Reaction Pathway Analysis - ACS Publications - American Chemical

Chapter 20

Reaction Pathway Analysis Global Molecular and Mechanistic Perspectives Michael T. Klein, Matthew Neurock, Linda Broadbelt, and Henry C. Foley

Downloaded by UNIV OF MINNESOTA on July 18, 2013 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0517.ch020

Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, DE 19716

Reaction pathway analysis and the closely associated topic of applied chemical kinetics are at the heart of both the practice and study of chemical reactions. They often form the basis of the process engineer's conservation equations and subsequent reactor design, but also can be the starting point for fundamental mechanistic inference. This centrality of reaction pathways guarantees their relevance and motivated this ACS symposium on the topic. Figure 1 summarizes three hierarchical levels at which reaction pathways can be studied experimentally and modelled. These levels collectively represent a connection between the mechanistic chemistry of elementary steps and industrially relevant process chemistry. Each has its own advantages and limitations, which are evident upon consid eration of related reaction models. Mechanistic models, i.e., those involving the elementary transitions (steps) of active centers and therefore often free-radical or ionic intermediates, have the advantage of being rich in chemical significance. The large number of parameters they contain is a true reflection of the diversity of nature'sreactions,but often imposes serious limitations related to the lack of a quantitative kinetics database. Moreover, these models are often numerically stiff and can impose a considerable CPU demand. An acceptable "research" CPU demand will often not meet the needs of industrial practice. This CPU issue along with historical limitations in analytical chemistry have often forced "practice-oriented" reaction models to be at a global level. These models tradition ally involve the relevant boiling point or solubility defined reactants, and not the control ling molecules or intermediates. Thus they will often appear to suffer from the point of view of chemical significance; however they are generally much easier to solve. Most importantly, these models are relevant; the reactants and products in these models are bought and sold! Increases in the molecular detail provided by modern analytical chemistry have motivated the formation of intermediate-level, molecular models. These hybrid models have molecules as reactants and products, not intermediate active centers (radicals or ions) or globally lumped product fractions. They embody more chemistry and rate constants

0097-6156/93/0517-0290$07.50/0 © 1993 American Chemical Society

In Selectivity in Catalysis; Davis, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

20. KLEIN ET AL.

291

Reaction Pathway Analysis

Gas Light Gasoline Heavy Gasoline Light Gas Oil Heavy Gas Oil Coke

GasOi

Mechanistic Simulation

*

1


Monte Carlo Simulation

paraffin(l) paraffin (1) + olefin (1)

• paraffin (2) + olefin (2) • paraffin (2) + olefin (2) k

Rate Expressions

Mechanistic Reactivity Rice Herzfeld Pyrolysis Cycles LHHW Kinetics Coupled Thermal & Catalytic Reaction Cycles

Figure 1.

J

Three hierarchical levels of reaction chemistry and reaction modelling.

than global models, but require subsequent product oriented lumping to ascertain relevant product fractions. These "pathways-level" models have two key advantages. First, they generally provide a better basis for extrapolation than global models because of their chemical richness. Second, and perhaps trivially obvious, they are fashioned to answer new ques tions concerning product quality and properties. For example, as environmental issues dictate the allowable aromatics content of gasoline, it is important to have models which can specify, e.g., the benzene content. The objective of this review is to consider each of these levels of modelling through the view of three important commercial chemistries: pyrolysis, hydrotreating and catalytic cracking. We first review the chemistry in each of the three levels in Figure 1. This is followed by a summary of available experimental information and kinetics data in terms of molecular pathway models. The hope is to expose this intermediate level molecular modelling as an attractive compromise between the chemically rich but CPU intensive mechanistic models and the CPU friendly but chemically poor global modelling approaches. Global Reaction Models Global reactions models are those where the reactants and products are defined by avail able analytical separation schemes. They generally represent the interconversion of lumps or pseudocomponents, i.e., aggregates of many molecules with common attributes. In general, the characteristics which assemble given sets of molecules into a lump will not be reactivity. For example, perhaps the two most commonly found globally lumped models are based on boiling point or solubility characteristics. Global reactions models have historical significance. Their formulation is largely due to the global nature of analytical output of hydrocarbon mixtures in the 1940s and 50s. Additional advantages of these models include their easy formulation and solution.


SELECTIVITY IN CATALYSIS


292

Often analytical solutions are permitted Moreover, on today's high-speed computers they impose only minor CPU requirements, and therefore could find considerable use in pro cess control, design and optimization. They also become very attractive models for solu tion on inexpensive personal computers which could be placed in a remote location or refinery. Asphaltene and resid pyrolysis provide two relevant examples of global pyrolysis models. The pyrolysis of an isolated asphaltene feedstock typically yields the type of data summarized in Figure 2, a plot of the temporal variation of weight based product fractions as a function of time (7). Thisfigureillustrates the exponential disappearance of asphaltene accompanied by the formation of coke, maltene and gas product fractions. Consideration of the initial slopes for the formation of coke, maltene and gas fractions led to the type of reaction network shown in Figure 3. Since resid and its reaction products can likewise be defined in terms of the solubility and volatility-based product groups asphaltene,

1.0 0.8

0 6

I· ?

0.4 0.2 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 t/hr

Figure 2. An example of the globalreactionproduct yields for the pyrolysis simulation of a Hondo-derived Asphaltene feedstock at 400°C [1]. Maltene Ji

il k1

k4 k2

k6

Asphaltene

ι» Coke

k3 ]

k5 }

Gas

Figure 3.

Global reaction network for asphaltene and resid pyrolysis systems [7].


20. KLEIN ET AL.

293


maltene, coke and gas, the reaction network of Figure 3 is applicable to resid as well as isolated asphaltenes (2). Table I summarizes relevant data for the pyrolysis of a series of isolated asphaltene feedstocks. This table highlights the dependence of reaction product yields and selectivi ties on the resid origin. Clearly the reactivity of each feed depends upon its source (7). Table I. Summary of global asphaltene pyrolysis experiments. The ultimate yields and final selectivities for a series of different asphaltene feedstocks which were pyrolyzed for two hours at 400°C (7) Downloaded by UNIV OF MINNESOTA on July 18, 2013 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0517.ch020

Product/Feedstock

Calif.

Jobo

Maya

Athab.

Hondo

Ultimate Yields Asphaltene Coke Maltene Gas

22 51 21 5

19 41 20 20

9 51 27 13

9 54 21 16

5 61 25 7

Final Selectivities Coke Maltene Gas

65 27 6

53 23 25

62 3 33

60 23 18

64 26 7

This point is underscored more dramatically in Figure 4, a compilation of the global rate parameters of Figure 3 for the various feedstocks of Table I. Clearly these parameters are feedstock dependent This tends to undermine the value of a reaction 2i

Asphaltene Pyrolysis @ 400 C

I California Q Mayan H Jobo 0 Athabasca • Hondo

I-HI k1

k2

k3

k4

Feedstocks

Figure 4. The effect of asphaltene source on the global kinetic rate parameters for the pathways illustrated in Figure 3 (7).


294



model, and highlights one of the main limitations of global reaction models. This may well be the strongest motivation for molecule-based reaction models. Hydrotreating reaction models are nicely exemplified by the Amoco Easy-Hard lump model of Figure 5. The global paths and the kinetics reported in this figure were taken from the review by Beaton and Bertolacini (3), which summarizes experimental results, the modelling, and commercialization of Amoco's hydrotreating unit In short, the subdivisions of resid into "hard" and "easy" fractions and gas oil into reactant and product fractions were required to account for kinetic nonlinearities. The seven lumps and 14 rate constants provided significant flexibility and were able to describe the kinetics of several relevant product fractions.

Gas Oil

Resid "Hard* 0.18

Gas

Naphtha

Distillate

Gas Oit

Figure 5. The Amoco Easy-Hard kinetic model for resid hydrotreating. The reaction products are characterized by seven distinct lumps, the three feedstock species which are shaded in gray (resid hard, resid easy, and gas oil) and the four new components which are not shaded (gas, naphtha, distillate, and gas oil). The kinetic interconversion of these groups is described by the 14 constants (3). Catalytic cracking modelling is nicely illustrated by a series of Mobil publications over the past 30 years. Chronologically, the Mobil models have increased in the number of model lumps, in an effort to describe more chemical detail. The early "three lump" cracking model followed the kinetic evolution of the reactant gas oil and the desirable gasoline product and the undesirable coke and gas fraction (4-6). This model is illustrated in Figure 6a. Weekman has published an extensive review of this approach (7). The need for additional information and better feedstock independence motivated the "ten lump model" (8), which is illustrated in Figure 6b. Finally, the molecule-based modelling approach articulated by Quann and Jaffe (9) follows the evolution of hundreds of pseudocomponents which are resolved from detailed modem analytical chemical analysis. In short, the identified trend is that as the sophistication in the available analytical tools


20. KLEIN ET AL.

295


Mobil 3-Lump Model (I960)

Gasoline

•


kj

Coke & Gas

Increasing Aromatics Decreasing Paraffins

Feed 1 2 3 4

Κ

Κ*

28.0 17.6 12.6 9.3

1.86 1.48 2.66 2.28

*3 6.0 4.5 2.9 3.0

Units of k (hr - Wt tract .)

_1

b) Mobil 10-Lump Model (1970) Naphthenesl 650F + Paraffins 650 F +

Carbons in Aromatic Side Chains 650 F 4 Carbon in Aromatic Rings 650 +

Carbon in Aromatic Rings 430-650

Figure 6. The Mobil series of catalytic cracking reaction models, a) The Mobil 3Lump model b) the Mobil 10-Lump model (4,5,7). and modelling issues rose, so too did the number of model lumps. In a sense, the increase in the number of lumps in global models showed the beginnings of the formulation of molecular models. It is these intermediate levels to which we now turn our attention. Molecular Reaction Models. Molecular reactions models are those in which the reactants and products are defined by actual molecules. The mechanistic chemistry is implicit, as active centers such as free radical and/or ion intermediates are not addressed explicitly. This pathway-oriented model is thus the "expression" of a sequence of elementary steps, governed by fundamental chemical phenomena such as the transition state activation barriers. The corresponding



296


kinetics for the mechanism are indirectly accounted for through the development of detailed rate expressions which are functions of the elementary rate constants. These models require an extensive data base. Often this will be compiled from pure component or model compound reaction pathways and kinetics. Model compound experiments allow for the quantitative deduction of "mtrinsic" reaction kinetics and, in favorable circumstances, reaction mechanism information. The true benefit from molecular models is that they permit the quantitative struc tural description of complex feeds. As noted above, this allows for easy extrapolation to unstudied systems and also permits product quality and properties issues to be addressed. The kinetics, while formulated at the molecular level, are nevertheless still implicitly tied to the mechanism. The CPU demands, however, are relatively small compared to those of mechanistic models. Returning to the asphaltene and resid pyrolysis chemistry, their molecular reaction models require a significant data base of hydrocarbon reaction pathways and kinetics. These feeds contain molecules that can be organized into one of five hydrocarbon types. In general, the reactants will contain alkylaromatics, alkyltetralins, alkylnaphthenes, par affins, and olefins (10,11). Figure 7 provides a succinct summary of representative reac tion pathways for each one of these families. Notice that the reactions are from one molecule to another, and that the selectivities are provided by stoichiometric coefficients. The controlling free-radical intermediates are well-known and have been modelled (7775), but are nevertheless only implicitly accounted for in the pathways and kinetics of Figure 7. Additional reaction paths not shown in Figure 7 but nevertheless relevant to asphaltene and resid pyrolysis include dehydrogenation, condensation, and ring opening. The first two paths lead to aromatization and coke formation, respectively, whereas the latter allows for additional molecular weight reduction. The kinetics associated with the reactions shown in Figure 7 are summarized in Table Π. Detailed mechanistic studies on the pyrolysis of alkylaromatics (72,73,75), alkylnaphthenes (14) and alkyltetralins (14) have allowed for the formulation of the Arrhenius parameters and stoichiometric coefficients shown. The kinetics for paraffin and olefin pyrolyses were extracted from the abundant literature data (16-18). Finally, the issue of kinetic interactions have been both theoretically and experimentally addressed (77,79). These interactions in general cause the reaction of the mixture to be different then the linear combination of the pure component rates. Resid hydrotreating contains a significant thermal component (3), but its key distinction from coking or asphaltene and resid pyrolysis is the inclusion of a catalytic hydrogénation component and, often, an acid cracking reaction. Thus hydrogénation re action pathways are of considerable relevance. Hydrogénation reaction pathways are typically reversible under relevant process conditions and thus the issues of not only activity but also thermodynamic limitations and hydrogen solubility are real. Table ΙΠ is a summary of the pathways and kinetics of hydrogénation of an exhaustive set of poly nuclear aromatic (PNA) hydrocarbons taken from the literature (20-34). A detailed re view and analysis of many of these paths has been provided by Girgis and Gates (24). The kinetics provided in Table ΙΠ are clearly dependent upon both process conditions, e.g., temperature, pressure, pellet size, and linear velocity, and catalyst properties. These issues suggest that perhaps not all the data in the literature can be considered intrinsic. Table IV is a concise summary of the effect of the number of aromatic rings and their configurational arrangement on the hydrogénation kinetics of relevant polynuclear



Olefins

i

Paraffins

i

Alkyltetralins

i

Alkylnaphthenes

i

Alkylaromatics

i

Components

Molecular

C4C20

C4C20

2ET

TDC

PDB

Expt. 1 0

A E*

V)

13.8 17.8

12.8 16.7

12.7

14.9

14.0

56.0 70.2

56.0 70.2

53.5

59.4

55.5

*2

v3

0.07

0.85

0.286 0.571 0.095 0.286 0.571 0.095

0.08

0.312 0.117 0.571 0.454 0.1 17 0.429 0.510 0.108 0.382

( 5- I ) (kcal/mole)

iog

T= 400°C j=425 °C j=450 °C

Conditions

0.048 CA=0.001 M 0.048 CA=1.0M

-

-

" -

v4

(i/13)

(1/15)1/2

W

Table Π. Kinetics for representative reaction pathways for hydrocarbon pyrolysis


6

16

1

14

14

12,13

Ref.


3

Η

^(ΘΓΟ

0

+ 3H>

©O—OO

(®Π^

+ 3B,

Reaction Pathway

185-190 atm

153 atm 185-190

380 C 300-400 C

atm atm atm atm atm atm atm

300-400 C

75 71 71 71 71 71 75

75 atm

C C C C C C C

Pressure (atm)

325 C

325 340 340 340 340 340 325

CO

Temp

2

Ni-W Ni-Mo

Ni-Mo

Co-Mo

NiW NiMo Co-Mo

C0M0

WS

M0S2

Co-Mo

Catalyst

Batch A C How

Flow

Batch A C

Batch A C Batch A C Batch A C Batch A C Batch A C Batch A C Batch A C

Reactor

0.023 0.365

cm-Vgcat-s IVgcat-hr

0.0739 IVgcat-hr

0.228

0.30 0.387

—

L/gcal-hr

0.0004 L/gcat-hr Relative Relative Relative Relative Relative — IVgcat-hr

Units ofk

0.005 σ) 0.015 (C)

0.0022

1***

0.0101 1*** 1*** 1*** 2***

kf

Table ΠΙ. The Pathways and Kinetics of Hydrogénation of Polynuclear Aromatic Hydrocarbons


Lapinas Girgis

Girgis

Sapre

Sapre Moreau Moreau Moreau Moreau Moreau Sapre

Ref.


^

+!H

+3H

2

^

^

ο

^

U y

^

Continued on next page

+3

(§O^ ^®O0

^

(§rO »"*0~0

®

\Av^

^v^v,

^

71 atm 71 atm 71 atm

340 C

340 C

75 atm

153 atm

153 atm

350 C

325 C

380 C

380 C

Batch A C

Batch A C

Batch A C

Batch A C

Sulfided Batch A C

Sulfided Batch A C

Ni-W

Co-Mo

Ni-W

Ni-W

— —

4 0***

—

0.13 15***

—

0.002 4***

relative

relative

cm /gcat-s

3

IVgcat-hr Relative

Moreau

Moreau

Lapinas

Sapre Moreau

Lapinas

cm /gcat-s

3

Lapinas

cm /gcat-s

3

Table ΠΙ. The Pathways and Kinetics of Hydrogénation of Polynuclear Aromatic Hydrocarbons (continued)



+ 2HJ:

2

OlOlOJ+xH :

s

Products

@@>-@Q

+2H

2

+ 3H *=*

+ 3H.

Reaction Pathway

300-400 C 200-380 C 402-543 C 430 C

325 C 340 C 340 C 340 C 340 C 340 C 220-435 C

300-400 C

340 C

Temp (°Q

185-190 102 atm 136-204 100 atm

75 atm 71 atm 71 atm 71 atm 71 atm 71 atm 102 atm

185-190

71 atm

Pressure (atm) Reactor

Batch A C Batch A C Batch A C Batch A C Batch A C

WS

Ni-Mo Ni-W Chromia Ni-Mo

NiW NiMo Ni-W

C0M0

Flow Batch A C Flow Batch

Batch A C

2

Batch A C

Co-Mo

Flow

M0S2

NiMo

Sulfided Batch A C

Catalyst

0.157 NA NA NA

0.2081 14 23 21 18 10 NA

0.387

5

2.0* *

kf

IVgcat-hr

relative

Units ofk

0.243

IVgcat-hr

0.0076 IVgcat-hr Relative Relative Relative Relative Relative

0.365

la-



Girgis Shabtai Haynes Lemberton

Sapre Moreau Moreau Moreau Moreau Moreau Wiser

Girgis

Moreau

Ref.



20. KLEIN ET AL.


301


5

98 atm

2

400 C

IQIOJL^ x H

+

69-205 72.5 35-137

341C 375 C 348-400 C

LOI01 ι γΤτ kv-g^ xH

2

^

ρ τ ^

^ ^ Products

185-190

300-400 C

t^-r^s

185-190

Pressure (atm)

185-190

/ \ / \

300-400 C

Temp (°Q

300-400 C

/ ^ N

^ N ^ s

Reaction Pathway

ZnCl2/ CuCo

Ni-W Co-Mo Ni-Mo

Ni-Mo

Ni-Mo

Ni-Mo

Catalyst

Batch A C

Batch A C Batch A C

Flow

How

How

Reactor

NA

NA NA NA

0.256

0.141

0.255

kf

—

— — —

—

—

—

kr

—

— — —

IVgcat-hr

IVgcat-hr

IVgcat-hr

Units ofk



Nakatsuji

Shabtai Johnston Stephans

Girgis

Girgis

Girgis

Ref.

20. KLEIN ET ΑΙ..

303


Ο Ο Ο

α

S SS σο

o

S

5 Downloaded by UNIV OF MINNESOTA on July 18, 2013 | http://pubs.acs.org Publication Date: May 5, 1993 | doi: 10.1021/bk-1993-0517.ch020

llll eu eu eu eu oo OO CN

* * * * * * *

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UUU

Reaction Pathway Analysis - ACS Publications - American Chemical

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