MERICON™: A Practical Low Cost Solution to Spent Caustics Treating

By James F. McGehee

Abstract

Refining and petrochemical operations often generate various spent caustic streams that are “high strength” and require pre-treatment before processing by a waste water treatment plant. Pre-treatment neutralizes spent caustic and removes compounds that are incompatible with the biological treatment facility. Merichem's solution to this problem is the MERICON™ technology. The process uses simple chemistry, operates at or near ambient pressure and temperature, and is flexible enough to handle phenolic, sulfidic, naphthenic or ethylene plant caustics individually or in mixed streams. MERICON operators benefit from a low total cost of ownership. Since the early 1990’s, MERICON units have pretreated spent caustics with contaminants that exhibit extreme Chemical Oxygen Demand (COD) up to 500,000 mg/L. This paper discusses how the MERICON process resolves the spent caustic treatment issue using examples from operating MERICON units and discusses other strategies for handling spent caustics.

The MERICON Process

Sodium hydroxide (caustic) is one of the most important and beneficial industrial chemicals. It is used not only in refining, but also in papermaking, soaps and detergents, foods, and many other industries. Today’s high cost of energy and environmental compliance compel both industry and the consumer to respect the US EPA slogan: “Reduce, Recycle, Reuse.”

For over 60 years, the Merichem Company has served refining and petrochemical customers by accepting used caustic streams that are processed to reclaim valuable materials and in many cases can be re-used in other industries; the most common industries being paper and aluminum production. In the 1980’s it became apparent that reusing caustic, while environmentally beneficial presented increasing logistical challenges due to the remote locations of refineries  far from processing facilities or other users. Merichem developed the MERICON process to address this challenge. Of the 31 units licensed to date worldwide, 13 customers operate Merichem’s latest generation technology, MERICON III.

MERICON III is a caustic pretreatment technology that detoxifies and neutralizes refinery and petrochemical plant spent caustic. The effluent of MERICON III is a neutral-pH brine that is suitable for biological treatment facilities and meets all US EPA standards for waste water pre-treatment in the petroleum refining and petrochemical categories [1]. Table 1 lists the common uses of caustic within the petroleum refining and petrochemical industries.

Table 1
Sources of spent caustic

Industry

Process Unit

Source of stream

Refining

Crude Distillation Unit

Kerosene or Diesel sweetening

 

Crude Distillation Unit

LPG treater

 

Coker

Coker LPG treater

 

Fluidized Catalytic Cracking (FCC) Unit

FCC LPG treater, FCC Naphtha treater, Alkylation feed pretreater

 

Others

Small H2S scrubbers, Reformer Vent scrubber

Ethylene manufacture

Product Recovery

Caustic Scrubber

Petrochemicals

Various

Vent scrubbers

The flow diagram in Figure 1 categorizes these caustics into three broad categories: sulfidic, naphthenic, and phenolic. Smaller streams exist, for example, in scrubbers used to neutralize acidic gas streams.

Figure 1
Spent Caustic Management

In 2000, Martinie et. al. [1] published a review of used caustic handling options and costs for a Saudi Arabian refinery. Options ranged from direct disposal or deep well injection to pretreatment and biological treatment processes. At the time of the publication, dilution and ocean disposal of low-organic brines was sometimes practiced but is now widely prohibited. In a 2011 review, Veerabhadraiah et. al. [2] give a hierarchy of preference for reducing and eliminating used caustic in today’s refining (Table 2). Applying “Good Engineering Design” to minimize the load is the preferred option. “Recovery and re-use” are then followed by treatment before disposal.

Table 2
Options for Used Caustic Management
 (adapted from Veerabhadraiah [2])

Action

By

Ranking of preference

Reduce

Source reduction

Good engineering design and operating efficiency

First option


Final option

Reuse

Use within source facility

Recycle

Use secondary value externally

Merichem Caustic Services

Recover

Extract the usable/saleable material

Remove

Treat to remove hazards before disposal

MERICON used caustic pretreatment + biological treatment facility

Spent caustic have the following environmental characteristics:

·         Used caustic streams are toxic to aquatic life and cannot be disposed directly to waterways, even after neutralization; and / or

·         Used caustic is unsuitable for direct addition to a biological treatment facility.

The impurities found in used caustic streams are listed in Table 3 along with how the MERICON process removes them. Standard operating procedures for modern plants require oxidation and neutralization with stripping for spent caustic before biological treatment.

Table 3
Common species in spent caustic and their removal by the MERICON process

Compound

Refining

Ethylene Manufacturing

Impact to Biological Treatment

Removal in MERICON

NaOH

X

X

pH upset

Neutralized to Na2SO4

Sodium Sulfide

X

X

Forms H2S

Oxidized and stripped

Sodium Bisulfide

X

X

Sodium Carbonate

X

X

None

--

Sodium Bicarbonate

X

X

Sodium Mercaptide

X

 

Odor, Volatile Organics

Removed in organic phase

Sodium Naphthenate

X

 

High COD load, foaming, odor

Sodium Phenolate/Cresylate (from phenols, cresols and xylenols)

X

 

Toxic to nitrifying organisms, low rate of biological degradation

Hydrocarbons, Gums, Polymers

 

X

High COD load, foaming, odor

The biological treatment facility receiving spent caustic can be either owned by the refinery, a chemical plant, or a publicly owned treatment works (POTW). These facilities are able to accept a stream based on several factors:

·         Overall flow rate

·         Treating load (BOD and COD)

·         Presence of a hazardous pollutant that could “pass through” into the effluent

·         Potential negative effect on the treatment process

Ø  Foaming

Ø  pH upset

Ø  Toxicity to the biological system

Typical flow rates of used caustic into a treatment facility range between 0.5-15 gpm. The rate can be larger for world-class ethylene facilities (up to hundreds of gpm). Although spent caustic are only a very small fraction of water treatment plants’ total flow, these spent caustic are categorized as a “high strength” waste stream. High strength implies that the stream has a high oxygen demand, complicated by a negative impact on the biological system. The chemical oxygen demand (COD) may be in the range of 50,000-75,000 or even higher (some highly spent naphthenic caustics from jet fuel treating can exhibit COD above 300,000 mg/L). COD is normally used to characterize the oxygen demand because the standard 5-day Biological Oxygen Demand (BOD-5) test gives low oxygen uptake results due to the toxicity of the used caustic. The ratio BOD/COD is sometimes used as a guide to assess the treatment plant’s ability to handle a particular waste material. A rule of thumb ratio when BOD/COD <0.5 signifies that the stream is not biodegradable.

THE PRETREATMENT CHALLENGE

Table 4 defines the minimum legal requirement in the US for pretreatment of refinery streams that are sent to downstream water treatment facilities. More stringent standards may be set in certain states or countries. Today, refineries and petrochemical facilities use high purity fresh caustic manufactured by one of two main electrolytic technologies, the diaphragm or membrane processes. These differ mainly in the amount of chloride present. As such, spent caustics pose no particular heavy metals concern.

Merichem meets the COD requirement through a combination of two chemical steps: (1) oxidizing inorganic compounds (primarily sodium sulfide and bisulfide), and (2) acidifying to a pH which allows the release of naphthenic acids and phenolics into a separate acid oil layer for either blending with fuel oil or upgrading in conversion units such as FCC or coking.

Table 4
Minimum pretreatment standards for new sources
in the petroleum refining category (40 CFR 419.16)[3]

Specification

Average of daily values for 30 day period not to exceed

kg/m3 (1)

weight ppm (2)

5 day Biological Oxygen Demand (BOD-5)

6.3

6250

Chemical Oxygen Demand (COD)

4.9

4860

Total Suspended Solids

32

31750

Oils and Grease

1.9

1890

Phenolic Compounds

0.043

43

Ammonia as N

1.3

1290

Sulfide

0.035

35

Total Chromium

0.105

104

Hexavalent Chromium

0.0068

7

pH

between 6 and 9

NOTES

(1) As written in standard

(2) Assumes brine density of 1008 kg/M3 based on neutralization of a 5 wt% NaOH caustic stream

As seen in Table 4, any pretreatment technology must eliminate phenolics at high efficiencies. Phenolics are particularly toxic to nitrifying organisms used in the waste water plants. The waste treatment system can acclimate to phenolics at dilute concentrations, but sudden increases shock the system and result in poor control of dissolved oxygen. Additionally, mercaptides and species that decompose to H2S (sodium sulfide and hydrosulfide) create hazardous air pollutants.

Used Caustic Pre-treatment Technologies

The most common technical approaches to the pre-treatment of spent caustic are listed in Table 5.

Table 5
Technical Solutions to Spent Caustic Pretreatment

Type

Approach

Technology

Advantages

Disadvantages

Thermal

Incineration

Special lined down-fired combustor

Low COD brine, high destruction efficiency

High capital and operating cost. Burners and lining systems need frequent maintenance and replacement

Wet Air Oxidation (WAO)

High temperature WAO for refining caustic

Medium-high temperature WAO for ethylene caustic

Heat of oxidation of organics provides most of thermal energy, high COD reduction

High capital cost, nickel lined high pressure vessel. Vent gas stream needs further treating.

COD reduction “floor” due to refractory organics

Chemical

Direct chemical oxidation

Hydrogen Peroxide or other chemical oxidant, with or without catalyst

Low temperature, liquid phase reaction

Cost of chemical reagents

Deep neutralization and separation

MERICON

Low capital cost, low pressure and temperature, reliable operation

COD reduction limited by solubility of organics in the brine. However add-on polishing allows even lower final COD

The technologies listed in Table 5 can be broadly categorized into two types, thermal and chemical. Each has unique attributes that are described below:

THERMAL TREATMENT

·         Incineration - In this technology, a fluid waste stream is sprayed into a zone having a temperature high enough to insure the total oxidation of all organics and sulfur species. Rather than injecting the stream directly into a burner that would cause corrosion and erosion, the used caustic is usually atomized into the treatment zone downstream of the flame at 850oC (1560oF) with a 1.5 – 2.0 sec residence time. Downstream of this zone is a water quench chamber. After the water is cooled to near ambient temperature, its low COD and near-neutral pH make it suitable for discharge. Unfortunately, sodium forms volatile oxides that can penetrate and destroy many conventional refractory linings. The spent caustic incineration technology resolves this problem through the use of special refractories. Despite these steps, incineration incurs significant maintenance costs and requires periodic lining replacement. Additionally, firing nozzles must be removed and inspected at regular intervals. Energy costs can become significant if a low-value fuel cannot be used to provide the heat input.

·         Wet air oxidation – As an alternative to the high energy costs of direct incineration, the wet air oxidation (WAO) technology was developed. WAO enjoys widespread use, especially in organic chemical manufacturing (explosives, fertilizers, etc.) with waste streams that are difficult or dangerous to handle. Kolaczkowski [4] published a detailed review of WAO, referencing available data on performance, relative capital cost and operating costs. WAO began in the 1950’s in the area of municipal sludge treating and was later extended to other applications. Maugans and Huaman [5] present a case history of this technology applied to refinery spent caustic. In this instance, WAO has overall 85% COD reduction and 36 ppm phenolics in the treated brine. WAO can be divided into three ranges of severity according to Table 6 [2].

Table 6
Wet Air Oxidation Process Conditions

Severity

Pressure, bar g

Temperature, °C

Performance

High

14-30

175-250+

Suitable for refinery caustics, however highest temperatures needed for destruction of phenolic compounds

Medium

7-10

100-120

Only converts about 70 % of sulfide to sulfate and rest to thiosulfate; does not take out organic COD

Low

4-7

75-100

Only converts about 50 % of sulfide to sulfate and rest to thiosulfate; does not take out organic COD

WAO achieves high COD reduction, but, in the case of refinery spent caustic, temperatures of at least 250 °C are needed due to the refractory nature of phenolics and naphthenic acids. Since unit pressure must be increased to suppress vaporization of water, the reactor is designed to withstand high pressure and temperature in the presence of strong alkaline conditions. This necessitates construction in nickel [6]. If the COD of the stream is above ~20,000, the heat of oxidation can sustain the process if a feed-effluent recovery heat exchanger is employed. Electric energy is necessary for pumping and air compression.

Treatment using WAO cannot lower the COD of refinery spent caustic to zero, because certain carboxylic acids are formed that resist further oxidation. Also, phenolics are not completely removed. This can require a downstream polishing unit in some instances. WAO also vents an oxygen-depleted offgas that may contain volatile organics and require incineration.

CHEMICAL TREATMENT

·         Direct Chemical Oxidation. Sulfidic spent caustic can be oxidized directly and efficiently with hydrogen peroxide without catalysts to form sulfate [7]. This can be attractive for small streams but not for larger ones since the peroxide requirement of ~2 lb H2O2 / lb S results in relatively high operating costs. Organic compounds that contribute to the used refinery caustic’s high COD do not react efficiently with H2O2 unless a catalyst is used (discussed below). For this reason, chemical oxidation is more often considered a polishing step rather than a direct pretreatment technology.

·         The MERICON Solution. As refining technologies changed through the 1990’s to accommodate heavier crude slates and more stringent fuel sulfur requirements, Merichem’s technology adapted to the changing spent caustic management requirements. The MERICON spent caustic pretreatment process evolved into the current generation, MERICON III. Figure 2 represents a block flow diagram of the process.

Figure 2
MERICON III Process

MERICON III can pretreat all spent caustic from refining and petrochemical operations. The complete process consists of five steps:

1. Deep neutralization. All the major organic components; Phenol, cresols, xylenols, mercaptans and naphthenic acids, are present as their organic salts at high pH. MERICON III uses acidification, usually with sulfuric acid, to lower the pH and “spring” these organics into a separate phase named acid oils. Neutral hydrocarbon carryovers in used caustic are also separated into this phase. Removing the organics lowers the COD substantially. In typical refinery spent caustic, this step results in a 2-2.6 ppm of COD reduction per ppm of organic material. Deep neutralization employs a relatively low cost reagent, sulfuric acid, that is already used in water treatment. No additional heat energy is needed.

2. Stripping. Deep neutralization also oxidizes sodium sulfide to H2S that is then removed by stripping using fuel gas or nitrogen. The vapors may be conveniently routed to the acid gas system and sulfur recovery unit.

3. Solvent wash. If requirements dictate, additional phenolic and other organic species can be further reduced by efficient contact with a suitable solvent.

4. Chemical Oxidation. MERICON III can be designed with an optional additional polishing step. Many refiners may wish to further reduce COD or phenolics to meet regulatory requirements or because wastewater treatment capacity is limited. If polishing is needed, a solvent wash followed by a final proprietary catalytic chemical oxidation system using hydrogen peroxide at low pH achieves the lowest possible phenolic level of any proven technology. In this step, safety and operability is paramount. The use of hydrogen peroxide is well proven. Peroxide has been in widespread use for many years as a pre-oxidant or to provide additional dissolved oxygen in municipal water treatment. The peroxide feed is normally purchased pre-diluted at specific concentrations that allow it to be handled as a low-reactivity oxidant (NFPA reactivity class 1). When properly stored, the solution has a degradation of on the order of 1% per year. Merichem’s delivery and control systems insure the optimal consumption of reagent for the final specified contaminant level.

MERICON III’s polishing step is optimized for pretreatment of spent caustic. Rather than consume additional energy or use treating chemicals to meet the desired standard for discharge to waterways, MERICON III’s polishing step is intended to:

·         Reduce phenolic levels to minimum; and,

·         Oxidize certain trace impurities that are still soluble in the acidic brine.

5. Re-neutralization. In this final step, the acidic effluent from either the solvent wash or polishing step is adjusted to the range of 6-8 pH. The neutral brine is further diluted when sent to the equalization basin and on to the biological treatment unit.

A key advantage of MERICON technology is that different spent caustics can be “blocked out” and segregated for the most economical treatment option. For example, the used caustic from Merichem kerosene treating units (NAPFININGTM, followed by MERICON) may produce an acid oil that has value for naphthenic acid reclamation. Sulfidic caustic from an LPG treating unit (THIOLEXTM) with minimal phenolics or naphthenic acids may require only neutralization and stripping. In this way, MERICON III adequately treats the contaminants for downstream waste water treatment and eliminates unnecessary operating costs.

CASE STUDY: EUROPEAN REFINER

Two MERICON III units were recently delivered to a European refiner for processing mixed refinery caustics. In this case, spent caustic from straight run LPG treating and catalytic cracking were mixed along with the spent caustic from a jet kerosene treatment train using Merichem’s NAPFINING and MERICATTM II processes. Table 7 contains the unit’s typical feedstock properties.

Table 7

MERICON III Design Basis – Case Study

Used caustic feeds

Source

Type

Current design

Kerosene treating (MERICAT II)

Naphthenic

Straight run naphtha treating

Sulfidic-naphthenic

Naphtha isomerization unit

Sulfidic

Straight run LPG

Sulfidic

Future

Coker LPG

Sulfidic

Vent scrubbing

Sulfidic

Throughput (each of two units)

m3/hr

2.3

Mixed feed properties

pH

13

NaOH

7.8 wt%

Naphthenic Acids

6.2 wt%

Phenolics

3600 ppm

COD

169,000 ppm

Other impurities

trace mercaptans and disulfides

The customer selected the patented Single Vessel MERICON III configuration [8] for both units that reduced capital cost and ISBL footprint compared to other offerings. Startup and commissioning of the units were straightforward and the performance of both units meets expectation. During test runs, the unit achieved an 81% reduction of phenolics and COD in the neutralization and solvent wash sections only.  No final brine polishing unit was needed to meet the customer’s pretreatment objective.  When polishing is included, MERICON can achieve reductions in excess of 90%.

 

Summary

MERICON III pretreatment process enables refining or petrochemical operators to handle spent caustic in an environmentally responsible way that, meets regulations with low capital and operating cost when compared to other options. MERICON III is the result of Merichem’s 65+ years of practical experience in the management of refinery spent caustics. In cases where used caustic cannot be reclaimed, MERICON III provides a low energy-intensive and straightforward option to pretreat the caustic before further handling in the waste water biological treatment units. Further information, including possible configurations and project planning details for MERICON III, is available upon request.



References

1. Martinie, G. D., Al-Kaltham, A., Al-Muaili, J., Al-Khateeri, M., Moran, S. and Jackson, B., “Investigation of Alternative Processes for Refinery Spent Caustic Treatment”, 16th World Petroleum Congress, June 11-15, 2000, Calgary, Alberta, Canada.

2. Veerabhadraiah, G., Mallika, N., and Jindal, S. Spent caustic Management: Remediation review, Hydrocarbon Processing, November 1, 2011

3. Code of Federal Regulations 419.16, Standards of performance for new sources (NSPS), petroleum refining point source category.

4. Kolaczkowski, P., Plucinski, P., Beltran, F. J., Rivas, F. J. and McLurgh, D. B., “Wet Air Oxidation: A Review of Process Technologies and Aspects in Reactor Design, Chemical Engineering Journal, 73 (2), May 1999, 143-160.

5. Maugans, Clay and Huaman, Felix, “Disposal of Spent Caustic at the Repsol YPF Refinery in La Pampilla, Peru”, NPRA Environmental Conference, Austin TX, September 24-25, 2007 paper ENV-07-133

6. Jobb, D., “Making the World a Safer Place”, Nickel: The Magazine of Nickel and its Applications”, 20 (3), July 2005, www.nickelinstitute.org

7. “Hydrogen Sulfide, Thiosulfates and Mercaptans Reduction for Refineries”, www.h2o2.com 

8. US Patent no7828962, November 9, 2010