Jul-2002

Vacuum unit design effect on operating variables

To revamp vacuum units, process modelling and equipment design know-how are needed, and the understanding of connected equipment performance can lead to higher gasoil quality and yields, with fewer unscheduled shutdowns

Gary R Martin, Process Consulting Services

Article Summary

Process modelling errors and failure to design vacuum unit equipment as an integrated system has caused yield loss, poor gasoil quality, and unscheduled shutdowns. The vacuum unit charge pump, fired heater, transfer line, column internals, and ejector system must be evaluated and designed together so that operating temperature and pressure can be optimised to meet economic goals. Revamps need to push major equipment to its intrinsic limits to minimise investment.

Real equipment performance should be the basis of a revamp, not office-based assumptions or cursory reviews of the original equipment manufacturer’s data sheets. Even though vacuum unit equipment is often highly constrained by existing equipment, an experienced revamp engineer can often manipulate heater outlet temperature, column flash zone pressure, coil steam injection, or vacuum bottoms stripping to achieve revamp yield and reliability targets.

The challenge is to accomplish these objectives while meeting stringent investment criteria or Capex restrictions. Vacuum unit heater-inlet-through-ejector-outlet (Figure 1) must be considered a single system when a practical, operable and cost-effective revamp is to be implemented. A successful revamp will meet all these standards.

Revamps should always start with a thorough test run gathering all necessary data on current unit and equipment performance. Next, the field data is used to calibrate a baseline process model and quantify current equipment performance. The calibrated model will be the basis of the revamp calculations. Only then can the revamp engineer apply equipment design know-how to identify all under-utilised equipment and exploit it to minimise investment.

Without this approach during the conceptual process design (CPD), all major cost bottlenecks may not be fully defined. This leads to scope growth and cost escalation during later stages of engineering.

Computer models are invaluable tools used during CPD to establish bottlenecks and define scope. But the process flow sheet models must represent true operation and account for the many equipment non-ideal situations encountered in a refinery vacuum unit. If not, inaccurate calculated process stream data will result in short run-lengths and low gasoil  product yields.

One such example is the influence of transfer line phase separation on predicted heater outlet temperature and the calculated wash section liquid flow rates. Unlike some theoretical modelling concerns that have few real penalties, failure to account for transfer line phase separation will lower the gasoil  product yield and result in a very low wash oil flow rate.

Wash bed packing requires adequate wetting in the middle of the bed to avoid stagnant zones where coking is initiated. In reality, accounting for transfer line phase separation raises the wash oil flow rate by 200–300 per cent over conventional modelling practices that assume liquid and vapour leaving the transfer line are in equilibrium. This one oversight alone has caused many unscheduled shutdowns after less than a year of operation. Process modelling plays a significant role in meeting overall refinery economics.

Rigorous equipment models need to account for the various designs that are encountered when revamping. Many vacuum unit fired heaters, transfer lines, columns and ejectors were designed and built based on a low capital investment strategy and not best practices methodology. Poorly designed equipment may operate well at low temperature and high pressure, but when severity is increased, the run-length or yield expectations are not met.

Many poorly designed vacuum heaters have been built because the selection criteria were based mainly on low price. Low-cost heaters have high heat flux imbalances among the passes that significantly reduce the maximum reliable heater firing.

Often, engineering approaches are based solely on superficial evaluations such as average radiant heat flux or vendor data sheet review. These methods minimise engineering cost but often make incorrect conclusions about future equipment performance. When equipment is pushed, fundamental design errors become apparent. Thorough heater analysis will identify heat flux imbalances allowing the experienced engineer to exploit under-utilised capacity. Potential minimum capital design changes, such as external radiant section jumper-overs can be used to correct the heat flux imbalances.

During CPD all under-performing equipment needs to be identified so that investment can be focused where it is really needed. Manipulating operating temperature and pressure to produce a reliable, operable, and cost effective revamp requires full utilisation of all existing equipment.

Critical operating variables
Operating temperature and pressure sets gasoil yield and unit run-length. The temperature required to meet the gasoil  product yield target is determined by the heater outlet oil partial pressure, column flash zone pressure, and stripping section efficiency. Heater outlet temperature is limited by the rate of coke formation inside the heater tubes or column internals.

Some refiners target a one-year run between decokings; therefore, high coke formation rates are acceptable. Yet, today, many refiners have set four to five-year run-length and higher HVGO product TBP cutpoints as goals. Increasing heater outlet temperature and run-length requires all equipment design optimised together to keep the rate of coke formation low. Only then can operating pressure and temperature be optimised to maximise performance within the investment guidelines and the real equipment performance.