PTQ Q3 2023 Issue

resid. The column is highly heat integrated. Kerosene, LGO and HGO pumparounds, and a portion of over- head heat are recov- ered to crude oil in the preheat train. A small portion of the LGO and HGO pump- around heat is used

Pumparound draw temps

Kero PA & LGO PA duty increased by the same amount as HGO PA duty reduction

Temperature °F

Crude

Kerosene

400 500 630 275 500 650

LGO (diesel)

HGO LVGO MVGO HVGO

Atmospheric column

Kero PA

400

Kerosene & LGO product yield increases by 10 kBPD

Table 3

LGO PA

500

LGO

to generate steam. From a heat recovery perspective, it is better to recover diesel in the atmospheric column vs the vacuum column. The LGO pumparound draw temperature is typically much higher than the diesel pumparound in the vacuum column. For CV1, the atmospheric diesel draw (LGO) is 500°F, whereas the LVGO draw (vacuum diesel) is 275°F. Higher pumparound draw temperature results in higher exchanger log mean temperature difference (LMTD). Therefore, a larger portion of the heat can be recovered to crude when draw temperatures are high. Most vacuum diesel pumparounds will be capable of recovering only 30-50% of the heat to crude because the draw temperature is low. The remainder is lost to air and water cooling. It was clear that diesel recovery needed to be maximised in the atmospheric column to improve final preheat temperature. Atmospheric column diesel recovery was maximised by increasing heater outlet temperature, improving bottoms stripping, processing a portion of the external RGO, and adjusting the column heat balance to increase reflux in the LGO/HGO fractionation section. The reflux rate in the LGO/HGO section of the atmos - pheric crude column is a function of the heat balance. The reflux rate depends on the amount of heat removal above the draw and the amount of product withdrawn. As an example, the amount of reflux below LGO product draw is set by the amount of flash zone vaporisation and the HGO pumparound duty. The HGO pumparound sets the vapour rate leaving the HGO pumparound section. Removing heat lower in the column increases the temperature level of heat available for exchange with crude oil, but lowers reflux between the products higher up in the column (see Table 3 ). This is the dilemma present in all crude unit designs. Maximum heat recovery hurts fractionation, and vice versa. 9 For this revamp, all the HGO pumparound duty was shifted to kerosene and LGO pumparounds (see Figure 8 ). The heat balance shift increased reflux in the LGO/HGO fractionation section and helped maximise atmospheric column LGO yield. MVGO preheat train exchangers were repurposed as kerosene PA to handle the significantly higher kerosene PA duty. Table 4 shows the pumparound duties before and after the modification. The kerosene PA duty increased from 10.5 MMBtu/h to 35 MMBtu/h. About 15 kBPD of RGO was routed to the atmospheric column between the LGO and HGO product draws. The RGO temperature is 410°F. A portion of the RGO vaporises as it is heated by rising vapour in the column, allowing a por- tion of it to be yielded as LGO product. An added benefit of routing a portion of the RGO to the atmospheric tower was

Diesel

RGO feed

Much higher reux

15 KBPD

HGO PA removed from service

HGO

CVI vacuum tower

Desalted crude

684

Steam

Temperature, ˚F

684

Figure 8 Atmospheric column heat balance modifications

Pumparound duties before and after

Service

Test run

Revamp

Crude ovhd

27.3 10.5 26.5 22.3

28.0 35.4 30.6

Kero PA LGO PA HGO PA

Table 4

a reduction in ejector load. 10 The light material in the RGO stream, which contributes to ejector condensable load, is stripped in the atmospheric bottoms stripping section. The atmospheric column still yielded about 15 kBPD of HGO product. Even though the HGO product was side stripped, it still contains diesel boiling range material. Improvements in the vacuum column enabled it to be routed there for ulti- mate diesel recovery. Vacuum column heat balance The CV1 vacuum unit is a deepcut vacuum unit. The col- umn has LVGO, MVGO, and HVGO pumparound heat removals and products. A fractionation section is located between LVGO and MVGO pumparounds. LVGO product is vacuum diesel. The column has HVGO wash zone and bottoms stripping sections. A three pumparound design produces a relatively high temperature HVGO pumpa- round, which helps optimise heat recovery to crude oil and, in this case, external AR. Overall, the vacuum col- umn is a good design but was plagued with a number of equipment shortcomings that resulted in poor LVGO recovery and low HVGO cutpoint. These shortcomings included high top pressure, top section corrosion with a leaky LVGO draw tray, and poor bottoms stripping. These

PTQ Q3 2023