Economic benefits of a dual stripper flow scheme vs a debutaniser-first flow scheme 1-8
Parameters
Unit
Debutaniser first
Dual stripper flow scheme
flow scheme (base case)
Debutaniser reboiler heater duty Product fractionator feed heater duty
MMKcal/hr MMKcal/hr tonne/hr $MM/year
55.0 24.2 Base Base Base Base Base Base
40.1
0.0
MP steam consumption
+5.0 (3.3) + 7.8
Utility cost
Estimated erected cost (EEC) for new equipment
$MM $MM
NPV from process improvements
+ 15.6
CO₂ iemission (Scope 1 & 2)
tonne/year $MM/year
(~87,000) (1)
CO₂ credit
+ 4.3
NPV including CO₂ credit + 45.7 Notes: 1. 358 m 3/hr (~54,000 BPSD) two-stage HCU with overall conversion of 99 vol%. 2. Fuel consumption is based on 90% heater efficiency. 3. CO₂ reduction credit $50/tonne. 4. Onstream hours: 8,400 hours per year. 5. NPV is for 15 years at 10% discount rate basis. 6. Simple payback period 28 months and with CO₂ credit payback period will further reduce to 12 months. 7. Utility price basis: • Fuel gas: $9.92/MMKcal. • MP Steam: $8/tonne. 8. Scope 1 & 2 CO₂ emission basis: • 0.262 tonne of CO₂/MMKcal of fuel gas • 0.208 tonne of CO₂/tonne of MP steam $MM Base
Table 2
as an overhead product, with naphtha and heavier fractions as a bottoms product. Bottoms liquid from the debutaniser column will be routed to the downstream product frac- tionator feed heater and subsequently to the flash zone of product fractionator with two or more side cuts to separate the naphtha, kerosene, and diesel products from the uncon- verted oil. A significant amount of energy is consumed in the debutaniser reboiler heater and the product fractionator feed heater to separate these different products. The dual stripper flow scheme provides a unique revamp opportu - nity to reduce fuel consumption in the fired heaters and will be discussed in more detail, focusing on the benefits of revamping the existing fractionation section with a dual stripper flow scheme. In the revamp flow scheme, the existing debutaniser will be utilised as the hot stripper with steam stripping, whereas a new cold stripper will be added to separate the bulk of naph- tha and light hydrocarbons as overhead material from the
cold flash drum liquid. The existing debutaniser reboiler will be repurposed as a product fractionator feed heater service by modifying the heater outlet piping, as shown in Figure 6 . The existing product fractionator heater will no longer be required in service during operation of the HCU and will be shut down or isolated from the system. The stripper overhead liquid will be stabilised in the small new stabiliser column. The proposed dual stripper flow scheme will be able to reduce total fuel consumption in the fractionation section of a two-stage HCU by approximately 50% compared to a conventional debutaniser-first flow scheme. Beyond reduc - ing total fuel consumption, another benefit is reduced CO2 emission from the existing two-stage HCU. Table 2 summa- rises the utility consumption, economic, and CO2 emission benefits of the proposed scheme as compared with a con - ventional debutaniser-first flow scheme. The summary in Table 2 is based on a recently proposed revamp solution, where the reduced total fuel consumption for this 358 m 3 /hr (~54,000 BPSD) two-stage HCU improves the facility’s NPV by $15.6 million. If CO₂ is valued at $50 per tonne, the NPV boost attributable to a dual stripper flow scheme approaches $45.7 million. Dual stripper with dual fractionator solution Similar to the dual stripper flow scheme, the dual fractionator flow scheme has been offered to several new HCUs and is a commercially proven solution. Apart from new HCUs, the dual fractionator flow scheme is also an excellent revamp solution to improve energy consumption from the existing HCU. The following discussion will elaborate on the applica- tion of the dual stripper with a dual fractionator flow scheme solution for HCUs. To further improve energy consumption in the HCU’s fractionation section, careful analysis of the compositions for cold and hot stripper bottoms streams were carried out. It was established that the cold stripper bottoms material predominantly contains naphtha and kerosene, whereas the hot stripper bottoms hydrocarbon stream predominantly contains diesel and unconverted oil. Thus, to improve the separation efficiency and take advantage of bulk product separation that already occurred at the cold stripper and
O - gas
Unstabilised naphtha
Naphtha
Cold stripper
Light fractionator
Kerosene
Cold ash drum liquid
MP steam
Heavy fractionator
LP steam
Hot stripper
Hot flash drum liquid
Diesel
MP steam
LP steam
Fractionator feed heater
Unconverted oil
Figure 7 Process flow scheme with the dual stripper and dual fractionator of an HCU
16
PTQ Q3 2023
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