In the Classroom
Steam Hydrocarbon Cracking and Reforming Michael Golombok Shell International Exploration and Production, Volmerlaan 8, 2288 GD Rijswijk, The Netherlands;
[email protected] Reactions
Steam Hydrocarbon Cracking The steam hydrocarbon cracking reaction can use a variety of feeds ranging from ethane gas to heavy gas oil (1, 2). In Europe, the predominant feed is light naphtha. For the purposes of a chemical equation, this can be represented by n-heptane. In its simplest form, the steam cracking reaction can be written as: C 7H16 C 2H4 (1) We have not included stoichiometric coefficients in eq 1 because the reaction is not driven to equilibrium and a range of fragmented products is obtained, including propylene, various C4 components, methane, higher hydrocarbons, and of course, coke. The formation of this last product is suppressed by introducing an inert gas into the reactants—steam. The introduction of steam kinetically controls the number of reactive collisions leading to hydrocarbon fragmentation. In particular, diffusion of the hydrocarbons to the wall is inhibited (wall effects can lead to catalyzed reforming). The reaction times in steam cracking are sufficiently short, around 0.3 s, so that no hydrocarbon steam reactions occur. Steam兾hydrocarbon mass ratios are typically 0.5 and the temperature is typically 800 ⬚C. 228
Journal of Chemical Education
•
Steam Hydrocarbon Reforming In steam hydrocarbon reforming (SHR), the reaction does go all the way to equilibrium and steam is one of the reactants rather than a buffer gas (3): C 7H16 + 7 H2O
7CO + 15 H2
(2)
Clearly this process involves cracking of the carbon backbone so a number of the reactions occurring for the steam cracking process are also relevant here. The equilibrium position for the stoichiometric reaction, of the type specified in eq 2, as a function of temperature is shown in Figure 1. The values are calculated from the hydrocarbon combustion program Gaseq (4) using full temperature dependent enthalpies and entropies. The calculations are based on a stoichiometrically balanced equation for full conversion to CO and H2, the synthesis gas. The reaction yield is presented as the mass fraction of CO in the reaction mixture. Reforming of three hydrocarbons is shown. Although we are comparing feeds used for naphtha cracking, methane is the principal industrial component that is steam reformed. The vast bulk of work has been on steam methane reforming (SMR):
CH4 + H2O
CO + 3 H2
(3)
The purpose of SHR in general is to generate CO and H2. Since these products are linked we may consider the reaction efficiency to be the mass yield of CO arising from equilibrium conversion. However, CO is no longer the principal fuel gas. Currently with the increasing importance of the hydrogen economy, the efficiency of the conversion is best defined by the amount of H2 that can be generated. For SMR the high hydrogen兾carbon ratio results in 3 moles H2 for ev-
100
Mass Fraction CO (%)
Many industrial processes are taught as distinct subjects, despite parallels in the chemical details. The reason for this lies in the positions of these processes within the current scheme of industrial chemical engineering. Such a schematic separation may, for example, arise from the differences between process and product engineering. An example is pyrolysis and combustion. In theory, the two conversion processes are very similar and parallel—the only difference being that the latter involves oxygen. Much of the kinetic decomposition is similar, and the formation of coke in engines is similar to that occurring in high temperature environments such as steam cracker reactors. A further conceptual split, apart from the process–product divide, is the fact that these two processes find themselves on opposite sides of the chemicals–oil industry divide. Pyrolysis is most commonly found as steam cracking at the oil–chemicals interface to generate the chemical feedstocks of ethylene and propylene. Combustion, on the other hand, sits in the oil–gas products area and is customer end-use oriented rather than a process as such. Another example of similar, yet separately treated processes across the oil–chemicals interface is steam hydrocarbon reforming and steam hydrocarbon cracking. In theory, both processes take a midrange hydrocarbon and convert it in the presence of steam, either by cracking to light olefins such as ethylene or by oxidizing to a mixture of carbon monoxide and hydrogen known as “synthesis gas” (usually abbreviated to syngas). In this article we examine these two processes focusing on their similarities and differences.
80
60
methane
40
n-heptane toluene
20
0 400
600
800
1000
1200
1400
Temperature / °C Figure 1. Equilibrium concentration of CO as a function of temperature for methane, n-heptane, and toluene. In each case the water/hydrocarbon ratio is stoichiometric, that is, for complete conversion to CO and H2.
Vol. 81 No. 2 February 2004
•
www.JCE.DivCHED.org
In the Classroom
ery 1 mole of CO. In general for an alkane of formula CmH2m+2, m moles of CO and 2m + 1 moles of H2 are generated so that the H2兾CO ratio is 2 + (1兾m). Thus SMR is clearly the most efficient hydrogen generator (Figure 1). nHeptane is shown for comparison with the steam cracking case above. (Toluene is shown for reasons to be discussed later.) The behavior with temperature is in any case roughly identical, with some slight differences in the position of the plateau owing to the different H兾C ratios of the species. The temperature needs to be higher than for the steam cracking process. High temperatures are required to dissociate the water molecules. In addition, in steam cracking the reaction does not proceed all the way to equilibrium so less severe (by comparison) conditions are used (3).
Cracking Kinetics Figure 1 indicates that the best place to carry out steam cracking controllably, so that it does not go all the way to equilibrium but gives reasonable rates, is somewhat before the equilibrium plateau is reached but at a temperature sufficient to give acceptably fast reaction rates—around 800 ⬚C. (The minimum temperature is set by the onset of cracking reactions, also the upper limit for unreactive distillation temperature, ca. 340 ⬚C.) The extent to which cracking occurs is known as the cracking severity. This is best characterized by a combination of temperature (determining the collisional energy and thus reactivity) and the residence time in the reactor (1, 5). The latter parameter represents the time the reactants experience these high temperature conditions; that is, the time to pass through the hot section of the reactor. If the volume of this hot section is V and the flow rate is Q, then the residence time, τ, is given by: τ =
V Q
(4)
The evolution of a number of key species during the steam cracking of n-heptane at 800 ⬚C at varying residence times in a plug flow reactor is shown in Figure 2. Such plots are easily obtained either from simple kinetic schemes or from
Yield (percent mass)
100
C7H16
80
C2H4 CH4
60
H2 CO
40
20
0 0.0
0.5
1.0
1.5
Time / s Figure 2. Variations in yields with increasing residence time at 800 ⬚C during steam cracking. The feedstock is n-heptane (C7H16). The steam兾hydrocarbon mass ratio is 0.5.
www.JCE.DivCHED.org
•
more complicated codes containing a large number of species (1). The steam兾hydrocarbon mass ratio is set to 0.5. We have selected two intermediate species between heptane and the equilibrium products of H2 and CO. The two most industrially important products are ethylene and methane (2). The maximum in ethylene indicates why the kinetics needs to be controlled if the cracking process is to be made most profitable: ethylene is the product with the highest market value. Figure 2 shows that n-heptane is rapidly depleted and ethylene rises to a maximum around 0.2 s; this is the reason that steam crackers typically have residence times in the range 0.1–0.3 s. Little hydrogen and CO are generated during the first second or so, although methane increases monotonically. For residence times greater than 1 s, the amount of hydrogen and CO begins to increase as the reaction begins to go from the kinetically-governed cracking regime to the equilibrium-governed reforming regime. In a full kinetics program it would, in theory, be possible to run all the way to equilibrium, but the number of equations and associated calculations would greatly impede the speed. However, full kinetics would involve a massive number of reactions. Because the objective of a kinetics program is to provide insight and rapid assessment for varying reactor conditions under the shorter time regime associated with steam cracking, then only a number of basic reforming reactions are included; the full kinetics is not presented, and indeed is superfluous for the purposes of the model. Comparison
Catalytic Effects Steam hydrocarbon reforming is actually a three stage process consisting of (i) desulfurization, (ii) primary reforming over a Ni兾K2O on alumina catalyst, and (iii) secondary reforming to complete the conversion to syngas (6). Figure 2 shows that the time scales of cracking and reforming are relatively well separated in the absence of a catalyst. Unfortunately this is not the case in the presence of a catalyst. The Ni walls of a cracking reactor can catalyze SHR so that it is considerably accelerated in comparison with the thermal conditions shown in Figure 2. This is a well-studied phenomenon. In order to suppress SHR during thermal cracking, sulfur is added to the feed typically in the form of dimethyl disulfide (DMDS). For a similar reason during SHR, the feedstock is first desulfurized, typically using a Co–Mo oxide catalyst. In contrast to the inhibiting role of sulfur on SHR, we can consider the common function of the group 1 and 2 metal oxides in the primary reforming step and also in catalytic steam cracking, where we have shown that potassium, for example, suppresses coking (7, 8). SHR will have a faster rate at a given temperature compared to SMR because it is easier to crack the carbon chain than to crack a C⫺H bond. Typically the hydrocarbon needs to dissociate on the metal surface (a pyrolysis reaction). In reforming, as opposed to cracking, the steam also needs to dissociate on the metal surface, that is, water activation (9). The efficacy of SHR thus depends on the reaction, H2O + surface
O surface + H2
(5)
where “O–surface” refers to oxygen atoms bound to the sur-
Vol. 81 No. 2 February 2004
•
Journal of Chemical Education
229
In the Classroom
Novel Feedstocks We stated earlier that the principal reforming feed is methane; however, other hydrocarbons can also be used and these require less severe conditions. Aromatics do require severe conditions because of the strength of the bonding structure. The same would apply to consideration of toluene for cracking. Although this may seem a rather strange feedstock for cracking there are in fact a number of valid reasons why it would be of interest to develop cracking (including hydrocracking) of aromatic rings. In particular current developments in the world of petrochemicals would make this an attractive option. There are two developments that make alternative utilization of aromatics a relevant issue in the medium term future. •
•
Aromatics are being removed from gasoline. Originally unrestricted, from 2000, a maximum of 42% volume has been imposed for European gasolines, and will be reduced to 35% volume in 2005 with possible reductions in 2008 to ca. 30%. The result is that the price of aromatic streams will drop. At the same time, demand for naphtha as a competing feedstock for steam hydrocarbon reforming and also for fuel cell hydrogen generation is increasingly coming to the fore. This means that demand for naphtha will increase with a corresponding increase in price.
With naphtha thus set to increase in price, and the price of aromatics going down, steam cracking of naphtha will become a somewhat less profitable proposition than it has been to date. From these considerations, it seems a small leap of imagination to go from cracking naphthas to yield aromatic containing products, to cracking unwanted aromatics. Of course, the equilibrium simply drives the reaction to coke formation, however applying the kinetic cracking regime (illustrated in Figure 2) one should still be able to derive some benefit and this has been experimentally verified recently (10). Potassium catalysts in the form of ionic potassium coupled to either an oxide, carbonate, or a metallic oxide such as VO3᎑, have been shown to produce yields of up to 10% ethylene from toluene cracking. These observations, along with our own of potassium as a coking inhibitor suggest a reexamination of the results shown for toluene in Figure 1. Rerunning the calculation under the assumption that the catalyst blocks coke formation while allowing all other reactions to reach equilibrium, we obtain the results shown in Figure 3. Note that unlike the kinetic calculations above, this reaction is run to equilibrium. A small amount of air is included to give the reaction a favorable Gibbs free energy change, ∆G. This small amount of air is shown to affect the reaction in such a way as to enhance the yield of ethylene. The amount of air is far too little
230
Journal of Chemical Education
•
50
Yield (percent mass)
face. Without a catalyst this is slow, so given that steam reforming is now particularly of interest because of its role in generating hydrogen for fuel cells, catalyst development has to take this last effect into account. This however does not rule out novel possibilities for treating hydrocarbon feedstocks.
40
30
20
10
0 0
5
10
15
20
Φ Figure 3. Theoretical ethylene yield during toluene cracking if route to coke is blocked by, for example, catalytic coking inhibition. Φ is the fuel/air equivalence ratio defined in the text.
for complete combustion; that is, the mixture is air “poor” or alternatively fuel “rich”. In the specific case of toluene, for total theoretical (stoichiometric) combustion we would have, C 7H8 + 9O2
7CO2 + 4H2O
(6)
so that 9 moles of O2 are required for complete combustion. The stoichiometric fuel兾oxygen ratio is thus 1兾9 or 0.11. In our equilibration calculation we use much less air than this. For moles of oxygen, nO, the fuel兾oxygen equivalence ratio is defined as the fuel兾oxygen ratio used divided by that for stoichiometry (11),
nf Φ =
no nf no
=
9 no
(7)
eq
where nf is the moles of fuel. This assumes that the number of moles of fuel in the two mixtures being compared is the same; this is reasonable if our independent variable is the amount of oxygen added to the system. Clearly for Φ = 1, we have stoichiometry. For Φ > 1, the mixture is fuel rich and if Φ < 1 the mixture is fuel lean. From Figure 3 it is clear that a very rich mixture at 7 times stoichiometry gives an ethylene yield comparable to that obtained from steam cracking of the more traditional naphtha feedstock. This explains the surprisingly good results obtained by other workers (10). (Note that the principal product is still syngas.) A rough economic enumeration based on current prices of the principal products indicates a feed upgrade of just over 3%. Moreover, the operating temperatures are around 250 ⬚C lower than for a typical steam cracker. When this is taken into account, the feed upgrade increases. However, the lower temperatures mean that a catalyst is probably necessary to ensure that the equilibrium mixture is attained on a reasonable time scale.
Vol. 81 No. 2 February 2004
•
www.JCE.DivCHED.org
In the Classroom
Conclusions Steam hydrocarbon reforming and steam hydrocarbon cracking are often perceived as separate industrial processes. As discussed in the introduction, the former occurs in the oil industry and the latter is usually, and rather arbitrarily, assigned to the chemical industry. In fact, reforming is just cracking carried to equilibrium. There is considerable overlap and complementarity in the concepts that are applied in analyzing the two processes. The two processes serve as good examples of equilibriumcontrolled and kinetically-controlled processes. Reforming starts off as a cracking process with the kinetics ultimately leading to the reforming products (syngas). The crackers’ art is determining when to fix the product slate by “freezing” the reaction (known as “quenching”). For optimal conditions in both reactions, the similarity is the desire to suppress coking; catalytic routes are favored. The interaction between economic factors, technology advancement, and chemistry is particularly well illustrated by an analysis of the comparative chemical features of steam hydrocarbon reforming and steam hydrocarbon cracking. We have used the European Union restrictions on aromatics to illustrate the drive to develop alternative processes. Evaluation of feedstocks for one process can lead via very simple concepts (such as equilibration combined with an inhibited coke formation mechanism) to predictions for alternative processes that give better feedstock upgrades than previously observed. There has been some preliminary work
www.JCE.DivCHED.org
•
in the field (8), but this work could be made more industrially attractive by combining it with insights derived from a consideration of the cracking processes discussed here. Literature Cited 1. Nowak, S.; Guenschel, H. In Pyrolysis: Theory and Industrial Practice; Albright, L. F., Crynes, B. L., Corcoran, W. H., Eds.; Academic Press: New York, 1983; pp 277. 2. Wiseman, P. J. Chem. Educ. 1977, 54, 154. 3. Weissermel, K.; Arpe, H-J. Industrial Organic Chemistry; VCH Wiley: New York, 1997. 4. Gaseq Chemical Equilibrium Program. http:// www.c.morley.ukgateway.net/ (accessed Oct 2003). 5. Golombok, M.; van der Bijl, J.; Kornegoor, M. Ind. Eng. Chem. Res. 2001, 40, 470. 6. Farrauto, R. J.; Bartholomew, C. H. Fundamentals of Industrial Catalytic Processes; Blackie, Chapman, and Hall: New York, 1997. 7. Golombok, M.; Kornegoor, M.; van den Brink, P.; Dierickx, J.; Grotenbreg, R. Ind. Eng. Chem. Res. 2000, 39, 285. 8. Golombok, M.; Dierickx, J. Ind. Eng. Chem. Res. 2000, 39, 3402 9. Rostrup-Nielsen, J. R. In Catalysis, Science and Technology; Anderson, J. R., Baudert, M., Eds.; Springer-Verlag: New York, 1984; p 3. 10. Pant, K. K.; Kunzru, D. Can. J. Chem. Eng. 1999, 77, 150. 11. Barnard, J. A.; Bradley, J. N. Flame and Combustion; Chapman and Hall: London, 1985.
Vol. 81 No. 2 February 2004
•
Journal of Chemical Education
231
