Partial oxidation (POX) processes and technology for clean fuel and chemical production

Partial oxidation (POX) processes and technology for clean fuel and chemical production

9 Partial oxidation (POX) processes and technology for clean fuel and chemical production R. L. KEISKI, S. OJALA, M. HUUHTANEN, T. KOLLI and K. LEIVIS...

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9 Partial oxidation (POX) processes and technology for clean fuel and chemical production R. L. KEISKI, S. OJALA, M. HUUHTANEN, T. KOLLI and K. LEIVISKÄ, University of Oulu, Finland

Abstract: Interest in partial oxidation (POX) has expanded because emissions reduction is a key topic in research and design of industrial processes. For example, hydrogen production from renewable sources is an important research area worldwide. POX processes operate with nonstoichiometric fuel-to-air mixtures, at high temperatures and elevated pressures. Catalytic partial oxidation (CPOX) enhances reactions; therefore operating temperature and pressure can be decreased. Problems include hot spots and poor heat conductivity in catalyst beds during very exothermic reactions. Efforts have been made to solve these problems in partial oxidation: fluidized bed reactors and using endothermic reactions such as steam reforming with POX. Catalysts based on nanomaterials and reactor systems using membranes, photocatalysis and microreactors have been developed to advance POX processes. Key words: partial oxidation, catalysis, reactors, membranes, microreactors.



Partial oxidation (POX) as an industrial process has been known for over 100 years, starting with the invention of synthesis gas (syngas) production. The POX method has been used in many industrial applications, e.g. in syngas production and ammonia synthesis as well as in production of valuable intermediates, e.g. maleic anhydride, ethylene oxide and phenols. Recently, POX has also been applied in connection with fuel cells and in catalytic processes following ‘green’ chemistry and engineering principles (Mattos and Noronha, 2005). Syngas contains varying amounts of carbon monoxide and hydrogen. These compounds are important reactants in the chemical industry. In the early years of the industry, going back over a century, gasification of coal and carbonaceous materials were often conducted using non-stoichiometric amounts of oxygen. The first attempt to formally produce water gas from carbon was not successful. In 1926, two engineers named Vandeveer and Parr from the University of Illinois found that in their experiments replacing air with oxygen the efficiency of syngas production was increased. It was noticed that the reaction of oxygen with carbon to carbon monoxide had to be well controlled. Theoretically, when this reaction is tripled, it produces enough heat for the decomposition of water to 262 © Woodhead Publishing Limited, 2011

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hydrogen. Thus, carbon, oxygen and steam in correct mixtures produce carbon monoxide and hydrogen. The main reason that the POX reaction was taken into industrial use twentyfive years later was the availability of air separation units for oxygen production that had sufficient capacity and were economical. The first process utilising the POX method for conversion of heavy hydrocarbons and liquids, like naphtha, was ammonia manufacturing, introduced by Texaco Development Corporation. Shell has also developed thermal processes for POX in 1945 (Rostrup-Nielsen, 2002; Kerry, 2007). Syngas production can be done without catalytic materials. In that case, the partial oxidation reaction depends on the air-to-fuel ratio. Reactions also proceed from reactants to carbon monoxide and hydrogen at relatively high temperatures, e.g. at 1200 °C, and at elevated pressures. This type of process is then called thermal partial oxidation (TPOX). When sulphur content of the fuel is under 50 ppm, a catalyst can be used and the temperature can be decreased to around 800 °C and pressure to 1 atm. However, the level of sulphur must be low, since it has a well-known deactivating effect on the catalytic materials. Non-catalytic POX is currently used to produce syngas from, for example, hydrocarbons. In addition, much interest has recently been paid to advance the use of, for example, natural gas as the feedstock in catalytic partial oxidation (Berrocal et al., 2010; Ma et al., 2006; Groppi et al., 2006). Utilization of partial oxidation in different chemical processes in gas and liquid phase is an intensively studied research area, worldwide. Based on the literature, the most studied reaction in recent years both in industry and among academic research groups has been partial oxidation of hydrocarbons, such as methane and ethane over different kinds of catalytic materials. Typically used catalytic materials in catalytic partial oxidation (CPOX) processes are nickel (Ni), palladium (Pd) and platinum (Pt) (Groppi et al., 2006). Other CPOX processes that have gained a lot of attention recently are utilization of various liquid fuels for hydrogen production and as direct feedstocks for fuel cells (MeOH-SOFC) (Cimenti and Hill 2009; Otsuka 2003). New catalytic nanomaterials such as carbon nanotubes (CNTs) are tested both in hydrogen production and as catalysts in fuel cell applications (Selvaraj and Alagar, 2007; Hou et al., 2009). In addition, the use of biomass in POX processes to minimize CO2 emissions is a very important and environmentally attractive research topic. Biomass-derived fuels (e.g. bio-alcohols, glycerol) have received a lot of research interest as they are renewable, clean, carbon neutral and widely available. In addition, biomass to produce methanol or ethanol for the catalytic partial oxidation for hydrogen generation has recently been studied. The catalysts used in the biomass-derived fuel processing can be divided into four categories: oxide catalysts (Al2O3, V2O5, ZnO), noble metal catalysts (Rh, Ru, Pt and Pd), base metal catalysts (for example Ni, Co, Cu) and enzymes (e.g. cellulase). The technology for production of hydrogen through the production and

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utilization of biomass-derived fuels is further advanced than direct utilization of solid biomass, such as wood and straw. Thus, the two-step utilization process (biomass → biomass-derived fuel → hydrogen) is more reasonable until a new, cost-effective and safe solution for hydrogen storage and transport is found (Xuan et al., 2009; Sukuraman et al., 2009; Hou et al., 2009). Gasification is a process that chemically and physically changes biomass or organics present in waste materials through the addition of heat in an oxygenstarved environment. The result of gasification includes solids, ash and slag, liquid and syngas. Therefore, gasification of plastics and waste sludges, etc, is an interesting solution to change waste to a monetary value product (Khan and Daugherty, 1992). For example, gasification of waste plastics occurred at lower temperatures than gasification of coal in a fluidized bed boiler, but the problem is polyvinylchloride (PVC), which is very detrimental for the boiler and may cause severe emissions (Nieminen, 1999). In addition, gasification of municipal wastes adds greenhouse gases, i.e. carbon dioxide. Ash that remains after gasification cannot be dumped into landfills because of its chemical properties (pH and toxicity) (The Blue Ridge Environmental Defense League, 2009).


Process technology and methods of partial oxidation (POX)

Generally speaking, in a partial oxidation process with a non-stoichiometric fuelair (or other oxidizer) mixture, the fuel is partially oxidized at elevated temperatures and pressures in a reactor to form the desired reaction products. The process may be catalytic or non-catalytic and before and after the reaction, depending on the feedstock and product contents, different separation and purification procedures can be applied. The basic technology is mature, but in recent years new patents have been applied concerning different improvements of the processes, such as catalyst development and manufacturing processes (Chen and Weissman, 2009; Kang et al., 2009) as well as novel reactor systems, e.g. for syngas production from liquids such as alcohol or oil derived from biomass, and for development of smaller-sized systems with improved process start-ups and phenomenological control (Institut Francais du Petrole, 2009; Cremer et al., 2009; Jung et al., 2009). Partial oxidation can be carried out in different catalytic reactor concepts, such as fixed bed and fluidized bed reactors, entrained flow reactors, catalytic membrane reactors and microreactors, to mention a few. The selection of the most suitable process technology depends on the reactants used and the desired products. Partial oxidation of hydrocarbons is a typical example of competing reactions; therefore special attention should be paid to the selectivity of the processes. The oxidizing reactant used is normally air or oxygen. In various industrial applications, the use of O2 as a reactant would be more economical than the additional costs caused by liquefaction and transportation of air that also contains N2. Usage of air as a

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reactant would, however, be better due to catalyst durability. Instead of the use of air, steam or CO2 can also be added to reduce overheating of the catalyst (de Smet et al., 2001; Brüggemann et al., 2010).

9.2.1 Fixed bed and fluidized bed reactors When heterogeneous catalysts are used, the CPOX reaction can be carried out for example in fixed bed or fluidized bed reactors. In a fixed bed reactor the catalyst is located in the reactor as a honeycomb or as granules fixed in a confined area of the reactor. For example, a conventional methanol reactor consists of a vertical shell and tube heat exchanger. The catalytic material, e.g. CuO/ZnO/Al2O3, is packed in vertical tubes and surrounded by boiling water (Rahimpour et al., 2008). In a fluidized bed reactor the catalyst is in a suspended layer of particulates floating inside the reactor. The catalyst can be fixed or introduced as particulates in both gas-phase and liquid-phase processes. In liquid phase, the process is not called a fluidized bed process, but may be titled, for example, a slurry reactor. In addition, in liquid phase processing more efficient mixing is needed to improve the contact between the catalyst and the reacting media. Catalytic partial oxidation has been explored in the reforming processes of alcohols and liquid fuels utilizing e.g. honeycomb type, oxide-based and CNTbased catalysts (e.g. Herbst et al., 2010; Cheetakamarla and Finnerty, 2008; Groppi et al., 2006, p. 268; Seelam et al., 2009). CNT materials can be made as membranes or as other structures that can be used in fixed bed processing (Halonen et al., 2008). The advantage of fixed bed reactors is that the separation of the catalyst from the reacting media is easy. However, in partial oxidation in adiabatic fixed bed reactors some concerns exist, namely the possibility of runaways due to gas-phase reactions occurring at high pressures (de Smet et al., 2001). In a fixed bed process there is always a temperature gradient. Similarly, it is possible to create hot spots in the reactor when the mass and heat transfer are not optimal. These hot spots may be harmful for the catalysts by causing sintering (i.e. thermal deactivation). One possibility to solve this overheating is the usage of a fluidized bed reactor. The fluidized bed reactor concept has not been intensively investigated, but it has been used, for example, in selective oxidation of propane to acrylic acid (Dubois, 2005).

9.2.2 Membrane reactors Catalytic membrane reactors have certain advantages over adiabatic fixed bed reactors, since they give higher conversions with higher selectivity. For example, Tian et al. (2010) have reported 74% selectivity to CO together with 75% selectivity to H2 with 17% CH4 conversion, when the SrTiO3 perovskite catalyst was used in connection with a dual-phase composite membrane that consisted of separate oxygen ion and electron conductor phases. The novel approach that has not yet been

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used on an industrial scale is the selective integral flow-through catalytic membrane reactor. In such a reactor the products formed are separated by a catalytic membrane, and the desired products are received either from both sides of the membrane or only from the permeate side of the membrane reactor. This type of reactor is mainly used in gas-phase reactions. It can, however, also be used in the liquid fuel conversion reactions by POX using, for example, glycerol as the feedstock (Westermann and Melin, 2009; Enger et al., 2008). Membrane-type reactors have also been tested in the methanol synthesis reaction from syngas (Rahimpour et al., 2008). Another type of membrane reactor is the distributor-type reactor where a membrane is used to control the addition of the reactant to the reaction mixture, which helps to avoid the hotspots and problems connected with oxygen usage. When the limiting reactant is fed evenly along the reactor, the existence of hotspots and unwanted side reactions can be minimized. In addition, air can be used as a reactant when the perm-selectivity of the membrane is suitable to allow only oxygen to pass through to the reacting zone (Westermann and Melin, 2009).

9.2.3 Microreactors Microreactors can provide significant advantages both in research by providing high-throughput experimentation and in processing by providing reaction conditions that may otherwise be difficult to achieve or maintain (Hessel et al., 2004). The use of microreactors in partial oxidation applications is in the development stage. Examples of research can be found from the literature and textbooks, e.g. hydrogen generation by partial oxidation in microreactors, methane to methanol, propene to acrolein, isoprene to cirtaconic anhydride utilizing microreactor technology (Hessel et al., 2004; Ehrfeld et al., 2000). The production capacity with microreactors is rather easy to increase by increasing the number of reactors instead of scaling up. Microreactors are also suitable for high-temperature oxidation in explosive regimes, since they provide safe operation due to extremely small reactor volumes, well-specified reaction conditions, low heating inertia allowing direct control of reaction temperature and efficient heat exchange due to large surface-to-volume ratios. The improvements in selectivity can be achieved by accurate setting of the gas residence time and avoiding hotspots in the reactor. Microreactors also provide less environmental impact, since, for example, they allow production of harmful reactants on-site and thus avoid transportation problems (Hessel et al., 2004; Ehrfeld et al., 2000). Microreactors have been tested in several applications including, for example, ethylene partial oxidation to ethylene oxide (e.g. Kestenbaum et al., 2000) and fuel processing for fuel cell applications. Application of microreactors in smallscale hydrogen production in connection with fuel cells for cars or electronic devices is also a very interesting application area. The use of microreactors on an industrial scale still requires development work, e.g. high manufacturing costs should be minimized, the adhesion of catalysts to the supports should be improved,

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possible corrosion problems should be solved, and particulate management in narrow channels as well as uniform mass and heat distribution should be developed. However, the precision of engineering tools has developed a lot during the past few years, and the application of microreactors is in a quite mature stage (Gavriilidis et al., 2002; Kolb et al., 2007; Becht et al., 2009).

9.2.4 Photocatalytic reactors Photocatalysis has been studied as a method for the selective oxidation of aromatics and alkanes to corresponding oxygenated compounds. The production of highvalue compounds from their relatively inexpensive raw materials has been studied recently more and more intensively. Partial photocatalytic oxidation has been reported to produce various intermediates, which can be utilized in, for example, fine chemicals production (Shimizu et al., 2002; Kluson et al., 2005). Photocatalysis is also studied in hydrogen and syngas production from hydrocarbons and alcohols as well as in the oxidation of aromatics (e.g. Palmisano et al., 2007; Wu et al., 2008). Purification of water by photocatalytic POX is one of the possible methods to use partial oxidation for higher hydrocarbons abatement (Manzavinos et al., 1996). Photocatalysis is used in purification of wastewaters and cleaning of air. In these application areas the amounts of organic pollutants are relatively low, and thus traditional methods are not suitable. TiO2 based catalysts are used in these reactions. In addition, photocatalytic reactors are used in drinking water purification (Du et al., 2006). A photocatalytic reactor system consists in its simplest form of a reactor vessel constructed from UV-transparent material with a UV light source outside. The UV source can also be set inside the reactor. The photocatalytically active materials (e.g. TiO2) can be immobilized on e.g. quartz or ceramic particles, or they can be used in the form of a powder in slurry reactors. However, the powder-like TiO2 may turn to a milky liquid and thus decrease the penetration of UV light into the liquid, leading to lower reaction rates. In addition, separation techniques are required to remove the powder-like TiO2. Immobilization of TiO2 is used to diminish the number of separation steps after the reaction and also to improve the UV radiation in the reactor (Ray and Beenackers, 1998; Ling et al., 2004; Kanki et al., 2005). Some studies have been carried out to utilize photocatalysis in catalytic partial oxidation reactions; however, the technique is not yet found to be technologically and economically feasible on a full commercial scale. Pilot-scale plant and microreactors have been applied to fine chemicals production, e.g. in photooxidations of quinones, a-terpinene, and citronellol (e.g. Stankiewicz, 2006; Schiel et al., 2001; Oelgemöller et al., 2007; Pennemann et al., 2004). Catalytic partial oxidation can be used as a pre-treatment method before the treatment of e.g. wastewater in a biological stage. Various organic compounds are not

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biodegradable and with partial oxidation these compounds can be transferred to, for example, phenols and organic acids, which are easier to oxidize by biological methods (Manzavinos et al., 1996; Carp et al., 2004).

9.2.5 Other suggested reactor systems In the literature, the partial oxidation method is also suggested for several other processes. For example, Smit et al. (2004) described the development of a reactor system where the partial oxidation reaction of methane is combined with air separation and a recuperative heat exchanger inside the same apparatus. The developed system is called the reverse-flow catalytic membrane reactor (RFCMR) method. Recently, Dobrego et al. (2008) have shown interest in reverse-flow operation, and they have modelled methane partial oxidation in such a process for reactor scale-up and optimization purposes. Partial oxidation can also be carried out with sub- and supercritical water. Armruster et al. (2001) have tested propane partial oxidation towards methanol, acetic acid and acetone, and found that transition of a reaction mixture from the sub- to the supercritical region enhances the reaction significantly. In this case, the effect of a catalyst was not that significant as the effect was caused by the supercritical conditions. The processing conditions in these experiments were 400–500 °C at maximum 245 bar pressure for six hours. The oxidant used in the process was synthetic air. Sato et al. (2003) have studied the upgrading of asphalt with partial oxidation in supercritical water. They found out that, in the studied conditions, sulphur can be removed. Desulphurization of residual oil by partial oxidation in supercritical water is also studied by Yuan et al. (2005). Plasma can be used to enhance the catalytic reactions in partial oxidation. Sobacci et al. (2002) have studied hydrogen production from liquid hydrocarbon fuels by a combined catalytic reactor and non-equilibrium plasma source, i.e. pulsed corona discharge achieving high hydrogen yield at 800 °C. The effect of plasma processing was more significant at lower temperatures when the catalytic reactions were not that important. The plasma reactors could be interesting in industrial applications, especially in remote areas. These reactors are more compact compared with the currently used technology. They also give greater variety in operating modes, which can be helpful when load variations are needed. One possible application could be in connection with fuel cells. However, plasma reactors should be developed further to minimize their high pressure and electrical energy demands (Pietruszka et al., 2004).


Basic partial oxidation reactions

The production of chemicals by partial oxidation reactions has been an essential process for several decades. Feedstock for partial oxidation reactions contains a variety of chemicals, biomass, crude oil etc. In the POX process the air–reactant

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ratio has to be strictly controlled. This means that the amount of oxygen is less than that of the feedstock reactant, and more CO is formed than CO2. Besides this, it has been proven that the product yield is higher when temperature is increased. In addition, the effect of pressure has been found to have an effect on the yield. Thus, the basic reactions are depending on the air–reactant ratio (for example for CH4:O2 this is 2:1), temperature and pressure. The general partial oxidation equation without a catalyst (POX) can be presented as (Rostrup-Nielsen, 2002): [Reaction 9.1] The general partial oxidation equation with a catalyst (CPOX) can be expressed as follows (de Smet et al., 2000): [Reaction 9.2] Selectivity toward CO and H2 can be lower at high temperatures because of the following exothermic non-selective secondary reactions (de Smet et al., 2000): [Reaction 9.3] H2 + ½O2 → H2O

[Reaction 9.4]

CO + ½O2 → CO2

[Reaction 9.5]

However, catalytic partial oxidation has a significant drawback, since there is a possibility of carbon deposition via the exothermic Boudouard reaction (de Smet, 2000): 2CO ↔ C + CO2

[Reaction 9.6]

Catalyst can be deactivated by endothermic decomposition of hydrocarbons (de Smet et al., 2000): [Reaction 9.7] Finally, carbon can also be formed on the catalyst surface when CO is reduced via the following exothermic reaction (de Smet et al., 2000): CO + H2

C + H2O

[Reaction 9.8]

The Boudouard and CO reduction reactions are favoured at low temperatures and high pressures, whereas for hydrocarbon decomposition the favourable temperature and pressure conditions are high temperatures and low pressures.

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9.3.1 Synthesis gas production Syngas and thus hydrogen can be produced commercially by three different ways: partial oxidation (POX), methane steam reforming (SMR) and autothermal reforming (ATR) (Papavassiliou et al., 2010; Silva et al., 2008). Syngas production can, for example, be carried out in an adiabatic fixed bed reactor with indirect formation of syngas via total oxidation followed by steam reforming and watergas shift reactions (de Smet et al., 2001). The current commercial POX process for syngas production is via the noncatalytic route. The process requires high temperatures (1200 °C) and elevated pressures. For the catalytic processes the reported temperatures are lower, ranging from 600 °C to about 900 °C and the pressure used is around 1 bar. Recently, research both in academia and industry dealing with syngas production has been focused on various light paraffin (CnHm+2) based raw materials such as C1−C3 (Groppi et al., 2006; Ferreira et al., 2010; Ma et al., 2006; Choque et al., 2010), higher liquid hydrocarbons e.g. isooctane and n-heptane (Cheetakamarla and Finnerty, 2008; Puolakka et al., 2006), and also alcohols like methanol (Yang and Liao, 2007), ethanol (Barthos et al., 2008; Silva et al., 2008; Liguras et al., 2004) and glycerol (Iulianelli et al., 2009, 2010). In addition, liquid fuels such as gasoline and diesel (Cheetakamarla and Finnerty, 2008; Mundschau et al., 2009) have been studied as feedstocks in the production of syngas. Partial oxidation of light hydrocarbons (e.g. methane) is found to be more energy efficient and more selective than steam reforming (Berrocal et al., 2010). Reaction mechanisms have been proposed for partial oxidation of liquid hydrocarbons and alcohols such as ethanol. According to Wang and Wang (2008) the main partial oxidation reactions in the syngas production by POX from partially oxidized hydrocarbons and ethanol can be presented as follows: C2H5OH + ½O2 → 2CO + 3H2 C2H4O + ½O2 → 2H2 + 2CO

[Reaction 9.9] [Reaction 9.10]

The optimal reaction temperatures in catalytic partial oxidation of higher liquid hydrocarbons and fuels such as bio-ethanol are found to be 600–800 °C. The feed is vaporized above the boiling point temperature of the inlet fuel, e.g. 250 °C for n-decane (b.p. 174 °C), 400 °C for n-hexadecane (b.p. 287 °C), and 150 °C for ethanol (b.p. 78 °C) (Krummenacher et al., 2003; Seelam et al., 2009).

9.3.2 Methane to methanol Methane to methanol is a very important reaction for producing methanol for use as a solvent and as a feedstock for the chemical industry. Methanol is used in various industrial processes as a chemical precursor. Partial oxidation of methane to produce methanol has attracted a lot of interest during the last decades. Partial

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oxidation of methane can take place both via homogeneous and heterogeneous reactions (Dallos et al., 2007; Zhang et al., 2008b). Various catalytic materials such as Mo, Cu, W, Ga and Zn oxides and the mixtures of oxides and various metals supported on oxides have been tested in the reaction (Taylor et al., 1998). The overall reaction from methane to methanol is presented below: CH4 + ½O2


∆H = −126 kJ mol−1

[Reaction 9.11]

Temperature and pressure used for the commercial methanol synthesis are below 300 °C and 50–100 bar, respectively. Methane can be oxidized also to CO and CO2 but, in addition, the formed methanol may further be oxidized to CO2, e.g. in environmental applications (Dallos et al., 2007; Zhang et al., 2008b). However, CO2 may also be used as a raw material in methanol production from CO2 and H2 (Toyir et al., 2009; Raudaskoski et al., 2009). Dallos et al. (2007) have reported derived kinetic models for methane partial oxidation reactions over various catalysts. They have used both homogeneous chemical reactions in gas phase and catalytic reactions over catalysts based on the findings and calculations from the literature. As a result, methane partial oxidation to methanol can be presented by the following eight reaction steps, where M presents reduced and MO oxidized catalytic sites (Dallos et al., 2007): CH4 + MO → MOCH3 + ½H2

[Reaction 9.12]

MOCH3 → MOCH2 + ½H2

[Reaction 9.13]

MOCH3 + MO → MOCH2OM + ½H2

[Reaction 9.14]


[Reaction 9.15]


[Reaction 9.16]


[Reaction 9.17]

MOCH2OM → 2M + CO2 + H2

[Reaction 9.18]

MOCH3 + ½H2 → M + CH3OH

[Reaction 9.19]

Many studies have agreed on the level of methanol selectivity of 30–40% when methane conversion of 5–10% (at 450–500 °C and 30–60 bar pressure) is achieved. At lower pressures catalysts are needed. However, the current process for the direct conversion of methane to methanol is not yet competitive with the indirect one that occurs via syngas (Otsuka and Wang, 2001). The kinetic modelling of methane to methanol reaction including formaldehyde as well as carbon monoxide formation has been studied by e.g. McCormick et al. (2002).

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9.3.3 Methanol and mercaptan to aldehydes Partial oxidation can be used also in emission abatement. The more economical and environmentally sound method compared to the total combustion of organic emissions is the partial oxidation of the gaseous emissions to valuable products. For example, the use of methanol and/or methyl mercaptan as a reactant to produce formaldehyde has been studied by many research groups (Burgess et al., 2002; Forzatti et al., 1997; Isaguliants et al., 2005; Wachs, 1999; Niskala et al. 2010). The reactions relevant for the preparation of valuable compounds in pulp and paper integrates instead of burning contaminated methanol involve the following main partial oxidation reactions (Burgess et al., 2002): CH3OH + ½O2 → CH2O + H2O

∆H = −156 kJ mol−1 [Reaction 9.20]

CH3SH + 2O2 → CH2O + SO2 + H2O

[Reaction 9.21]

The byproduct formation, such as CH3SSCH3, is taking place during the processing of the reactants: 2CH3SH + ½O2 → CH3SSCH3 + H2O

[Reaction 9.22]

In addition, formaldehyde may be oxidized towards the total oxidation products (CO2 and H2O) or partially by the following reaction: CH2O + ½O2 → CO + H2O

[Reaction 9.23]

The commercial formaldehyde process uses methanol and air as the feed mixture and either silver or metal oxides are applied as catalysts (Gerberich and Seaman et al., 1994). When using the silver catalyst the reactions occur at atmospheric pressure at temperatures between 600–650 °C. The overall yield is reported to be around 86–90%. The formaldehyde process, which uses a metal oxide catalyst, can also be operated at atmospheric pressure and the temperatures are then remarkably lower, being between 300–400 °C. The overall plant yield is 88–92%. Various catalysts have been widely tested for methanol partial oxidation reactions. Vanadium-based catalysts, particularly, have been under intensive investigation (Ai, 1978; Deo and Wachs et al., 1994a, b; Forzatti et al., 1997; Isaguliants et al., 2005; Kijenski et al., 1986; Mann and Dosi et al., 1973; Roozeboom et al., 1981). For example, V2O5/TiO2, V-Mg-O and silica-supported Sb-V mixed oxide catalysts have been studied (Forzatti et al., 1997; Isaguliants and Belomestnykh, 2005; Zhang et al., 2008a). Oxidation of methanol over vanadium oxide based catalysts follows the Mars–Van Krevelen mechanism. First, methane is adsorbed dissociatively on the surface of the catalyst to form surface methoxy and hydroxyl intermediates. The methoxy intermediate then decomposes to gas-phase formaldehyde and another hydroxyl that can further react with other hydroxyls to form water. When

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water is desorbed from the catalyst surface, it reozidizes with oxygen atoms in the gas phase. Non-selective side reactions include e.g. readsorption of formaldehyde as formates that can decompose to CO and CO2. The rate-determining step is the decomposition of surface methoxy to formaldehyde. According to Holstein and Machiels et al. (1996) it gives the following rate equation, where the rate is first order to methanol concentration and negative half-order to water [9.1] where krds is the rate constant for the surface decomposition, K1 and K2 are adsorption equilibrium constants and pi is gas-phase partial pressure of compound i (Burcham and Wachs et al., 1999). Only a few articles can be found describing the reaction mechanism and kinetics of the selective partial oxidation of methyl mercaptan to formaldehyde. More studies related to desulphurization exist. For example, Dvorak et al. (2001) have studied the methanethiol reaction on ZnO and Cs/ZnO. They found out that methane reacts on ZnO to from mainly CO and methane, but also formaldehyde, ethane and a mixture of ethylene and acetylene are formed. At temperatures less than −173 °C, methyl mercaptan adsorbs on Zn2+ as a thiolate intermediate, which is stable up to 227 °C and then breaks to a methyl intermediate and atomic sulphur. Above 227 °C carbon is removed from the surface as gaseous products, but sulphur remains bound to the zinc sites on the surface. The decomposition of methyl mercaptan is very different from methanol. For methyl mercaptan, the C–S bonds are almost completely broken, whereas for methanol the C–O bond is not cleaved.

9.3.4 Other partial oxidation reactions in chemicals production Maleic anhydride production is done using n-butane, pentane and pentene, benzene or propane as starting materials. The process using n-butane as the starting molecule is, however, nowadays the only industrially used reaction for maleic anhydride production (Fernández et al., 2010; Chen et al., 2007; Gascón et al., 2006; Ivars et al., 2010; Ozkan et al., 1997). Gascón et al. (2006) have proposed a reaction mechanism for the partial oxidation of n-butane to maleic anhydride (MA). The proposed mechanism is divided into nine reactions steps showing the complicated nature of the partial oxidation reaction: O2 (g)

O2 (a)


[Reaction 9.24]

O2 (a) → O(surf)


[Reaction 9.25]

C4H10 (g)


[Reaction 9.26]

C4H10 (a)

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[Reaction 9.27]

O2 (a) + C4H10 (g) → CO2 (g)


[Reaction 9.28]

O2 (a) + C4H10 (g) → MA(g)


[Reaction 9.29]

O(surf) + C4H10 (a) → MA(g)


[Reaction 9.30]

O(surf) + C4H10 (a) → CO(g)


[Reaction 9.31]

O(surf) + C4H10 (a) → CO2 (g)


[Reaction 9.31]

MA(g) + O(surf) → CO2 (g)


[Reaction 9.33]

The first three mechanistic steps are the reactions between the active surface of the catalyst and oxygen. Then adsorbed surface oxygen and gas phase butane may react to maleic anhydride and CO2. Maleic anhydride can also be oxidized in the presence of surface oxygen to CO2.

9.3.5 Partial oxidation intermediates, reaction pathways and catalyst leaching Most of the studies done in the field of chemical production concentrate on the oxidation reactions of organic matter; however, knowledge about the intermediates formed during the reaction by partial oxidation gives valuable information for the further development of processes by integration of, e.g., biological and chemical treatments. The catalytic partial oxidation can be utilized as a pre-treatment method before the biological treatment stage of, e.g., organic pollutants in water. Various organic compounds are not biodegradable as such and with partial oxidation these compounds can be transferred, for example, to phenols and organic acids, which are much easier to oxidize by biological methods (Manzavinos et al., 1996). One example of environmentally benign fine and intermediate chemicals manufacturing is citral production from isobutene and formaldehyde (BASF process). In this process, silver-based catalysts are applied. Citral is a valuable intermediate for the production of carotenoids and vitamin A. In addition, citral is used as a starting material for ionones for perfumes and cleaning substances (Hoelderich, 2000). Based on the reaction intermediates that have been analyzed during the process, the hypothesis for the reaction pathways can be made. This is important when reaction kinetics is evaluated. Catalyst leaching has to be taken into account for three main reasons. According to Manzavinos et al. (1996) an additional separation step may be necessary for the removal of leached metal ions; secondly, the continuous leaching of heterogeneous catalysts may lead to the complete deactivation and activity loss; and thirdly, the

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metal ions dissolved to the liquid mixture may be responsible for the reactions occurring in the reactor instead of reactions over the heterogeneous catalyst itself (Manzavinos et al., 1996).


Catalysts utilized

CPOX technology for methane is in a pre-commercialization stage even though the catalysts for the process were invented almost 100 years ago (Enger, 2008). Schmidt and co-workers have done pioneering work in CPOX monolithic catalysts development (Kestenbaum et al., 2000; Krummenacher et al., 2003; Wanat et al., 2005). Typically, the catalysts widely tested in methane-to-syngas processes are various supported Ni catalysts. However, even if Ni is highly active, it suffers from carbon deposition and metal sintering. In addition to Ni, platinum group metals, Pt, Rh, Ru, Pd, have been tested for the CPOX reactions (Enger, 2008; Tian et al., 2010; Rabe et al., 2005; Papavassiliou et al., 2010). Zeolites, silica and alumina based oxide catalysts, and CNT-based catalysts are studied by many research groups. CNT materials are developed and studied in partial oxidation reactions, e.g. in fuel cells, in n-butane CPOX reaction to maleic anhydride and in oxidation of cyclic alkanes. Metal-loaded CNTs have shown good activity in partial oxidation reactions and, in addition, as novel support materials providing a support into which the active metal can be impregnated as remarkably small nanoparticles. In addition, titanium oxide-based catalysts have been used in photocatalytic partial oxidation reactions (Selvaraj and Alagar, 2007; Chen et al., 2007; Li et al., 2003; Kluson et al., 2005). In Table 9.1 catalysts used in some CPOX reactions are presented.


Process control and modelling techniques

A steady-state, one-dimensional heterogeneous reactor model has been created to design the adiabatic fixed bed catalytic (supported Ni) partial oxidation reactor for methanol and hydrogen production by de Smet et al. (2001). It has been found that the adiabatic reactor should be feasible in hydrogen-to-fuel cell applications due to the low pressure and safer operation. Dallos et al. (2007) have modelled the direct synthesis of partial oxidation of methanol in a gas–solid–solid reactor. An extensive numerical simulation of a prototype TPOX reformer operating with methane has been done by Vourliotakisa et al. (2008). This prototype hydrogen production system involved POX of methane in an inert porous material. Computations were performed on the basis of CFD simulations in a reactor network incorporating full gas-phase chemistry and the results were successfully compared against hydrocarbon species determined experimentally. Computational results were used to identify the kinetic pathways for hydrocarbon partial oxidation, molecular growth and pollutant formation as well as to identify optimum reformer operating conditions.

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Table 9.1 Catalytic materials used in some CPOX reactions CPOX reaction



Methane to methanol

FePO4, Mo, Cu, V, Zn, Ga2O3 Taylor et al., 1998; Dallos W 2 O3 et al., 2007

H2 production by POX

Pt/CeO2, metal loaded carbon nanotubes (CNTs)

Mattos and Noronha, 2005; Seelam et al., 2009; Eswaramoorthi et al., 2006

Methanol mercaptan to formaldehyde

V2O5/TiO2, V-Mg-O and silica supported Sb-V mixed oxide catalysts

Forzatti et al., 1997; Isaguliants and Belomestnykh, 2005; Zhang et al., 2008a

n-Butane or n-pentane to maleic anhydride

V-based catalysts, vanadium-phosphorusoxide (VPO) CNT-based vanadium

Chen et al., 2007; Ivars et al., 2010; Ozkan, 1997

Ethane to ethylene oxide (EO)

Ag/Al2O3 and Pt, Pd, Ir, Ru-based catalysts

Hoelderich, 2000

Wastewater purification

Heterogeneous catalysts: Fe oxide, Cu/Zn/Al2O3, Co/Bi oxide, Pt/Al2O3

Manzavinos et al., 1996; Carp et al., 2004

Homogeneous catalysts: Zn(NO3)2, Cu(NO3)2.3H2O, (NH4)2SO4·FeSO4·6H2O, Co(NO3)2·6H2O Biomass-derived fuel process

Oxide catalyst, noble metals, base metals, enzymes

Photocatalytic oxidation Metal-loaded TiO2 of aromatics and alcohols

Xuan et al., 2009

Carp et al., 2004

CPOX has potential for producing hydrogen that can be fed to fuel cells for portable power generation. In order to be used for this purpose, CPOX must be combined with other processes, e.g. water–gas shift and preferential oxidation, to produce hydrogen with a minimal carbon monoxide content (Hohna and DuBois, 2008). Active research has been carried out in optimizing the production routes utilizing partial oxidation of hydrocarbons as one reaction step, e.g. in H2 production for fuel cells. The optimization of the combined carbon dioxide reforming and partial methane oxidation processes over a 1% Pt/Al2O3 catalyst has been studied by Larentis et al. (2001) using empirical and phenomenological modelling approaches and statistical experimental design. The aim was to optimize the production of syngas with the H2-to-CO ratio close to 1 for the further use of syngas. Empirical polynomial models were used to analyze the effects of process variables on the response factors. The final correlation coefficients obtained were

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above 95%. The phenomenological model was created based on mass balances, and the obtained correlation coefficients were above 95% for CH4 and N2, 90% for CO2 and H2O and 70% for H2 and CO. The empirical modelling approach was found to be more efficient and simpler. It also led to better results compared to those obtained by the phenomenological model approach. Concerning the control of the fuel processor system (FPS) consisting of hydrogen production (e.g. CPOX) and purification connected to utilization processes, at least two control options are of utmost importance: assuring the required hydrogen production for the FPS as a whole, and keeping the CPOX at optimum temperature (Pukrushpan et al., 2006). The system should react fast to changes in load to guarantee correct system operation. Undersupply might lead to stack starvation that can damage the stack (Song et al., 2000). Oversupply, on the other hand, leads to wasting hydrogen (Song et al., 2000). A too-high CPOX temperature will destroy the catalyst while a too-low temperature slows down the reaction (Pukrushpan et al., 2006; Zhu et al., 2001). The importance of temperature control and thermal management as a whole is also discussed in Qi et al. (2007). Dynamic models are needed for control design and production optimization because of variable interactions and a high degree of process integration. The processes are usually tightly connected with each other with no intermediary storages. Pukrushpan et al. (2005, 2006) have presented a dynamic multivariable model for an FPS connected to a proton exchange membrane fuel cell (PEMFC). They have also shown how control engineering tools (Bristol’s relative gain array (RGA), and observability measures) are used in systems analysis and controller design. Sopeña et al. (2007) have reported on optimization and control of an FPS consisting of an oxidative steam reformer, water gas shift (WGS) and preferential oxidation PrOX connected to a 5 kW PEMFC. Conventional proportional/derivative/integral (PID) controllers have been used in flow and temperature controls. Lin et al. (2006) tested two control strategies for an FPS consisting of autothermal reforming (ATR), water gas shift (WGS) and preferential oxidation PrOX reactor producing hydrogen for a 2–3 kW PEMFC. The first strategy is based on manipulating methane feed flow rates (on-supply control), and the second on manipulation of the product hydrogen flow (on-demand control). Both provide the system with reasonable responses, but the on-demand control seems to respond faster in load-change situations. Wu and Pai (2009) introduce a Matlab/SimulinkTM model for a PEMFC and FPS consisting of combined steam and PO-reforming of methanol, heat exchangers, water gas shift (WGS) and preferential oxidation PrOX reactor. They also apply a multi-loop fuzzy incremental control for the system and test it with simulations.


Advantages, limitations and optimization

CPOX has been investigated for many years since it has many advantages compared to POX. In CPOX, the reaction is initiated catalytically and thus is flameless as opposed to ATR and POX. The reactions have extremely short

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residence times, in the order of milliseconds. In CPOX processes, selectivities and conversions up to 90% can be achieved. The advantage in the CPOX reaction is that it has a low energy demand (mildly exothermic) and it is kinetically controlled (Joensen and Rostrup-Nielsen, 2002). The partial oxidation reactor is more economical with heat, and can be combined with endothermic reactors such as steam reforming or dry reforming using carbon dioxide to make these reactions more energy efficient (York, 2003). In the process where syngas is produced by steam reforming and steam cracking of higher alkanes, e.g. diesel fuel or methane, some obstacles for industrial application exist, such as flames during vaporization and mixing, soot formation associated with combustion of fuel-rich gases, and coke formation on reactor walls and on catalysts. The most important is the knowledge about how to prevent hotspots in the catalyst bed caused by exothermic heat of reaction and poor heat conductivity. These hotspots have a negative effect on the catalyst stability. The problem can be avoided by using fluidized bed reactors or combining the partial oxidation with some endothermic processes such as CO2 or steam reforming (de Smet et al., 2001; Krummenacher et al., 2003; Groppi et al., 2006, p. 268). The advantages of the gas–solid–gas type reactor in methane to methanol reaction are the high yields and that the whole process is performed in a single step. However, the limitation of this application is the complexity and interactions between the catalyst particles and the gas phase (Dallos et al., 2007). Limitations of methanol production in membrane reactors are thermodynamic equilibrium limitations, catalysts deactivation and large variation in stoichiometry. However, the advantages of this kind of a procedure are overcoming the limitations set by thermodynamics leading to remarkably higher conversions. In addition, selectivity can be improved by membrane reactor technology (Rahimpour et al., 2008).


Future trends

The trend in the chemical industry to benefit from green chemistry and engineering approaches and to act towards resources use optimization and waste minimization are important issues when POX reactions are designed and utilized. The increasing role of biomass as a starting material, the innovative and optimal integration of different reaction steps and the efforts to combine the best features of biocatalysis, homogeneous and heterogeneous catalysis into one pot are also the driving forces for innovations in the use of POX methods. Some of the newest research results and conclusions have already been highlighted in the earlier sections, e.g. use of membrane and microreactor concepts, CNT based catalysts and photocatalysis in CPOX reactions. The driving force is also to produce new innovative and valuable partial oxidation products and intermediates from biomass and waste materials. One future trend is to decrease the formation of greenhouse gases in hydrogen production and consumption processes. Hydrogen is expected to become an

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important energy carrier for sustainable energy consumption due to its clean-burning nature. Hydrogen can be used as a fuel either directly in internal combustion engines (ICE) or, indirectly to supply electricity using PEMFCs (Eswaramoorthi et al., 2006). Presently used hydrogen production methods, such as steam reforming of natural gas or oil-derived naphtha, partial oxidation of heavy oil residues and coal gasification, utilize fossil resources and produce large amounts of CO2. Biomass is carbon neutral, thus, utilization of biomass as a hydrogen resource is a method for suppressing greenhouse gas emissions. In addition, biomass-derived liquids contain many oxygenates that affect the performance of the reforming catalyst. For hydrogen generation, the current biomass technologies include gasification, pyrolysis, conversion to liquid fuels by supercritical extraction, liquefaction, hydrolysis and so on (Holladay et al., 2009). However, direct biomass utilization to hydrogen needs more research, since unwanted CO2 is formed during the reactions. One solution is to use biomass gasification coupled with dry reforming in a ‘Y’-type reactor with Ni-based catalysts (Hu and Lu, 2010). This is a green and effective method of storage and utilizing CO2. Actually, CO2 is a valuable chemical feedstock, which competes with the CO-based C1 building blocks, that is CO, phosgene and methanol. The utilization of secondary CO2 as a feedstock for producing chemicals is an interesting challenge to catalysis and industrial chemistry (Centi and Perathoner, 2004; Gong et al., 2007; Ballivet-Tkatchenko and Sorokina, 2003; Sakakura et al., 2007; Lee et al., 1989; Raudaskoski et al., 2009; Ballivet-Tkatchenko et al., 2006a, 2006b). The key motivations to produce chemicals from CO2 is the possibility that CO2 can lead to totally new and innovative polymeric materials. The new routes to existing chemical intermediates and products could also be more efficient and economic than the current methods. Hydrogen as a fuel in mobile fuel cell applications has technical limitations associated with hydrogen storage, safety and refuelling restrictions. Methanol is an H2-rich liquid fuel, which can easily be converted into a hydrogen-rich gas in the temperature range 200–300 °C using a catalytic reactor. The other advantage is that the hydrogen/carbon ratio is high, and there is no carbon–carbon bond, minimizing the chance of coke formation (Eswaramoorthi et al., 2006). Hydrogen can be produced from methanol by steam reforming, decomposition, POX or oxidative steam reforming techniques. CPOX has some obvious advantages, since it is an exothermic reaction, which is excellent in cold-start conditions. The most widely used catalysts in partial oxidation of methanol (POM) are Cu–Zn-based catalysts over Al2O3 and SiO2 supports. The new carbon forms such as CNTs and carbon nanofibres (CNFs) have shown their potential as POM catalyst materials, since they have many advantages such as electronic properties, high mechanical strength and thermal stability and the possibility to create anchoring sites over the conventionally used supports, such as carbon, Al2O3 and SiO2. Finally, the new and improved partial oxidation processes, i.e. CPOX and POX, need sustainability analysis. Economic, ecological and social sustainability need

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to be taken into account when designing new and innovative CPOX and POX processes. This is to guarantee that the resources of our globe will be used economically and ecologically and the new processes are non-polluting, conserving energy and natural resources, and that they are economically efficient, safe and healthy for workers, communities and consumers, and also socially and creatively rewarding for all employees.



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POX processes for clean fuel and chemical production


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