Effluent Caustic Treating Systems Utilizing MERICON^SM Technologies (cont)

6. MERICON^SM Technologies

It is evident from Table 5 that the MERICON III^SM technology applies to treating all types of caustic combinations given certain parameters. Basically, this deep neutralization technology is required for treatment of naphthenates, but at the same time can handle the other two common types of effluent caustic. However, MERICON I^SM and MERICON II^SM technologies can be utilized just as effectively for used caustic streams that are sulfidic in nature. The advantage of MERICON II^SM over MERICON I^SM is its ability to provide total oxidation of sulfidic and cresylic compounds.

 

6.1 MERICON I^SM - Partial Oxidation

The MERICON I^SM system is a non-catalytic process designed to treat caustic effluent streams containing sodium sulfide, sodium bisulfide and sodium mercaptide by partially oxidizing these compounds to sodium sulfate, sodium thiosulfate and DSO. The oxidized caustic solution is then neutralized with acid to obtain a neutral pH prior to being discharged into the WWTP. An offgas stream and an acid oil stream are the only two byproducts generated by the MERICON I^SM process.

6.1.1 Process Description

6.1.1.1 Oxidizer and Scrubber Systems (Figure 1)

Sulfidic caustic is pumped from tankage to the oxidizer tower. Prior to entering the tower the caustic is first preheated through feed/product exchangers and then contacted with steam in a proprietary static mixer. Steam is added on automatic temperature control to raise the temperature of the sulfidic caustic solution to promote oxidation.

After being adequately heated, the sulfidic caustic enters the bottom of the oxidation tower where it is aerated via a proprietary air distributor. Oxidation air first passes through a set of air filters to prevent solids from plugging the air distributor. The oxygen reacts with sodium sulfide and sodium bisulfide, converting them to sodium thiosulfate and sodium sulfate according to reactions 1 and 2. The extent of total oxidation to sodium sulfate is related to the severity of oxidation. Since these are highly exothermic reactions, the caustic heats up as it flows upward through the oxidizer tower. The maximum operating temperature is limited by equipment metallurgy in terms of design temperature. The temperature rise in the tower is quenched, if necessary, by adding process water ahead of the inlet air distributor.

Any sodium mercaptide present will be oxidized to DSO as expressed by reaction 3. A small amount of this DSO is stripped out of the caustic by the oxidation air. DSO is only slightly soluble in caustic, however, the insoluble DSO requires long periods of time to separate because of the slight density difference between DSO and caustic. The DSO remaining in the caustic is removed in the neutralization system.

Oxidation air is added through an inlet air distributor. Proprietary air/caustic redistributors provide recontacting for highly efficient oxidation. The desired degree of oxidation dictates how much recontacting is used. The oxidized caustic exits through a chimney tray situated at the top of the tower. This tray assembly is designed to provide a good separation between the liquid and gas phases. The chimney tray level is maintained by adjusting the flow of oxidized caustic leaving the oxidizer tower. Before proceeding to the neutralization system, the oxidized caustic is cooled by the feed/product exchanger and water cooler.

After disengagement from the caustic in the chimney tray, the offgas is vented to a caustic scrubbing system on manual differential pressure control. The offgas is mainly composed of nitrogen and unreacted oxygen. Some hydrocarbons, mercaptides and disulfide oils may be present as a result of being stripped from the oxidized caustic by the oxidation air. The offgas enters the caustic scrubber through a proprietary FIBER-FILM^TM Contactor where it is contacted continuously with recycled caustic for the purpose of cooling the offgas and removing odorous trace impurities. Caustic is recycled from the bottom of the scrubber on manual flow control and any accumulation in aqueous level is removed on automatic control. A recycle caustic cooler transfers the heat duty removed from the offgas. A demister pad is installed in the top of the scrubber to remove caustic droplets entrained in the offgas. The scrubbed offgas is vented through a backpressure control valve to a safe and elevated location consistent with plant standards.

Caustic is replenished in the scrubber on a batch basis using fresh or oxidized caustic on a frequency to prevent odors in the offgas. Upon changeout, the caustic from the scrubber is sent back to sulfidic caustic tankage.

6.1.1.2 Neutralization System (Figure 2)

After cooling, the oxidized caustic is contacted with fresh sulfuric acid via an inline static mixer to neutralize the treated caustic to a pH between 6 and 8. This mixture is routed to a brine/acid oil phase separator to provide sufficient residence time for the neutralization of free caustic and any remaining mercaptides (refer to reactions 9 and 10).

Depending on the required specifications of the brine effluent stream, DSO can be removed from the treated brine by gravity separation or by washing the treated brine with a suitable solvent. DSO accumulation in the neutralizer tower is withdrawn on a batch basis and utilized as a fuel oil blendstock or sent to a hydrotreating unit for future processing.

The brine effluent leaving the separator is pumped through a product cooler to remove the heat of the exothermic neutralization reactions. A portion of the cooled brine effluent is recycled back and combined with the incoming oxidized caustic to dampen pH fluctuations. MCRS employs a proprietary pH control scheme to maintain the brine effluent between a pH of 6 and 8. The remaining portion of the cooled brine effluent stream is routed to the WWTP on level-to-flow control.

Nitrogen or a fuel gas stream on manual flow control is added to the separator to purge acid gases to the flare or incineration as they accumulate in the top of the separator.

6.2 MERICON II^SM—Total Oxidation

The MERICON II^SM system, also non-catalytic, is the technology MCRS recommends for total oxidation of sulfidic and cresylic caustic solutions. This process is a high temperature, high pressure and high efficiency air oxidation operation that converts sodium sulfides and mercaptides to sodium sulfate and bisulfate. It also transforms cresylics and light aromatics to organic acid salts, in the presence of excess NaOH, to reduce COD and BOD levels.

6.2.1 Process Description

6.2.1.1 Reactor System (Figure 3)

The sulfide- and cresylate-bearing caustic solution is pumped from tankage on flow control through a set of aqueous basket strainers to remove large particles. The caustic solution is then mixed with oxidation air using a proprietary air sparger and sent to the feed/product exchangers. The oxidation air is filtered of any large particulates prior to contacting the caustic.

The preheated solution is further heated in the first oxidation reactor to a temperature that initiates the oxidation reaction. For this purpose, a steam heater comprises the lower section of the first stage oxidation reactor. Steam is automatically injected as needed to maintain the temperature required for complete oxidation to sodium sulfate. Steam addition depends on the incoming caustic sulfide concentration and the resulting exothermic temperature rise. The oxygen reacts with sulfidic compounds, cresylates and trace hydrocarbon according to reactions 4 through 8. Since these are highly exothermic reactions, the caustic heats up as it flows through the oxidation reactors.

The partially oxidized caustic enters the second oxidation reactor in series to complete the oxidation reactions. A proprietary air/caustic redistributor is installed in the bottom of the second reactor to increase the efficiency of the reaction. The completely oxidized caustic solution then overflows out of the second reactor and is cooled by feed/product exchangers and a water cooler before entering a separator vessel.

Before the offgas leaves the separator, it passes through a specially designed coalescer located in the upper section of the separator to remove any entrained liquid. The offgas will contain mainly unreacted oxygen and nitrogen and will be saturated with water. The offgas is vented on backpressure control and, alternatively, can be sent to a FIBER-FILM^TM water scrubber to remove any traces of hydrocarbon and odorous compounds before releasing it to a safe disposition.

6.2.1.2 Neutralization System (Figure 2)

The oxidized caustic, from the bottom of the offgas separator, is contacted with fresh sulfuric acid via an inline static mixer to neutralize the treated caustic to a pH between 6 and 8. This mixture is routed to a phase separator to allow sufficient residence time for the neutralization of free caustic. A cooler is also provided to remove the heat generated by the neutralization reactions to ensure the brine effluent does not exceed the maximum rundown temperature. The brine effluent then proceeds to the WWTP on level control.

A fuel gas purge on manual flow control is provided to sweep any acid gas accumulation at the top of the neutralizer tower.

6.3 MERICON III^SM—Deep Neutralization

Used caustic solutions containing sodium naphthenates are not candidates for oxidation because of the foaming tendency of naphthenates. For these types of solutions, MCRS recommends the use of its MERICON III^SM—Deep Neutralization technology. MERICON III^SM is also used to treat mixed used caustic streams. This process uses sulfuric or hydrochloric acid and treats these streams in a low pH reactor. Hydrogen sulfide and mercaptans are liberated in the reactor and sent to an incinerator or sulfur plant. The cresylic and naphthenic acids sprung in the reactor can be skimmed off or washed with a suitable solvent to meet the required effluent brine specifications.

MCRS also provides an enhanced phenol removal process. This technology utilizes a proprietary solvent that is regenerative and capable of extracting phenols to significantly lower levels compared to conventional solvents. This technology is discussed further in section 6.3.1.3.

6.3.1 Process Description

6.3.1.1 Deep Neutralization and 3—Phase Separation (Figure 4)

The feed caustic stream first passes through one side of a parallel set of basket strainers to remove relatively large particles. The removal of particulates prevents accumulation of solids within the separator and the stripping column, and reduces the maintenance associated with this equipment.

The feed caustic stream then enters the reactor through a nozzle located on the bottom head of the reactor. Sulfuric acid flows into the reactor through a separate nozzle also located on the bottom reactor head. The acid addition rate is controlled to maintain a reactor outlet pH of between 2 and 4. A sufficient amount of acid is injected into the reactor to both neutralize the free NaOH and to drop the acidity of the outlet brine. Naphthenic and other organic acids are reconstituted from their sodium salts in this pH range. These neutralization and springing reactions are exothermic and are expressed by reactions 9, 11 and 16. Thorough mixing is required in the reactor to ensure the acid/base reactions are completed within the reactor residence time. A motorized agitator provides the necessary mixing.

Solvent is injected on manual flow control and is adjusted to optimize the extraction of acid oils from the caustic. Downstream of the solvent injection point is a mix valve that provides the contacting necessary to solvent-extract acid oils from the aqueous phase.

The acidified stream then enters a 3-phase separator that has sufficient residence time to allow separation of the acidified brine, solvent with extracted acid oils and acid gas. The organic acids float to the top of the acidified brine solution and are removed on level control. A level-to-flow cascade control is used to maintain the proper aqueous phase level within the separator, and to deliver a steady flow rate of acidic brine to the stripping column. The operating pressure of the separator is maintained by the addition of fuel gas, which also serves to absorb the acid gases generated. The rich fuel gas can be routed to the sulfur plant, a heater or an incinerator, depending on plant requirements.

6.3.1.2 Stripping Column and Acidified Brine Neutralization (Figure 5)

The acidified brine from the separator enters the stripping column at an elevated position above a fixed packing bed. The acidified brine flows downward through the packed bed. A measured amount of fuel gas is introduced below the packing within the stripping column, and flows upward through the packed bed. Within the packed bed, the counter-current contacting between the acidified brine and fuel gas removes trace quantities of residual acid gas from the brine. Level-to-flow control is used to maintain the acidified brine level in the bottom of the stripping column. The offgas passes through a demister pad as it leaves the stripping column to remove entrained liquid before proceeding to the sulfur plant, a heater or an incinerator, depending on plant requirements.

The pH of the acidified brine is adjusted to between 6 and 8 by the addition of fresh caustic. A controlled flow rate of fresh caustic is added to the stripped acidic brine stream just upstream of a static mixer. The neutral brine effluent is then discharged to the WWTP.

6.3.1.3 Enhanced Phenol Extraction

MCRS has developed a process that uses a proprietary solvent to extract phenols. The resulting brine contains low levels of this contaminant resulting in minimal COD and BOD levels. The solvent is regenerated internally for reuse in the phenol extraction process. MCRS is the sole-supplier of the proprietary solvent.