EmmanuelTop
Chemical
- Sep 28, 2006
- 1,237
From the first start-up of Crude Unit, heat exchangers in the so-called "hot preheat train" (upstream of process heater) experience intensive and very progressive fouling. In these exchangers, crude is placed on the shellside, while atmospheric residue flows through the tubes.
Since in all other heat exchangers in CDU crude is placed on the tubeside, I wonder why the designer had choosen this configuration in hot preheat train.
Atmospheric residue does not have any hydraulic demands - it has sufficient pressure head to overcome all resistances before entering vacuum unit feed drum. Even if it was placed on the shellside instead of crude oil, for the given fouling rate, calculations show that we could easilly achieve 4 year run-length without shutting down the unit and cleaning the exchangers - as far as process hydraulic is concerned.
So my question is: what if we change flow pattern through these exchangers and place the crude oil on the tubeside? The initial pressure drop through the hot preheat train, in such case, would be higher, but I think the rate of fouling will be much much lower, because of velocity profile; so when the terminal tube velocity is reached, no significant fouling cannot take place.
Process heater is available to deliver additional duty if heat transfer rate drops (for crude deltaT = 25C = start of run - end of run), because of fouling from the atmospheric residue side.
In most of crude units, crude oil is placed on the shell side (in the hot preheat train), and they all suffer from the same disease: crude heater inlet pass vaporization, caused by big pressure drop through the train. So why, then, all designers always place the crude oil on the shellside? And what about switching flow configuration?
Thank you in advance.
Since in all other heat exchangers in CDU crude is placed on the tubeside, I wonder why the designer had choosen this configuration in hot preheat train.
Atmospheric residue does not have any hydraulic demands - it has sufficient pressure head to overcome all resistances before entering vacuum unit feed drum. Even if it was placed on the shellside instead of crude oil, for the given fouling rate, calculations show that we could easilly achieve 4 year run-length without shutting down the unit and cleaning the exchangers - as far as process hydraulic is concerned.
So my question is: what if we change flow pattern through these exchangers and place the crude oil on the tubeside? The initial pressure drop through the hot preheat train, in such case, would be higher, but I think the rate of fouling will be much much lower, because of velocity profile; so when the terminal tube velocity is reached, no significant fouling cannot take place.
Process heater is available to deliver additional duty if heat transfer rate drops (for crude deltaT = 25C = start of run - end of run), because of fouling from the atmospheric residue side.
In most of crude units, crude oil is placed on the shell side (in the hot preheat train), and they all suffer from the same disease: crude heater inlet pass vaporization, caused by big pressure drop through the train. So why, then, all designers always place the crude oil on the shellside? And what about switching flow configuration?
Thank you in advance.
