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Oct-1998

Practical considerations for crude unit revamps

Two examples are used in this article to illustrate how changes in the process flow scheme can increase preheat train heat recovery

Tony Barletta, Process Consulting Services
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Article Summary
Increasing crude oil heat input is often the key element in a successful crude unit revamp. When crude units are revamped to increase throughput or distillate yield, crude oil heat input must be increased. Crude oil is heated in the crude preheat train and the crude heater. A simplified diagram of a typical crude unit is shown in Figure 1. In an integrated crude unit, the preheat train recovers heat from crude and vacuum column products and circulating pumparound streams, reducing the amount of fuel burned in the crude unit.

When crude oil heat input must be increased, the crude heater is the first piece of major equipment to evaluate. Even when crude heaters operate at maximum capacity and limit crude oil heat input, they can be revamped to increase heat input. However, the extent to which a crude heater can be revamped  economically must be compared against modifications to the preheat train. In some cases, modifying both the preheat train and the crude heater will be most economical. This article is focused on increasing crude preheat train heat recovery.

Revamping an integrated crude oil preheat train to increase heat input is more difficult than the designing of a new crude unit. The revamp engineer must find a practical, low-cost design that best utilises not only existing exchanger surface area, but all existing equipment. Many additional constraints imposed by the existing equipment make this a difficult task.

In most cases, a conventional process design approach fails to identify low-cost solutions for crude unit revamps. With this approach, first the column fractionation objectives are satisfied, then the preheat train is designed with exchanger network optimisation tools. Often, the conventional approach results in unnecessary capital expenditure.

In fact, low-cost solutions to preheat train revamps often require changes to the process flow scheme. A more practical approach, which facilitates evaluation of different process flow schemes, is needed to identify low-cost revamp solutions. To increase preheat train heat recovery with minimal capital expenditure, the exchanger network design must be performed together with the design of the rest of the process equipment.

Crude unit revamps which increase throughput or distillate yield require higher crude oil heat input. The question that the revamp engineer must ask is, “Can heat input be increased with less capital expenditure by modifying the preheat train?”

Sometimes, heater capacity can be increased economically. Other times it cannot. In some cases, modifying both the heater and the preheat train will be most economical.

Revamping preheat trains
The crude preheat train is the heart of a crude unit revamp. Every crude unit is different and must be evaluated on an individual basis to arrive at the most economical process flow scheme. A low-cost revamp design is a function of the existing crude unit equipment limitations.

The revamp engineer must consider all equipment constraints while evaluating new preheat train configurations. Therefore, the design approach needed to implement low-cost crude unit revamps is very different from a grassroots design approach.

Conventional approach
In the conventional approach, first, the fractionation objectives set the distillation column heat and material balance (H&MB). This fixes product and pumparound temperatures and the amount of heat available in each stream for preheating crude oil. Next, the exchanger network is optimised by reconfiguring the preheat train exchangers and adding new surface area.

While the configuration of the exchanger network may change, the column H&MB does not change. Then, the “optimised” process flow scheme that is generated with exchanger network optimisation tools is used for the rest of the design. Although this approach works well for grassroots design, it is difficult to apply to revamps.

The network optimisation approach can result in improved heat recovery, but it does not always guarantee a low-cost design. A disconnect exists between the preheat train design and the attached distillation columns’ heat and material balance. The conventional design approach ignores the interdependencies of the integrated crude unit equipment. Additionally, preheat train optimisation tools concentrate on exchanger thermal requirements.

The mechanical limitations of the exchangers and other connected equipment are evaluated after the process flow scheme is selected. Eventually, all of the equipment and its mechanical limitations are evaluated (during detailed design) and often, optimised preheat train designs are not feasible without additional capital expenditure.

Practical approach
The solution to most exchanger network revamps is more deeply rooted in the process flow scheme. Altering the unit H&MB, a process flow scheme change will improve the driving force for preheat train heat exchange and will result in fewer modifications to the unit. To evaluate the impact of process flow scheme changes on preheat train design, a different design approach is needed.

A practical approach to crude unit revamps evaluates preheat train and process flow scheme modifications together to find a minimum cost solution. All major equipment must be evaluated in sufficient detail to compare the relative costs of various process flow scheme options. For a crude unit revamp, the process equipment must be evaluated in enough detail before the process flow scheme is selected; otherwise, the solution is probably not the lowest cost.
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