February 2021

Process Optimization

Optimizing for viability—VDU revamp in a brownfield project

Revamping an existing column in an operating plant for higher throughput is a challenge, especially when the column is already operating at its rated capacity.

Revamping an existing column in an operating plant for higher throughput is a challenge, especially when the column is already operating at its rated capacity. This is particularly true for a vacuum distillation unit (VDU) column, which involves multiple product and pumparound streams and associated auxiliary units, such as a steam ejector system.

Optimizing the design and operation of the column is inevitable to ensure the viability of a brownfield project, as it avoids major modifications to the column and its associated auxiliary systems. This article discusses how optimization was carried out on the design and operation of an existing VDU column in an oil refinery, avoiding major modifications of the associated ejector, steam, cooling water, sour water systems and heat exchanger network and allowing a brownfield project to remain commercially viable.

Case study background

A case study of an existing VDU column at a refinery in Malaysia is included in this article. The VDU column was designed to operate at 9 mmHg operating pressure with a throughput capacity of 3,500 bpd of low-sulfur waxy residue (LSWR) from an upstream crude distillation unit (CDU).

As part of the refinery’s business growth strategy, the VDU column was considered for debottlenecking to allow additional throughput of LSWR, which is lighter than the existing throughput at 23,000 bpd to the unit. The lighter, higher LSWR throughput led to higher non-condensable and condensable vapor flows at the top of the column, thereby massively increasing the column internals loading and producing higher column operating pressure, which placed additional burden on the overhead steam ejector system.

Major modifications to the column shell diameters, steam ejectors and condensers were expected, and the replacement of the equipment would require massive structural modification within a congested, operating plant. On top of that, with the larger steam ejectors and condensers, major modifications to the associated steam generation system, cooling water system and sour water treating unit were expected.

The total cost of the required modifications was estimated at approximately $25 MM, which would render the project uneconomical. Replacing the VDU column itself was not an option in view of the sheer complexity of the construction work required in the operating plant, which would require extended downtime of the refinery. The economic loss associated with the prolonged downtime would outweigh the benefit of debottlenecking the column. A creative solution was needed to economically optimize the column operation.

Solution for brownfield optimization of the VDU

One way to make the project economically viable is to optimize the column operating conditions and column internals design efficiencies and to exhaust all available heat duties within the pumparound and heat integration network to reduce the impact to the existing ejector system and minimize the modifications to the VDU unit. This plan was outlined using industrially acceptable process simulators and an in-house sizing software for column internals.

Optimizing the column operating conditions includes increasing column bed efficiencies, adjusting the operating pressure, redistributing heat duties in the heat exchanger network, optimizing pumparound flow, and adjusting the column temperature profile and inlet temperature. These modifications led to changes for both the mass and heat transfer profiles within the column. These changes resulted in the reduction of column internals liquid and vapor traffic, thereby decreasing the total load at the top of the column to the overhead steam ejector system.

Debottlenecking a VDU column

Understanding how a VDU column works and the influence of the auxiliary systems to the VDU column operation are key to optimizing both the design and operation of the column itself. The VDU column under study was a typical design of five packed beds, one flashing section and one stripping section, operated under vacuum conditions at 9 mmHg. The VDU is designed to fractionate heavy LSWR into product streams including light vacuum gasoil (LVGO), medium VGO (MVGO), heavy VGO (HVGO) and vacuum residue (VR). FIG. 1 shows the schematic of the VDU under study.

FIG. 1. Schematic of the case study VDU column.

A two-phase feed enters the VDU column flashing section via an upstream fired heater. Lighter vapor components will rise up the column to the wash, bottom pumparound (BPA), middle pumparound (MPA), fractionation and top pumparound (TPA) sections to be further distillated to product streams through both the heat recovery pumparound beds and product purification beds. The heavier liquid components will go down the column bottom through the stripping section to ensure that the lighter portion can still be recovered before coming out of the VDU column as the VR product stream.

The VDU debottlenecking project will introduce lighter LSWR at an increased capacity of 23,000 bpd into the VDU column. Due to the higher throughput and to allow for adequate separation within the column, a new fired heater was added upstream of the VDU column to provide an additional 20 MW of heat duty. To meet the debottlenecking objectives, the column not only receives additional mass throughput but also receives additional heat load to the column heat balance.

Through detailed tray-to-tray simulation and hydraulic sizing calculations performed at each of the column internals, it was observed that introducing additional mass throughput and heat load to the column caused the following:

1. Substantial vapor traffic rising up the column from the flashing section to the TPA section
2. Greater pumparound duty requirement for all pumparound sections due to higher vapor traffic inside the column (this is to balance out the additional heat introduced by the new fired heater)
3. Lighter components slipping down to the VR section due to higher vapor traffic in the column, thereby compromising the VR product initial boiling point (IBP) specification
4. Additional column pumpdown requirement at fractionation and wash oil sections due to an increase in liquid load
5. Higher pressure drop across the column due to higher holdups caused by greater liquid and vapor traffic.

These changes to the column operating conditions will translate into major modifications to the VDU column and its auxiliary systems if no optimization is performed on the column internals design and operation.

VDU column modification without optimization

The conventional way of debottlenecking a VDU column involves a total revamp of the column, which could include the total replacement of the column internals to higher-performance internals; an increase in the column inlet and outlet nozzles, thereby allowing a larger column diameter; or, at the extreme end, the total replacement of the column itself. To manage the additional heat load introduced into the VDU column heat balance, additional external cooling or heat sinks must be introduced, either through the introduction of additional heat exchangers in the heat exchanger network or the provision of new water or air coolers.

In addition to these items, higher vapor load (exiting from the top of the column and consisting of both condensable and noncondensable hydrocarbon components) could result in a massive, multistage ejector modification requirement. To support the ejector modification, major modifications to the associated steam generation system, cooling water system and sour water treating unit were expected.

VDU column modification with optimization

To prevent major modifications to the VDU column, the column design and operation were optimized before the modification scope was identified. The VDU column operating parameters and each section within the VDU column were examined in detail, and iterative simulations were performed to ensure that each bed within the VDU column could be fully optimized.

The following optimization approach was applied to minimize the scope of the VDU column modifications:

1. The fired heater coil outlet temperature (COT) into the VDU column was reduced from the design operating temperature to the lowest operating temperature at which the column is still able to allow good separation to occur within the column. This optimizes the heat balance within the VDU column and the capital investment required to handle the additional heat within the heat exchanger network.
2. The column top vacuum operating pressure was increased from the design operating pressure to the maximum operating pressure at which good separation can still occur in the column. This reduces the amount of condensable and noncondensable hydrocarbon components at the vapor outlet of the VDU column to the ejector system, avoiding major modifications to the ejector system and associated steam, cooling water and sour water treating units.
3. For all heat recovery pumparound beds within the VDU column (i.e., TPA, MPA and BPA), high-efficiency packing with additional surface area was selected. This type of packing does not sacrifice the void fraction (i.e., the capability to withstand higher load).
4. The heat recovery through the heat exchanger network was optimized to achieve maximum heat integration within the VDU unit and minimize capital investment for the modification of existing heat exchangers, except for additional water or air coolers for final heat rejection. This is achievable through the use of a higher-efficiency bed at both the TPA and MPA sections, allowing better heat recovery at lower pumparound flow, which reduces both the column internals vapor and liquid traffic and improves heat recovery in the highly integrated heat exchanger network.
5. For all fractionation packing beds within the VDU unit (i.e., the fractionation and wash sections), hybrid packed beds were chosen to provide better performance (i.e., a higher void fraction without sacrificing efficiency). By using the high-efficiency packed bed, the hydraulic limitation was reduced without having to increase the column diameter, while still meeting separation and product quality requirements. This choice avoids the need to modify the VDU column shell.

With the considered optimization approaches, it was observed that the VDU column total internals vapor and liquid traffic was greatly reduced throughout the column, while maintaining the products separation and quality. The optimization also resulted in lower vapor condensable and noncondensable flows at the top of the column. These approaches avoid major modifications to the VDU column, its auxiliary systems and the heat exchanger network, significantly reducing the capital investment of the brownfield project.

Results and discussion

The modifications required for the VDU column with and without the optimization are listed in TABLE 1.

The optimization of the VDU column operating conditions and internals design avoids the replacement of the first-stage ejector (the largest ejector system), the associated steel structure for the ejector, the requirement for a new cooling water package and associated interconnected piping, a new steam generation package and associated interconnected piping, a new sour water treating unit, additional heat exchangers, and cooling water or air coolers in the heat exchanger network. The total cost avoidance as a result of the optimization work is estimated at approximately $25 MM, which allows the project to stay economically viable.

Additionally, the project recently underwent a performance test run and was able to meet the desired performance guarantees.

Takeaway

In conclusion, optimization of column operating conditions and design is critical when the column is already operating at its rated capacity in a brownfield project. A case study of a VDU column in a Malaysian oil refinery indicates that massive modifications are required if optimization is not carried out on the column operating conditions, which add unnecessary capital expenditure and may render the project economically unviable. HP

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