Quench water pretreat process

Details Quench water pretreat process Quench water pretreat process

2002-02-12

2003-06-10

Drodge, Joseph (Department: 1723)

Liquid purification or separation

Processes

Liquid/liquid solvent or colloidal extraction or diffusing...

C210S774000, C210S806000, C095S264000, C585S809000, C585S833000

Reexamination Certificate

active

06576132

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a method for treatment of a net quench water stream and in particular to treatment of same stream for removal of organic contaminants prior to its disposal to the environment or its reuse for dilution steam generation required in ethylene plants.
BACKGROUND OF THE INVENTION
Base petrochemicals, such as ethylene and propylene, are largely produced by steam cracking of saturated hydrocarbon (H/C) feedstocks. In the endothermic cracking process, H/C plus steam diluent are decomposed between 750° C. and 900° C. by fundamental free radical reactions initiated by the rupture of the C—C covalent bond. Beyond the primary reaction products of ethylene and propylene, many other co-products are also produced in varying quantities including H
2
, paraffins, olefins, acetylenes, diolefins, cyclics, aromatic compounds and coke together with CO, CO
2
, H
2
S and a series of organic sulfur compounds. The composition of the pyrolysis effluent cracked gas (CG) varies with feedstock composition and severity of steam cracking.
The low molecular weight pyrolysis effluent products are reactive at high temperatures and will undergo further reaction to less desirable secondary reaction products unless the reaction temperature is rapidly reduced to below about 200° C. The hot furnace CG is therefore rapidly cooled in Transfer Line Exchanger (TLE) that generates very high pressure (VHP) steam used for power production within the ethylene plant.
For gaseous feedstocks (ethane, propane and butanes), a Quench Oil Tower (QOT) is not required because only small amounts of C5
+
liquids are produced. For these feedstock types, a simple Quench Water Tower (QWT) is used to cool the effluent gas from the TLE.
The CG is cooled in the bottom of the QWT to near its adiabatic saturation temperature causing condensation of tars and other heavy oily components in the CG. The CG is further cooled by contact with recirculating quench water (QW) as it flows up the QWT, thereby condensing most of the dilution steam and part of the H/C in the CG. The recirculating QW leaving the QWT carries all condensed H/C components both dissolved and separate phase in the form of tars and oils as well as coke and complex oligomers and emulsions.
Water is highly suitable for quenching purposes because it is both an effective heat transfer media and inexpensive. The employment of water in the quenching operation, however, has one great attendant disadvantage, after treating the furnace cracked gas with water, the quench medium contains significant amounts of dissolved and emulsified hydrocarbon oils, as well as heavy tar-like polymers and coke particulate matter. The oils are comprised of aromatic hydrocarbons and light polymers. These materials form stable oil/water emulsions when the cracked gas stream is intimately mixed with the quench water. The resulting emulsions comprise from about 2000 to more than 6000 parts oil per million parts emulsion. The stability of the emulsion is apparently due, at least in part, to a mutual affinity between the unsaturated hydrocarbon components in the dispersed oil phase and the continuous aqueous phase. Thus, the emulsion will resist efforts to separate it sharply into its various phases.
The QW from the bottom of the QWT is settled in an Oil-Water Separator (O/WS) that has three compartments in series separated by weirs, the heavy tar and solids is withdrawn from the 1
St
compartment, the raw QW from the 2
nd
and the light pyrolysis gasoline from the 3
rd
respectively.
The raw QW, from the O/WS, still contains residual fine particle solids, unsettled free oil, emulsified oil, and dissolved H/C's. Most (90-95%) of this raw QW at 90° C. is recirculated for low-level heat recovery in the plant before returning to the QWT. The net (discharge) raw QW is either: (1) used to generate dilution steam for steam cracking as a closed loop system, or (2) purged to battery limits as an open-loop system.
This net raw QW discharge can be pretreated to remove the residual suspended solids, and free and emulsified oil in order to prevent and/or reduce fouling in a downstream closed dilution steam generation system. On the other hand, if the excess raw water were simply purged to battery limits, it would be desirable to purify this water to such an extent that it could be discharged into local streams without causing pollution. Sufficient impurities present in the wastewater would adversely affect riverways, oceans, aquifers, fish and other wildlife.
Because ethylene plants cracking gas feedstocks do not have a QOT prior to the QWT, quench water in these plants is characterized by being more fouling service and more susceptible to emulsion formation than its counterpart in liquid cracking plants. A particular problem is the entrainment of fouling species in the quench water slipstream to the dilution steam generator (DSG).
The feed to the QWT is the furnace cracked gas. The QWT is also a dump for many other recycled streams, both continuous and intermittent, which may cause changes in the surface properties of the water as well as its pH. A low pH (<4.5) or a high pH (>9.5) makes it difficult to separate the emulsified oil. In addition, a low pH raises corrosion concerns, and a high pH increases foaming tendencies and causes difficulties in oil/water separation.
Spalled coke and coke fines from furnace transient (decoking) conditions reaches the QWT, which suspends in both the oil and water phases. Tars and heavy oil in furnace effluent streams are also contained in the bottom section of the QWT. They are heavier than water and settle down. In the upper section of the QWT, lower MW hydrocarbons condense and separate as light oil. The combination of the tar, heavy oil, and polymers with the coke fines makes a gummy agglomerate that causes fouling and blockage of the trays and other internals.
Unsaturated reactive polymer precursors such as styrenes, indenes and dienes have appreciable solubility in the water phase, making them difficult to separate from the quench water using conventional separation techniques. Further, these components tend to polymerize when exposed to high temperatures encountered in downstream systems. Thus, it would solve a long felt need in the art if an effective method for removing these soluble components from the QW could be found.
In conventional systems, the condensed dilution steam/hydrocarbons and circulating quench water from the QWT are phase separated in an Oil/Water Separator. In gas crackers this separation is difficult because of small difference in specific gravity and large potential for emulsion formation. Free and emulsified oil carried with the water to the low pressure water stripper (LPWS) and dilution steam generator (DSG) contain polymer precursors that cause fouling of these towers.
To minimize heavy oil/tar carryover with the QW to the LPWS and DSG one or more of the following traditional systems has typically been used in the past:
Addition of gasoline to enhance phase separation (emulsion breaking).
Hydro-cyclone.
Filter—Coalescer.
Dispersed Oil Extractor (DOX) system.
Induced Gas Floatation (IGF) system.
The Dispersed Oil Extractor (DOX) system is an industrial system used to remove emulsified oil and suspended solids from the quench water. The system consists of a primary granular media coalescer filled with a multi-layer of different size granular material, followed by a vertical coalescer filled with carbon media that further coalesce the oil. The oil coalescence is finished in a horizontal performax separator containing a matrix plate section and a separation section that allows the separation of the three phases (light oil, treated QW and heavy oil). This system does not remove dissolved hydrocarbons from the QW.
Strausser et al., U.S. Pat. No. 3,507,782, describes a process for the purification of plant process wastewater by separation of dissolved and emulsified hydrocarbon from aqueous media. The dispersed phase of stable emulsions comprising aromatic hydrocarbon-containing oils in aqueous m

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