Mar-2008

Improving crude vacuum 
unit performance

When simulating or designing an unconventional heavy oil/bitumen vacuum crude unit, the sharing of operating data on cracked gas make, stripping section performance and indirect entrainment can help ensure designers make the right design choices.

Darius Remesat
Koch-Glitsch Canada LP

Article Summary

The refining industry is becoming more experienced at operating vacuum units that process unconventional heavy oil/bitumen- derived crudes. New vacuum units are being planned, and the knowledge gained from existing operations could improve the economics of these projects by incorporating the nuances in processing these crudes to create robust designs that represent actual operation.

This article focuses on sharing data specifically from unconventional heavy crude vacuum units operating in wet mode (stripping and velocity steam)¹ to provide some insight into using a proven vacuum distillation tower simulation topology² with heavy oil/bitumen upgrader feeds.
When simulating or designing a heavy oil/bitumen upgrader vacuum column, the following design factors need to be incorporated, in order of importance:
1    Crude characterisation^3,4
2    Thermodynamics choice^3,4
3    Simulation topology (non-equilibrium).²

Each subsequent design factor cannot compensate for poor crude characterisation or thermodynamics choices. Assuming the heavy oil/bitumen crude has been characterised appropriately for the higher boiling point components (eg, using both ASTM 2887 and HTSD),⁵ and the representative thermodynamic package is chosen (eg, variation of Peng-Robinson, Grayson-Streed, Esso Tubular, BK10), the focus for the vacuum column design shifts to simulation development. Table 1 illustrates the use of different default thermodynamic equations in a vacuum column, and the difference can be significant. The thermodynamic package must be appropriately set before the simulation topology and any simulation enhancements can be considered.

Figure 1 shows a proven vacuum column simulation topology² that addresses the realities of the physical system built. The bottoms of the atmospheric crude column is heated (partially vapourised) in a vacuum heater to provide the necessary energy to separate the higher boiling components at a reduced pressure in the vacuum column. In the simulation, the column is divided into two main sections (stripping and above the flash zone). The transfer line is partitioned into separate vapour and liquid streams to address the non-equilibrium situation, and various flashes  address entrainment and the specifics of the flash zone and slop wax collector tray. It has been shown that the predictions on HVGO yield and the liquid rate to the top of the wash section from Figure 1’s topology match up well with the plant’s data.^2,3,6

After reviewing operating data from various units, Figure 1’s simulation topology has been used to enhance the representation of a bitumen-based feed vacuum unit by capturing the thermal and variable stability characteristics of the crude. The cracked gas generated, the entrainment formed and the performance of the stripping section should be critically evaluated to ensure these factors represent the bitumen’s characteristics.

Cracked gas make
The operational and thus economic impact of underestimating crude cracking in the vacuum heater is large due to increased light hydrocarbon molecules, which can reduce the effectiveness of the vacuum ejector system, and increased coke particle generation, which can reduce column run length and throughput capacity,⁷ and result in a reduced maximum film temperature and a low residence time. These heater design guidelines are a function of the crude being processed, and typically the impact of the heater design is not fully included in the simulation.
Figure 2 shows a published relationship (Lieberman) that uses readily available measured plant data that relate the transfer line temperature to the amount of off-gas (scfd) leaving the vacuum unit⁸ (assumed downstream of the vacuum hot well) as a ratio to the crude flow (bpd) to the unit. Basically, the trend is that as the heater’s temperature increases, the quantity of off-gas per crude feed increases as well. This relationship is only as good as the field measurements, but provides a tangible appreciation of the degree of cracked gas generated from an unknown heater design. This off-gas value is lumped, consisting of air ingress, dissolved hydrocarbons in the feed, residual water and cracked gas. From the data gathered, a reasonable assertion appeared to be that the cracked gas comprised the majority of the off-gas and was the prominent contributor to the increase in off-gas flow when the heater’s temperature increased.

Figure 3 shows two data sets  taken from vacuum units processing crudes with vacuum feed APIs of 13 and 16. Both data sets support the Lieberman relationship trend that as the transfer line temperature increases, the off-gas flow-to-crude feed ratio increases as well. The API 13 data set seems to match the Lieberman relationship well, while the API 16 data set seems to generate far less cracked gas. Trend lines were added to Figure 3 for each data set to reflect the Lieberman relationship representation.

Figure 4 includes trended data from vacuum units processing more heavy (API 6–10) unconventional crude. Again, the relationship where the amount of off-gas flow increases as the transfer-line temperature increases (from increased heater firing) is reflected in these data sets. In general, vacuum units processing heavier crudes will operate at lower heater outlet temperatures than lighter crudes, primarily due to the tendency for the heavier crudes to be more unstable at higher temperatures. The instability translates to increased coke generation and light hydrocarbon formation. Also, obviously as the API decreases, the degree of off-gas increases. However, many vacuum unit designs do not reflect this situation. For example, at a temperature of 700°F, the API 13 crude tends to exhibit an off-gas-to-crude flow of 2–3 scf/bbl, while for the API 10 crude (eg, Cold Lake) the scf/bbl would be 6–7 (ie, more than double).

This type of simplified relationship between API (primarily bitumen derived) and off-gas flow (cracked gas make) has implications on vacuum column unit design. Assuming a desire to operate the vacuum unit at as low a pressure as possible (to maximise HVGO product yield), the overhead vacuum ejector design/size and the heat-transfer capabilities of the quench/pumparound zones will be different if the data in Figure 4 is used rather than the data available in literature.⁸

Figure 5 shows one operating example and one redesign example (not in operation yet). Point A reflects a design point now in operation for an unconventional crude with a feed API 10, where the actual off-gas flow was significantly more than expected for the designed heater outlet temperature and HVGO yield. The vacuum ejector system was overwhelmed, and the heater 
outlet temperature was reduced by approximately 35°F (point B), thereby reducing the HVGO yield significantly. It appears that the design point A for the off-gas flow (cracked gas make) was based on the cracking characteristics of a more conventional crude. Point B’ is a suggested design point, using the benefits of an operating experience database. Understanding and applying the specific crude characteristics (crude characterisation and validating with similar data points) is crucial if a design is to operate as intended.