Article | October 17, 2018

Water Recycling Efficiency In Ethylene Facilities Producing Spent Caustic, Part II: Process

EthyleneSpentCaustic

After addressing the business, financial, and operating benefits of segregated ethylene spent caustic treatment in Water Recycling Efficiency In Ethylene Facilities Producing Spent Caustic, Part I: Cost, this conclusion to the story delves deeper into the process involved.

The Treatment Challenges

As one of the most widely used commercially produced organic chemicals, ethylene is a well-known commodity worldwide. Unfortunately, its production yields a lot of sulfur in the form of toxic and very malodorous hydrogen sulfide gas that gets captured in a sodium hydroxide solution and turned into sodium sulfide and mercaptans.

Ethylene producers have developed a variety of approaches for breaking down their sulfide-laden spent caustic waste streams. Each approach offers different physical or economic advantages and disadvantages, and not all of them are compatible with water-reuse applications.

  • Incineration. Due to the nature of ethylene spent caustic being a high pH, high total dissolved solids (TDS) sodium hydroxide solution, burning it places a lot of stress on the refractory lining of a furnace or incinerator. This leads to frequent, disruptive, and costly maintenance and repair (M&R) issues such as the added expense of having redundant incinerators to provide continuity in the case of M&R downtime. Also, the incineration process generates sulfur dioxide (SO2), which creates further environmental and regulatory concerns.
  • Offsite Shipping. Shipping spent caustic offsite for processing — sometimes halfway around the world — offers the psychological advantage of being out-of-sight and out-of-mind but carries a high cost. Worse yet, the potential liability for a spill of hazardous material is far from ideal at any cost.
  • Combined Onsite Wastewater Treatment. Another way to deal with ethylene spent caustic disposal is to blend it with other wastewater into one combined stream (following pretreatment) to be processed through the site’s wastewater treatment plant. This approach, however, requires the additional pretreatment process to avoid issues with the biological treatment system and with odors. Also, high levels of salt impose new problems on the consolidated wastewater stream, problems that must be resolved before the water can be recycled.
  • Dedicated Stream Processing. Processing the relatively small volume of ethylene spent caustic wastewater separately from larger onsite wastewater treatment streams in the petrochemical plant simplifies the job of recycling the majority of wastewater following conventional biological treatment processes. The right dedicated-stream process can completely treat the effluent such that it can be discharged to the environment.

A Balance Of Strategies

A dedicated-stream alternative for ethylene spent caustic processing uses finesse, not force, to deconstruct the most problematic properties of ethylene spent caustic. Combining proven technologies — for example, Zimpro® wet air oxidation (WAO) integrated with biological/powdered activated carbon treatment (PACT®) and membrane bioreactor (MBR) technology — mitigates undesirable attributes and sets the stage for more desirable and economical disposal.

The WAO process completely destroys sulfides while converting the remaining organics to biodegradable short-chain compounds by heating the sulfide-laden caustic stream in a heat exchanger, putting it into a reactor vessel for a prescribed residence time, cooling it back down, and separating the off-gas and oxidized liquid effluent (Figure 1). It is a simple, reliable process with virtually no downtime.

Figure 1. WAO’s capability to convert sulfides into sodium sulfate and mercaptans into sulfonates eliminates odor issues and breaks down large organics into smaller ones that are easier to treat.

In this process, oxygen reacts with sodium sulfide (NaHS and Na2S) to form sodium thiosulfate (Na₂S₂O₃), then further reacts to form sodium sulfate (Na2SO4), reducing odor problems. Mercaptans (C2H5SH or CH3SH) are turned into sulfonates (CH3-SO3Na). Both of these conversions eliminate odor issues and break down any larger organics into smaller ones that are easier to treat. By contrast, if the stream was not pretreated, neutralization and aeration in a biological wastewater system would release malodorous hydrogen sulfide and mercaptans into the air.

PACT MBR technology combines three distinct treatment processes — biological, physio/chemical, and ultrafiltration liquid-solids separation — in a single, compact, and robust treatment technology. It achieves some of the lowest chemical oxygen demand (COD) and total organic carbon (TOC) effluents in refining and petrochemical applications. It also yields best-in-class performance for treatment of water in recovery and reuse programs.

The PACT stage integrates the biological reaction steps of conventional activated sludge with the mass transfer kinetics of carbon adsorption. This reduces footprint and capital investment requirements, as compared to conventional discrete biological treatment coupled with activated carbon filtration.  The PAC buffers the biomass from toxins including the high TDS environment of oxidized spent caustic, delivering high-quality effluent where conventional activated sludge treatment cannot satisfy (Figure 2).

Figure 2. Even with high TDS levels encountered to treat oxidized spent caustic, the PACT system maintains a stable effluent with TOC levels 10 to 30 mg/L, well under the regulatory limit of 75 mg/L. Conventional activated sludge treatment exceeds the effluent limit at TDS levels as low as 50 g/L. 

The MBR stage is used to clarify the final water discharge. Biological treatment of highly saline solutions hinders gravity clarification for solids separation. Membrane filtration is key to complementing biological treatment of solutions with high salt concentrations in oxidized spent caustic. The membrane provides a physical barrier for complete solids removal without polymer.

Top-Of-The-Line Technology, Bottom-Line Results

Wet air oxidation processes treating ethylene spent caustic are designed at conditions that destroy sulfur compounds and are not typically designed for organic destruction.  The addition of biological degradation and carbon adsorption features in the WAO and advanced biological polishing treatment scheme handle the remaining organics to deliver the low-level biochemical oxygen demand (BOD), TOC, and COD required of the effluent (Table 1).

Table 1. Even at well over 100 g/L of TDS in oxidized spent caustic, the PACT system maintains high effluent quality, suitable for discharge to sea.

Under stressful high-sulfide and salt conditions, this total treatment approach is able to deliver effluent quality that alternative wastewater treatment processes typically cannot match, including:

  • Guaranteed sulfide and mercaptan levels of less than 1 mg/L,
  • COD below 150 mg/L,
  • TOC below 75 mg/L,
  • TSS below 1 mg/L, and
  • Turbidity below 0.1 NTU.

Equally important, WAO and advanced biological polishing does it at a lower OPEX than alternative ethylene spent caustic treatments (Figure 3).

Figure 3. In addition to clean water effluent and odor control, WAO and advanced biological polishing delivers onsite performance at operating costs well below incineration, sodium hypochlorite, or hydrogen peroxide processes. (NOTE: Cost comparison does not include maintenance costs.)