Practical Process Engineering

36- Catalyst deactivation can be expressed quantitatively by a new variable called a(t) that is the ratio of current reation rate to the original reaction rate. Then this variable incorporated to the reaction rate equation: So: -r(A)= a(t) K(T) fn(Ca, Cb,...) a(t) is only time dependant K(T) is only temperature dependant fn(Ca, Cb,...) is concentration depandant 37- The rate of catalyst activity is named "Rate of catalyst decay". Rd= d(a)/ dt

35- How to quantify catalyst deactivation

Let's move forward to catalyst deactivation. What do we mean by deactivation and what is the mechanism? 30- Suppose comonent A on a catalyst surface reacts to produce B (A——>B). I f Ca and Cb are the composition of A and B in the bulk fluid, Cas and Cbswould be composition on the surface or on the active site. 31- The total molar concentration of active sites per unit mass of catalyst is equal to the number of active sites per unit mass divided by Avogadro’s number and will be labeled Ct (mol/g-cat). 32- The molar concentration of vacant sites, Cv (mol/g-cat), is the number of vacant sites per unit mass of catalyst divided by Avogadro’s number. 33- There is a valid asuumtion as a starting point to derive equations for catalytic reactions: TOTAL CONCENTRATION of ACTIVE SITES IS CONSTANT; it means: Ct= Cv+ Cas+Cbs: site balance 34- For an active catalyst, above equation is always valid. But, due to some mechanisms (will be discussed later in this discussion), catalyst looses its activity in the course of time. So, deactivation means the Ct is no longer constant. That is, the total concentration of active sites, Ct, accessible to the reaction does change with time.

28- There is a analogy between catalytic reaction rate and electrical the amount of electrical current in a circut: Reaction rate or electrical current= (Driving Force)/ (Total Resistance) driving for in an electrical circut is voltage but for a catalytic reaction is reactant concentrations. Total resistance for a chemical reaction is the summation of steps 3, 4 and 5 resistances. We want to know which step/ steps in the adsorption–reaction–desorption series is limiting the overall rate of reaction. 29- This approach in determining catalytic and heterogeneous mechanisms is usually termed the Langmuir–Hinshelwood- Hougen- Watson or LHHW approach.

24- For large catalyst pellets, it takes a long time for the reactants to diffuse into the interior, compared to the time that it takes for the reaction to occur on the interior pore surface. Under these circumstances, the reactants are only consumed near the exterior surface of the pellet and the catalyst near the center of the pellet is wasted catalyst. On the other hand, for very small pellets it takes very little time to diffuse into and out of the pellet interior and, as a result, internal diffusion no longer limits the rate of reaction. At small particle sizes, internal diffusion is no longer the slow step. 25- Extenal diffusion is in steps 1 and 7 and they are contollable by fluid velocity (fluid velocity can't affect internal diffusion and surface reaction). Internal diffusion is in steps 2 and 6 and they are influenced by particle size. So, we are going to choose our pellet size and external fluid velocity such that neither external diffusion nor internal diffusion is limiting. 26- Thersfore, the surface reaction sequence of adsorption, surface reaction, and desorption (Steps 3, 4, and 5 ) limit the overall rate of the reaction. 27- The rates of each of these three reaction steps in series (adsorption, surface reaction, and desorption) are equal to one another for a steady-state hetrogeneous catalytic reactiom on a solid catalyst. However, one particular step in the series is usually found to be rate-limiting or rate-controlling. That is, if we could make that particular step go faster, the entire reaction would proceed at an accelerated rate.

Effect of particle size on steps 1 & 7

22- Talking about steps 1 & 7: Higher fluid velocity over the catalyst particle reduces boundry layer thickness around it and it no longer offers any resistance to the diffusion across the boundry layer. 23- Steps 1 & 7 resistances also decrease as the particle size is decreased. As the fluid velocity increases and/or the particle diameter decreases, the mass-transfer coefficient increases until a plateau is reached.

19- When the diffusion steps (1 , 2, 6, 7) are very fast compared with the surface reaction-rate steps (3, 4, 5) , , the concentrations in the immediate vicinity of the active sites are indistinguishable from those in the bulk fluid. In this situation, the transport or diffusion steps do not affect the overall rate of the reaction. 20- If the reaction steps are very fast compared with the diffusion steps, mass transport does affect the reaction rate. 21- In systems where diffusion from the bulk gas or liquid to the external catalyst surface or to the mouths of catalyst pores affects the rate (steps 1 and 7) changing the flow conditions past the catalyst should change the overall reaction.

18- Seven steps.

17- Seven steps in a hetrogeneous catalytic reaction. The overall rate of reaction is limited by the rate of the slowest step in the sequence.

13- a reaction is not catalyzed over the entire solid surface but only at certain active sites or centers. 14- an active site as a point on the catalyst surface that can form strong chemical bonds with an adsorbed atom or molecule. 15- One parameter used to quantify the activity of a catalyst is the turnover frequency (TOF), f. It is the number of molecules reacting per active site per second at the conditions of the experiment. When a metal catalyst such as platinum is deposited on a support, the metal atoms are considered active sites. 16- Catalyst classification based on the reaction type👇🏼👇🏼👇🏼

Bonding from the adsorption of ethylene on a platinum surface to form chemisorbed ethylidyne.

8- For a catalytic reaction to occur, at least one and frequently all of the reactants must become attached to the surface. This attachment is known as adsorption and takes place by two different processes: physical adsorption and chemisorption. 9- Physical adsorption is similar to condensation. The process is exothermic, and the heat of adsorption is relatively small, being on the order of 1 to 15 kcal/ mol. The forces of attraction between the gas molecules and the solid surface are weak. These van der Waals forces consist of interaction between permanent dipoles, between a permanent dipole and an induced dipole, and/or between neutral atoms and molecules. The amount of gas physically adsorbed decreases rapidly with increasing temperature, and above its critical temperature only very small amounts of a substance are physically adsorbed. 10- The type of adsorption that affects the rate of a chemical reaction is chemisorption. Here, the adsorbed atoms or molecules are held to the surface by valence forces of the same type as those that occur between bonded atoms in molecules. As a result, the electronic structure of the chemisorbed molecule is perturbed significantly, causing it to be extremely reactive. Interaction with the catalyst causes bonds of the adsorbed reactant to be stretched, making them easier to break. 11- Like physical adsorption, chemisorption is an exothermic process, but the heats of adsorption are generally of the same magnitude as the heat of a chemical reaction (i.e., 40 to 400 kJ/mol). 12- If a catalytic reaction involves chemisorption, it must be carriedout within the temperature range where chemisorption of the reactants is appreciable.

1- Because a catalyst makes it possible to obtain an end product by a different pathway with a lower energy barrier, it can affect both the yield and the selectivity. 2- A catalyst changes only the rate of a reaction; it does not affect the equilibrium. 3-A heterogeneous catalytic reaction involves more than one phase; usually the catalyst is a solid and the reactants and products are in liquid or gaseous form. 4- A heterogeneous catalytic reaction occurs at or very near the fluid–solid interface. 5- Because a catalytic reaction occurs at the fluid–solid interface, a large interfacial area is almost always essential in attaining a significant reaction rate. In many catalysts, this area is provided by an inner porous structure (i.e., the solid contains many fine pores, and the surface of these pores supplies the area needed for the high rate of reaction). 6- Sometimes pores are so small that they will admit small molecules but prevent large ones from entering. Materials with this type of pore are called molecular sieves, and they may be derived from natural substances such as certain clays and zeolites, or they may be totally synthetic, such as some crystalline aluminosilicates. 7- In some cases a catalyst consists of minute particles of an active material dispersed over a less-active substance called a support. The active material is frequently a pure metal or metal alloy. Such catalysts are called supported catalysts, as distinguished from unsupported catalysts. Catalysts can also have small amounts of active ingredients added called promoters, which increase their activity.

#Catalysts #Catalytic_Reactions #Catalyst_Deactivation A catalyst is a substance that affects the rate of a reaction but emerges from the process unchanged. A catalyst usually changes a reaction rate by promoting a different molecular path (“mechanism”) for the reaction. For example, gaseous hydrogen and oxygen are virtually inert at room temperature, but react rapidly when exposed to platinum. The reaction coordinate shown in bellow figure is a measure of the progress along the reaction path as H2 and O2 approach each other and pass over the activation energy barrier to form H2O.

We all know how a chimney works: Hot air rises, carrying smoke and soot up and out through the roof. The vacuum draws cooler air at hearth level to feed the flames. Most people agree that fireplaces are charming and cozy, and do a lot for a home’s ambiance, but the term “chimney effect” also describes how your home leaks air and feels drafty. The chimney effect (also called the “stack effect”), as applied to home heating and cooling, explains how in winter, our heated air rises through gaps in the roof and upper floor, drawing cooler air inside from gaps along the foundation due to pressure imbalances. In summer, the process is reversed, though the pressure differences occur to a lesser degree. The taller your home, the more it acts like a chimney; the taller the column of air, the more pressure the column exerts on air exchange.