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Apr-2009

Reliability vs recovery for delayed coking fractionators

When selecting tower internals, numerous choices are available for coker main fractionator configurations, each offering benefits and trade-offs. Some of these choices are discussed along with insight into the internals features that can be provided in any coker fractionator

Darius Remesat
Koch-Glitsch Canada
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Article Summary
The increased processing, now and in the future, of lower quality, heavier, more sour, less stable crudes, together with the rise in demand for refined products, makes it necessary to upgrade as many of these crudes as possible to maximise refinery economics.

Delayed cokers have proven to be effective at decarbonising and demetallising heavy petroleum residues. This refining process can provide 20–40% of the downstream hydroprocessing feedstocks. In general, cokers producing fuel-quality by-product coke are operated to maximise liquids and minimise coke. Optimisation of the coker operation is crucial to any clean fuels programme. Having the delayed coker produce clean hydroprocessing feedstocks may compete with the need to improve coker operations to process heavier feedstocks and/or increase capacity. The economic and reliable operation of these units requires a review of feedstocks, including the properties of cracked feeds from delayed coking operations, and the right choice of internals.

The coker fractionator plays a key role in ensuring the unit delivers the products as desired and operates reliably for the required run length.1

Main fractionator
A coker usually consists of one or two sets of coke drums and a main fractionator (Figure 1). The coker main fractionator separates the unstabilised naphtha and heavier products. Figure 2 illustrates a typical main fractionator configuration including pumparounds and side strippers. There are two main sections in the main fractionator: the wash zone and the fractionation zone.2 The upper section of the column is the fractionation zone used for separating the main fractionator products, such as naphtha, kerosene, light gas oil (LGO) and heavy gas oil (HGO).

The lower section of the column (below the HGO draw tray) is the wash zone. An additional sub-section, called the quench zone, can be provided in the wash zone, to deal with high severity coke drum designs for very heavy, unstable crudes that generates a high proportion of coke fines. The quench zone protects the internals in the wash zone from the coke fines in the feed vapour.
    
Wash zone considerations
The wash zone has three objectives:
—  To control the heavy tail of the HGO distillation
—  To minimise entrained coke fines in the main fractionator products (mainly HGO)
—  To optimise product yield by setting the recycle cut point.

The design of the wash zone requires trade-offs between HGO product quality, unit reliability, yields and unit throughput, and involves two basic choices: wash oil rate and fractionation hardware. High wash oil rates improve HGO product quality by removing more of the heavy tail and coke fines, and improve column reliability by mitigating the tendency for coking. However, high wash oil rates reduce unit capacity, since the wash material ends up in the product bottoms and contributes to the recycle load on the heater charge.

The wash zone can have various configurations, with Figure 3 illustrating four typical arrangements. Higher wash zone efficiency is countered by reduced fouling resistance, with the spray chamber (Design A) considered the highest fouling-resistant and lowest efficiency configuration. There is minimum opportunity for coking, since this design provides a minimum surface area for coke particles to adhere to.  Reliability is normally better than with the other designs, but the recovery of heavy coker gas oil (HCGO) is reduced. Design B, with shed decks, attempts to use a rudimentary separation device (eg, 10–20% tray efficiency per shed row) to gain efficiency while maintaining run length. As with Design B, Design C uses a grid to improve wash zone separation efficiency.

Design D shows a traditional high-efficiency approach, with trays in the wash zone requiring a high recycle rate (typically maintained above 15% to prevent very short run lengths), and produces the best quality HCGO. This design has the lowest yield and is the most prone to coking. A quench zone is considered most often with trays due to the low reliability of the trays.

Data has been gathered and compiled from operating units for the various configurations used in the coker fractionator wash zone. Figure 4 takes this industrial feedback, plotting observed run lengths against average wash zone efficiency, measured as vanadium in the HGO product stream. Although there are many variables (type of crude [eg, contaminants, stability], coke drum operation and wash rates) that impact the efficiency and reliability of an operating unit, the data shown in Figure 4 provide some insight into each configuration. All the designs show  both successes and failures.

The underlining message is that if the design objectives are clear and match the actual operation, a wash zone configuration can be successful. One design is not better than any other; they just address different design objectives. However, you should not necessarily settle for low wash zone efficiency to increase your chances of a desired run length. It appears that each design can create a desired run length, and there is a difference in efficiency among the offerings. Spray chambers and shed decks offer comparable performance, while grid and trays provide improved wash zone efficiency. The impact of wash oil flow rates needs to be factored into this assertion. Included in the data are cases where unconventional, unstable crudes (such as oil sands) have a grid in the wash zone and have provided favourable results. The operator’s experience level, economic objective and risk/reward philosophy dictate which configuration is used and it is the design team’s responsibility to maximise the economic objective.

Wash zone section designs depend on the recycle and HCGO quality targets. Increasing the liquid volume yield requires minimum recycle. Therefore, for high reliability, high yield, with minimum concern for product quality, spray chambers are used. However, as noted in Figure 4, they have very low efficiency and, once column vapour capacity (noted as C-factor) rises above 0.26–0.28 ft/s, entrainment of non-distillable material and coke fines into the HCGO increases.

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