Selective Oxidation of Hydrocarbons by Supercritical Wet Oxidation

Chapter 34

Selective Oxidation of Hydrocarbons by Supercritical Wet Oxidation

Downloaded by CORNELL UNIV on September 1, 2016 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch034

Fouad O. Azzam and Sunggyu Lee Process Research Center, Department of Chemical Engineering, The University of Akron, Akron, OH 44325-3906

Wet oxidation is a process in which partial or complete oxidation reactions occur in a supercritical water medium. Supercritical water exhibits drastically enhanced solvent power toward both oxygen and hydrocarbons, therefore permitting oxidation reactions to be carried out in a nearly homogeneous phase. The selectivity of this partial destructive oxidation can be controlled by varying the process operating parameters, thereby creating a favorable environment for desired byproducts while simultaneously reducing unwanted products. The advantages of these types of processes include; no use of classical conventional catalyst, milder temperature conditions for certain applications, very short residence time, high selectivity, and in most cases a "near zero" discharge. The selective partial oxidation of chlorinated hydrocarbons (CHC) is discussed in this paper, with particular emphasis on the recovery of reaction byproducts for reuse or further processing. BACKGROUND Wet Oxidation Wet oxidation is a chemical process in which oxidation reactions take place under a water blanket. This process utilizes water in its supercritical state as the reaction medium, and high pressure oxygen as the oxidizing agent. Wet oxidation makes use of the increased solubility of oxygen in the supercritical water phase. While only 9.2 mg of oxygen at room temperature dissolve into a liter of water, the level of dissolved oxygen in the supercritical medium is practically infinite, which results in a homogeneous mixture of oxygen and supercritical water (1,2). This permits the process to utilize a lesser amount of excess oxygen in order to achieve the maximum destruction of the waste. Supercritical water also exhibits a very strong solvent power toward most chemical species. This dramatically increased solvating power is due to the sharp increase of the fluid density as well as the polar nature of the fluid. Also, since many organics are completely miscible in supercritical water, the problem of mass transport 0097-6156/93/0523-0438$06.00/0 © 1993 American Chemical Society Oyama and Hightower; Catalytic Selective Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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resistances can be eliminated, thereby achieving a complete and unhindered oxidation reaction. In the wet oxidation process, materials partially or completely dissolve into a homogeneous, condensed-phase mixture of oxygen and water, and chemical reactions between the material and oxygen take place in the bulk water phase. This condensed-phase makes wet oxidation an ideal process to transform materials which would otherwise be non-soluble in water to a harmless mixture of carbon dioxide and water. Since oxidation reactions are also exothermic, the high thermal mass of supercritical water makes this reaction medium better suited for thermal control, reactor stability, and heat dissipation. The purpose of this research was to establish a new method for selectively oxidizing waste hydrocarbons into new and reusable products. Downloaded by CORNELL UNIV on September 1, 2016 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0523.ch034

Wet Oxidation vs. Catalytic Oxidation Partial wet oxidation or controlled wet oxidation is, in a sense, similar to that of catalytic oxidation. Catalytic oxidation provides a conventional catalyst in order to boost and control the oxidative reaction, whereas wet oxidation provides a favored atmosphere for the reaction to occur. More accurately, catalytic oxidation provides a surface upon which intimate contact between the reactants takes place compared to the thermodynamic (or fluid dynamic) contact provided by wet oxidation. In wet oxidation, it can be said that the supercritical water phase acts as the "catalyst" for the reaction. Generally, the time required for catalytic oxidation is greater than that for wet oxidation. This is commonly due to the mass transport resistances which are encountered by the bulk phase reactants as surface reactions proceed. This is particularly true in commercial applications of catalytic oxidation. On the other hand, wet oxidation occurs at a molecular scale where only a controlled contact time is required. Since wet oxidation reactions occur in a homogeneous or semihomogeneous phase, mass transport resistances are practically eliminated thereby permitting a more uniform distribution of reactants at any point in the reactor. This lack of resistance allows for more throughput per pass for wet oxidation which translates to a smaller recycling stream and therefore a more efficient process. Furthermore, by eliminating the use of conventional catalyst, the regeneration time associated with catalytic oxidation is eliminated. In wet oxidation, water can easily be recycled back into the oxidation vessel for further reuse. PILOT PLANT LAYOUT Figure 1 is a schematic representation of the wet oxidation micro-pilot plant. The system consists of three sections, an electrically heated oxidation vessel, a high pressure solvent delivery system, and a water cooled depressurization and collection chamber. A more detailed description of the pilot plant can be found in previous publications (5,4). Primary Oxidation Vessel The heart of the system is the oxidation vessel depicted in Figure 1. This vessel is a high pressure, 1000 ce, Hastelloy C-276, bolted closure reactor manufactured by Autoclave Engineers Inc. Hastelloy C-276 was chosen as the material of construction due to its excellent corrosion resistance to a wide variety of chemical process environments, which include processes utilizing strong oxidizers (5,6,7). The unit is fitted with 1/8" and 1/4" Hastelloy C-276 feed delivery and product outlet lines

Oyama and Hightower; Catalytic Selective Oxidation ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

440

CATALYTIC SELECTIVE OXIDATION


SOLVENT RESERVOIR

(?)

0®

HIGH PRESSURE STORAGE

WET OXIDATION VESSEL OXYGEN SOURCE

Figure 1. Schematic process flow diagram of wet oxidation mini-pilot plant.


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respectively. The reactor is equipped with a thermowell, cooling coil, and a top mounted air driven agitator (Magnedrive).


Solvent Delivery System Solvent (water) is delivered to the reactor via a high pressure micro-metering pump. This pump is capable of precisely delivering the solvent against a 5000 psi back pressure. The flow capacity of this unit is 6000 cc (STP)/hr at a motor speed of 85 RPM. Oxygen is delivered to the reactor via a high pressure oxygen compressor (Haskel AGT 30/75). This air driven unit is capable of pressurizing pure oxygen to a maximum pressure of 5000 psig. The compressor is also equipped with a variable pressure safety relief valve and an automated air pilot switch. Both of these safety features make it practically impossible to overpressurize the oxygen storage cylinder, thereby decreasing the chances of failure due to spontaneous oxidation caused by overpressurization of the components (8,9,10). Effluent Depressurization Depressurization of the oxidation unit is achieved with the use of a high pressure control valve (Annin Wee Willie). This control valve can be set to operate in any one of three control actions: proportional, reset, or derivative. Following the control valve, the hot products are directed to a water cooled high pressure condenser. Here they are cooled to ambient conditions before being sent to a holding vessel for GC analysis. WET OXIDATION OF CHLORINATED HYDROCARBONS Chemistry In the absence of oxygen, the thermal degradation of p o l y v i n y l chloride) involves dehydrochlorination, which gives polyene sequences followed by crosslinking. The dehydrochlorination also takes place even in the presence of oxygen or in other oxidative environments. When oxygen is present, chain scissions involving C-C bond breakages as well as dehydrochlorination take place. The relative rates of these two modes of reactions depend upon the concentration of oxygen, the temperature, the pressure in the case of supercritical wet oxidation, etc. In the supercritical wet oxidation, the concentration of oxygen in the reactive system is high and the contact between the reactants is more intimate, thus making chain scission reactions much more active. This appears to be a major reason for more rapid degradation of PVC in an oxygen environment and production of monomers and dimers. The following reaction mechanisms are proposed (efforts are currently being made to experimentally confirm these mechanisms): I. Dehydrochlorination and Oxidation RCK>

~CH -CHC1-CH -CHC1~ 2

•

2

~CH-CHC1-CH -CHC1~ 2

+o

2

~CH=CH-CH -CHC1 ~ 2

•

2C0 + H 0 + CH =CHC1 2

2

2


442


The dehydrochlorination rate increases substantially in a wet oxidation environment and this acceleration is believed to be due to peroxy radicals, which are formed by the straight oxidation of a hydrocarbon or a fraction of polymer. However, this also is yet to be confirmed experimentally. II. Dehydrochlorination and Chain Scissions ~CH -CHC1-CH2-CHC1~

•

2

CH =CH-CH=CHC1 + HC1

~CH -CHC1-CH -CHC1~


2

(2)

2

•

2

2CH =CHC1

(3)

2

α ~CH -CHC1-CH -CHC1~ 2

•

2

CH=C-CH=CHC1 + H

(4)

2

ΙΠ. Oxidation (Free-Radical Initiated) RCK> · • ~CH-CHC1-CH -CHC1~

~CH -CHC1-CH -CHC1~ 2

2

2

~CH=CH-CH -CHC1~ _ 2

+0 — ^

H

Q

.RQOCI*

_

Q #

»

~CH=CH-CH=CH~

2

CH2=CHCl + 5/2 02

C0 + H 0 2

•

(5)

2

2C02 + H 0 + HC1

(6)

2

The final products of Route III are C02, H2O, and HC1 in their stoichiometric amounts. IV. Hydrochlorination CH =CHC1 + HC1 2

•

CH C1-CH C1

(7) (8)

2

2

CH =CHC1 + HC1

•

CH -CHC1

CH =CH-CH=CH + 2HC1

•

CH3-CHCI-CHCI-CH3

2

2

2

3

2

(9)

It is conceivable that all these reactions, included in Routes I, II, II, and IV, take place competitively in the system, even though their relative kinetic rates depend on various operating parameters, in particular the oxygen concentration, the reactor residence time, and the pressure and temperature. For example, if the reaction mixture is left too long in a wet oxidation environment, the reaction would proceed to completion, resulting in producing only H 0, C0 , and HC1. Therefore, in order to 2

2


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maximize the production of vinyl chloride monomer or dimers, an optimal process condition must be sought, especially in terms of the residence time, percent excess of oxygen, temperature, and pressure.


Experimental Selective partial wet oxidation of p o l y v i n y l chloride) (PVC) was performed using the wet oxidation pilot plant described in earlier sections. Experiments were initiated by charging the primary oxidation vessel with a preweighed amount of PVC resin. A typical oxidation utilized 0.5 to 2 g of PVC. Once charged, the oxidation vessel was brought up to the desired extraction/reaction temperature and pressure by heating and the constant addition of preheated supercritical water. Oxidation temperatures ranged between 390 and 440 °C with pressures ranging from 230 to 275 atm. Once the operating parameters were established, the injection of high pressure oxygen was initiated. Oxygen flow rates ranged between 50 and 200 scc/min. Reactor effluents were then collected at preset intervals and were directed to a gas chromatograph for analysis. Results and Discussion The results of partial wet oxidation of PVC were quite encouraging. The results of these experiments are summarized in Figure 2. For this particular case, PVC was oxidized to give monomers, dimers and lighter hydrocarbons. The percent recovery of select hydrocarbons have been plotted with respect to reactor residence time. It should be noted that the percent recovery is the amount of a particular hydrocarbon which is present in the reactor at the time of sampling. This amount is based on a gas chromatograph signal which has been precalibrated and is coupled with a mass balance which accounts for the theoretical amount of carbon which has been combusted. From the figure, it can be shown that the recovery of 1,2-dichloroethane increases as the reaction time progresses. This trend continues until a recovery of 2.5% is achieved. The same is also true for 1,1-dichloroethane, however, the change in the rate of recovery is not as dramatic until approximately 1.5 minutes into the reaction. At this point, the percentage of recovery increases at a much more significant pace until a maximum of 9 percent is achieved at 1.75 minutes. Perhaps a more interesting compound is vinyl chloride. With this compound, the percent recovery goes through several changes before reaching a maximum. These changes are attributed to several different reactions which take place during the course of partial oxidation. Initially, as PVC is thermally degraded, there is a buildup of hydrochloric acid and vinyl chloride monomer. As the wet oxidation process continues, the existing monomer is destroyed by the incoming oxygen stream which causes the recovery of vinyl chloride to decrease. This is clearly illustrated in the figure between 0.75 and 1.25 minutes. This explains the decreasing nature of the recovery curve. Once the majority of this monomer is destroyed, the oxygen begins to attack the polymer and smaller molecular weight fractions thereby releasing more vinyl chloride. Since the oxygen concentration in the reactor is not sufficient to destroy this newly formed monomer, a rise in the recovery percentage is noticed. This occurs between 1.25 and 1.75 minutes until a maximum recovery of 8 percent is achieved. It should be noted that increasing the reaction time past 1.75 minutes results in a complete wet oxidation and the destruction of the hydrocarbons. Safety Safety must be an integral part of any oxidation process. Safety begins with sound equipment, such as ASME code vessels and continues through the operating and shut



444


Time (min) Figure 2. Recovery of select hydrocarbonsfromwet oxidation of chloride).

polyvinyl



34. AZZAM AND LEE


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down procedures for the equipment. This is particularly true for processes utilizing unusually oxidative materials coupled with high temperature and pressure operations. For this reason, a brief background into the safety considerations which were utilized in the undertaking of this research are included here. In the construction of the wet oxidation unit, several areas of safety were considered. Of utmost importance was that of personal safety. Since this type of operation demands the use of high pressures and temperatures, operator contact with the high pressure vessels had to be limited. To accommodate this criterion, a barrier was constructed to shield the operator from any unforeseen releases from the reactor. This barrier was constructed from 1/4 inch steel and is designed in a manner that will fully contain any releases. This barrier is also equipped with two explosion vents to direct the force of any explosions away from the main walls and into a safe area. To further maximize personnel safety, all operator assisted controls are mounted on the outside of the unit. A policy of a three level safety design was also adopted. This includes the use of utilities and safety devices. An orthogonal design was used such that interruption of any part of these services would not affect the remaining sections. Manual bypasses were also provided in order to permit operator intervention at any point in the process. Finally, any releases of material from any of the relief devices were directed into secondary holding vessels in order to prevent releases into the atmosphere. A brief but important point must also be made regarding the use of oxygen as the primary oxidant. Oxygen in itself is an oxidizer. However, high pressure oxygen (greater than 2800 psig) becomes extremely oxidative and the possibility of spontaneous detonations increases significantly should it come in contact with a hydrocarbon. For this reason, special handling techniques must be employed. All oxygen process lines, fittings, holding vessels and any other part which is in direct contact with the oxygen must be specially cleaned for oxygen service. This can be done either commercially or in house, however, the final result must be a hydrocarbon free system. Any lubrication needed in the assembly of the oxygen systems must be hydrocarbon free and specially rated for this particular application. With these types of precautions, supercritical wet oxidation reactions can be made safer, and spontaneous detonations can be eliminated. Further Potential Applications The principle of supercritical wet oxidation can be applied to several areas, including municipal waste treatment, chemical waste treatment, polymeric waste treatment, and the treatment of mildly radioactive waste. Since the basic principle of wet oxidation involves the rapid oxidation of organic material, any substance which is mildly oxidative can be subjected to this process. With such high temperature operation, the remainingfractionof inorganic material can be simply precipitated as a salt and then easily collectedfromthe bottom of the oxidation vessel. Conclusions Several conclusions can be drawn from this experimental work. 1. The operability of the wet oxidation pilot plant was successfully demonstrated, particularly from a standpoint of reaction controllability. 2. The system was successfully operated for both complete and partial oxidation of a chlorinated hydrocarbon (PVC). 3. Selectivity can be controlled by variation of process parameters and reactor residence time.


446


4. The research confirms the viability of wet oxidation as a new method of selectively oxidizing hydrocarbons into fresh and usable products. ACKNOWLEDGMENTS The authors are grateful for a generous fellowship support provided by BF Goodrich Co.


LITERATURE CITED 1. Pray, Η. Α., Schweickert, C. E., and Minnich, Β. H., "Solubility of Hydrogen, Oxygen, and Helium in Water at Elevated Temperatures", Industrial and Engineering Chemistry, 44:5, pp. 1146-1151, 1951. 2. Himmelblau, D. M., "Solubilities of Inert Gases in Water 0 C to Near the Critical Point of Water", Journal of Chemical and Engineering Data, 5, pp. 10-15, 1959. 3. Azzam, F. 0., Fullerton, K. F., Vamosi, J. E. and Lee, S. "Application of Wet Oxidation for Waste Treatment", invited paper for Symposium on The Environmental Issues of Energy Conversion Technology, paper No 87G, AIChE Summer National Meeting, Pittsburgh, PA, Aug 18-21, 1991. 4. Azzam, F. 0. and Lee, S., "Design and Operation of Wet Oxidation Mini-Pilot Plant for Complete and Partial Combustion", invited paper for Symposium on The Role of Pilot Plants in Commercialization of Processes, paper No 36E, AIChE Spring National Meeting, New Orleans, LA, Mar 29 - Apr 2, 1992. 5. Corrosion Resistance of Hastelloy Alloys, Properties Data Booklet, Cabot Corporation, Stellite Division, 1978. 6. Corrosion Resistance of Tantalum and Niobium Metals, NRC Inc., Bulletin No. 3000. 7. Metals Handbook, 8th Ed., Vol. 1, Properties Selection of Metals, American Society of Metals, Novelty, Ohio, 1961. 8. Fryer, D. M., "High Pressure Safety System Analysis- A Proposed Method", Design, Inspection, and Operation of High Pressure Vessels and Piping Systems, PVP, Vol. 48. 9. Loving, F. Α., "Barricading Hazardous Reactions", Industrial and Engineering Chemistry, Vol. 49, October 1957. 10. Pohto, Η. Α., "Energy Release from Rupturing High-Pressure Vessels - A Possible Code Consideration", Journal of Pressure Vessel Technology, Vol 101, May 1979. RECEIVED

October 30, 1992


Selective Oxidation of Hydrocarbons by Supercritical Wet Oxidation

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