Naphtha cracking for light olefins production
As an alternative to steam cracking, an FCC-type process provides on-purpose production of propylene
Michael J Tallman and Curtis Eng, KBR
Sun Choi and Deuk Soo Park, SK Energy
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Annual worldwide growth in the demand for propylene is expected to exceed 5% over the next several years. Steam crackers currently produce approximately 60% of the world’s propylene as a by-product of ethylene production. The amount of propylene available to be produced, however, is â€¨limited to a typical weight ratio of approximately 0.4 to 0.6 parts of propylene per part of ethylene, and only when cracking heavier feeds such as naphtha and gas oil. The balance of propylene is primarily supplied from refinery sources, mostly as a by-product from FCC units producing fuels (gasoline and diesel).
Since the ethylene market is expected to grow at a slower pace than that of propylene, and since many of the new steam crackers being built utilise ethane as a feedstock (mostly in the Middle East), which does not produce any propylene, propylene supply from ethylene expansion is not expected to meet demand. Similarly, FCC operations are driven by fuel demands, and new FCC units will not fill the demand either, although some refiners will gravitate toward higher severity operations to increase production and fill a portion of the need. Therefore, new sources of propylene will be needed to meet expected future demand.
Catalytic processes for propylene
Currently, steam cracking and â€¨refinery operations account for approximately 94% of the propylene produced today. Refinery FCC units can boost propylene production through the use of catalyst additives and by higher severity operations.
KBR has a suite of technologies that target propylene as a primary product; the technology of choice is dependent on the type of feed available. These include Superflex technology, a commercialised process originally developed by LyondellBasell for increasing propylene production from olefinic by-product streams from steam crackers or refinery processes; Maxofin, a high-severity FCC process for increased propylene production from traditional refinery sources such as gas oils and resides; and the Advanced Catalytic Olefins (ACO) process for enabling increased propylene production from straight-run paraffinic feeds. This article will focus primarily on the ACO process.
Features of KBR FCC
KBR’s catalytic olefins processes, such as ACO, utilise hardware similar to the company’s refinery FCC units. These units catalytically crack heavy feeds such as gas oil and resid in a riser to lower molecular weight products, such as gasoline, diesel and kerosene.
The reactor (converter) comprises four sections:
• Riser/reactor, where the cracking reactions take place in the presence of catalyst
• Disengager, where catalyst is separated from product gas through the use of cyclones
• Stripper, where cracked gas contained in catalyst pores is removed via stripping with steam or nitrogen and routed with the other product gas
• Regenerator, where coke formed on the catalyst during the cracking process is removed by combustion with oxygen, supplying heat of reaction for the cracking process.
Although mechanical modifications to KBR’s FCC system are made to accommodate the particular operating conditions for ACO, the functionality is not changed. Note that no feed pretreatment is required because of the nature of the feed utilised and the catalyst system employed.
Accessory systems for the reactor are standard FCC systems and include catalyst storage, air supply, flue gas handling and heat recovery.
The feed is introduced at the bottom of the riser and mixed with hot, regenerated catalyst. The feed is vapourised and the reactions take place as the feed gas and catalyst flow upward in the riser. At the end of the riser, the product gas and catalyst are separated in cyclones, housed in the disengager. The catalyst is then routed to the stripper, where product gases still entrained in the catalyst pores are stripped with steam or nitrogen and routed with the reactor effluent. Stripped catalyst is then routed to the regenerator, where air is introduced and coke that has formed on the catalyst during the cracking operation is burned, regenerating the catalyst for reuse in the riser and supplying the heat of vapourisation and heat of reaction in the riser. Accessory systems for the FCC unit include air supply, flue gas handling, and heat recovery and catalyst storage. Reactor overheads are typically routed to the primary fractionator and subsequent product separation and recovery.
KBR’s FCC units use a stacked configuration known as Orthoflow. In this configuration, the disengager is stacked above the regenerator rather than side-by-side as with other designers’ units. The advantages of the Orthoflow configuration are several: first of all, the plot space required is much smaller for this configuration compared to the side-by-side unit. Foundation and structural costs are also reduced. The Orthoflow configuration enables the unit to be fabricated and dressed off-site and set into place with one lift, saving on construction costs and requiring less welding in the field. The stripper is submerged within the regenerator, which reduces the vertical height of the unit and thus the cost. A schematic of a typical FCC unit is shown in Figure 1.
There are several other features of KBR’s FCC reactor system that can also be used in the ACO process, including the dual riser, closed cyclones and third-stage separator:
Dual risers The use of dual risers was quite common during the early stages of FCC development. The primary reason for dual risers at that time was one of scale-up; that is, to design commercial risers that had similar flow characteristics as tested in the pilot plant. For several process reasons, dual risers can be used in ACO and Maxofin applications
Closed cyclones Closed cyclones minimise the residence time of hydrocarbon vapour and catalyst in the disengager, thereby eliminating post-riser thermal and catalytic cracking. Less valuable products are destroyed, leading to more valuable products.
Third-stage separator In regions where there is a stringent requirement to control particulate emissions, the third-stage separator has proven to be effective.
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