Cost-effective and reliable revamps
Test run hydraulic measurements and analysis, together with equipment know-how, can help refiners identify profitable, low-cost revamp opportunities
Scott W Golden, Tony Barletta and Steve White, Process Consulting Services
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With worldwide petroleum product demand and refinery margins dropping and project financing disappearing, many grassroots projects have been postponed or cancelled. Yet there continues to be highly profitable revamps that increase product yield or feedstock flexibility. For example, many refiners have 25–30% diesel boiling range material in their FCC feed, creating an opportunity to increase their refinery diesel yield by 5–6% on whole crude. Several refiners have already realised this benefit by revamping vacuum columns to recover diesel from light vacuum gas oil (LVGO) FCC feed (Figure 1). In the past few years, grassroots projects have dominated refinery and E&C industries’ focus. These large projects use a highly structured linear work approach, where one specialised group passes its work to another without much interaction. As long as the design basis does not change, this system can work.
Unfortunately, grassroots method-ology simply is not ideal when developing cost-effective revamps. The revamp engineer needs to identify opportunities and quickly assess feasibility and constructability without involving multiple specialised departments that focus on one thing. In the diesel recovery example, it is not necessary for the distillation specialist, systems group, vessel engineer and exchanger expert to be involved early on. An experienced revamp engineer can do the process modelling, determine whether there is sufficient room in the existing vessel to install a fractionating bed, evaluate the top pumparound circuit limits and decide whether a heat balance shift will be needed. Cost-effective revamps require a hands-on approach, utilising the experience of seasoned revamp engineers.
The revamp should always begin with a test run and data analysis to determine real existing plant limitations. All too often, however, a revamp begins with the process engineer developing a process model using little or no plant data other than a printout of the DCS. The main premise behind relying solely on process models with little data is that refinery units behave ideally based on the original equipment data sheets. In this approach, avoiding extensive unit baseline testing reduces engineering costs and shortens schedules, but fails to address the ultimate goal, which is a cost-effective and reliable revamp. Consequently, there is little if any effort spent gathering field data. For example, increased pressure drop caused by fouling in a heat exchanger network must be estimated to correctly assess hydraulics. But how is this done without field measurements?
Unfortunately, many design engineers have not worked in a refinery or ever taken any field measurements. As a result, they cannot appreciate the value of real data. System hydraulics is one of the common and potentially costly unit limits that must be overcome. Field measurements are the only accurate way to quickly generate cost-effective hydraulic system solutions. Refinery crude charge and product rundown system hydraulics cannot be accurately assessed with models, as a large part of the system pressure loss is caused by fouling. Poor equipment selection and design are the major sources of rapid and excessive fouling in the refining industry. Field pressure measurements, in conjunction with fundamental principles and equipment know-how, are used to identify profitable revamp opportunities. In each case, the most cost-effective solution does not come from sophisticated models and tools, but from field data and the application of fundamental system and equipment design principles. Throughout the authors’ careers revamping refinery units, the starting point has always been the gathering of accurate field measurements and product stream analysis, not the process models. Yet, hierarchical organisational and planning strengths and cutting-edge simulation tools continue to be toted as keys to increasing refinery profitability. While modelling tools are useful and necessary, alone they will not produce a reliable and cost-effective revamp. Low cost is easy to achieve using such constructs as the value process — there are many examples. But successful revamps that actually work the first time require a different approach!
Revamps: why they are different
Grassroots projects start with a clean sheet of paper and no constraints, and there are no existing process flow schemes and equipment designs that must be considered. The grassroots methodology starts with process modelling (heat and material balances), these results are then sent to the systems group (hydraulics, P&IDs), and this group’s results move on to the equipment specialists until front end engineering (FEED) is complete. Since there is no existing process flow scheme or equipment to constrain the work, this linear approach can be scheduled and controlled by the project manager with minimal interface between groups. Command and control are the primary drivers. The process engineer is just one cog in the wheel as opposed to the decision maker helping to establish the most cost-effective and reliable revamp. There are no messy existing process systems or equipment operating problems to overcome, and there is no need for field measurements and data interpretation.
Revamp process design
Revamps should always begin with a test run and data analysis to determine real existing plant limitations. Unfortunately, comprehensive test runs are rarely done because they are viewed as unnecessary costs. Many process design engineers believe they can calculate anything with models, whereas the experienced revamp engineer knows that solutions that actually work flow from unit measurements and analysis. As process flow sheet models become easier to use, many users have come to believe they represent reality. However, process flow sheet model results are not reality. The measurements gathered during the test run are the reality even if they are not ideal or what one expects. Test run planning and execution should be conducted by revamp engineers performing the work so they become familiar with the layout of the unit and problems that must be dealt with. First-hand knowledge of operating limitations and problems is critical to success. Cost-effective revamps should include:
• Planning and organising a comprehensive test run, gathering all necessary data for the particular unit being revamped. Each different type of unit will require customised data gathering
• Identifying existing system and major equipment limitations
• Circumventing all bottlenecks cost-effectively.
The revamp engineer must evaluate test run data to determine how the unit operates before generating options or running sophisticated process and equipment models. For instance, a vacuum distillation unit (VDU) revamp to increase heavy vacuum gas oil (HVGO) product yield requires more HVGO pumparound duty to condense the incremental distillate yield.
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