Jun-2016

Improved stripper efficiency raises upgrader production

Revamping a heavy oil upgrader’s steam stripping tower increased tray efficiency and achieved record throughput.

Mike Goulding and Fred Zhang, Suncor
Michael Krela, Koch-Glitsch

Article Summary

Suncor Energy is Canada’s leading integrated energy company, with a focus in the oil sands, and has been involved in heavy oil extraction and refining dating back to the Great Canadian Oil Sands project in the 1960s. At the time this was the largest private investment in Canadian history. The Base Plant Upgrader, located in Fort McMurray, Alberta, has been in operation since 1967.

This article reviews the use of Superfrac trays in a successful revamp of an underperforming steam stripping tower in the Base Plant. The increased tray efficiency provided by the trays allowed for debottlenecking of the downstream vacuum overhead condenser system, resulting in an overall upgrader production increase of 5% and record throughput.

Background
Suncor’s Base Plant Upgrader operation recovers diluent from diluted bitumen feed in diluent recovery units (DRU). The recovered diluent is then sent to a vacuum distillation unit (VDU). In 1998, a steam stripping tower with six trays was put into operation in an effort to minimise diluent slip from the DRU distillation tower and to reduce the load on the downstream vacuum overhead ejectors (see Figure 1). The vacuum tower is a dry tower design with a pre-condenser and is particularly sensitive to slippage of light naphtha (diluent).

Performance of the stripper has been poor from the outset, with very little, if any, stripping being measured. Operations saw a diluent slip of 0.85%, corresponding to 857 b/d diluent slipped to the vacuum unit and recovered in the overhead system as vacuum overhead kero (VOK). The pre-condenser is not designed to condense this amount of light naphtha, with most of it slipping to the first-stage ejector and impacting vacuum. As a result, the existing overhead system places a constraint on total upgrader production.

Technical evaluation
In 2012, Suncor and Koch-Glitsch performed a technical evaluation of the tower with a view towards improving performance. A simulation study was done to assess the impact of improving the stripping tray efficiency on the overall unit. The existing tray efficiency (~0%) was modelled by treating the stripping section as a flash column with no steam flow. In addition, a typical efficiency range for this service was modelled. The evaluation also reviewed the stripping tower operation and existing mass transfer equipment, identifying serious operational and design issues. Both the bitumen and steam feeds were found to be areas of concern.

The existing bitumen feed distributor is a 12in (305mm) open nozzle flowing into a centre false downcomer above the top tray (see Figure 2). The existing nozzle was undersized for this type of feed arrangement, with a false downcomer inlet velocity of nearly 14 ft/s (4.2 m/s). With this existing arrangement, hydraulic calculations showed that the jump height would far exceed the open channel (false downcomer) height. The calculated liquid height after the first jump was 20.9in (0.53m), which is well in excess of the 16in (0.4m) tall false downcomer (Figure 3). As a result of the hydraulic jump, the performance of the top tray would be poor due to the majority of the liquid short- circuiting the tray flow path.

To improve liquid distribution, a perforated feed pipe distributor was installed (see Figure 4). The use of a feed pipe would ensure that all feed liquid would remain within the false downcomer, and that there would be uniform distribution feeding onto both active area panels. A new false downcomer was designed to match the inlet panel dimensions of the Superfrac tray. To minimise the turnaround time, a stab-in arrangement was used to eliminate welding to the vessel wall.

The existing steam distributor was modified from a V-baffle deflector plate to a perforated pipe distributor (see Figure 5). While the velocity through the nozzle is relatively low, it was determined that the increase in pressure drop associated with flow through an orifice would help ensure uniform vapour flow to both active area panels of the bottom tray. The new feed distributor design eliminated welding to the vessel wall.
 
Tray efficiency
Inherently low tray efficiency in the stripping section of heavy oil towers has been well documented in previous literature.^1,2 A well designed tray for this service could obtain upwards of 25-40% efficiency; however, values below 10% are common.^3,4 A typical grassroots project will specify 4-8 trays based on an assumed 25% efficiency.

There are many factors that contribute to stripping of hydrocarbons having an inherently low efficiency, including:
• Insufficient steam/oil ratio
• High relative volatility
• High liquid viscosity.

Because grassroots projects will often have multiple equipment vendors bidding, an effort is made to standardise the offering and compare based on price, working off little more than data sheets, and without proper review of overall tower layout, diameter and feed arrangements. This process leads to inadequate communication between the selected vendor and the EPC company, resulting in poor equipment selection and inadequate design. Inherent low tray efficiency is further compromised.

The following are common design errors that could lead to reduced efficiency:
• Having the same diameter for the flash zone and stripping section, which leads to severely oversized trays
• Applying a single valve layout (open area) across the entire zone even though there are large changes in vapour flow from tray to tray
• Poor liquid feed distribution to the top stripping tray
• Poor steam distribution under the bottom stripping tray
• Fouling that can negatively affect performance if not accounted for in tray design (valve selection, orifice size, and so on)
• Tray mechanical design not suitable for potential upsets, which can result in loss of trays.

Applying the high performance Superfrac tray to heavy hydrocarbon stripping
High performance trays traditionally are only considered for increasing capacity, with a reluctance to use in grassroots projects for the majority of refinery columns. However, application of technology should be based on economic impact. A hydrocarbon stripping tower is the perfect example of this concept. From a purely hydraulic capacity standpoint, it is rare that a high capacity tray is justified. That being said, if the maximum obtainable tray efficiency is considered when doing the grassroots design, their use would be much more commonplace. While the operating point of a typical hydrocarbon stripper tray is in the range of 30-60% flood (including the tower in this article), the following conditions conspire to reduce the efficiency of conventional trays:
• Long residence times and stagnant areas at high liquid flow rates resulting from use of conventional tray technology
• Excessive weir loadings
• High downcomer exit velocities leading to hydraulic jump across the active area
• Short flow path lengths through the use of large, straight downcomers
• Fouling due to coke formation, which is a function of long residence times.