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In Diesel Hydrotreater, what factors decides to go with Hot HP Separator / Cold HP Separator / Cold LP Separtor configuration or only CHPS/CLPS configuration or only CHPS configuration?

Jan-2023

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Marcio Wagner da Silva, Petrobras, marciows@petrobras.com.br

For hydrotreating units processing lighter feeds, like naphtha, the hydrotreated stream tends to present physical properties very different from the water and a single separation vessel can be applied. For heavier feeds like LCO (Light Cycle Oil), and Gasoils two separating vessels are applied once the hydrotreated feed present physical properties closer with the water leading to a harder separation process which demands higher interface area, in these cases are applied a High Pressure (HP) separation vessel (Usually a vertical vessel) where it's separated the recycle gas (H2, H2S, and NH3) from the liquid phase which concentrates water, hydrocarbons and dissolved H2S and NH3. The liquid phase is fed to a Low Pressure (LP) separation vessel (Usually a horizontal vessel), as aforementioned this is the common configuration for high severity hydrotreating units which is normally the case of modern Diesel Hydrotreating units that process unstable feeds (LCO, Coker Gasoil, etc.)

Regarding the configuration of HHPS (Hot High Pressure Separator), CHPS (Cold High Pressure Separator), CLPS (Cold Low Pressure Separator) the choice tends to be driven by energy consumption analysis. The use of HHPS configuration offers the possibility to energy savings once the hydrotreated stream can be fed to the stripping section without reheating need. Furthermore, the HHPS configuration allows a reduction in the dimensions of air-cooler system and the water injection can be made only after the hot separator which allows the use of noblest metallurgy materials in a small section of the processing unit. The advantages of a CHPS is the low contaminants concentration in the recycle gas which can affect the performance of the hydrotreating reactions due to the reduction of hydrogen partial pressure, in some cases this fact tends to be balanced through the higher consumption of make-up hydrogen.

Considering only the performance, the configuration of HHPS and CLPS tends to be chosen for high severity processing units which process heavier feeds, but it's important to consider another factors like dimensions of the processing unit, composition of make-up hydrogen, etc. Furthermore, according to the historic of the refining asset, the use of low pressure separators tends to be chosen to avoid to expose the operators to high pressure systems.

Jan-2023