In the life of a petroleum or chemical process facility, it is common to discover in operation that the facility can perform somewhat beyond its original design capacity. This may be due to the design margins in the original design, or in changes to the feedstock or operating conditions from the design basis. It is usually the case that some parts of the plant have more excess capacity than others, and the components that are limiting the possible increase in throughput are called “bottlenecks”. The process of finding and removing these is often called “debottlenecking”.
Unrealized plant capacity and performance can be achieved by replacing or retrofitting a few pieces of equipment, adjusting process conditions or both. Increases to the productivity of the plant, while still maintaining much of its existing infrastructure, can produce a large return on investment (ROI) for the owner. This can provide a much-needed edge in today’s competitive economic climate. A successful debottlenecking project requires the project to be completed with the best ratio of cost per unit of increased capacity. To achieve this, it is necessary to balance the extent of modifications to the existing facility with the desired capacity, while considering constraints related to the duration of any shutdown necessary to make the changes. Ideally, the duration of the shutdown requirement should be no longer than a normal maintenance turnaround period so there is no additional loss of revenue associated with the project. However, there will be cases where an extended shutdown to implement the changes may be warranted. Rework, unexpected constraints or longer shutdown times will only take away from the increased ROI that can be seen from the debottleneck.
Many facilities are built with some surplus capacity due to the implementation of design margins, equipment overdesign, or process changes after startup. These margins can often have accumulated in the original design, where there are margins hidden in the design components defined by technology licensors, the owner’s own specifications, typical margins used by engineering contractors and the margins included by equipment manufacturers in its designs. All these margins have a purpose, but when combined can result in excess processing capability; the challenge is that these margins are not applied equally through a facility design, which leads to some components having more excess capacity than others.
A specific piece of equipment will reach its capacity first and therefore limit the remainder of the plant. Debottlenecking aims at gaining production increases by utilizing the surplus capacity that exists through removal of that current limitation. There are many areas in which a plant can be constrained, but all constraints are driven by energy flow or hydraulic limits or both. Usually, these constraints will be interdependent, where removing a bottleneck in one can relieve a bottleneck in another. However, if the plant is not looked at holistically, where one considers the relationship between the different process unit operations, additional constraints that were not previously understood will soon appear and the anticipated capacity to be gained may be unachievable.
A debottlenecking project should be conducted over several phases including a process analysis, feasibility study and detailed engineering. It is important to look at the entire facility from a high level at the beginning of a debottleneck. With a broader initial view of the plant, the different limitations and interdependence of the process unit operations can be better understood. This will allow the team to narrow down the key areas that will make the largest impact to the plant capacity and understand the knock-on effects that changes here will cause. By starting with the more holistic approach, potential additional bottlenecks can be examined, and the limitations of the plant can be fully understood.
Missing the limitations for interdependent unit operations or how changes impact different aspects of the unit can quickly lead to key scope for the debottleneck project being overlooked. For example, increasing the heat duty of a reboiler can quickly lead to reaching the hydraulic limits of the circuit’s piping due to the higher volumetric flow of vapor coming from the reboiler, or the need for more heating medium flow. Higher flows or a heavier composition to a distillation column will require increased duty in the reboiler. Even if there is sufficient energy to supply the heat in the plant, the exchanger where transfer is required maybe still constrained. It may not have enough surface area, a high enough LMTD or a change to the heat transfer coefficient if the heat transfer regime is altered due to changes in fluid viscosity, any of which would prevent the unit from reaching its energy requirements and thus limiting the capacity. Changes to gain efficiency upstream in a facility can also lead to potential downstream efficiency losses; this is why it is key to fully understand how the process works and know the key requirements for all pieces of equipment. If energy is found within the process that can be recovered at one section of the plant, that can be viewed as an efficiency gain in that area. But if that energy was required further down in the process, and now it is missing, that section might no longer work and lead to loss of production or even equipment damage in that area.
Since the plant is already operating, getting operational insight and plant data can provide valuable information for engineering and help govern the debottleneck project scope. If there are issues with operations currently, those need to be understood and conveyed to the engineering team to be addressed. Equipment may not operate exactly as the datasheet or simulation may predict. If a piece of equipment operates well below its current expected performance, then it is unlikely it will operate significantly better without the root of the problem being addressed. Operations personnel can provide insight into where possible gains can be made above what the listed data may show. Additional tests run by operators can provide insight into how equipment will react to potential changes.
The best practice is to build new simulations or modify the original design models if they are available, to match the plant operating conditions when the plant was running at its maximum capacity. Each piece of equipment should be check-rated against the operating conditions using the simulation to fill in details that were not measured in the plant. This will provide details about the equipment beyond the design parameters. It will be possible to determine if heat exchanger fouling factors can be adjusted from the original design or if the efficiency of a compressor is slightly different from the original design. It can also identify issues where reality is not what was imagined in the original design. A good example of this is where fluid viscosities are different than imagined in the design, which can have a significant impact on hydraulics, heat transfer and mass transfer performance. Another example is where the kinetics in the reactor can be back calculated, which can discover that catalyst performance has improved from what the design was based on.
Once all the bottlenecks are identified, the simulation models can be run at incremental throughput increases and each piece of equipment can be checked at each step to determine at which step each piece would need modification to achieve the new capacity. A route to simplifying this engineering exercise is to look at the biggest and most expensive parts of the facility to change reactors, distillation columns and compressors. It is possible to use the plant data and equipment designs to determine the maximum capability of each component because adding additional parallel parts, or replacing these parts, is usually difficult to justify. This can help reduce the number of steps you need to model and assess.
Debottlenecks require a complete understanding of the process. From this understanding, all the limitations of the facility can be defined and properly accounted for. It is when they are missed in the early phases, and the project moves to the detailed engineering and procurement phase, that the project can quickly go sideways. Having a clear understanding of the process can also lead to more than one solution to complete the debottleneck, allowing it to be completed in phases (if desired) and limiting the amount of rework that may be required.
Being realistic and taking a pragmatic approach to the project is also required. Cutting new nozzles and re-traying a 50-year-old distillation column may seem like a feasible option on paper, but is it practical? Retrofitting old equipment and piping can create many problems that cannot be fully accounted for, even with proper planning, and possibly should have been avoided through other means. Inspection reports from recent shutdowns should always be reviewed to understand the condition of the plant before betting on making modifications to equipment, especially those that are of older vintage. It may be more cost effective over the life of the project and plant to replace rather than retrofit.
Because of the many pitfalls and complicated nature of debottlenecking, having a qualified engineering contractor with the right experience in the area can lead to a very effective outcome. Fluor has the necessary experience and knowledgeable personnel that will be able to more efficiently achieve the debottleneck goals and limit potential long-term issues. Fluor has a long history of working with owners and operators to develop fit-for-purpose designs to debottleneck their facilities efficiently. From this experience, Fluor can quickly draw on its global team of experts and know where potential pain areas can arise. It is always better to eliminate pain early rather than find out after startup that something does not work. This will allow an owner to optimize its capital investment and meet the contractual obligations surrounding the debottlenecking project.