Crude unit revamp to increase diesel yield
Account of a crude vacuum unit revamp where detailed calculation of the true performance of existing equipment raised diesel production by 14.5 vol% on crude
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Capital is scarce in today’s low margin, highly competitive refining industry. This is a fact of life. For revamps to be approved they must fit into constrained capital budgets. Revamp costs must match financial objectives. This requires revamps to be minimum cost. A minimum cost revamp improves existing equipment utilisation and pushes equipment to its ultimate capacity. There
is no room for large design tolerances or “design fat”.
Although the revamp approval process varies from company to company, most revamps are funded based on cost/benefit analysis. Cost/benefit methodologies vary and are not important in this discussion. What is important, however, is that the benefits are estimated accurately, and they are realised.
Under- or over-predicting revamp benefits can lead to undesirable outcomes. Under-predicting revamp benefits, for instance, can result in a revamp that will not be funded from a perceived lack of benefits. This represents a lost business opportunity. On the other hand, over-predicting can result in a revamp that will not meet financial expectations.
In this article, we examine a crude vacuum unit revamp to illustrate two important revamp considerations: prediction of revamp benefits and minimising capital expenditure. This crude unit revamp increased diesel production by 14.5 vol% on crude oil. Crude characterisation and diesel cloud point prediction methods permit accurate estimation of diesel yield. Also, by changing the process flow scheme, crude preheat was increased by 108ºF (60ºC) and costly fired heater modifications were avoided.
The distinguishing characteristic of this crude unit is the requirement to operate in two modes of operation. Conventionally, the vacuum unit is operated in series with the crude unit. In this case, however, the vacuum unit is operated in series and in parallel with the crude unit. The crude unit and vacuum unit modes of operation are:
1. Crude oil is processed through the crude unit and reduced crude oil is fed directly to the vacuum unit. This is referred to as series operation, and is illustrated in Figure 1.
2. Crude oil is processed in the crude unit and reduced crude oil is routed directly to the FCC unit. The vacuum unit operates independently of the crude unit and processes a special feedstock to produce lube oil refinery feedstock and bitumen. This is referred to as parallel operation (Figure 2).
The crude column has light and heavy diesel side products. The heavy diesel product is the lowest side product. When the unit is operated in parallel mode, reduced crude product is sent to the FCC unit as feedstock. The unit is operated in the parallel mode 60% of the time.
During parallel operation diesel boiling range material in the reduced crude product is lost to FCC unit feed, which results in loss of revenue based on overall refinery economics.
The primary revamp objectives were to increase crude unit diesel yield and keep revamp costs to a minimum. Secondary objectives were to increase crude throughput by 10 per cent, process a range of “light and heavy” crudes, and improve energy efficiency. To accomplish these objectives, a performance test was conducted to uncover unit constraints and identify opportunities.
Constraints and opportunities
Revamp design is inherently more difficult than grassroots design. By definition, revamps start with an existing plant, and most of the existing equipment is re-used. Therefore, existing equipment performance is critical. If a specific piece of existing equipment is under-performing and goes unnoticed, then a revamp can fail to meet its objectives.
Office-based assumptions about existing equipment performance are often wrong, so it is therefore better to use actual performance tests. These make no assumptions and minimise unwanted surprises. Properly executed, performance tests identify unit constraints, unit capabilities, and under-performing equipment.
In terms of an analogy, if a revamp is a house, then a performance test is its foundation. A performance test can make the difference between a failed revamp and a successful one, or a minimum cost revamp and one that wastes capital. To many people, “performance test” implies a single event but, in fact, it involves preparation, execution, and flowsheet modelling.
Preparation is key to a performance test. A significant amount of planning and preparation is required to execute a successful performance test. For example, flow meters must be calibrated, material balance checked, sample schedule prepared, and P&IDs worked out. Only after the necessary preparation is complete can the actual field work take place.
Performance test execution can begin after all necessary preparation is complete. It takes place in the field, not in the control room. Field measurements supplement and confirm process computer data. Temperature and pressure measurements are used to develop a complete temperature and pressure profile of the unit.
Field observations often give clues to problems that may be otherwise difficult to detect, like a cavitating pump or a thermocouple that is located in the wrong place and is supplying misleading information.
Then, rigorous flow sheet modelling establishes a baseline process model. The flowsheet model is adjusted to match performance test data. All pertinent equipment is rigorously modelled. This includes distillation columns, fired heaters, and heat exchangers. Here, actual equipment performance is determined. No assumptions are made. For example, actual heat exchanger fouling factors are calculated, measured exchanger pressure drops are compared to calculated clean pressure drops, and distillation tray efficiencies are determined. The calibrated process model is used as the basis for subsequent revamp simulation.
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