question 9

In which direction does heat flow below the pinch?

Answer 9

From the hot streams into the cold utility.

Question 10

List the six steps of the problem table method.

Answer 10

1. Shift the cold temperatures up by half of Tmin, shift the hot temperatures down by half of Tmin.
2. In each shifted temperature interval, calculate the total duty. CP here will be the sum of the cold stream CP minus the sum of the hot stream CP.
3. Cascade the surplus heat from high to low temperature. Find the lowest duty and add this to the top of the cascade to remove negative heat flows and to find the minimum hot and cold utility duties.
4. Start at the pinch (the most constrained heat flow) and match streams so that above the pinch, the cold CP is greater than the hot CP, and below the pinch the hot CP is greater than the cold CP.
5. Transfer as much hheat as is possible in each match to complete the heat requirements for one of the streams in the match. Then move onto other matches and try to complete as many of the heat transfer requirements as possible.
6. Draw the heat exchanger network showing heat exchangers between each stream with their loads labelled, all intermediate and final stream temperatures and the hot and cold utility requirements left for each stream.

Question 11

Does pinch analysis give the optimum design in terms of costs?

Answer 11

No, it gives the optimum design in terms of energy.

Question 12

What things should be considered when designing a heat exchanger network for optimum cost?

Answer 12

Both capital and operating costs.

Is it cheaper to cross the pinch?

Stream location.

Whether it is dangerous to mix stream materials.

Question 13

How do you construct the graph for finding the optimum point between capital and operating costs of a heat exchanger network, and draw a sketch example.

Answer 13

The energy curve is obtained by finding the total required energy for a range of Tmin values.

The optimum Tmin value lies where the sum of the energy curve and the capital cost curve lies at a minimum.

Question 14

Give five complications that can come into play when designing a heat exchanger network that could change the design.

Answer 14

1. Non-global minimum temperature differences
2. Process constraints on stream matches
3. Utility selection
4. Multiple pinches
5. Stream splitting