High performance internals in severe service

Application of a severe service grid achieved 10% higher throughput with improved reliability in a vacuum tower revamp.

Phillippe Weiss, Consumers Co-operative Refinery Limited

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Article Summary

While the recent shale energy revolution and supply infrastructure issues from heavy oil producing countries have steered the majority of US refineries towards being set up for processing light, sweet crude oils, the situation in Western Canada is starkly different.1 Refineries there were originally designed to process light sweet crude oils, but over time have had to make the necessary adjustments to process heavier crude oils due to growing oil sands production.2

As the operator of Canada’s first heavy oil upgrader, the Consumers Co-operative Refinery Limited (CCRL) refinery has pioneered its way through the ever-changing Canadian energy landscape. Founded by eight enterprising farmers amid the great depression to reduce their reliance on major oil companies, the refinery has since gone through numerous upgrades and expansions.3 CCRL is a wholly owned subsidiary of Federated Co-operatives Limited (FCL) and owns and operates the refinery and upgrader facilities.

While major capital projects are one way to increase value to refinery operations, opportunities such as end of life vessel replacement or five-year maintenance shutdowns should not be overlooked with a replacement in kind (RIK) scope. This is an opportune time to improve unit performance without incurring significant costs, and often can be done without incurring any incremental cost when compared to a RIK scope.

This article goes on to discuss the changes made during a vacuum tower replacement project, where new technology was incorporated instead of taking the RIK approach. The changes made increased throughput by 10% while reducing pressure drop, resulting in significant savings in operating costs as well as a more reliable operation for a typically severe service. CCRL became the first refinery in the world to use Proflux severe service grid in a vacuum column wash bed, and the article will discuss the reliable operation achieved and performance benefits realised since start-up in 2013.

Project overview
Crude vacuum distillation units are responsible for maximising gasoil recovery from atmospheric residue. While this service is generally severe due to the origin of the feedstock, the importance and severity are further underscored in the Western Canada region where the front-end feed to the refinery is already a low quality, heavy crude.4

The feed to the vacuum column is heated to temperatures in the range 700-760°F which can result in cracking and subsequent carbon residue formation, known as coking. The transfer line is sized so that the two-phase stream enters the column at close to critical velocity through a feed device designed for severe service applications. A well designed wash bed should reduce and withstand fouling in order to increase column run life, as well as protect the quality of the HVGO product from entrainment from the flash zone. The low operating pressure is instrumental to extracting gasoils from the atmospheric residue.

As part of the company’s initial RIK equipment inquiry, CCRL communicated a desire to increase the column throughput. This opened discussions to look for higher performance tower internals within the framework of an RIK vessel project so that throughput could be increased while maintaining product quality. The column operates as a dry vacuum column (see Figure 1).

Enhanced process technology
Utilising new technology to increase capacity, reliability, efficiency and minimise pressure drop across the tower provides benefits that can be realised in a variety of ways. These include:
- Reduced operating costs from the auxiliary equipment
- Increased product recovery
- Stable operating conditions
- Reduced equipment replacement cycle

While this article specifically talks about the enhancements added in this vessel replacement project, it is important to realise that typical five-year maintenance outages are an opportune time to replace old technology with new designs — and reap some of the benefits mentioned above. Many design enhancements can be explored for vacuum column operation, however focus here is on three areas: high performance packing — grid and structured — and a high performance inlet feed device.

The wash bed, along with the flash zone, is the most critical section of the vacuum column and the optimal design approach can be a source of rigorous debate within the industry. 

Wash bed — Proflux severe service grid
The function of the wash bed is to eliminate entrainment of residue in the feed to the HVGO product, and to provide some fractionation to improve the HVGO end point. The main entrainment concern to be addressed is minimising the amount of micro Concarbon residue (MCR) and heavy metals that end up in the HVGO. The results section of this article will discuss the impact of high performance tower internals on the MCR found in the HVGO product — a lower MCR was obtained at higher throughput for similar feedstocks. 

There are many competing interests in the design of the wash bed and the packing types traditionally used in this application; first generation grid packing and structured packing have limitations. Maximising gasoil product can potentially come at the expense of insufficient wash oil to maintain adequate packing wetting, a key requirement of preventing coke formation. The use of an open, low surface area, first generation grid packing for severe service offers good fouling resistance but does not provide adequate de-entrainment in vessels that are pushed past a moderate operating point (Cs > 0.35 ft/s). The use of a medium crimp structured packing offers high de-entrainment properties and provides good fractionation, but it comes at the expense of less reliability — it is susceptible to fouling, and too much fractionation can cause the bed to dry out the wash oil and promote coke formation. A deep bed provides more efficiency than a short bed does, but it also increases residence time which can lead to increased fouling. The ideal approach is to balance the de-entrainment and fractionation requirements while maintaining reliability and maximising gasoil yield.

In 2009, Koch-Glitsch set out to provide the industry with a better packing product for fouling applications. With hundreds of existing vacuum column installations to draw from, the company looked to provide a packing that offered the de-entrainment characteristics of structured packing while improving upon the reliability of a traditional grid packing. De-entrainment tests that emulated vacuum column wash bed operating conditions were performed in the Koch-Glitsch pilot plant. The results confirmed what operating engineers have experienced in vacuum column wash beds around the world:
- Structured packing provides better de-entrainment than traditional grid packing at moderate and high gas velocities (Cs > 0.35 ft/s).
- For a given packing style, the amount of fouling is proportional to the surface area — higher surface area increases the fouling tendency.
- For the same surface area, the shape of the packing will influence the pressure drop and amount of fouling.

With respect to the last point, testing also confirmed that the structured packing geometry itself contributes to the fouling tendency. Contact points between sheets provide locations for solids to bridge and propagate. Experiments showed a novel structure that employs sheets that are shaped to prevent material from building up, and spacing these sheets apart would minimise the fouling potential.5

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