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3. Chemical Process 3
4. Considerations in Chemical Processing Several questions related to different aspect of chemical processing arise when we deal with chemical processing:
Synthesis: What sequence of processes are required (mixer, heater, reactor, separator) ?
Design: What type and size of equipment?
Operation: What operating conditions will yield desirable product?
Control: What process input can be manipulated ?
Safety: What If a process unit fails?
Environmental: How can we operate the system to minimize pollutants ? 4
5. Process Model
6. Model, Process Model & Chemical Process/System Model
A mathematical or physical system obeying certain specified conditions, whose behavior is used to understand a physical, biological, or social system to which it is analogous to.
McGraw-Hill Dictionary of Scientific and Technical Terms
A process model is a set of equations (+ necessary input data to solve the equations) that allows us to predict the behavior of a chemical process system
Chemical Process/System: A single or a combination of chemical unit operations that cause physical and/or chemical change in a substance or mixture of substances, e.g.
Separator (distillation column) 6
7. Basic Approach to Process Modeling 7
8. Types of Process Models Fundamental Model
These models are based on known physical-chemical relationships
Ideal gas law
Reaction kinetics or rate law
Heat transfer rate
These models are usually employed when the processes are too complex or poorly understood
Least-square fit of experimental data 8
9. Classification of Chemical Process Systems
10. Classification Based on Spatial Homogeneity Lumped Parameter System
A system wherein process variables are spatially homogeneous (do not vary in spatial dimension) but may vary with time.
Distributed Parameter System
A system wherein process variables are spatially heterogeneous (vary with space) and may vary with time. 10
11. Classification Based on Temporal Variation Steady-State System
Chemical processes that operate at steady-state conditions, i.e. the inputs and the outputs of the process(es) or process variables do not change with time.
Chemical processes for which inputs and the outputs of the process(es) change with time, i.e. they have a dynamic behavior.
e.g. batch reactors for pharmaceutical chemicals
Note: Dynamic behavior can also be encountered during start-ups and shut-downs of processes or when certain inputs are changed either deliberately or accidentally.
12. Types of System and Resulting Equations 12
13. Mathematical Modeling of Chemical Processes:Balance Equations
14. Process Modeling: General Discussion Mathematical equations constituting process models arise from the fundamental laws of mass, energy, and momentum conservation.
In CHEE 222, we will focus primarily on Mass and Energy (M&E) balances
For systems undergoing changes in chemical composition, species balance equations must be included in the process model.
In CHEE 223, you will learn about momentum balance
For reliable outputs from a process model, we must ensure that the model constitutes correct equations describing the system; as well, we must be confident that we are solving the equations correctly. 14
15. Differential and Integral Balance Equations Differential Balance Equations
Equations are derived by considering what is happening in a system at any instance in time. Each term of the equation is a RATE (rate of input, output etc.)
Integral Balance Equation
Equations are derived by considering the state of a system at two distinct time. Each term of the equation is an AMOUNT of the balance quantity
16. Differential Balance Equation General Form 16
17. Mass Conservation: Material Balance Equation 17
18. Energy Conservation: Energy Balance Equation 18
19. Mole Balance Equation 19
20. Transient Mass Balance Example 20
21. Transient Energy Balance
22. Differential Energy Balance Equation 22
23. Energy Balance Equation (Cont.) Let us examine each term of the energy balance equation. 23
24. Energy Balance Equation (Cont.) 24
25. Energy Balance Equation (Cont.) 25
26. Energy Balance Equation (cont.) For most of the chemical processes, the kinetic and potential energy changes in the system and between the inlet and outlet streams are negligible
27. Energy Balance Equation (cont.) Substituting equations (9), (10), and (11) in equation (8), we get
28. Energy Balance Equation (cont.) Simplification of eqn (14) below depends on what valid assumptions can made
29. Case1: If msys, P and r are constant
30. Energy Balance Equation (cont.) Case1: If msys, P and r are constant, we can write
The above assumption may hold true for constant pressure liquid-system (r=constant) for which the input mass flow rate is equal to output mass flow rate (msys=constant)
Applying eqn (15) in overall energy balance equation (12)
31. Case2: msys varies with time; P and r are constant
32. Energy Balance Equation (cont.) Case2: If msys is NOT constant but P and r are. Also, if the system is a lumped-parameter system
For lumped-parameter system, because of the well-mixed assumption, we can define the properties of the fluid in the system to be equal to that of the outlet fluid stream, therefore 32
33. Applying eqn (17) in overall energy balance equation (12)
From mass balance, we can also write,
Substituting eqn (19) in (18), we get
Energy Balance Equation (cont.) 33
34. Expanding eqn (20)
Rearranging we get,
For liquids, we can further simplify
Energy Balance Equation (cont.) 34
35. Transient Energy Balance Example 35
36. Constitutive Relationships
37. Constitutive Relationships 37
38. Types of Variables State and Design Variables
Measurable variable that indicates the state of a system
A variable that normally must be specified to solve a problem
Must be known to mathematically solve a problem
Often fixed by nature