CA2826715A1 - Ozone-ultrasonic treatment of spent caustic wastewater - Google Patents

Abstract

A process for treating spent caustic comprises steps of oxidation of the spent caustic while agitating the spent caustic using ultrasonic vibration, where sulfur-based compounds are converted into benign compounds (mostly sulfates), chemical adjustment/treatment, where wastewater pH is adjusted to meet downstream treatment and handling requirements, and additional treatment and/or polishing, where residual contaminants are removed to meet wastewater discharge criteria.

Description

Ozone-Ultrasonic Treatment of Spent Caustic Wastewater Technical Field [0001] The present disclosure relates to wastewater treatment, and more particularly to treatment of spent caustic.
Background [0002] Spent caustic is an aqueous waste stream in oil refineries and petrochemical plants when petroleum derived fluids are processed with aqueous sodium hydroxide. It is formed out of scrubbing processes where excess sulfur compounds are removed from refined mid and final products, creating a stream with very high amounts of hydrogen-sulfide, organic disulfides, phenolics, mercaptans, and other hydrocarbon compounds. In addition, high residual sodium-hydroxide makes pH range from 11-14. The spent caustic wastewater produced from this processing is typically dark brown in color, turbid, highly alkaline, contains high levels of sulfides and has a pungent odor characteristic of olefins and sulfides.

[0003] Although both oil refineries and petrochemical plants generate a wastewater stream , that belongs to this category, the actual chemical composition of these wastewater streams varies significantly from one plant to another depending on site deployed refining/purification processes. For example, the oil refining spent caustic stream comes from multiple sources, and includes sulfidic, naphthenic, and cresylic spent caustic waters. Sulfidic spent caustic is generated by a scrubbing process of liquefied petroleum gas (LPG) and pentane from catalytic cracker (FCC), as well as continuous distillation unit (CDU). Naphthenic spent caustic comes from the Merox type treatment of kerosene. On the other hand, cresylic spent caustic comes from the Merox type treatment of visbreaker gasoline. Typical chemical composition ranges for these streams is shown in Table 1.

Table 1: Typical Spent Caustic Chemical Composition Parameter [rig/L1 Sulfidic spent caustic Napthenic spent Cresylic spent caustic caustic pH 11-13 11-14 ___ 11-14 COD 8,000-100,000 60,000-100,000 130,000-220,000 TOC 500-50,000 10,000-30,000 25,000-60,000 Sulfides 2,000-50,000 <1 0-60,000 Sulfite ______________ 0.2-500 0.5-1 500-1,500 Mercaptans 0-30,000 <25 0-5,000 Thiosulfate 0-4,000 50-150 10,000-15,000 Phenols 0.3-30 2,000-10,000 15,000-20,000 [0004] The spent caustic is considered one of the most difficult streams to handle by wastewater treatment industry professionals. Typical conventional treatment options range from steam and/or air stripping, chemical oxidation to oxidation supported by high pressure and incineration. Disadvantages of using these techniques relate to high capital deployment per unit basis, high operating costs, incomplete treatment requiring additional treatment steps and associated safety concerns.
Summary [0005] A process for treating spent caustic comprises steps of oxidation of the spent caustic while agitating the spent caustic using ultrasonic vibration, where sulfur-based compounds are converted into benign compounds (mostly sulfates), chemical adjustment/treatment, where wastewater pH is adjusted to meet downstream treatment and handling requirements, and additional treatment and/or polishing, where residual contaminants are removed to meet wastewater discharge criteria.
Brief Description of the Drawings [0006] These and other features will become more apparent from the following description in which reference is made to the appended drawings wherein:
Figure 1 is an overview of an exemplary treatment process for treating spent caustic;
Figure 2 is a system flow diagram for an exemplary treatment system for treating spent caustic;
Figure 3 shows a US03 system on a base frame;

Figure 4 shows a side view of the U503 system of Figure 3;
Figures SA and 5B show, respectively, front and rear views of a U503 system container; and Figures 6 and 7 show an overview of the US03 system.
Detailed Description [00071 As shown in Figure 1, in operation, wastewater is pumped from the wastewater tank through the process piping by a feed pump. The wastewater is filtered to remove solids from it.
The filtered wastewater then passes through the treatment system and then returns to the holding tank. This is done continuously during the treatment process.
[0008] An exemplary system for treating spent caustic comprises a plurality of modular components that are designed to be transported to a client's facility, assembled and operated there temporarily to treat wastewater that has accumulated and is stored there. Upon project completion, the system is disassembled and removed. By treating the wastewater at the site, the risk of a potentially hazardous wastewater spill during highway transport is obviated.
[0009] The exemplary system comprises at least one wastewater tank, a main equipment enclosure, a process feed pump, filter and heat exchanger module, at least one hydrogen peroxide tank, at least one acid tank, a gas catalyzer module, an oxygen gas supply module and a chiller, as well as suitable piping and hoses.
[0010] The wastewater tank contains the wastewater that is treated. In one exemplary embodiment, the volume of the wastewater tank is approximately 50,000 liters;
the wastewater tank may be any suitable size. Carbon pillows are positioned to obstruct openings at the top of the tank and inhibit the emission of offensive odors.
[0011] The main equipment enclosure contains the equipment that chemically treats the wastewater, namely, hydrogen peroxide and acid pumps that inject these chemicals into the process stream, and ozone gas diffusers that introduce ozone gas into the wastewater. It also houses the ozone gas generator that feeds the diffusers, and its coolant pump.
[0012] Ultrasonic transducers that agitate the wastewater stream during oxidation, accelerating the chemical reactions, are plumbed downstream of each ozone diffuser.

[0013] In a preferred embodiment, a 12-inch US03 system offered by Ultrasonic Systems GmbH, having an address at Gemeindewald 7a, 86672 Thierhaupten, Germany, is used to deliver ozone gas and provide ultrasonic agitation. The power connection is 128 amp Ceekon 400V, and the US03 system supports remote monitoring through a GSM or LAN
interface and uses IPC control with WAGO SPS. Figures 3 to 7 show various aspects of the US03 system.
[0014] The US03 for the treatment of a fluid contaminated with H2S is equipped with an additional outside pump/filter module and a plate heat exchanger downstream of the treatment cycle. The US03 unit itself is implemented in an insulated 40-foot machine container. The container also includes the main control center operating and controlling the unit. For security reasons the control center is separated from the actual treatment area through a wall including a gas-tight lockable door.
[0015] While filling the US03 a self cleaning filter system (downstream of the pump) removes larger impurities from the water. In order to make sure the unit is filled completely, a level sensor is placed at the end of the treatment system. Once the system is completely filled, the oxidation process is started by feeding ozone through the injection points at the six included OptimiXers, The six OptimiXer units are fed by three water/air cooled ozone generators each supporting 2 OptimXers. The desired amount of ozone gas is controlled and set through mass flow controllers (MFC). Each of the six OptimiXers has its own MFC. To protect the MFCs against a backflow of liquid, each OptimiXer injection nozzle is equipped with a directly actuated solenoid valve and a cone check valve.
[0016] In parallel to the ozone injection the ultrasound generators are activated. The high intensity ultrasound accelerates and improves the oxidation process.
[0017] In front of each OptimiXer a collector pipe removes residue ozone gas to a catalyst unit placed at the outside of the container. The catalyst unit consists of two independent systems, one for residual ozone and one for H2S gas. Therefore the residue gas streams are guided out of the US03 system and are converted into environmentally non-harmful substances.
[0018] Four roller shutters alongside the complete container length (two on each side) protect the six ultrasound sections and The six OptimiXer gas injection points against human contact.

The roller shutters can be opened electrically or, in case of a power failure, manually, for maintenance or repair work.
[0019] Before the treated liquid is discharged, it is led through a safety plate heat exchanger.
The main function of this heat exchanger is an immediate cooling of the liquid in the event of an excessive heat dissipation of the ozonized H2S medium. In this case a 2/2-way ball valve automatically opens the cooling water connection to the plate heat exchanger and the heat dissipation is reduced to an acceptable level. In order to control the temperature curve of the H2S-liquid on a constant level, each ultrasound section is equipped with a temperature sensor PT100. By reducing the ozone amounts or ozone concentrations an uncontrolled oxidation process is avoided. The adjustment of the ozone amounts and ozone concentrations is preferably automatically controlled by a computer control unit.
[0020] After the plate heat exchanger the treated liquid can be discharged to a storage tank, placed on a lower level than the pump level, and can be circulated back into the wastewater tank until the desired result is achieved.
[0021] An on-board power supply (battery) enables the system to be drained, purged and put into stand-by mode in a controlled manner in the event of a power failure (indicated by an orange light on the container roof).
[0022] A 100mm floor pan is positioned below the OptimiXers and ultrasound sections; in case of leakage the liquid is collected in the floor pan and can be drained by a 3 inch floor connection in the container bottom.
[0023] In other embodiments, other oxidation and ultrasonic agitation systems other than a MOS system may be used. A treatment system may other comprise a plurality of individual systems and components connected together, for example as a permanent on site treatment facility.
[0024] In a preferred embodiment, the treatment system's computer control and electrical distribution systems are located inside the equipment enclosure. The electrical distribution system provides electrical power and overcurrent protection for all of the system's electrical devices, [0025) The control system continuously monitors data from the treatment system's temperature, pressure, level and flow sensors, chemical analyzers and inputs from the user. It controls flow of process fluids and progression of the treatment process. It also enunciates warnings to the user and shuts the system down if an unsafe condition exists.
[0026] Hydrogen sulfide and ozone gas detectors that are located in the main equipment enclosure are monitored by the control system. They enunciate an alarm and shut down the treatment system if hydrogen sulfide or ozone gas is detected outside of the process piping in excess of predetermined levels (i.e. leakage); in a preferred embodiment this will occur if hydrogen sulfide or ozone gas is detected outside of the process piping at levels over 10 parts per million (ppm) or 0.1 ppm, respectively. If leakage is detected the system is stopped, drained and purged immediately (indicated by a red signal light on the container roof). In addition, ventilators instantaneously start to remove the toxic gases and ventilate all rooms inside the container. The ventilated air is fed through carbon filters to limit harmful gas escape into The environment.
[0027] The process feed pump, filter and heat exchanger module contains the pump that circulates the wastewater through the treatment system, a filter that removes suspended solids from the wastewater, and a heat exchanger that removes the heat that is generated from chemical reactions in the treatment process, preferably maintaining a process temperature below 30 C.
[0028] The hydrogen peroxide tank contains an aqueous solution of hydrogen peroxide, and the add tank contains an acid solution. The particular acid that is utilized varies, depending upon availability and the chemical characteristics of the wastewater being treated. In one preferred embodiment, the volume of the hydrogen peroxide tank and the volume of the acid tank are each 1,000 liters.
[0029] In a preferred embodiment, the gas catalyzer module comprises two catalyzers that utilize oxygen gas and a granular catalyst to remove ozone and hydrogen sulfide gases from the process and convert them to benign gases before releasing them to the atmosphere.
6.

[0030] The oxygen gas supply module comprises a machine that extracts oxygen from the surrounding atmosphere to supply the oxygen gas that feeds the ozone gas generator and cataiyzers.
[0031] A compression chiller rejects process-generated heat to the surrounding atmosphere and allows for the process to operate at temperatures lower than ambient. This accelerates the process and increases its efficiency.
[0032] Process piping and hoses convey the wastewater stream from the wastewater tank, through the treatment system and return it to the wastewater tank. All materials that are in contact with the wastewater are either stainless steel or non-metallic materials that have been designed to convey the corrosive chemicals present in the wastewater.
[0033] Figure 2 shows a detailed system flow diagram for an exemplary treatment system for treating spent caustic.
[0034] The treatment system has 3 specific operating functions. During each function, the wastewater is sampled and analyzed, at regular intervals, for total sulfide content and pH. The function that is utilized to treat the wastewater is dependent upon the particular chemical characteristics of the wastewater being treated.
[0035] The first function is a peroxone reaction which decreases the total sulfides in the wastewater by oxidizing them with a combination of hydrogen peroxide (H202), ozone gas (03) and ultrasonic agitation. This function is utilized where the wastewater has a total sulfide content of over 10,000 ppm.
(0036] Generally, the composition of a spent caustic stream is based on sulfides, mercaptans, thiosulfate, and phenols. The oxidation reactions of sulfide and other reduced sulfur compounds by ozone and hydrogen peroxide 03/H202 (peroxone) can be used for industrial wastewater treatment. The peroxone reaction can generate the formation of hydroxyl radicals (ON) during the reaction. The relative oxidation power of hydroxyl radical is higher (2.05) than ozone (1.52) and hydrogen peroxide (1.31) independently [1]. The addition of H202 to %can initiate the decomposition of 03, resulting in the formation of 'OH radicals:

203 + H202 42 'OH + 302 [0037] The formation of 'OH during the peroxone reaction is controlled by a number of variables, including PH, temperature, peroxide concentration, ozone concentration and reaction time, [0038] The typical reactions occurring during the oxidation of a spent caustic wastewater stream include the following [2]:
+ 4H2024 SO; + 4H20 (sulfides to sulfates at alkaline pH) 2RSH + H2024 RSSR + 2H20 (thiols to disulfides at alkaline pH) RSSR + 5 H202+ 20H-4 212503- + 6 H20 (disulfides to sulfonic acids at alkaline pH) [0039] Carrying the reaction to sulfonic acid and/or sulfates is generally enough to control odors and reduce the amount of sulfides to acceptable levels, [0040] To start the peroxone reaction, a metered amount of hydrogen peroxide is added to the wastewater, which oxidizes the sulfide contaminants in it. The hydrogen peroxide is added by a variable speed pump. This allows the rate of hydrogen peroxide addition to be adjusted. The flow rate of ozone gas into the system is also adjustable. The rate of hydrogen peroxide addition and the concentration of hydrogen peroxide solution being added are dependent upon the particular chemical characteristics of the wastewater being treated.
[0041] The second function decreases the total sulfides in the wastewater to acceptable levels with a combination of ozone gas and ultrasonic agitation. This function is utilized where the wastewater has a total sulfide content of less than 10,000 ppm. The rate of ozone gas addition and the concentration of ozone gas being added are dependent upon the particular chemical characteristics of the wastewater being treated.
[0042] The third function is to lower the pH of the wastewater. This function is only utilized when the total sulfide content of the wastewater has been decreased to 10 ppm or less. This pH
adjustment is done by adding a metered amount of acid to the wastewater to lower its pH, preferably to approximately 7. The acid is added by a variable speed pump.
This allows the rate of acid addition to be adjusted, if needed. The concentration of acid and the rate of addition of the acid are dependent upon the particular chemical characteristics of the wastewater being treated.
[0043] A typical caustic neutralization reaction using hydrochloric acid is as follows:
NaOH +1-1C1 -) NaC1+ H20 [0044] Because pH adjustment is only done when the wastewater no longer contains high levels of sulfides, the release of harmful gasses into the environment can be effectively limited.
[0045] Typical sulfidic spent caustic chemical composition and treatment results are given in Table 2.
Table 2: Influent Sulfidic Spent Caustic Composition and Treatment results Parameter [mg/Li Sulfidic Influent Sulfidic Effluent before additional treatment/polishing PH _________________________________ 12-12.7 82-9.0 COD 30,000-70,000 1,100-6,700 TOC 7,200-14,800 1,180-2,050 Sulfides 27,000-42,000 <1 Sulfite 30-74 <1 Mercaptans _________________________ 3,800-6,900 <10 Thiosulfate 420-710 <20 Phenols 0-12 <1 [0046] The present description is provided by way of example. It will be apparent to persons skilled in the art that a number of variations and modifications can be made without departing from the scope of the claims.
[0047] The following references were referred to in the above description:
[1] Munter, R., Advanced Oxidation Process ¨ Current Status and Prospects.
Proc.
Estonian Acad. Sci, Chem., 2001, 50, 2, 59-80.
[2] Solvay Interox, Pty. Ltd. Hydrogen Peroxide Controlling Reduced Sulfur Compounds.
www.solvayinterox.com.au 2001, 1-9,

Claims (2)

WHAT IS CLAIMED IS:

1. Treatment of spent caustic comprising oxidation of the spent caustic while agitating the spent caustic using ultrasonic vibration, where sulfur-based compounds are converted into benign compounds.
2. The treatment of claim 1, further comprising chemical adjustment/treatment, where wastewater pH is adjusted to meet downstream treatment and handling requirements.

2. The treatment of claim 2, further comprising and additional treatment and/or polishing, where residual contaminants are removed to meet wastewater discharge criteria.