Wet Air Oxidation Process

Introduction

The Spent Caustics Treatment Unit (SCTU) is typical for a refinery ot cracker plant, treating high molecular weight organic acids in the feed. An important feature will be the ability to convert them to simple organic acids such as formic or acetic acid, and thereby avoid severe operational problems in the down stream Effluent Treatment Unit. Wet Air Oxidation is one process that can be utilized to treat this spent caustic.

For a refinery, Wet Air Oxidation Process is usually for treating spent caustic containing Mercaptides (RSH), Sodium Bisulfate (NaSH), Sodium Sulfides (Na2S), undissolved oils, acid oils, phenols, and etc. from LPG Merox and Jet Fuel Caustics Merox Units, and to meet treated spent caustic as a feed stock of Effluent Treatment Plant (ETP) for further treatment which used biological treatment technology.

Wet Air Oxidation (WAO) Process is widely used the spent caustic. There are many types of WAO process commercialized in the market as follow:

1. Non catalytic WAO such as The Zimpro process, and The VerTech process
2. Heterogeneous CWAO processes such as The NS-LC process, The Osaka Gas process, and The Kurita process
3. Homogeneous catalytic wet oxidation processes such as The Ciba–Geigy process, The LOPROX process, and The WPO process
4. Recent developments such as The ORCAN process, and The ATHOS process

In this article, Zimpro Wet Air Oxidation (WAO) Process will be discussed

Wet Air Oxidation Process Flow Scheme

Figure 1: Typical Process Flow Diagram (Zimpro WAO)

Figure 2: Flow Diagram for LPG and Gasoline Merox Spent Caustic Treatment (Zimpro WAO)

WAO Reactions and Chemistry

Oxidation reactions take place in the aqueous environment where the water behaves much like a catalyst and is an integral part of the reaction. Air (oxygen) is bubbled through the reactor to provide a supply of oxidant.

Primary end-products from WAO are CO2 and H2O with additional by-products, carboxylic acids and other partially oxidized short chain organics. The yield of these compounds varies greatly depending on system design, but typically 5-25% of the TOC (Total Organic Carbon) from the feed remains as by-products, predominantly acetic acid. Carboxylic acids such as acetic acid are readily degradable in conventional biological treatment facilities. Organic forms of the elements: nitrogen, phosphorous, sulfur, and chlorine which enter the reactor bonded to organic molecules are reacted to NH3, PO43-, SO42-, and Cl- respectively.

The chemistry of WAO has some advantageous properties with regards to the off-gas produced. The off-gas from a WAO reaction has negligible NOx, and SOx.

By using WAO in pretreatment of Spent Caustic from LPG Caustic Merox and Jet Fuel Caustic Merox, a summary of reactions occurring in the reactor can be expressed as follow:

1) NaSH + 2O2 --> NaHSO4 + H2O
2) NaSR + 2O2 --> NaHSO4 + CO2 + R’COONa (unbalanced)
3) [Naphthenics] + O2 --> CO2 + R’COONa (unbalanced)
4) [Cresylic] + O2 --> CO2 + R’COONa (unbalanced)

where R’ is typically CH3

Process Description

Wet oxidation is the oxidation of soluble or suspended components in an aqueous environment using oxygen as the oxidizing agent. When air is used as the source of oxygen the process is referred to as Wet Air Oxidation (WAO). The oxidation reactions occur at temperatures of 150°C to 320°C and at pressures from 10 to 220 bar. The required operating temperature is determined by the treatment objectives. Higher temperatures require higher pressure to maintain a liquid phase in the system. Wet Air Oxidation typical operating condition are shown in figure 3

The typical WAO system is continuous process. High pressure pump and compressor is required to feed spent caustics and air (oxygen) to the unit. Trim heater is provided to heat up feed during start up and provide trim heat if required during normal operation. Since the oxidation reactions are exothermic, sufficient energy may be released in the reactor to allow the wet oxidation system to operate without any additional heat input. In some case, Reactor feed/effluent heat exchanger may provide to allow heat recovery from reactor effluent.

In case of high concentration of spent caustic stream, additional fresh water will be blended with the spent caustic stream and feed to the reactor to maintain a controllable exotherm in the reactor.

The hot effluent from the reactor is cooled using a cooling water heat exchanger before it is depressurized. The pressure control valve is a proprietary erosion resistant valve for reducing the pressure from reactor pressure to 2 - 5 bar. The depressurized effluent is then phase separated in a flash tank with the liquid effluent consisting mainly of sodium sulfate and sodium acetate in water routed to the biological treatment facility. The off-gas is vented to the atmosphere.

From the reaction summary in reaction 1) and 2), section 3.2, as oxygen reacted, acids are produced. This is an important consideration as NaOH depletion can lead to acidic conditions (formation of sulfite and sulfate), resulting in slower reactions and corrosion in the reactor. Then additional caustic may be added to the system to prevent acidic condition and maintain alkalinity of the system.

In operation, the feed heat exchanger may require cleaning to remove a calcium carbonate scale which built up over time on the cold side. This scale results from the plant water used during start-up. The high pH of the spent caustic should sufficiently soften and eliminate any inorganic fouling tendencies.

Material of construction

Typically high nickel alloy (alloy 600) is recommended for the reactor and process sides of heat exchangers in order to withstand the corrosive environment during normal operating conditions, as well as during upset.

Spent caustic from LPG and Jet Fuel Merox Unit can have varying amounts of alkalinity depending on the operation of the Units. However, it is possible to have spent caustic that does not have enough alkalinity to buffer the acid formed during oxidation. As an operational parameter, alkalinity in the spent caustic must be monitored and maintained to assure that the pH of the oxidized spent caustic remains alkaline. In general, the materials of construction that are used for these wet air oxidation systems are not necessarily resistant to corrosion under acid conditions especially in the presence of chloride ion.

Design variables

The primary variables considered in the design of a wet air oxidation system are as follows:

o Reactor temperature
o Reactor pressure (function of reactor temperature)
o Hydraulic detention time
o Oxygen partial pressure

1. Reactor temperature: Of these variables, the degree of oxidation is most sensitive to temperature. A higher degree of oxidation is achieved as the temperature is increased. Operating temperature from 240°C to 260°C, are usually applied to the naphthalenic and cresylic spent caustic found in refineries.


2. Reactor pressure: The system pressure is directly related to the system temperature and the air rate. The system is pressurized to control evaporation. As temperature and/or air rate increase, the required system operating pressure will increase.


3. Hydraulic retention times: The hydraulic retention times of between 45 and 90 minutes are typical of wet air oxidation system designs with operating temperatures above 150°C. At lower operating temperatures, longer retention times are typically required. Under these conditions, only sulfide oxidation can reasonably be achieved, and the retention time will be dependent on the total sulfur oxygen demand. Increasing retention time will positively affect the degree of sulfide destruction (oxidation) at these low temperatures.


4. Oxygen partial pressure (Oxygen solubility in the liquid): The degree of sulfide oxidation is also affected by the oxygen transfer rate from the gas to the liquid phase which is strongly dependent upon the solubility of oxygen in the liquid which, in turn, is a function of the system temperature and system oxygen partial pressure. The oxygen solubility decreases with increasing temperature to a minimum value near 100°C, then increases with increasing temperature to an infinite value at 374.1°C, the critical point of water. The oxygen solubility is also affected by the oxygen partial pressure, which is a function of the system pressure and the oxygen concentration in the gas phase. Therefore, the oxygen partial pressure can be increased at a given system temperature by increasing system pressure or gas phase oxygen concentration. As discussed above, the wet air oxidation system pressure increases as the temperature increases. This results in higher oxygen partial pressures at a given gas phase oxygen concentration. This increase in oxygen partial pressure will increase the oxygen transfer rate to the liquid phase. To attain the same level of oxygen partial pressure, a wet air oxidation system operating at a lower temperature, 150°C, will require approximately 200 percent of the stoichiometric oxygen in the gas phase compared to a system operating at 200°C which would require a gas phase oxygen concentration of approximately 120 percent of the stoichiometric amount.
   Monitoring and maintaining sufficient residual oxygen assures that treatment is taking place. Some WAO process, included Zimpro Process, will provides an on-line oxygen analyzer to allow easy monitoring of residual oxygen levels in the off gas from the system. Typically, a change in residual oxygen will be seen within a few minutes, reflecting a change in feed characteristics. When operating a system on spent caustic, residual oxygen indication will quickly tell if the oil/polymer layer in the feed tank is being pulled into the pump inlet. The change in COD between the spent caustic and the polymer layer is very large and this difference is quickly seen in terms of reduced residual oxygen.