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Severity Parameters for Steam Cracking Michael Golombok,*,†,‡ Joop van der Bijl, and Marcel Kornegoor Shell International Chemicals, Badhuisweg 3, 1031 CM Amsterdam, The Netherlands
There are several ways to measure severity in steam cracking which are all a function of residence time, temperature, and pressure. Many measures of severity are not practicable for experimental purposes. Our experimental study shows that methane make is the best measure of severity because it is an independent (and thus controllable) variable, is easy to measure, and provides the most scientific insight to current efforts to boost steam cracker performance by strategies such as feedstock manipulation and catalytic steam cracking. 1. Introduction Steam cracking is the process by which hydrocarbon feeds are broken down to form lower olefins. The feedstock can vary from ethane gas to a heavy gas oil resembling a crude oil. Generally these feedstockssand flexibility of cracking reactors is increasingly requireds pass once through a hot reaction zone, and the extent to which they are cracked is defined by a parameter called the severity. There is no unique definition of this quantity.1,2 In this study, we contrast a number of practical definitions and critically experimentally compare them for materials spanning the range of liquid feedstocks which are currently used in the industry for the production of ethylene. 2. Background The severity of steam cracking is a measure of how much the feed molecules are broken down. For a pure component such as ethane gas cracker systems, this is the extent to which feed conversion X has taken place. However, in most cracking systems the feed is not a pure component, so the classic chemical engineering definition of conversion as the fraction of feed which has reacted is not a useful or appropriate measure. Clearly, the higher the temperature and the longer the residence time in the hot reactor, then the more reaction will occur. For thermodynamic reasons, one generally wants to keep pressure low so that one can match temperatures T and residence times τ to a given severity
W ) Tτa
(1)
where a is typically around 0.05. However, in a heated reactor, it is the outside wall temperature which is controlled and the temperature profile of the gas inside the reactor cannot be directly measured.2 There are a number of other measures of severity; however, these often require a detailed knowledge of either the pyrolysis kinetics or the analytical relationship between T, τ, and hydrocarbon partial pressure. * To whom correspondence should be addressed. E-mail:
[email protected]. Tel: 31 70 311 2327. Fax: 31 70 311 3366. † Also with the Department of Applied Physics, Technical University of Delft, Lorentzweg 1, 2600 GA Delft, The Netherlands. ‡ Currently with Shell International Exploration and Production, Volmerlaan 8, 2288 GD Rijswijk, The Netherlands.
From a practical point of view, in attempts to assess how far a reaction has gone, these definitions are difficult to usesparticularly when one is comparing a wide variety of feeds all the way from gases to hydrowax. Experimentally, the relationship of eq 1 is the most practical; however, the index a is clearly feed-dependent. Ideally, one would like to have some independent controllable measure of severity which would function as an independent variable to be preset before a test run. Froment et al. identified a number of measures with an emphasis on the C3 fraction normalized to propylene yieldschosen for its pressure independence and industrial severity measure.3,4 There are three such possible measures which allow one to assess the effects of widely differing mixed feeds while at the same time allowing differing amounts of control of the reactor conditions. These are (1) the methane make, (2) the propylene/ethylene ratio (PER), and (3) the hydrogen/ carbon ratio (HCR) of the C5+ cracked effluent. These may be practicable, but only one of them is an independent variable (the methane make), with the other two being dependent variables which can only be determined after a cracking run, which we discuss in the following. 2.1. Methane Make. The extent to which a feed has been cracked is experimentally well characterized by methane yield. This parameter has two advantages. First, it is intuitively clear that the more cracking of hydrocarbons that has occurred, the more that molecular chains have been split up, generating methane. Second, methane make with its high HCR (25% compared to typical feed values of 15%) is a good indicator of effective hydrogen stripping from the liquid feed, and this correlates well with the formation of the low H/C liquid (i.e., C5+) precursors for coke formation. Methane yield correlates with coke formation and run length. Whereas species of size propane and higher are depleted by secondary reactions and ethylene reaches a plateau at moderate severity, methane (along with hydrogen) continues to monotonically increase with the extent to which the overall reaction has proceeded, i.e., the severity. Methane make has been shown to be a good experimental corollary to other severity functions,4 although because it is a product from severe cracking we may expect it to respond differently to different feedsssomething we examine in the Experimental Section. It also has the advantage of being easily measured on a laboratory-scale unit, although not so easily in full-scale plants. 2.2. PER. Commercially, the favored system for assessing cracking severity has been the PER. The
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Figure 1. Comparison of a dependent variable and independent variable severity: methane yield as a function of the W parameter defined in text from temperature and residence time. The comparison is for a range of four feeds. Lines are least-squares fits to data. Table 1. Hydrogen Content (% H) Details of Heavy Residue (VGO) and Hydrotreated Heavy Residue (H-VGO) with Steam Cracking Yields at 780 °Ca VGO H-VGO
% H in feed
% pygas
% FO
13.28 13.79
24.0 25.9
9.1 6.4
a Pygas is the pyrolysis gasoline (C5-C12) fraction, and FO is the fuel oil heavier component.
reason for using PER is that on-line gas chromatographs (GCs) cannot measure the absolute yields of ethylene or propylene, but ratios can be easily determined. These are usually kept constant at around 0.55. At low severities (before propylene yield has maximized), PER is around 0.7, so that not much cracking has occurred and a large amount of liquid byproducts (i.e., pyrolysis gasoline) is formed. This can be limiting on the backend handling capability of the postreactor processing and separation and is not optimal for ethylene make. At the other extreme, at high severities PER is around 0.48, and despite the fact that more ethylene is being made, the HCR in the C5+ fraction has gone down. This correlates with the increased formation of coke at the heavy ends, leading to shorter run lengths of the furnace. 2.3. HCR in the C5+ Fraction. Under high-severity conditions, the HCR in the C5+ fraction goes down, giving a measure of the coke which will be formed, as described previously. The reason for defining severity in terms of HCR C5+ is that it measures how much hydrogen remains in the liquid after pyrolysis has stripped off hydrogen in the feed to make light products. As described previously, it is also a good measure of coking propensity with cracking severity for feeds in general. An example is if one cracks a vacuum gas oil (VGO) before and after hydrotreating. Table 1 shows the hydrogen content of a raw and hydrotreated gas oil which was run on our experimental unit (described later). This clearly shows that increasing the hydrogen content increases the amount of pyrolysis gasoline (the C5-C12 fraction) and suppresses the heavier fuel oil yield. This kind of approach is very useful if one wants
to obtain an idea of coking propensities from the distribution of liquid products. The HCR in the product is clearly, however, a highly dependent variable; the process severity can only be assessed after the experimental run has been completed. It requires not only a detailed knowledge of the HCR of the feed but also accurate assessment of a host of minor gaseous components from hydrogen to C4’s in order to then be able to calculate the severity parameter, and this can lead to significant error accumulation. 3. Experiment Our experimental runs were carried out in a pyrolysis milliscale unit (PMSU). This consists of a single-pass tube mounted in an oven with a sequence of three independently heatable radiant coils. This unit is a downflow milliscale reactor where evaporated feeds are cracked in a 3 mm diameter quartz tube of length 78 cm which has been heated using radiant electrical resistance coils. Preheated steam is mixed with the feed to give a steam oil mass ratio (STOR) of 0.5 (the typical industrial value), and the pressure was fixed at 1.5 bar. Helium is doped into the feed to act as a reference for mass balance and yield purposes. Set points for the cracking temperature are between 760 and 840 °C. After the single-pass cracking run, there is a sequence of stepped condensing units for collecting different fractions of liquid product. A GC analysis is used to obtain a detailed split for the C5-C12 components into paraffins, isoparaffins, olefins, naphthenes, and aromaticss the so-called PIONA analysis. A more detailed molecular analysis is carried out on the C1-C4 fraction using a Carlo-Erba GC which determines the mount of light olefins and associated gases and resolves the mass balances using the prefed tracer gas. Figure 1 shows the relationship for four widely varying cracker feeds: a light naphtha, a gas oil, a hydrowax, and the VGO mentioned previously. The linearity of the response between methane yield and the cracking severity as measured by eq 1 is very good. The higher slope of the naphtha reveals the greater sensitivity of this feed to the reactor conditions of temperature
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with and without catalyst are seen to lie on the same response curve. The effect of the catalyst is actually merely to increase the overall severity of the reaction by shifting the reaction regime to an effectively higher value of W in eq 1. In terms of the HCR C5+, coking is suppressed, and this seems to be the main function of a number of catalysts claimed to catalytically enhance the steam cracking effect.5 We have shown that the main effect of the catalyst is to suppress coking. This is an example where the coking severity and overall reaction severity have to be decoupled from the amount of methane make and the amount of coke. This is because not all routes to coke formation run via methane, although, as we have shown elsewhere, the two are correlated.6 4. Conclusion
Figure 2. Comparison of ethylene yield obtained for an empty reactor and filled with catalyst using two different severity measures: (a) methane make; (b) HCR in the C5+ effluent fraction.
and residence time. The greater methane offset for the hydrowax compared to the gas oil is a reflection of the higher hydrogen content of the latter feed. Note that the methane make can be directly obtained from the offgas GC measurement and is thus the most practicable measurement. It is also the one which is most closely related to the temperature and residence times and thus the nearest thing that there is to a uniformly agreed measure of severity. Nonetheless, there are other parameters of severity, despite the fact that they are dependent variables. Thus, PER simply requires ratioing the propylene and ethylene obtained from an experimental run; however, the HCR for the C5+ fraction requires a large number of experimental steps. These are (1) separation of the C5+ fraction, (2) speciation by carbon number, and (3) determination of the HCR for each of these carbon number fractions; inevitably, all of these steps lead to error accumulation. Nonetheless, if accurately carried out, these can still be useful for providing insight into the chemistry associated with the kinetic variations associated with different feedstock and process innovations. Figure 2 shows an example where we have cracked a gas oil in an empty reactor tube and also in a reactor tube filled with a catalytic material. In both cases the residence time was 0.6 s. (As opposed to Figure 1, STOR here is 1 in order to suppress coking.) Figure 2a shows the results where the ethylene yield is plotted as a function of methane make. We see that, at a particular methane yield, the catalyst augments the amount of ethylene. However, the nature of the promotion is better seen if we use another severity parameter: the HCR for the C5+ fraction as outlined above. This is shown in Figure 2b, where the ethylene yields
In steam cracking, severity has to be separately defined from conversion because of the wide range of components in the reactant stream. Different underlying mechanisms for observed phenomena are derived, dependent on which parameter is observed. We showed this for two aspects of steam cracking of current interest: the capacity to handle widely varying feedstocks and catalytic steam cracking. The readiest method for assessing how far the overall reaction has proceeded and the easiest to assess is the methane yield. This correlates well with temperature and residence times, the most readily controlled independent variables. Process improvements are then characterized by a greater ethylene yield for a particular methane make. When it comes to understanding the performance of different feedstocks and coking or studying reaction mechanistic aspects such as catalytic vs noncatalytic, the other measures of severity which we have outlined can also be illuminating. Literature Cited (1) Wiseman, P. Ethylene by naphtha cracking: free radicals in action. J. Chem. Educ. 1977, 54 (3), 154. (2) Nowak, S.; Gunschel, H. Pyrolysis of petroleum liquids: naphthas to crudes. In Pyrolysis: theory and industrial practice; Albright, L. F., Crynes, B. L., Corcoran, W. H., Eds.; Academic Press: New York, 1983. (3) van Camp, C. E.; van Damme, P. S.; Willems, P. A.; Clymans, P. J.; Froment, G. F. Severity in the pyrolysis of petroleum fraction: fundamentals and industrial applicaiton. Ind. Eng. Chem. Process Des. Dev. 1985, 24, 561. (4) Froment, G. F. Thermal cracking for olefins production: fundamentals and their application to industrial problems. Chem. Eng. Sci. 1981, 36, 1271. (5) Golombok, M.; Kornegoor, M.; van den Brink, P.; Dierickx, J.; Grotenbreg, R. Surface enhanced light olefins yields during steam cracking. Ind. Eng. Chem. Res. 2000, 39, 285. (6) Golombok, M.; Dierickx, J. Reply to Comments on “Surfaceenhanced light olefins yields during steam cracking”. Ind. Eng. Chem. Res. 2000, 39, 3402.
Received for review June 16, 1999 Revised manuscript received October 10, 2000 Accepted October 20, 2000 IE990436R
