A Highly Refined Effort

By Felix Davila Huaman, Nicolas Villar, Chad Felch, Clay Maugans and Steve Olsen

Siemens Water Technologies provides operating notes on how wet air oxidation at Peruvian refinery generates biodegradable, odor-free effluent from spent caustic

The Repsol YPF refinery in La Pampilla is the largest in Peru. Its hydrocarbon streams contain sulfides and organic acids – all undesirable contaminants. These are removed by caustic treatment. Several different caustic treatment steps are performed through the refinery, making numerous spent caustic waste streams. About 3,000 m³/yr of combined spent caustic is generated. Spent caustic is malodorous and toxic, and difficult to treat by conventional biological means. Since 2005, Zimpro® wet air oxidation (WAO) from Siemens Water Technologies has been used to treat these streams prior to disposal. WAO allows on-site treatment without odorous off-gas.

The technology

Wet oxidation is an aqueous phase process that operates at elevated temperature and pressure. Dissolved or entrained contaminants are oxidized in the liquid phase water. The process relies on the liquid water molecules to catalyze the oxidation. Oxygen is used as the oxidant and is supplied by mixing air with the spent caustic. For refinery spent caustic, the design operating temperature was 260°C (500°F), well above the normal boiling point. System pressure is 88 kg/cm² (1,250 psi), keeping most of the water in the liquid phase and minimizing energy consumption. Typical reactions during WAO of refinery spent caustic include:

Description of wastewater

The multiple waste streams include sulfidic, naphthenic and cresylic spent caustics. Sulfidic spent caustics come from scrubbing of liquefied petroleum gas (LPG) and pentane from a fluid catalytic cracker (FCC) and continuous distillation unit (CDU). Naphthenic spent caustics come from the Merox® type treatment of kerosene. Cresylic spent caustics come from this same type treatment of visbreaker gasoline. Compositions are shown in Table 1.

About 85% (by volume) of the spent caustic is produced continuously in treatment of kerosene. The remaining 15% of the flow is produced in batches of varying quantities and compositions. Two storage tanks are used as equalization tanks. One large batch of sulfidic spent caustic is segregated into one storage tank. Remaining spent caustics (sulfidic, naphthenic and cresylic) are combined and held in the other storage tank (called the main spent caustic).

The partial segregation is done so the sulfide concentration fed to the WAO unit can be better regulated. This is done to control oxygen uptake and corrosion that can result from uncontrolled sulfide concentration spikes.

Metering pumps are used to control the flow rate of spent caustic from the two storage tanks and the fresh caustic. Dilution water is added using a centrifugal pump to control heat release from the oxidation. Caustic is consumed by the oxidation products. If there’s insufficient caustic, pH of the oxidized effluent will become acidic, which can damage nickel alloys used in the high temperature portion of the WAO system. Fresh caustic is added to the feed to maintain the discharge pH above 8.

Description of the oxidation system

A block flow diagram of the wet oxidation system is shown in Figure 1. The operating conditions for the system are shown in Table 2. The four feed streams are combined and pass through a high pressure feed pump. Compressed air is added to the feed and this mixture is heated in the feed/effluent heat exchanger (F/E HX). The F/E HX is a concentric tube exchanger with feed passing through inner tubes and the hot reactor effluent through annular space of the outer tube. The feed is heated further in the trim heat exchanger. System operating temperature is regulated by steam flow to the trim heat exchanger. Oxidation reactions begin with heat addition in the heat exchangers. The hot fluid enters the bottom of the reactor. The exothermic reaction heats the fluid to the final operating temperature in the reactor. The reactor is a bubble column, with gas bubbles rising through the liquid phase. It’s sized to provide the retention time necessary to achieve the desired degree of treatment. Fluid exits the top of the reactor and passes through the shell of the F/E HX for heat recovery. After partial cooling in the F/E HX, oxidized effluent passes through the process cooler and then to the process separator. Off-gas is separated from the oxidized liquor by gravity in the separator. The liquid effluent is discharged to the sea. The off-gas is scrubbed and sent to a burner used to fire a distillation column.

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Results

System performance is shown in Table 3. The combined spent caustic and fresh caustic composition is reported in the feed column of Table 3. Dilution water is added to the feed after the feed sample point. In addition, scrubber water is added to oxidized effluent in the process separator. These water streams dilute the oxidized effluent to about 60% strength, reflected in the effluent column in Table 3. After correcting for the dilution effect, the WAO system destroyed 85% of COD, 73% of TOC, and >99.97% of sulfide. Figure 2 is a photograph showing the sulfidic spent caustic, the main spent caustic (mostly naphthenic with some cresylic components), and the oxidized WAO treated effluent.

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Safety

The oxygen supply is limited by the capacity of the compressor. This prevents excessive temperature increase in the event of a COD excursion in the feed. Liquid water and air in the reactor is an energy sink to further dampen temperature excursions. Pressure in the reactor is regulated, so an excessive energy release in the reactor creates more steam at the process temperature, rather than excessively increasing the temperature or pressure. The off-gas may contain 5-21% O₂ and some VOCs. To prevent a fire hazard in a flare system (due to the oxygen), a dedicated off-gas line is used to take this gas to the firebox of a burner. The system processes spent caustic at elevated temperature and pressure. Typical spray and spill protections are used to protect personnel and the environment. There have been no safety incidents with the WAO system.

Reliability

Since unit start-up in 2005, the WAO system has proven to be reliable. The only item of note is fouling in the heat transfer equipment. After about 12 months of operation the heater exchanger tubes became fouled, causing an increase in pressure drop and a loss of heat transfer efficiency. According to process provider Siemens Water Technologies, this is unusual for a spent caustic application. The scale was found to be mostly iron and is formed from unusually high concentrations of iron in the feed. There was also an organic fraction to the scale. Due to the small size of the tubes, it wasn’t practical to hydroblast the tubes, which were partially cleaned using steam.

This didn’t remove very much of the scale, though, and a better solution was needed. The process provider conducted a series of lab tests to identify a solvent and cleaning procedure that would be effective at removing the scale, but not damaging to the metallurgy. The best compromise found for this scale was a pickling solution of the following composition:

• 535 mL 20° Baume HCl
• 33 g CuCl₂
• 1,000 mL H₂O

This mixture dissolves the scale at room temperature. On corrosion coupons, it was found the solution has a general corrosion rate of 38 MPY (mils per year) at 25°C (77°F). Elevating temperature to 80°C (176°F) increased effectiveness of the solvent at scale removal, but also greatly increased the general corrosion rate to 600 MPY. Based on the lab tests, it was concluded this solvent can be used for up to a maximum of seven total days a year at 25°C (77°F) to achieve suitable scale removal while maintaining acceptable corrosion from the solvent to the piping.

This solution was cycled through the heat exchangers and found to be effective at scale removal without damaging the equipment. The heat exchanger has been cleaned once using this method, with an exposure time of eight hours. It’s anticipated it will need to be cleaned every six months. In addition, flanged unions may be installed on some of the exchanger’s loops to allow access for mechanical cleaning of the equipment as well.

Conclusion

The WAO system is an effective approach for treating refinery spent caustic. Sulfides and mercaptans are destroyed to below analytical detection limits. TOC and COD are greatly diminished and oxidation products neutralize the pH to between 8 and 10. Gaseous and liquid effluents are not malodorous. The oxidized spent caustic is discharged to a holding tank, and then directly to the sea. Off gas is of sufficient quality that it’s directed to a burner to destroy volatile compounds. System operation has proven reliable. After a year of operation an iron/organic scale had accumulated in the heat transfer equipment due to unusually high iron concentrations in the spent caustic feed. This accumulation was successfully removed using a pickling solution solvent.