PTQ Q3 2023 Issue

O - gas

Unstabilised naphtha

Naphtha

Stripper

Kerosene

Product fractionator

Cold ash drum liquid

MP steam

Diesel

Hot flash drum liquid

Product fractionator feed heater

LP steam

Unconverted oil

Figure 2 Process flow scheme for single stripper first fractionation section of an HCU

passing through the product fractionator feed heater, thereby avoiding a mixing of the hot and cold flash drum liquid. This has the potential to save on fired duty, thus making the HCU more energy efficient while reducing carbon emissions. In the dual stripper design, the hot and cold flash drum liquid streams from the reactor section are fed into separate strippers: a hot stripper and a cold stripper. Both columns are steam stripped. The overhead vapour from the hot strip- per is routed to the cold stripper, whereas the liquid from the hot stripper is sent to the product fractionator feed heater and subsequently to the flash zone of product fractionator. In this arrangement, only liquid from the hot stripper bottom is being sent to a product fractionator feed heater, unlike in a single stripper arrangement where all the liquid is sent to the product fractionator feed heater, thereby demanding higher energy input and creating higher resultant flue gas emissions. The cold stripper bottoms liquid is preheated with availa- ble process heat in the fractionation section to reach a certain Figure 3 Shandong Super Energy two-stage HCU in China employing the dual stripper flow scheme

with the dual stripper flow scheme and put into operation, making this a commercially proven solution (see Figure 3 ). By implementing this flow scheme, refiners will be able to reduce the product fractionator feed heater duty by 20-40% compared to a conventional single stripper flow scheme, depending upon HCU conversion. Apart from new HCUs, the dual stripper flow scheme provides an excellent revamp solution to reduce energy con- sumption in an existing HCU. The novel flow scheme (see Figure 4 ) was developed to reduce product fractionator feed heater duty and deliver a reduction in operating costs and furnace stack emissions. For reactor section flow schemes incorporating a hot separator, it is noted that the composi- tion of the cold separator hydrocarbon stream, and therefore the cold flash drum hydrocarbon stream, is much lighter than that of the hot flash drum liquid. This makes it possible to heat the cold flash drum hydrocarbon stream using low-end process heat, available in the fractionation section, without

O gas

O - gas

Debutaniser

Unstabilised naphtha

Naphtha

LPG

Cold stripper

Product fractionator

Kerosene

Product fractionator

Kerosene

Cold ash drum liquid

MP steam

Debutaniser reboiler

Cold ash drum liquid

Diesel

Hot stripper

Hot ash drum liquid

MP steam

Product fractionator feed heater

LP steam

Unconverted oil

Product fractionator feed heater

New / modications

Figure 4 Revamp process flow scheme with dual stripper fractionation section of an HCU

Figure 5 Process flow scheme with the debutaniser-first fractionation section of an HCU

PTQ Q3 2023