1. CHAPTER 2 Description of Chemical Processes
CHEM2003/Dr. Ahmed J. Ali/

2. Objectives To be able to define different streams in a process.
To describe the meaning of standard abbreviations and symbols used on process flowsheets.
Write a description of a process flowsheet.
Draw a process flowsheet from a written description.

3. What is Process? A process is some operation carried out to modify input(s) to output(s) – based on physical and/or chemical changes
Outputs
Inputs
Process
“feeds”
“products”

4. Classification of Processes
An integrated series of operations through which materials and/or energy
are converted from one form to another.
Batch process:
Has a definite end
Material is put in, processed, and discharged
Applies to more than just reactors (washing machine for example)
Continuous process
Materials enter and leave in uninterrupted streams
Periodic shutdown is required
Semi-batch or Semi-continuous
Some materials are charged/discharged at intervals while some
enter/exit continuously
Biotechnology industry, to add nutrients

5. Process Variables
The variables that describe the condition of a process fall into two categories:
Extensive variables: which depend on the size of the system (mass, volume)
Intensive variables: do not depend on the size of the system (e.g. T, p, , mass and mole fractions)
Often on a process flowsheet, designers would like to specify process variables such as size, composition and temperature, etc.

6. Flowsheet conventions
Chemical processes often involve substances of:
1-high chemical reactivity,
2-high toxicity, &
3-high corrosivity operating at high pressures &
temperatures.
Thus, visual information is the clearest way to present material & is least likely to be misinterpreted. For these reasons, it is essential that chemical engineers be able to formulate appropriate process diagrams & be skilled in analyzing & interpreting diagrams prepared by others.

7. Flowsheet conventions
Flowsheet: is a Diagram for Understanding
Chemical Processes.
the commonly used by chemical engineers are:
Block Flow Diagram (BFD)
Process Flow Diagram (PFD)
Piping and Instrumentation Diagram (P&ID)
Plant Layout

8. Block Flow Diagram1/4
In the courses of material & energy balances, often the initial step was to convert a word problem into a simple visual block flow diagram.
This diagram is a series of blocks connected with input & output flow streams. It included operating conditions (temperature & pressure) & other important information such as conversion & recovery, given in the problem statement.
But, it did not provide details regarding what was involved within the blocks, but concentrated on the main flow of streams through the process

9. Block Flow Diagram2/4
A block diagram is the simplest form of presentation.
Each block represents a single piece of equipment or a complete stage in the process.

10. Block Flow Diagram3/4 Operations shown by blocks
Major flow lines shown with arrows giving direction of flow
Flow goes from left to right whenever possible
Light stream (gases) toward top with heavy stream (liquids and solids) toward bottom.
Critical information unique to process supplied.
If lines cross, then the horizontal line in continuous and the vertical line is broken.
Simplified material balance provided.

11. Block Flow Diagram4/4 Combiner and splitter:
Avoid crossing streams. If streams must cross, you need to indicate whether they mix or not.
streams combine and split
streams cross without mixing
Ambiguous

12. Example of a block flow process diagram is shown in Figure 1.1
Toluene & hydrogen are converted in a reactor to produce benzene & a methane. The reaction does not go to completion, & excess toluene is required. The non-condensable gases are separated & discharged. The benzene product & the unreacted toluene are then separated by distillation. The toluene is then recycled back to the reactor & the benzene removed in the product stream.

13. Example: Synthesis of Ammonia1/5
Type of process?
Reactor and condenser are continuous
Storage tank is semi-continuous
Reaction: N H2 2NH3

14. Example: Synthesis of Ammonia2/5
A mixture of hydrogen and nitrogen is fed to a catalytic reactor where some of the hydrogen and nitrogen is converted to ammonia. The reactor effluent is sent to a condenser where all of the ammonia is condensed. The condensed ammonia is sent to product storage. The uncondensed hydrogen and nitrogen are recycled by being mixed with a fresh feed of the same composition. The resulting mixture is fed to the reactor.
Step 1 is the feed to the reactor:
“feed” N2 H2
“effluent”
NH3 N2 H2
reactor

15. Example: Synthesis of Ammonia3/5
A mixture of hydrogen and nitrogen is fed to a catalytic reactor where some of the hydrogen and nitrogen is converted to ammonia. The reactor effluent is sent to a condenser where all of the ammonia is condensed. The condensed ammonia is sent to product storage. The uncondensed hydrogen and nitrogen are recycled by being mixed with a fresh feed of the same composition. The resulting mixture is fed to the reactor.
Step 2 is to purify the product:
“feed” N2 H2
reactor
condenser
“effluent”
NH3 N2 H2
NH3
liquid

16. Example: Synthesis of Ammonia4/5
A mixture of hydrogen and nitrogen is fed to a catalytic reactor where some of the hydrogen and nitrogen is converted to ammonia. The reactor effluent is sent to a condenser where all of the ammonia is condensed. The condensed ammonia is sent to product storage. The uncondensed hydrogen and nitrogen are recycled by being mixed with a fresh feed of the same composition. The resulting mixture is fed to the reactor.
Step 3 is to send product to storage:
“feed” N2 H2
reactor
condenser
“effluent”
NH3 N2 H2
NH3
liquid
Storage tank

17. Example: Synthesis of Ammonia5/5
A mixture of hydrogen and nitrogen is fed to a catalytic reactor where some of the hydrogen and nitrogen is converted to ammonia. The reactor effluent is sent to a condenser where all of the ammonia is condensed. The condensed ammonia is sent to product storage. The uncondensed hydrogen and nitrogen are recycled by being mixed with a fresh feed of the same composition. The resulting mixture is fed to the reactor.
purge
Step 4 is to recycle unreacted feeds:
recycle
N2, H2
“feed” N2 H2
reactor
condenser
“effluent”
NH3 N2 H2
NH3
liquid
Storage tank