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Pinch Technology |
Chapter 1 |
3.For a petro-chemical process the method will identify sharp changes in the plot of Energy Cost vs. DTmin, which is the method described in "Retrofit targeting based on DTmin - Energy curves".
Retrofit targeting based on experience DTmin values
It is expected that retrofit projects involving similar cost scenarios (fuel and capital costs etc.), and similar levels of process technology may result in similar target DTmin values. In such cases previous applications experience provides a useful source of information for setting the target DTmin for the process.
Usually similar processes have similar shapes of composite curves. For example for atmospheric distillation units, the composite curves tend be "parallel" to each other due to the similarity of the mass flows between the feed and the products of distillation. The shape of the composite curves influences the temperature driving force distribution in the process and therefore the heat exchanger network capital cost. The figure below illustrates the impact of the shape of the composite curves on the target DTmin value.
Effect of shape of composite curves on optimum process DTmin
For wide (or divergent) composite curves, even at low values of DTmin, the overall temperature driving force is quite high. Conversely for tight (or parallel) composite curves the heat exchanger capital cost will be quite high at low DTmin values. Such an understanding coupled with previous applications experience can be quite useful in setting practical retrofit targets.
The following tables detail KBC Energy Service's experience DTmin values. It is important to note that although experience based DTmin values can provide practical targets for retrofit modifications, in certain situations it may result in non-optimal solutions and therefore loss of potential opportunities. It is therefore recommended that the use of experience based DTmin is treated with caution and that as much as possible the choice is backed up by quantitative information (such as DTmin versus energy plot etc.).
Chapter 1 |
Pinch Technology |
21 |
Typical DTmin values for various types of processes
Industrial Sector |
Experience DTmin Values |
Comments |
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Oil Refining |
20-40ºC |
Relatively |
low |
heat |
transfer |
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coefficients, |
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parallel |
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composite |
curves |
in |
many |
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applications, fouling of |
heat |
||||
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exchangers |
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Petrochemical |
10-20ºC |
Reboiling |
and |
condensing |
|||
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duties |
provide |
better |
heat |
||
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|
transfer |
|
coefficients, |
low |
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|
fouling |
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Chemical |
10-20ºC |
As for Petrochemicals |
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Low Temperature Processes |
3-5ºC |
Power |
|
requirement |
for |
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refrigeration system is |
very |
||||
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|
expensive. DTmin decreases |
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with |
low |
refrigeration |
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temperatures |
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Typical DTmin values used for matching utility levels against process streams
Match |
DTmin |
Comments |
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|
Steam against Process Stream |
10-20ºC |
Good heat transfer coefficient for steam |
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condensing or evaporation |
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Refrigeration against Process Stream |
3-5ºC |
Refrigeration is expensive |
|
|
Flue gas against Process Stream |
40ºC |
Low heat transfer coefficient for flue gas |
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Flue gas against Steam Generation |
25-40ºC |
Good heat transfer coefficient for steam |
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Flue gas against Air (e.g. air preheat) |
50ºC |
Air on both sides. Depends on acid dew |
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|
|
point temperature |
|
|
CW against Process Stream |
15-20ºC |
Depends on whether or not CW is |
||
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competing |
against |
refrigeration. |
|
|
Summer/Winter |
operations |
should be |
|
|
considered |
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Pinch Technology Introduction