Improve energy efficiency of your hydrocracking unit
Reduced fuel firing in HCU product fractionators enables higher diesel yields and improved product properties while reducing Scope 1 and 2 emissions.
Kiran Ladkat, Jan De Ren and Kiran Kashibhatla
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Refinery Scope 1 and 2 emissions represent 3% of the global anthropogenic CO₂ emissions, which equates to 1,124 million tonnes annually.1 For a typical refinery configuration that has hydrocracking and delayed coking units, 9%² of these emissions originate from hydroprocessing units, where the major contributor for an individual unit is the hydrocracking unit (HCU) because of the higher operating severity as compared to hydrotreating units.
Figure 1 provides an overview of the three GHG protocol scopes (1, 2, and 3) and categories for each of the scope emissions. Improving energy efficiency and reducing CO₂ emissions from existing HCUs is a key focus area for improving refinery profitability and reducing emissions. Within the HCU, the main contributors to carbon emissions are fired heaters and rotating equipment.
Against this backdrop, Honeywell UOP’s dual stripper and dual fractionator solutions for HCUs have demonstrated reduced fuel firing in the product fractionator feed heater by ~50-55%, enabling higher diesel yield and improved diesel product properties. Together, this delivers improved refinery economics and reduced Scope 1 and 2 emissions. These are solutions that can be applied to new and existing HCUs.
The fractionation section of an HCU is designed to separate the net reactor effluent from the reactor section into the desired products: LPG, naphtha, kerosene, diesel, and unconverted oil. A simplified flow diagram of a single stripper first HCU fractionation section is shown in Figure 2. The fractionation section typically includes a stripper, a product fractionator with two or more side cuts, side-cut strippers, a debutaniser, a naphtha splitter, and other columns, depending on the required product recovery.
Dual stripper flow scheme for existing HCUs
Several new HCUs have been designed by Honeywell UOP 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 consumption 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 composition 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 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 stripper 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 available process heat in the fractionation section to reach a certain vaporisation and then fed directly to the product fractionator between the flash zone and diesel product draw stage. Apart from being applicable to new HCUs, the dual stripper flow scheme is a good revamp solution to reduce the existing product fractionator feed heater duty. In the revamp flow scheme, the existing stripper will be utilised as hot stripper service, whereas a new cold stripper will be added in the flow scheme (see Figure 4).
The dual stripper flow scheme can reduce the product fractionator feed heater duty by 20% compared with the existing conventional single stripper flow scheme. Beyond reducing product fractionator feed heater duty, another benefit is that it provides a reduction of carbon emissions from the existing HCU. Table 1 provides a summary of the utility consumption, economic and CO2 emission benefits with a dual stripper flow scheme as compared to a conventional single stripper first flow scheme.
The benefits described in Table 1 are based on a recently proposed revamp solution and subsequent basic engineering work, which has been completed. The revamp project is currently in the detailed design phase. As summarised in Table 1, the reduction in fuel gas consumption for this 835 m3/hr (~126,000 BPSD) mild HCU improves the facility’s net present value (NPV) by $29.5 million. If CO₂ is valued at $50 per tonne, the NPV boost attributable to the dual stripper flow scheme approaches $32.9 million.
Dual stripper scheme applied to a debutaniser-first flow scheme
Some of the earlier designed HCUs are operating with a debutaniser-first flow scheme. A simplified flow diagram of the debutaniser-first fractionation section flow scheme is shown in Figure 5. This scheme typically includes a debutaniser column to separate LPG and lighter sour off-gases 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 fractionator 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 unconverted 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 opportunity 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 naphtha 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 reducing total fuel consumption, another benefit is reduced CO2 emission from the existing two-stage HCU. Table 2 summarises the utility consumption, economic, and CO2 emission benefits of the proposed scheme as compared with a conventional debutaniser-first flow scheme.
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