Low molecular weight hydrocarbons and olefins can be produced in a pyrolysis furnace which provides heat sufficient to break chemical bonds in higher molecular weight hydrocarbons. The hydrocarbon feedstock to be cracked in a pyrolysis furnace usually consists of hydrocarbon gases such as ethane, propane, butane, or hydrocarbon liquids such as naphtha, kerosene, gas oil, or other available hydrocarbon feedstock. Hydrocarbon products produced when cracking gas feeds can include olefins such as ethylene and propylene, coke, and gasoline range hydrocarbons (C5+). Hydrocarbon products produced when cracking liquid feeds and heavier feedstocks can also include light and mid-range hydrocarbons, as well as coke and other heavy oils. To help control the cracking process, steam is typically used to dilute the feedstock hydrocarbon in the pyrolysis furnace; the amount of steam used can be characterized by the ratio of steam to total hydrocarbon fed to the pyrolysis furnace, hereinafter referred to as the steam to hydrocarbon weight ratio. After the cracking reaction, the resulting pyrolysis furnace effluent is typically cooled through indirect heat exchange to produce high pressure steam, and can also undergo a second indirect heat exchange to produce medium or low pressure steam, or other methods of heat recovery.
Typical downstream processing for cracked ethane/propane feed includes a water quench tower to cool the cracked gases and to condense and separate the dilution steam and gasoline from the lighter hydrocarbons. Typical downstream processing for cracked naphtha or other liquid hydrocarbon feedstock can also include a fractionator to separate the heavy and mid-range hydrocarbons from the steam, gasoline, and light-end hydrocarbons, upstream from the water quench tower. The water quench towers are also used as a source of low level heat to supply hot water for process heating.
One example of the downstream processing of effluent from a hydrocarbon cracker includes a steam diluted cracker effluent immediately cooled to a temperature below 650° C. (1200° F.), sufficient to stop the cracking reaction, through direct heat exchange with water, steam, or oil introduced through a primary ejector. One or more indirect heat exchangers are then used to recover heat and to produce high, medium, or low pressure steam prior to feeding the effluent to a fractionation tower or a quench tower. A secondary ejector is also disclosed, which can be used to cool the process stream to the desired fractionation tower or quench tower inlet temperature downstream of the indirect heat exchangers.
Another second example of the downstream processing of effluent from a hydrocarbon cracker includes a steam diluted cracker effluent cooled by direct and indirect heat exchange upstream of a fractionation tower. The overhead vapor product from the fractionation tower is then fed directly to a water quench tower to separate gasoline range hydrocarbons from ethylene and propylene.
Due to the conventional design of the water quench process, existing water quench towers are capacity-limited. A need exists to increase the capacity of new or existing water quench towers.
The embodiments are detailed below with reference to the listed Figures.
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