Jul-1997

Revamp process design — optimum crude unit product yields

Designing a new crude unit is an unconstrained process, limited only by capital
investment targets, while revamping an existing unit to optimise product yields is constrained by the existing equipment performance and design

Scott W Golden, Process Consulting Services

Article Summary

A low cost revamp requires pushing the plant to the limit of the existing equipment and minimising equipment changes. Optimised revamp product yields require that the existing equipment performance be quantified. Quantifying equipment performance is not the same as saying that the crude feed system hydraulics are limiting unit charge.

What specific equipment or system causes the limitation? Is the limitation the crude charge pumps, high exchanger pressure drop, heat exchanger network split flow stream imbalances, or a fundamental system design problem?

Measuring current unit performance is one of the critical components of the revamp process prior to optimising product yields. Revamps have a higher incidence of failure than new unit designs. Revamping process units for optimised product yields requires a different approach to the process design.

A conventional project’s process design executes a series of activities that are often performed by different groups of people (Figure 1). First, a heat-and-material balance based on predetermined revamp objectives is completed. Existing equipment evaluation then follows. Generally, all process work is completed in the design office. If the calculated existing equipment capacity is insufficient, the equipment is replaced or paralleled.

Engineering calculations are performed based on presumed equipment performance, vendor-supplied estimates, industry averages or the specific E&C’s design standards. In meeting the revamp objective, the equipment design assumptions often force unnecessary equipment modifications or underestimate the existing equipment capacity.

To prevent revamp failures, safety margins are added. The safety margins required to adequately cover the errors in the conventional project's approach are often greater than the real changes required.

Optimised product yields
Revamp process design requires a different approach. The revamp optimisation must focus resources on the following phases of the process design:
- Test-run: quantifying existing equipment performance
- Alternative flow schemes: maximising existing equipment utilisation
- Reliable revamps: detail equipment design.

All phases must be addressed to meet the revamp optimisation criteria of improved product yields, minimum capital cost, and a reliable operating unit. Shortcutting this process results in either higher investment costs due to unnecessary equipment modifications or an unreliable process being installed because existing equipment problems are not identified.

The most important part of a revamp is the test-run, because it determines crude unit performance at both a general level (product cutpoint) and a very detailed level. The test-run data gathering and subsequent analysis quantifies major equipment performance. Without the test-run data and analysis, the alternative flow scheme evaluations are based on presumed equipment performance.

Often, presumed equipment performance is significantly different from the operating performance. For instance, measured heat exchanger fouling factors and pressure drop are higher than computer calculations or presumed industry standards for the service. Existing equipment performance must be established before alternative process flow schemes can be evaluated.

Conceptual process design explores the practical flowsheet alternatives to find the least-cost means of circumventing the unit bottlenecks. The conceptual process design is the single most important factor in realising the targeted product yields, cost, and reliability objectives. For instance, reboiling the FCC debutaniser with main fractionator 500°F pumparound material is feasible when the debutaniser bottom temperature is 360°F. However, when the debutaniser gasoline RVP specification has to be reduced from 9 to 6.5, the debutaniser bottom temperature increases to 390°F.

While it is still feasible to use the 500°F pumparound, the circulation rate and exchanger surface become impractical. Thorough conceptual process design will not create an unworkable process flow scheme like this one.

Revamp process design work is highly front-end loaded, with the test-run and alternative flowsheet analysis constituting the high resource and technically challenging portion of the work. Although implementing a reliable revamp requires a process design package containing the detail equipment design, implementing the wrong flowsheet can be costly and unreliable. Once the minimum cost flow scheme is determined the detail equipment design must be performed. While details are often considered the equipment vendor's responsibility, the installed equipment must support the theory. Often, important detail equipment design errors prevent the revamp from meeting its goals. The process design package is important; however, it is the lowest resource portion of a revamp.

While all revamp process design phases are important, in this article it is the conceptual aspect of evaluating flow scheme alternatives for a crude unit revamp that is addressed.

A conceptual process of any kind is often viewed as being general and non-specific. The term feasibility study is used as a synonym for conceptual process design. However, revamp process design bases the alternative process flow scheme evaluations on test-run data and analysis of equipment performance. Analysis of major equipment, such as the fired heaters and heat exchanger networks, are performed with rigorous equipment models, utilising the equipment’s specific mechanical configuration.