Mar-2021

Delayed coker revamp for a capacity increase

Revamping to a three-drum coker arrangement delivered improved product yields, higher throughput, and greater feedstock flexibility.

KEITH MAGDOZA and VIRENDRA MANRAL
Chevron Lummus Global

Article Summary

Crude price volatility and current over-capacity in the global refining sector have made refiners wary of taking up new greenfield projects. However, refiners in emerging economies continue to seek the ability to scale up capacity and to switch their feed slate to heavier crudes based on economics. A delayed coking unit (DCU) revamp in such scenarios can enable refiners to process higher bottom-of-the-barrel capacity, improve the ability to shift the product slate, or even change the coke type in line with market demand. Moreover, revamp projects are more likely to be funded because of relatively low capital expenditure with a desirable internal rate of return (IRR). This article describes the recent revamp of an existing DCU aimed at achieving capacity expansion, feedstock flexibility, and energy efficiency. It discusses the meticulous planning and importance of innovative design for the successful execution of the project in a limited timeframe.

Background
The subject DCU is located in central India and was initially designed to process vacuum residue from high sulphur crude using a two-drum configuration. After successful operation at design conditions, the refiner wanted to increase the DCU’s capacity by 33.8% in line with refinery expansion plans. As a result, a revamp project was implemented in three phases.

The first phase of the project was a detailed feasibility study performed by Chevron Lummus Global (CLG). During this phase, CLG validated the feasibility of capacity expansion while considering the constraints on plot space and maximising the use of existing equipment.

The feasibility study’s results were encouraging, and further economic analysis enabled the project to progress to the next phase: the development of a basic engineering design package (BEDP) by CLG. Innovative design solutions, such as adding a third coke drum, a parallel wet gas compressor, a steam feed preheater, and a water ring compressor were all implemented to overcome the challenges faced during this revamp. These design improvements helped further optimise cost and supported the project to be approved for execution with full funding.

The third and final phase of the project was detailed engineering and execution. The original detailed engineering contractor (DEC), which performed the detailed engineering design of the original unit, was selected as the DEC for this phase. This phase was carefully planned to achieve the goal of completing the revamp during the refinery turnaround period.

The unit’s revamp modifications were completed in November 2018, with the third coke drum installed but not yet tied into the existing coke drum structure package. In September 2019, the third coke drum was successfully commissioned, and the unit operated at its revamp capacity.

Revamp phase 1: feasibility study
The first step of any revamp project is the feasibility study. The feasibility study is crucial as it helps determine whether the project is economically feasible before further capital is invested in the project. A feasibility study typically involves benchmarking the unit’s yields and current operation, modelling the test run data, determining key revamp operating parameters and product yield structure, identifying bottlenecks and potential modifications, and preparing a preliminary equipment layout and budgetary revamp capital estimate.

During this phase of the project, the existing equipment was evaluated to determine its adequacy for the revamp conditions. For the equipment found inadequate, potential modifications, replacements and, in some cases, altogether new equipment was proposed. Plot availability for all changes was validated, and all major changes were marked on a preliminary revamp plot plan indicating approximate locations for new and modified equipment with high-level constructability considerations.

For the feasibility study, the unit’s operating parameters were optimised to achieve increased revamp throughput. The coke drum pressure was increased, and the cycle time was lowered in order to mitigate the increase in coke drum superficial vapour velocity associated with the higher revamp throughput. The pumparound duties and heat recovery of the unit were optimised to ensure maximum energy efficiency. The revamp conditions were designed such that no change would be required for the two existing coke drums.

CLG’s in-house coking yield correlations were fine-tuned by the results of a benchmark test run in order to match plant performance closely. Various parameters in the simulation model were adjusted to represent the benchmark operation of the DCU. Based on this model, the product yields for the high pressure operating cases were estimated and provided as part of the feasibility study.

After a review of the feasibility study, it was noted that the higher pressure operation resulted in an increase in coke yield. To reduce the coke yield, two options were investigated. The first option was the installation of an additional pair of coke drums and heater operating at the original design pressure. However, this option would be capital intensive and would require plot space greater than was available. The second option was a three coke drum configuration operating at a slightly higher pressure than the original design. This configuration would eliminate concerns about coke drum ullage by further lowering the coking cycle time. Such a configuration would be the first of its kind in India, and the feasibility of its design was carefully studied by CLG and the refiner. Figure 1 and Table 1 show a diagram of the three coke drum configuration and its coking and decoking schedule, respectively.

The configuration was designed such that one coke drum would be in coking mode while the other two coke drums would be in decoking mode. Along with saving on both plot space and cost, this option required no change to the blowdown tower due to the increase in decoking times for each drum. Feed flexibility would also increase as this configuration can handle higher CCR by further lowering coking cycle times. With this encouraging proposal, the three coke drum configuration was determined as cost-viable, and the project moved forward to the next phase of the revamp, preparation of the BEDP.

Revamp phase 2: basic engineering design package
Using the results of the feasibility study and the three coke drum configuration as a basis, a BEDP was prepared for the DCU revamp. A typical revamp BEDP includes equipment datasheets for all existing, new, and modified equipment. Marked-up PFDs and P&IDs are provided as well with their respective demolishing drawings. The BEDP contains all information necessary for the selected detailed engineering contractor to proceed with the detailed engineering of the project.