Ch E 542 – Intermediate Reactor Analysis & Design

Ch E 542 – Intermediate Reactor Analysis & Design Catalyst Deactivation

Catalytic Reactors Continuous Stirred Tank Reactor Moving Bed Reactor Packed Bed Reactor Straight Through Transport Reactor

Packed Bed Reactor Packed Bed Reactor in use for a Fisher-Tropsch synthesis reaction at Sasol Limited Chemical.

Moving Bed Reactor disengager boiler regenerator This photo is of a catalytic cracker moving bed reactor. Only the disengager section of the reactor is visible in the picture - the reaction section is hidden behind the stair structure beneath the disengager. The cracker is used for the catalytic cracking of gas oil into light aromatics and straight chain hydrocarbons, which are then separated in the distillation tower to the right of the photo. still The white unit to the left of the cat cracker is the catalyst regenerator,where coke deposits are burned off the catalyst. This is a highly exothermic operation, (1400°F), and so the boiler unit at the far left recovers the sensible heat of the regenerator exhaust to produce steam.

Stacked CSTRs The unit shown in this photo is a stacked reactor platform, which (in this case) is a set of four reactors in series. The reactors are stacked on top of one another to minimize the amount of ground space required for the reactors. Because this reaction requires high temperatures, the furnace at the left of the photo is used to heat the initial feed material to around 1000°F. The reaction is also endothermic, which means the material cools as it passes through each reactor, and must be re-heated back up to proper reaction temperature between each pair of reactors. The large silver (insulated) pipes on the left side of the reactors transport material through the furnace between each pair of reactors

Straight Through Transport Reactor Straight Through Transport Reactor (STTR) in use for a Fisher-Tropsch synthesis reaction at Sasol Limited Chemical.

Catalyst Deactivation • When the kinetics are separable… …it is possible to study catalyst decay and reaction kinetics independently. • When the kinetics are non-separable…

Catalyst Deactivation • separable kinetics • Commercial reactors maintain constant production rate by increasing T (reaction rate constant increases), as catalyst decays (catalyst activity a decreases). • experimental analysis of the decay rate is as:

Catalyst Deactivation • Sintering (aging) • Activity loss by loss of active surface caused by prolonged exposure to elevated gas-phase reaction temperatures. • Mechanistically… • Crystal agglomeration/growth, reducing internal surface area accompanied by narrowing/blocking of pore cross section. • Change in surface structure through recrystallization or other modes of defect elimination (active site loss). • Typically a 2nd order process;

Catalyst Deactivation • Fouling/Coking • Deposition of carbonaceous material on catalyst surface • Catalyst activity level is a function of the amount of carbon deposited on the catalyst surface (Cc): where A and n are fouling parameters dependent on the type of gas being processed. • Activity is expressed as f(Cc) by one of the following:

Catalyst Deactivation A + S  A•S A•S  B•S B•S  B + S constant impurity partial P P + S  P•S upon integration • Poisoning • Mechanism involving the irreversible chemisorption of “poison” molecules to active sites. • Poisons may be reactants, products or impurities (sulfur is a common petrochemical processing catalyst poison).

Uniform Poisoning Define  as fraction of sites remaining activewhere CPI is the concentration of poisoned sites Ignore mass-transfer effects, only consider deactivation function; other effects can be added later. Define CPI in terms of CPs, poison conc in feed Since rate constant is proportional to number of available sites, activity decreases linearly with poison concentration

Poisoning with diffusion limitations Consider diffusion limitations in a first-order reaction Substitute krA based on poison concentration

Poisoning with diffusion limitations • Taking the ratio of rates at fixed CA, • Limiting cases • No diffusion limitations • Strong diffusion limitations

Shell-Progressive Poisoning rate of poison adsorption external interfacial transport steady diffusion through spherical shell deposition rate at boundary poison concentration in bulk, at solid surface, at core boundary poison concentration on solid at saturation Adsorbed poison boundary moves as a shell through catalyst, around an unpoisoned core.

Shell-Progressive Poisoning modified Sherwood number for poison Damköhler number For a flow reactor, reference time is the mean residence time; reference poison concentration is that of the feed. Pellet average poison concentration as related to the unpoisoned core radius Eliminate intermediate concentrations CsPs and CcPs using rate equations

Shell-Progressive Poisoning result predicts poison deposition (<CPI>/CPI) as function of time (implicit in dependent variable) For CP = f(L) as in a PBR, equation is a PDF coupled to the mass balance of the flowing fluid (solved in Chapter 11). For the case where CP is constant, integration is as

Shell-Progressive Poisoning B.C.s • Poison concentration known, effect on chemical reaction must be derived. • Assume poisoned shell is completely inactive, and first order catalytic reaction occurs elsewhere, • D'eA is the effective diffusivity of A in the poisoned shell • DeA is the effective diffusivity of A in the unpoisoned core • krA is the rate constant of the desired chemical reaction

Shell-Progressive Poisoning assumes pseudo steady-state for which it must be true that the deactivation rate is much slower than reaction rate Solve and substitute into the definition of the , For which… selectivity effects demonstrated in Mathcad

Coking • Coking is not understood mechanistically. • Coking essentially has two effects on the catalyst • Site coverage (coke covers the active catalytic site) • Pore blockage (coke creates diffusional limitations by changing pore size distribution) • The growth of coke in a pore network leads to a probabilistic formulation of the problem, by which the probability that a site is accessible (P) is related to a probability density q(D), which is expressed in terms of the pore network structure (such as the -tree discussed in Chapter 3 of Froment and Bischoff).

Catalyst deactivation by site coverage A + S  A•S A•S  B•S B•S  B + S C + SC•S Consider reversible isomerization, where C is a precursor to coke formation that is strongly absorbed and not found in the gas phase.

Catalyst deactivation by site coverage A + S  A•S Neither term measureable, thus some empirical correlation is necessary A•S  B•S B•S  B + S C + S P•S Consider reversible isomerization, where C is a precursor to coke formation that is strongly absorbed and not found in the gas phase.

Catalyst deactivation by site coverage A + S  A•S A•S  B•S B•S  B + S C + S P•S A is a deactivation function.

Catalyst deactivation by site coverage • Mechanism of coke formation is not understood. An empirical expression () is typically applied. • Some commonly employed expressions include:

Example of deactivation by coking gas-oil(g) products(g) + coke(s) • The gas-phase cracking of a Salina light gas-oil reaction is to be carried out in a straight-through transport reactorat 750°F with a catalyst that decays by coking. • CAo is 0.2 kmol/m3. Catalyst particles are assumed to move with the mean gas velocity (Ug =Us = 7.5 m/s). The rate law is • Bk = 8 s-1 • KA = 3 m3/kmol • KB = 0.01 m3 /kmol • Catalyst activity for light gas-oil over this catalyst for short contact times (i.e., less than 100 s) at 750F is (A = 7.6 s-1/2) • Neglecting volume change with reaction, pressure drop, and temperature variations, determine the conversion as a function of distance down the reactor. The reactor length is 6 m.

Example of deactivation by coking gas-phase reaction  = 0, T = T0, P = P0. Mole Balance Stoichiometry Rate Law Combining Decay Law A catalyst particle traveling with velocity U, the time the particle has been in the reactor when it reaches a height Z is t = z/U

Example of deactivation by coking

Decaying Catalyst in a CSTR gas-oil(g) products(g) + coke(s) Mole Balance Stoichiometry gas-phase reaction  = 0, T = T0, P = P0. Rate Law Combining Decay Law

Decaying Catalyst in a CSTR gas-oil(g) products(g) + coke(s) Define ODEs become:

Temperature/Time Trajectories A control strategy involves maintaining a constant conversion with catalyst decay by increasing operating temperature. To achieve this result, one must develop a temperature/time trajectory that follows:

Temperature/Time Trajectories decay law is:

Temperature/Time Trajectories decay law is: for

Temperature/Time Trajectories

D A U Selectivity Effects Consider the parallel reactions:

Ch E 542 – Intermediate Reactor Analysis & Design