22 |
Pinch Technology |
Chapter 1 |
Typical DTmin values used in retrofit targeting of various refinery processes
Process |
|
DTmin |
Comments |
CDU |
|
30-40ºC |
Parallel (tight) composites |
VDU |
|
20-30ºC |
Relatively wider composites (compared to |
|
|
|
CDU) but lower heat transfer coefficients |
Naphtha |
Reformer/Hydrotreater |
30-40ºC |
Heat exchanger network dominated by feed- |
Unit |
|
|
effluent exchanger with DP limitations and |
|
|
|
parallel temperature driving forces. Can get |
|
|
|
closer DTmin with Packinox exchangers (up to |
|
|
|
10-20º) |
FCC |
|
30-40ºC |
Similar to CDU and VDU |
Gas Oil Hydrotreater/Hydrotreater |
30-40ºC |
Feed-effluent exchanger dominant. Expensive |
|
|
|
|
high pressure exchangers required. Need to |
|
|
|
target separately for high pressure section |
|
|
|
(40ºC) and low pressure section (30ºC). |
Residue Hydrotreating |
40ºC |
As above for Gas Oil Hydrotreater/Hydrotreater |
|
Hydrogen Production Unit |
20-30ºC |
Reformer furnace requires high DT (30-50ºC). |
|
|
|
|
Rest of the process: 10-20ºC. |
1.8 Process Modifications
The minimum energy requirements set by the composite curves are based on a given process heat and material balance. By changing the heat and material balance, it is possible to further reduce the process energy requirement. There are several parameters that could be changed such as distillation column operating pressures and reflux ratios, feed vaporisation pressures, pump-around flowrates, reactor conversion etc. The number of choices is so large that it seems impossible to confidently predict the parameters that could be changed to reduce energy consumption. However, by applying the thermodynamic rules based on Pinch Analysis, it is possible to identify changes in the appropriate process parameter that will have a favourable impact on energy consumption. This is called the "plus-minus principle".
1.8.1 The plus-minus principle for process modifications
The heat and material balance of the process determines the composite curves of the process. As the heat and material balance change, so do the composite curves. The figure below summarises the impact of these changes on the process energy targets.
In general, hot utility target is reduced by any:
•Increase in hot stream duty above the pinch.
•Decrease in cold stream duty above the pinch. and cold utility target is reduced by any: