AECH 1103 - Topic 2 PDF

Topic 2 Representing flow diagrams of industrial processes AECH 1103 Industrial Process Overview  What is a Chemical Processing? A chemical process is a combination of steps in which starting materials are converted into desired products using equipment and conditions that...

Topic 2 Representing flow diagrams of industrial processes AECH 1103 Industrial Process Overview  What is a Chemical Processing? A chemical process is a combination of steps in which starting materials are converted into desired products using equipment and conditions that facilitate that conversion. Ethylene cracker complex in Freeport, Texas, 2 Credit: Dow  Lab Scale A chemical reaction : A+B→C Industrial Process 3  Outline Section A. Describing Chemical Processes Section B. Flow Diagrams Section C. Material Balances 4  Section A. Describing Chemical Processes 1. Motivation 2. Definition of a System 3. Open and Closed Systems 4. Batch, Semi-Batch and Continuous Systems 5. Steady State and Transient Systems 5  Motivation It’s important to learn some of the jargon used to classify chemical processes Starting now, we’ll be applying these concepts in this course, other courses, and throughout our careers Investigating types of systems will enable us to have a better understanding of where the molecules are going in any given process Prerequisite for nearly all analyses and calculations in chemical engineering 6  Definition of a System/System Boundary A system can be any process, plant, unit, or piece of equipment that you wish to consider for analysis E.g., an individual chemical reactor or a whole refinery The system is defined by the system boundary System Boundary: an imaginary line or box that encloses the portion of the process you wish to analyze Example System: a well mixed tank System Boundary 7 Gas Dehydration Unit TEG (l) Example System includes the Absorber, Distillation Column, Pump, Reboiler and Condenser TEG (l) Example System only includes the Absorber 8  Black Box Representation Defining the system and system boundary allows for a “Black Box” representation We’re focused on the system inputs and system outputs We’re not overly concerned with details of what’s inside the system Black Box 9  “Black Box” TEG (l) Dried Gas Wet Natural Gas Gas Dehydration Unit Water 10  “Black Box” TEG (l) Dried Gas Wet Natural Gas Absorber TEG TEG + Water 11  Open and Closed Systems A Closed System is one where no material crosses the system boundary Closed System Changes may occur within the system E.g., chemical reactions, phase changes but no material enters or exits 12  Open and Closed Systems An Open System, or flow system is one where material crosses the system boundary Open System Changes may occur within the system Material may enter, exit, or accumulate within the system 13  Open or Closed System? A Microwave Oven A Boiling Tea Kettle 14  Open or Closed System? A Distillation Column A Storage Tank at a Refinery 15  Batch, Semi-Batch, and Continuous Processes Systems can be classified as open or closed Systems can also be classified as either a batch, semi-batch or continuous process To classify a process as Batch, Semi-Batch, or Continuous we consider the nature in which streams enter and exit the system  Batch, Semi-Batch, and Continuous Processes Batch Process: The feed is charged into a vessel at the beginning of the process. No material crosses the system boundary between the time the system is charged and the time the products are removed. Step 1: Step 2: Step 3: Add the Feed Remove Products 17  Batch, Semi-Batch, and Continuous Processes Continuous Process: Material continuously flows into and out of the system throughout the duration of the process. A B A+B →C C 18  Batch, Semi-Batch, and Continuous Processes Semi-Batch Process: Any process that is neither batch nor continuous. Most semi-batch processes can be described as either of the following: Feed is continuously added to the system, but product is not removed (e.g., filling a blending tank) The process begins with a full vessel and products are continuously removed (e.g., allowing the contents of a pressurized gas container to escape) A B 19  Batch, Semi-Batch, or Continuous? A Microwave Oven A Boiling Tea Kettle 20  Batch, Semi-Batch, or Continuous? A Distillation Column A Storage Tank at a Refinery 21  Steady State and Transient Systems Do the process variables (e.g., temperatures, pressures, flow rates, volumes, levels, concentrations) change over time in a given process? Steady State System – process variables do not change with time Explain why the system below is at steady state 50 kg/min H2O 50 kg/min H2O H2O  Steady State and Transient Systems Do the process variables (e.g., temperatures, pressures, flow rates, volumes, concentrations) change over time in a given process? Unsteady State or Transient System – process variables change with time Explain why the system below is at unsteady state 50 kg/min H2O 70 kg/min H2O H2O 23  Steady State and Transient Systems By definition, batch and semi-batch processes are transient (unsteady state) systems. The values of process variables change over time (An exception is a storage tank that is neither being filled nor emptied) Continuous processes may operate at steady state or unsteady state It is desirable to operate continuous industrial processes at steady state in order to make product of a consistent quality Starting up, shutting down, or making changes to a continuous process results in a transient or period of unsteady state 24  Example: Chemical Reactor Is this as an open system or a closed system? 25 kg/hr of A 50 kg/hr of B Is this a batch, semi-batch or continuous system? A+B →C 75 kg/hr of C Is this operating at steady state or unsteady state? 25  Example: Semi Batch Chemical Reactor Describe how the process variables (e.g., temperature, volume, flow rates, 25 kg/hr of A 50 kg/hr of B concentrations) change over time A+B →C + heat 26  Summary: True or False 1. All continuous systems are steady state systems 2. All steady state systems are continuous systems 3. A continuous system may be steady state or transient 4. All batch and semi-batch systems are transient systems 27  Flow of A (kg/hr) Reactor Temp (deg C) 60 80 70 50 60 40 50 30 40 30 20 20 10 10 0 0 0 30 60 90 120 150 180 0 30 60 90 120 150 180 Time (minutes) Time (minutes) Flow of B (kg/hr) Tank Volume (L) 60 50 50 40 40 30 30 20 20 10 10 0 0 0 30 60 90 120 150 180 0 30 60 90 120 150 180 Time (minutes) Time (minutes) A→B 28  Section B. Flow Diagrams 1. Block Diagram (BD) 2. Process Flow Diagram (PFD) 3. Piping & Instrumentation Diagram (P&ID) 29  1. Block Diagrams A block diagram provides a simple representation of a chemical process in which a box or block is used to represent either a single equipment item or a combination of equipment items that collectively accomplish one of the principal steps in the process. Block diagram for a low-pressure process to produce nitric acid 30 Block diagram as a simple representation of the human digestion process 31  2. Process Flow Diagrams (PFD) This type of diagram shows the arrangement and interconnection of all the major pieces of equipment and all flow streams, and the equipment is represented by symbols or icons that “look like” the actual equipment. Each stream is identified with a number or letter, and the data for the streams are usually compiled in a stream table at the bottom of the flowsheet. The amount of information given in the stream table will vary but usually includes: Stream composition. Most commonly, this is given as either of the flow rate of each individual chemical species the percentage or fraction of each species in the stream Total stream flow Stream temperature Normal operating pressure of the stream 32 33 34 35 36 37  3. Piping & Instrumentation Diagram (P&ID) Provides greater details for a process than the PFD. Prepared to be consistent with PFDs, usually using the same numbers or letters as in the PFD to represent streams and equipment. Includes engineering details of equipment, instrumentation, piping, valves and fitting, piping size, material specification, control lines, … etc. 38  Differences between P&ID and PFD P&IDs provide more details about the process including instrumentation details (flow instruments, temperature, pressure, pressure safety valves, meters and control valves), pipe routing conditions (minimum distance, slope, free flow, etc.), and piping details (size, specifications, service, insulation, rating). PFD is a simplified version of P&ID. 39