NHT/CCR Combined Pinch Analysis – The following are the cold streams to be heated and the hot streams to be cooled used for energy assessment task:
The global ΔTmin used for existing system can be obtained via drawing the hot and cold composite curves and moving it until the existing hot utility (459MMbtu/hr) and cold utility (264MMBtu/hr) are attained. By doing that, the current example global ΔTmin used is72°F.
Another graph for typical Hot and Cold utilities targeting using the composite carves at ΔTmin equal to 30 °F is shown below:
Pinch analysis shows that 70 MMbtu/hr can be saved by modifying the existing HXN. From the grand composite curve, we see that there is a possibility to vary the hot and cold utilities used too.
Looking to the NHT process as a stand alone plant we can also find opportunities for energy consumption savings.
Most of the old and the recently built NHT plant uses either Axens or UOP processes.
These two licensed processes configurations are almost the same. They do not exhibit
direct integration between the two stripping and Naphtha splitting sections, the feed is
preheated by the bottom product and the extra waste heat in the heavy Naphtha stream is sent to the air and water coolers. One of them is a little more efficient from energy point of view than the other since it sends the heavy Naphtha stream to the air coolers at a little lower temperature.
It is beneficial to the oil refining industry to hydrotreat different Naphtha feed stocks to refining reforming unit and other applications with less energy consumption. It is also
beneficial to the oil refining industry to hydrotreat the Naphtha feed stocks to refining
reforming unit and other applications while producing less green house gas emissions. It is definitely beneficial to the oil refining industry to hydrotreat the Naphtha feed stocks to refining reforming unit and other applications using less number of heaters and , air and water coolers.
The energy balance for the Naphtha Hydro-treating unit includes the feed preheater, furnace and exchanger, reactor and separators. The criteria we use for efficient NHT unit to define energy efficient operation is as before in furnaces; flue gases at 300ºF, hot streams before going to air and/or water coolers at most is less than 250ºF via intra-process integration.
It is important to note that, inter-process integration among NHT, CCR, Isomerization units and may be other refinery units may also enable us beat the target/benchmark we obtain from tight NHT intra-process integration.
The difference in the recommended/guideline value of main NHT variables and the currently used ones can be used to estimate the gap and the potential opportunity in energy saving in the unit.
It is instructive to mention here that above guidelines can be even beaten up by some new NHT designs as will be demonstrated in the flowsheet (s) below.