Dilution steam is an integral component in the process of production of ethylene, propylene and other byproducts via the pyrolysis of hydrocarbon feedstock. Dilution steam promotes the formation of desired olefins by reducing the hydrocarbon partial pressure in the pyrolysis furnace and it extends the run length of the furnace by slowing the rate of coke deposition.
After the hydrocarbon feedstock is pyrolyzed in the cracking furnace, the effluent gases must be rapidly cooled, i.e., quenched, in order to prevent the recombination of the reactive olefins into unwanted mixtures of oligomers, polymers and fused aromatic structures. During this quenching process, steam is condensed and the resultant hot water is used for heat recovery, the water condensate is cooled further to be used in the quenching process, and a portion of the condensate is processed for re-use as dilution steam.
This “steam cracking” process (pyrolysis of hydrocarbon feedstock in the presence of dilution steam) also produces a small quantity of less desirable by-products such as carbon monoxide, carbon dioxide, acetaldehyde, and acetic acid. The organic acids, acetic acid, propionic acid, formic acid, and to a lesser extent higher C4-C6 organic acids promote corrosion in the aqueous environs of the quench water system, the quench water cleaning vessels (oil/water separator, coalescers, process water stripper) and the dilution steam generator. Another contributor to acidic conditions in the “dilution steam system” (a system which includes the quench water system, oil/water separator, process water stripper, dilution steam generator and dilution steam piping) are sulfur-based acids, formed from cracking of sulfur compounds that come with or are added to the hydrocarbon feedstock. These acidic byproducts are neutralized with an alkaline agent.
In many systems, the neutralizing agent of choice for dilution steam systems was caustic, NaOH, and this alkalizer is cost-effective provided that the dilution steam generator has sufficient size or design features that prevent the incidental carry-over of sodium ions with the dilution steam. Low levels of carry-over of sodium with the dilution steam can cause a greater degree of furnace coking and shorter furnace run length, while high levels of carry-over of sodium can destroy the mechanical properties of the furnace radiant tubes (e.g., sodium embrittlement).
To circumvent the hazards associated with sodium carry-over, a large number of ethylene producers have opted to control pH in the dilution steam system with the use of neutralizing amines. Although monoethanolamine (MEA) is a cost-effective amine, it reacts with acetic acid in dilution steam condensate to form MEA-acetate salt. In an aqueous solution, this salt generates a buffered pH condition wherein a small addition of acid does not greatly decrease the pH and a small addition of base does not greatly increase the pH. Even though this buffering condition protects against pH shifts into the more corrosive acidic regime, it also requires use of large amounts of MEA to raise the pH into the protective pH range to avoid solubilization of iron oxides and thereby prevent corrosion.
Even though MEA has a relatively low volatility ratio in a steam boiler, some amine will carry over with the steam phase in the boiler. When an amine, such as MEA, goes to the pyrolysis furnace, the amine is cracked to form ammonia and hydrocarbon by-products. Ammonia is a contaminant for the ethylene product because it poisons the catalysts that are used to produce polyethylene and ethylene copolymers. When ethylene product is off-specification due to ammonia, the ethylene product is sent directly to the flaring system until the product is back on specification. Since ammonia is a base, it can raise the pH in the quench water of the dilution steam system. If the addition of the ammonia is uncontrolled, then the quench water can become too alkaline and can promote stabilized emulsions in the quench oil/water separator, causing premature fouling of the dilution steam generator.
Thus, a need for a more effective neutralizer exists.