Delayed coker design and project execution
This article presents insights into the key considerations for residue upgading and details some of the advances in technology, including online computer control and an innovative water management/coke recovery system
Ram Malik and Gary L Hamilton, ABB Lummus Global
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Crude slates have and will continue to change in both quality and residue content, resulting in a decline in the quality of distillates and resid, while final product specifications are becoming more stringent. For competitiveness, refiners must convert more of the bottoms material to higher value distillate. This requires the proper selection of bottom-of-the-barrel upgrading technology while maximising use of installed investment. A prime focus in selecting a bottom-of-the-barrel processing scheme, such as delayed coking, is flexibility.
Delayed coking units can handle a variety of feedstocks such as petroleum-derived resids, cracked materials (pyrolysis tar and cycle oils), tar sands bitumen, and liquid feedstocks derived from coal. Products have included needle, anode, and other qualities of coke. Typical unit sizes have ranged between 15000 and 50000bpd, with the size of the units trending upward. Lummus was recently awarded the contract to design a unit to handle a feed rate of 124000bpd in its first phase with an ultimate capacity of approximately 180000bpd.
As shown in Figure 1, delayed coking produces LPG, naphtha, middle distillates and coke. The coke can be either fuel or anode grade, depending on the feedstock characteristics. Naphthas, light gasoils and heavy gasoils produced from thermal processes, like delayed coking, require considerable hydrotreating before further use as either feed to downstream process units or as a final product (diesel) in the case of the light gas oils produced.
With advanced hydroprocessing technologies, a refiner can upgrade the delayed coking heavy oils and distillates more easily than previously possible. Coupled with the drive for clean fuels, many refiners are now considering a combination of delayed coking and hydroprocessing to upgrade heavier crude slates to cleaner fuels.
Key delayed coking design features now include online computer control, automatic unheading, double fired heaters, state-of-the-art coke drum mechanical design, and an innovative water management/coke recovery system. Some of the advances in the technology include:
- Special coking heater design that maximises run length and provides high efficiency
- Online heater decoking that provides higher on-stream factor
- Automated flange unheading system to enhance operational safety
- Coke drum mechanical design that maximises drum life even for larger drum sizes and shorter cycles
- Low recycle design for maximum distillate production
- Coke pit/pad and coke drum structure designs to reduce investment and maintenance costs
- API sludge disposal process for sludge disposal capability
- Advanced control systems that result in operating cost savings and recovery of higher value products
- Increased use of interlocks and automation to enhance operational safety and avoid any mishaps.
All processes/technologies by nature have a certain degree of flexibility in their ability to cope with feedstock and product yield and quality changes. This is accomplished through the use of operating variables. Delayed cokers are inherently flexible due to the batch, cyclic nature of the coke drum operation and a design that must accommodate the swings associated with the coking/decoking cycle of each drum.
Delayed cokers, including downstream treating units (distillate hydrotreaters, cat crackers, and hydrocrackers), all need to be capable of addressing the following processing issues: processing different and changing feeds, processing different feed rates, maximising distillate production and producing improved quality products.
To achieve these capabilities, the selection of the unit’s design basis requires a different approach or methodology in order to provide for the desired operating flexibility and to maximise the overall economic benefit. In many cases, this has meant designing to address those previously noted processing issues and others requiring specific attention to unique design considerations. The following presents highlights of some of these key considerations:
Capability to initially process resid from current crude runs but capable of possibly processing more resid later while producing and handling considerably more coke production due to likely higher resid CCR content.
Ability to optimise the process conditions (i.e., temperature, pressure, recycle ratio) required if processing lighter, sweet feedstocks that can be used to produce anode grade coke. Range of process conditions must be such that the design can achieve the proper balance between distillate and coke production for the most economic operation.
To optimise the process conditions required when processing heavier, sour feedstocks in order to maximise distillate production and minimise coke yield. Such operation requires designing for low pressure, higher temperature, and lower recycle ratio. To adjust operating conditions over the coking cycle to suppress shot coke formation on heavier feedstocks when desired, and to handle shot coke when required.
To change (increase) pressure to reduce coke drum superficial velocity, thereby allowing higher feed rate as required.
To set operating pressure to achieve overall optimum economics. Pressure selection must balance lowest pressure to maximise distillate production versus the number of coke drums. Low pressure operation requires careful evaluation of the consequently higher superficial velocities in the coke drums, the requirement for larger equipment and piping, and system pressure drop and pressure profile.
To set recycle ratio to achieve maximum distillate production consistent with meeting the heavy coker gasoil specification (CCR, metals, end point). In designing with low recycle operation capability, the coker design must include special fractionator and internals design.
To provide the required flexibility in the coke drum design to achieve increased capacity via shorter cycle time, proper drum life through consideration for the range of heating/cooling frequencies expected and design drum accordingly, and optimised design for best results, taking into account coking cycle lengths, operating conditions, feed rate, coke yield, and the number and size of the drums.
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