CHAPTER 2: Selective Hydrocarbon Oxidation 2.1 Introduction
2.5 Oxidative dehydrogenation of paraffins
The oxidative dehydrogenation of paraffins to olefins and aromatics has much potential to change the way in which many important organic chemicals are manufactured. The increasing demand for olefins has stimulated considerable research into this new technology.
The increased use of olefins in the polymer industry and a relative decrease in their production, arising from changes in operating conditions in steam and catalytic cracking units, accounts for their global shortage [24]. Catalytic paraffin dehydrogenation for the production of olefins has been in commercial use since the late 1930's. Catalytic dehydrogenation is considered to be a relatively mature and well-established technology, whilst oxidative dehydrogenation is regarded as being in its infancy [14].
2.5.1 Oxidative dehydrogenation versus dehydrogenation
Catalytic dehydrogenation of paraffins operates commercially through processes such as UOP's (Universal Oil Products) Oleflex™ and Pacol™, Lummus Catofin™, Snamprogetti- Yarsintez fluidized bed dehydrogenation, Phillips STAR™ and Linde™-BASF. These processes use platinum-based or chromium-based catalysts. The performance of two UOP processes are shown in Table 2.5 below [14,25]:
Process
Oleflex™
Pacol™
Description
Catalytic dehydrogenation technology for production of light olefins from paraffins Dehydrogenates M-paraffins in a vapor phase reaction to corresponding mono-olefins
Feed
Propane
«-Butane Zso-butane M-Heptane w-Cio-Cu
«-Cu"Ci4
Conversion (%)
40 50 50 20 13 13
Olefin Selectivity (%)
90 85 92 90 90 90 Table 2.5: Performance of UOP's Pacol™ and Oleflex™ dehydrogenation processes [14]
In 2002 there were ten Oleflex™ process units in operation and an additional unit was under construction [14]. Approximately one million metric tons per annum of propylene and over two million metric tons of isobutylene were produced by these units [14]. Linear mono- olefins produced by the Pacol™ process are largely used to manufacture linear alkylbenzenes, which are used as detergents.
Catalytic dehydrogenation processes have several limitations [24]:
- Reactions are thermodynamically restricted - Thermal cracking is a significant side reaction
- Difficulty in separation of paraffins, olefins and by-products - Strongly endothermic reaction is energy intensive
- Short catalyst lifespan necessitates frequent regeneration
- Harsh reaction conditions may render catalyst irreversibly deactivated.
CnH2n+2 <- CnH2n H2
The thermodynamics of the above reaction are such that the paraffin is favoured by the equilibrium at relatively low temperatures and at atmospheric pressure [21,26]. The equilibrium constants for «-paraffin dehydrogenation at 500 °C are shown in Figure 2.1 below. The temperatures required to achieve ten and forty percent conversion of «-paraffins at one atmospheric pressure are shown in Figure 2.2 [14].
Figure 2.1: Equilibrium constants for n- paraffin dehydrogenation at 500 °C [14].
700 650
^ 600
£ 550 S 500 S 450
H 400 350 300
\ ^ - ^ _ ^ 40% Paraffin Conversion
10% Paraffin Conversion '—'—.
. 3 4 5 6 7 8 9 10 11 12 13 14 15 Carbon Number
Figure 2.2: Temperatures for 10 and 40%
conversion of C2-Q5 «-paraffins at 1 atm [14]
Paraffin dehydrogenation reactions are strongly endothermic. From Figures 2.1. and 2.2, it can be seen that reaction conditions become more severe with decreasing carbon chain lengths.
Ethane dehydrogenation is the least thermodynamically favoured reaction.
The use of high temperatures to increase the equilibrium conversion in catalytic dehydrogenation results in an increase of side reactions such as paraffin cracking. Coke formation and catalyst deactivation is also accelerated. Coke formation can be a very serious problem, requiring frequent catalyst regeneration. Regeneration may even be required after only minutes of operation [14,28]. Catalysts are regenerated by burning the coke of with oxygen and the heat produced by this reaction can be incorporated into the dehydrogenation process. Nevertheless, additional energy is also required. Although deactivation by coking may be reversible, some catalysts may become irreversibly deactivated by the severe reaction conditions of dehydrogenation processes.
There are four lines of research to overcome these limitations [24]:
1. Development of catalysts with better selectivity and greater resistance to deactivation 2. Oxygen-assisted dehydrogenation i.e. coupling of dehydrogenation (endothermic) with
hydrogen oxidation (exothermic)
3. Use of catalytic membranes to remove hydrogen and hence shift the equilibrium towards product formation
4. Oxidative dehydrogenation.
In oxygen assisted dehydrogenation, deactivation through coking is still a problem. It is also difficult to find a hydrogen oxidation catalyst over which all products and reactants are stable.
The use of catalytic membranes in dehydrogenation is unfavourable due the costs of these membranes. Oxidative dehydrogenation is regarded as the leading pathway to overcome the limitations of catalytic dehydrogenation [14]. In this reaction, oxygen reacts directly with the paraffin molecule on the surface of a catalyst.
C„H2n+2 + V202 • CnH2n + H20
The products of oxidative dehydrogenation reactions are olefins and water. The formation of water makes this reaction very thermodynamically favourable. In principle, almost complete conversion can occur even at low temperatures and high pressures, and this can have considerable advantages from an economic and process engineering point of view [21].
Oxidative dehydrogenation reactions are exothermic and they can be carried out at lower temperatures than dehydrogenation reactions. Thus the formation of coke and cracked products is relatively insignificant [21,26]. Frequent catalyst regeneration is not required in oxidative dehydrogenation. Oxygen may also help to remove any coke or its precursors which may form during the reaction [28].
The main limitation of oxidative dehydrogenation of paraffins is selectivity. Carbon dioxide and carbon monoxide are the most thermodynamically stable products. Suitable catalysts must therefore be found which are capable of minimizing total oxidation products. A secondary limitation of oxidative dehydrogenation involves reactor operation. These reactions are highly exothermic and special care is required during operation to prevent the possibility of reaction runaway. Another secondary limitation concerns the feed composition.
Certain feed compositions may be explosive. Safe feed compositions may not yield optimal productivity. Specially developed reactor configurations such as the fluidized-bed reactor may allow for operation within the explosion limits. Here the continuous movement of catalyst mass efficiently inhibits radical chain propagation [24]. Even so, safety is still an issue here, especially in the case of a malfunction.
Notwithstanding safety issues, selectivity problems and the economics of loosing hydrogen as a by-product, oxidative dehydrogenation is regarded as more promising than dehydrogenation [21]. It is expected that capital and operation efficiencies would be gained by eliminating the need for a furnace and for decoking shutdowns, lowering operating temperatures, lessening material demands and conducting less maintenance operations [29]. Its outlook for development on an industrial scale is thus viewed as having much potential [24].
2.5.2 Catalytic systems used for oxidative dehydrogenation
Catalysts containing either vanadium or nickel oxides or both have been reported to give the highest yields to oxidatively dehydrogenated products [21]. Unsupported vanadium pentoxide, vanadia supported on silica, alumina or titania and nickel molybdates have been extensively researched on lower paraffins [21,26].