Crude oil is a complex mixture of hydrocarbons, sometimes characterized as "a useless mixture of useful products." Petroleum coke is, essentially, the "bottom of the crude barrel" - the carbon in the crude charge that cannot be recovered in normal refining processes - comprising about 5-7 wt% of each barrel of crude.
Crude Selection & Calcined Coke Quality
A refiner must carefully balance crude characteristics against refining unit capacities and product slates. If the facility produces anode-grade carbon, other impacts must be considered. Consistent quality coke begins with consistent quality crudes; "trim" crudes introduced at the "front end" of the refining process to control eventual carbon quality results in much more consistent finished carbon product to the end user/smelter as opposed to trying to blend solids (green and/or calcined) to spec. "after the fact".
The first step in any refining process is to fractionate the crude oil charge into intermediate and finished products:
The crude charge is first water-washed in a "desalter(s)" to remove solids and salts, and then separated into various fractions (by boiling point) in an "atmospheric distillation" unit - here, the Crude Unit. The desalted crude charge is preheated, and is then vaporized in a fired heater. The vaporized charge is fed to a distillation tower, where it cools and condenses as it flows up the tower. "Intermediates" (feedstock's for further processing in downstream units) and finished products are withdrawn from the tower at various points.
The "bottoms" from the first distillation of the crude charge ("reduced crude") are routed to another unit, where the charge is reheated, and subjected to a vacuum in another distillation tower. The vacuum causes the charge to boil at a lower temperature than would be the case at atmospheric or positive pressure; this allows distillation of additional intermediates without thermally decomposing the still-valuable reduced crude to carbon (or coke).
The bottoms from the vacuum distillation operation cannot be further distilled; instead, the "vacuum residuum" is routed to the next unit in the process, the Coker.
The operation of the Crude and Vacuum units can have a tremendous impact on finished calcined coke quality:
Desalting: While desalting the incoming crude charge is intended primarily to prevent corrosion in downstream piping, equipment, and process units, finished carbon products benefit from reduced levels of iron (less corrosion products = less iron in the finished calcined coke).
Distillation: Proper distillation in the Vacuum Unit is critical to resid (Coker feed) quality control. Specifically, the amount of vacuum gas oil sent to the Coker with the vacuum residuum (determined by the efficiency of the fractionation in the Vacuum tower itself) must be carefully monitored and controlled to insure adequate hardness of the green coke produced by the Coker. An "optimal" resid yield for a refiner more concerned about downstream catalytic units may not necessarily be optimal for eventual calcined coke properties!
In a delayed coker, the incoming resid charge is mixed with coker gas oils (CGO), and the resultant bottoms stream is fed to a fired heater(s). In the heater(s), this heavy oil charge is thermally cracked into vapor and liquid. The vapor/liquid effluent from the heater(s) flows into a coke drum, where the liquid drops out and solidifies (eventually filling the drum), and the vapors are returned to the fractionation process. By progressively condensing the vapor coming from the Coke Drums in a distillation tower(s) (here, the Fractionators), useful intermediates are collected for further processing.
Coke drums operate in pairs with an associated heater. While one coke drum is filling, its twin is being drilled. When one of the drums is full, the effluent from its associated heater is rerouted into the empty drum of the pair. The full drum is isolated and cooled, and the collected "green coke" is drilled out of the drum with high-pressure water. Once the drum has been completely drilled out, it is preheated with effluent (vapors) from the associated heater, and the heater effluent is switched into it so the other drum can be drilled.
The green coke is collected in a containment basin, where it is allowed to drain. The green coke is then reclaimed from the "coke pit" with clamshell bucket ganty cranes, and conveyed to covered storage facilities.
The best last step in calcined product quality control is control of the green coke feed quality. Coker operations can impact coke quality in a number of ways:
Coking is essentially a time-temperature phenomena; the longer the coking cycle, and the higher the temperature, the harder the resulting green coke. Harder green feed translates to larger calcined product. However, longer cycle times limit Coker (and, consequently, crude) rates, and higher heater outlet temperatures result in shorter run lengths for the Coker heaters (which must be periodically taken off line for decoking of the heater tubes).
A Coker that is more concerned with liquid yields than with green coke quality will operate at lower heater outlet temperatures (to minimize wear and tear on the heaters and maximize run lengths between decokings) and shorter cycles (to maximize crude and coker feed rates), both of which adversely affect finished coke properties. Heater outlet temperatures must also be carefully adjusted throughout each fill cycle to maximize consistency (specifically, hardness) of the green coke in the drum from bottom to top.
Throughput (or Recycle) Ratio
The ratio of heater feed (combined heavy coker gas oil and fresh resid) to fresh resid feed (only) is referred to as throughput, or recycle, ratio. If a Coker is optimized around liquid yields (gas oils, for downstream upgrading in a hydrocracker and/or fluid catalytic cracker), this ratio is minimized (to minimize the amount of valuable gas oil coked). However, higher recycle ratios may be desirable if coke quality control is the end goal. Higher recycle ratios, while "destroying" highly-valued (gas oils) feedstocks, also reduce metals and sulfur levels in the green coke.
Consistency of drilling technique is another key to consistent coke quality, specifically, sizing. Longer drilling times result in smaller, easier-to-handle green product. Shorter drilling times, conversely, result in larger green coke feed, which translates into larger calcined product.
Surface contaminants can have a tremendous impact on air reactivity and other coke characteristics. Often, cokers use drilling as a means to recycle or dispose of waste water. Careful control of drilling water chemistry and scheduled replenishment of the drilling water supply with clean, fresh water is crucial to controlling surface contaminants.
In transferring green coke to handling, and on to calcining, several things can impact the quality of the final (calcined) product. If the green coke is handled repeatedly by mobile equipment, sizing can be negatively impacted by crushing, dropping, and the like.
If the green coke feed is stockpiled in the open, surface contaminants can also become an issue. In addition to windborne silica and other pollutants, any material applied to the coke pile(s) for dust control, such as water sprays, can introduce sodium and other contaminants.
To protect final product quality, green feed is best handled by dedicated equipment (e.g. gantry cranes and conveying the feed to storage, as opposed to reclaiming and transporting via front-end loaders), and should be conveyed and stored under cover from the environment (e.g. enclosed conveyors and inside storage).
In the calcining process, the green coke feed is heated to a sufficiently high temperature to drive off any residual moisture, and to drive off and combust any residual hydrocarbons (the combustion of the evolved volatile materials provides the necessary heat for the calcination process) in the green coke feed.
At Cherry Point, this is accomplished in three rotary hearths. The hearths are comprised of fixed roofs over rotating tables. Fixed ploughs ("rabbles") in the hearth roofs gently move the material in a spiral fashion from the perimeter of the hearth, where the green feed is introduced, to the center of the hearth, where the product is withdrawn. Unlike kilns, rotary hearths rely solely on volatiles in the green feed and preheated combustion air to calcine the green feed; no "external" fuel (i.e. burners) is used. After cooling (and, for customers requiring it, oiling for dust control), the calcined product is routed to weather-tight silos for storage prior to shipment to the end user.
Cherry Point produces high RD (2.070 g/cc), high bulk density calcined coke.
At Wilmington, Gelsenkirchen and Lingen, this is accomplished in conventional rotary kilns. For each Calciner, green coke feed, produced by BP's nearby Refineries, is stored under cover in a Coke Storage Barn and at Wilmington and Gelsenkirchen a feed reclaimer is used to ensure a consistent feed size is processed by the calciner. At Lingen, the green coke is segregated by size in various silos and reblended as calciner feed for feed consistency. As opposed to rotary hearths, which rely solely on volatiles in the green feed and preheated combustion air for calcination, rotary kilns commonly employ large fuel gas burners or oxygen injection at the "downhill" end of the kiln to calcine the green feed. The finished product is in turn transported by truck, conveyor or rail to the Port for export, or is loaded into railcars for shipments to domestic customers.
The calcining operation is the last, and arguably one of the least, influential tool to control the eventual quality of the carbon product. No calcining operation can compensate for poor feed resulting from upstream operations! There are several considerations involved in any calcining operation:
Feed consistency: Blending solids to meet green feed and product specifications (vs. blending crudes at the "front end" of the refining process), and careful control of upstream (crude distillation and coking) operations to insure consistent, high-quality green feed.
Feed handling: Reclaiming feed with dedicated equipment (e.g. bridge reclaiming from covered storage) vs. front end loaders or other heavy equipment, enclosed handling systems, covered feed storage to prevent atmospheric contamination, etc.
Calciner operation: Consistency and control of operations, application of statistical process controls, quality and intrinsic process control capabilities of the equipment and units.
Consistent quality carbon begins with consistent quality feedstocks and upstream process unit operations;no calcining operation can turn poor quality or inconsistent feed into consistent or high quality calcined product! The most leveraging controls for eventual calcined coke quality are all upstream of the calcining operation; only an integrated refining operation - where coke quality is considered, day-in and day-out, right along with liquid product yields and quality - offers the most consistent carbon.