Jul-1998

Troubleshooting vacuum unit revamps

While vacuum unit revamps increase gasoil yield, management is pushing the intervals between turnarounds to four or five years

Scott W Golden
Process Consulting Services

Article Summary

Revamps of crude or lube vacuum units to increase the yield of the gasoil product rely on appropriate feed characterisation, process modelling and equipment design to meet the processing objectives. Successful revamps require the economic objectives to be realised over the run length. Many revamps operate for a six- to 24-month period before a shutdown is required to make equipment repairs. Improving gasoil yield requires a higher operating temperature at the heater outlet, within the heater, and in the column.

Many refiners have planned run lengths of four to five years while they are attempting to increase operating temperature. Feed characterisation methods, process modelling and equipment design should reflect the refiner’s gasoil yield and run length expectations.

Historically, vacuum units have operated at flash zone conditions of 385°C (725°F) or less, whereas some refiners are attempting to operate at 418°C (785°F) today. A few are successful, but many refiners operating at elevated temperatures have had problems. Proper equipment design is essential to achieve a four or five year run at temperatures above 390°C (735°F). Often, the temperatures in the fired heater and distillation equipment exceed the thermal stability of the oil.

Fired heater tube, wash zone packing, collector tray, and stripping section tray coking are the primary causes of unscheduled shutdowns. Oil residence time must be kept to a minimum to meet both high operating temperatures and extended unit run length.

The emphasis in this article is on equipment design and its impact on coke formation. Feed characterisation is discussed because it affects equipment design and operation. Accurate feed characterisation is essential to properly evaluate equipment performance. System pressure and its impact on operating temperature is briefly reviewed. Vacuum unit fired heater and column internal coking problems are discussed.

Vacuum gasoil yield
Feed composition and system pressure
A vacuum unit consists of a fired heater, transfer line, column and internals, and ejector system (Figure 1). Heat is added to the feed to vaporise the oil. Increasing heater outlet temperature or reducing operating pressure will increase the amount of oil vaporised.

Steam can be added to the system to increase oil vaporisation. Ultimately, oil temperature and residence time must be controlled so that the thermal stability of the oil is not exceeded. For a given oil residence time, there is a temperature at which the oil will begin to crack. When the oil cracks, gas and coke are made. The rate of cracking (time and temperature) must be kept sufficiently low so that gasoil yields and product quality are maintained over the run length.

The vacuum unit recovers 343–595°C (650–1100°F+) gasoil boiling range material of adequate quality for downstream processing. Vacuum gasoil is used as feed to a hydro cracker or fluid catalytic cracking (FCC) unit. Oil composition and system pressure affect oil residence time. Coking is a function of equipment operating temperature and oil residence time. For example, when revamping a unit from light crude of 0.825–0.865 specific gravity (32°–40° API gravity) to heavy crude of 0.887–0.922 specific gravity (22°–28° API) gravity, the oil residence time in the heater changes.

The vacuum heater tube size and transition locations are selected based on the vaporisation profile of the feed. A light crude will vaporise sooner in the heater, therefore, the radiant section tube layout is selected to minimise oil residence time and oil temperature. When heavier oil is processed in the same heater, the oil residence time increases because the heavy oil vaporisation profile is different.

Increased oil residence time can cause coke formation in the heater tubes, thereby increasing the tube metal temperature. Once the maximum tube metal temperature is reached, the heater must be shutdown to remove the coke.

Vacuum unit feed composition
Vacuum unit feed composition establishes how much recoverable gasoil is available, the approximate quality of an incremental barrel of oil, and the equipment performance.

Proper vacuum unit feed true boiling point (TBP), specific (or API) gravity, molecular weight, and contaminants distribution curves are essential for analysing performance problems. The TBP curve determines the available gasoil product. The feed contaminants distribution determines the product contaminants for a given gasoil yield and unit equipment performance.

Vacuum unit feed is composed of thousands of large molecules of unknown chemical composition. The large number of molecules must be reduced to a reasonable number of pseudocomponents for process and equipment modelling. Pseudocomponent properties are determined by performing various laboratory tests on the feed. These tests are used to generate sufficient data to develop accurate TBP, specific gravity, and contaminants distribution curves. Measuring an accurate vacuum unit feed TBP and contaminants' distribution curves is not a trivial task.