Lecture Script Flashcards
(26 cards)
What is the focus of the module?
The module focuses on heterogeneous and multiphase reactors. Through understanding the underlying physics of the different reactor types, the student will be equipped to carry out reactor design tasks for conventional and novel reactors in a systematic way. Particular focus is on teaching a generally applicable problem solution approach, which is of relevance to the fourth-year design project as well as professional practice.
How is the module structured?
The module consists of the following components:
1. Fundamentals of transport processes in heterogeneous reactors
2. Fixed bed catalytic reactors
3. Fluidized bed reactors
4. Gas-Liquid and Gas-Liquid-Solid reactors
What are the five learning outcomes?
By the end of the module students should be able to:
- Establish and follow a selection process to determine the most appropriate reactor type for a specific process.
- Apply a general problem-solving approach to design heterogeneous and multi-phase reactors.
- Carry out reactor sizing calculations to the level of detail required.
- Identify critical parameters affecting the performance of heterogeneous and multi-phase reactors.
- Apply engineering judgement to assess the validity of and limitations in their calculations.
What did we learn in reaction engineering fundamentals?
In reaction engineering fundamentals: material and energy balances for ideal PFR, CSTR and batch reactors were performed.
What is meant by pseudo-homogeneous conditions?
Mass transfer and heat transfer resistances between different phases are neglected, such that the reactor contents can be treated as a single phase. Useful for preliminary design or truly homogeneous systems.
When are heterogeneous models used?
Heterogeneous models are used when temperature and composition need to be distinguished between phases.
What is meant by multiphase reactors?
Real reactors may involve multiple phases (i.e. multiphase reactors), which will often need to be considered as heterogeneous.
However, the phrase “multiphase reactors” is usually used for systems involving fluid-fluid interactions and liquid-liquid systems.
For systems involving solids, what are the two general cases that exist?
For systems involving solids, two general cases exist:
1.Solid as porous catalyst pellet
Solid is not consumed in reaction, but its physical and chemical nature may change.
Examples
(1) Pore blocking due to deposits of carbonaceous by-products of reaction, i.e. coking.
(2) Metal particles (the active catalyst) may coalesce at high temperature, reducing the overall surface area and hence the rate constant, i.e. sintering
- Solids as non-catalyst
Examples
(1) Dissolution of solid through reaction with a fluid.
(2) Burning off of coke in a catalyst pellet for its regeneration.
Where is the most practical use of solid catalyst?
Most practical and common utilization of solid catalysts is in a fixed bed catalytic reactor (FBCR), i.e. a tubular reactor packed with catalyst, through which the fluid reaction species flow.
What are the advantages of FBCR?
No solids handling
Little solids attrition
High surface area through the use of porous catalyst
Plug flow operation can be approached
No separation of catalyst from reaction products is needed.
What are the disadvantages of FBCR?
Pressure drop
Complex (multi-tubular) arrangements for reactions that require high-heat exchange duties
Large downtime for catalyst which deactivate rapidly
Where disadvantages of FBCR are prohibitive, what do we use?
Where disadvantages of FBCR are prohibitive, reactors involving the fluidization of the catalyst, or the flow (transport) of the catalyst is some way, are employed.
Such operation enables better heat transfer between the fluid-solid and the fluid-heat exchange surface and provides a means for the continuous removal of catalyst for regeneration, and feed of fresh catalyst.
What is the general performance equation to design heterogeneous and multiphase reactors?
The general performance equation to design heterogeneous and multiphase reactors is quite complex:
Output = f (input, material balance, equilibrium, kinetics, flow & contacting pattern, phase aggregation)
Hence, we always search for simplifying approximations
What convention do we take for the rate of reaction?
To model our reactors, we will use the rate of consumption rA (or rate of reaction for short). It is a positive number! Unless otherwise stated, we will also use the convention of positive stochiometric coefficients.
aA + bB –> cC + dD, with a,b,c,d > 0 so that rA/a = rB/b = rC/c = rD/d
What approximations do reactor models use? Write down the material balances for the ideal cases.
All reactor models use approximations, modification or combinations of the ideal cases: batch, PFR, differential reactor, and CSTR. Therefore, it is important to know these reactor design equations.
Batch reactor
rA = -1/V dNA/dt
Integral Reactor PFR
rA = - dnA/dV
Differential Reactor
rA = - nA-nA0/ v
CSTR
rA exit = - nA exit - nA0/V
What knowledge is needed to design reactors?
To design a reactor, knowledge of the rate of reaction is key. For heterogeneous and multiphase reactors, the rate of reaction can be expressed in many different ways. We have to use the rate, which is most convenient for our design, as we can always convert one into the other:
rA x V reactor = rA’ x m catalyst = rAs A surface.
What does the choice of a reactor for a specific application depend on?
The choice of a reactor for a specific application depends on the reaction rate, the catalyst stability and product distribution considerations. The selectivity of the process is a key factor for selecting the desired mixing environment we require for our process.
What is the general solution approach to design heterogeneous and multiphase reactors.
Regardless of the problem type, the general solution approach to design heterogeneous and multiphase reactors remains the same:
- Assemble all relevant physico-chemical data for the process.
- Quantify the relationship between intrinsic and mass transfer rate.
- Draw a model sketch.
- Define the balance equations, model parameters, and boundary conditions.
- Look for model simplification opportunities, taking into account step (2) above.
- Solve the model by combining equations to arrive at XA = f (t or tau)
- Use the equation to determine the expected conversion for a given reactor volume or calculate the reactor volume for a desired conversion.
What is a differential reactor?
Data acquisition using the method of initial rates and a differential reactor are similar in that the rate of reaction is determined for a specified number of predetermined initial or entering reactant concentrations. A differential reactor is normally used to determine the rate of reaction as a function of either concentration or partial pressure. It consists of a tube containing a very small amount of catalyst usually arranged in the form of a thin wafer or disk. A typical arrangement is shown schematically in Figure 5-8. The criterion for a reactor being differential is that the conversion of the reactants in the bed is extremely small, as is that change in reactant concentration through the bed. As a result, the reactant concentration through the reactor is essentially constant and approximately equal to the inlet concentration. That is the reactor is considered to be gradientless, and the reaction rate is considered spatially uniform within the bed.
Describe the operating condition and construction of the differential reactor.
The differential reactor is relatively easy to construct at low cost. Owing to the low conversion achieved in this reactor; heat released per unit volume is small (or can be made small by diluting the bed with inert solids) so that the reactor operates essentially in an isothermal manner. When operating this reactor, precautions must be taken so that the reactant gas or liquid does not bypass or channel through the packed catalyst but instead flows uniformly across the catalyst.
When is the differential reactor not an appropriate choice?
If the catalyst under investigation decays rapidly, the differential reactor is not a good choice because the reaction rate parameters at the start of the run will be different from those at the end of the run. In some cases, sampling and analysis of the product may be difficult for small conversions in multicomponent systems.
Establish the design equation for a differential reactor.
The volumetric flow rate through the catalyst bed is monitored, as are the entering and exiting concentrations. Therefore, if the weight of catalyst, W is known, the rate of reaction per unit mass of catalyst, rA’, can be calculated. Since the differential reactor is assumed to be gradientless, the design equation will be similar to the CSTR design equation. A steady-state mole balance on reactant A gives:
FA0 - FAe + rA’ W = 0
The subscript e refers to the exit of the reactor. Solving for -rA’, we have
-rA’ = FA0 - FAe/ W
- rA’ = voCA0 - vCAe / W
In terms of conversion or product flow rate Fp:
-rA’ = FA0/X = Fp/W
The term FA0X gives the rate of formation of the product Fp, when the stoichiometric coefficient of A and of P are identical.
For constant volumetric flow rate:
-rA’ = voCp / W
Establish the expression for the rate of reaction in the catalyst bed.
By using very little catalyst and large volumetric flow rates, the concentration difference (CA0 - CAe), can be made quite small. The rate of reaction can be obtained as a function of the reactant concertation in the catalyst bed, CAb :
-rA’ = -rA’ (CAb)
by varying the inlet concentration. One approximation of the concentration of A within the bed, CAb, would be the arithmetic mean of the inlet and outlet concentrations:
CAb = CA0 + CAe/ 2
However, since very little reaction takes place within the bed, the bed concentration is essentially equal to the inlet concentration,
CA0 = CAe
so -rA’ is a function of CA0:
-rA’ = -rA’ (CA0)
As with the method of initial rates, various numerical and graphical techniques can be used to determine the appropriate algebraic equation for the rate law.
Example 5.4
Solve Example 5.4
