10 Questions
What is the primary advantage of the simultaneous non-modular approach in process simulation, and what is the drawback of this method?
The primary advantage is that it takes full advantage of the specific features of the equations, making it efficient in terms of computation time. The drawback is that it can be tedious and error-prone to set up.
What is the purpose of tear streams in the sequential modular approach, and how do they affect the number of flowsheet iterations?
Tear streams are introduced to break cycles, and they increase the number of flowsheet iterations needed to reach the solution.
What is the primary difference between the simultaneous modular approach and the sequential modular approach in process simulation?
The simultaneous modular approach combines the modularizing of equations related to specific equipment with the efficient solution algorithms for the simultaneous equation solving technique, whereas the sequential modular approach involves grouping equations describing units together and solving them in modules.
What is the role of the Barkley and Motard algorithm in network decomposition, and how does it work?
The Barkley and Motard algorithm is used to look for the fewest tear streams, and it involves listing each stream and its inputs in a table and removing streams that have only one input stream feeding them.
What are the common causes of convergence faults in process simulation, and how can they be addressed?
Common causes of convergence faults include ill-posed problems, incorrect equipment specifications, and tolerance set too tightly. These can be addressed by revising the problem formulation, checking equipment specifications, and adjusting the tolerance settings.
What is the primary difference between equations of state models and activity coefficient models in thermodynamic modeling, and when are they recommended?
Equations of state models are algebraic equations for the pressure of a mixture as a function of composition, volume, and temperature, and are recommended for simple systems and regions where activity coefficient models are inappropriate. Activity coefficient models are algebraic equations for the activity coefficients as a function of composition and temperature, and are recommended for complex liquid mixtures if all binary interaction parameters are available.
What are the guidelines for selecting equipment parameters for a level 1 simulation, and what information is required for different types of equipment?
For level 1 simulations, some guidelines are useful for selecting equipment parameters. For example, valves require either the outlet pressure or pressure drop to be specified, pumps require either the desired pressure increase or the desired pressure of the fluid leaving the pump, and heat exchangers require different specifications depending on the type of heat exchange.
What is the role of the phase rule in specifying feed stream properties, and how does it define the number of variables that must be specified?
The phase rule defines the number of variables that must be specified to define completely any feed stream, and it is used to specify the composition, temperature, and pressure of the feed stream.
What is the purpose of network decomposition algorithms in process simulation, and how do they work?
Network decomposition algorithms are used to solve a flowsheet in the most efficient manner possible, involving two steps: partitioning and precedence ordering. Partitioning involves finding the units that must be solved as a group, and precedence ordering defines the order for solving the partitions.
What are the different types of reactors that can be modeled in process simulation, and what are the required specifications for each type?
The different types of reactors that can be modeled include stoichiometric reactors, kinetic reactors, equilibrium reactors, and minimum Gibbs free energy reactors. Each type requires specific specifications, such as the number and stoichiometry of the reactions, temperature, conversion, and pressure of the limiting reactant, and thermal mode of operation.
Study Notes
Steps to Set up a Process Simulation
- Select chemical components from a component database
- Select thermodynamic models required for the simulation
- Input topology of flowsheet by specifying the input and output streams of each piece of equipment
- Select feed stream properties (T, P, cji, Vi…)
- Select equipment parameters
- Select output display options
- Select convergence criteria and run simulation
Structure of a Process Simulator
- Component database: contains constants required to calculate physical properties from thermodynamic models
- Thermodynamic model solver: provides various options for vapour-liquid equilibrium (VLE), liquid-liquid equilibrium (LLE), and other thermodynamic properties
- Flowsheet builder: keeps track of the flow of streams and equipment in the process being simulated
- Unit operation block solver: provides computational modules for material and energy balances, and design calculations for various process equipment
- Data output generator: customizes the results of the simulation in terms of output report and graphical displays
- Flowsheet solver: controls the sequence of calculations and overall convergence of the simulation
Approximations to Simulation
- Stochastic simulation: identifies principles or laws related to the process evolution, and uses mathematical equations to characterize it
- Stochastic models: based on the theory of Markov chains, can be transferred to numerical models, and can solve complex systems
- Statistical simulation: applied in three cases: incomplete process information, complex process states, and limited researcher ability to develop a deterministic or stochastic model
- Statistical models: require inputs and outputs of the process, and the validity of the simulation is conditioned by the experimental database employed to develop the model
Process Simulation vs Pilot Plant
- Traditional development of a chemical process: laboratory, pilot plant, factory
- Process simulation: substitutes pilot plant stage to save money and time, requires experimental data and knowledge
- Pilot plant: provides relatively easy access to actual process conditions, validates home-made models, and permits addition of equations describing non-ideal process hardware to compute efficiencyHere are the study notes for the text:
4. Structure of a Process Simulator
- A process simulator consists of a:
- Component database
- Thermodynamic model
- Flowsheet builder
- Solver
- Output generator
- The sequence of operations in a process simulator is:
- Select chemical components
- Select thermodynamic model
- Input topology of flowsheet
- Select units and feed stream properties
- Select equipment parameters
- Select output display options
- Select convergence criteria and run simulation
4.1 Solution Algorithms
- There are three types of solution algorithms:
- Sequential Modular (Sequential and Simultaneous)
- Simultaneous Non-Modular (Equation-Solving)
- Simultaneous Modular
- Sequential Modular algorithm:
- Equations describing units are grouped together and solved in modules (unit by unit)
- Each unit is solved in sequence, starting with the first unit
- Simultaneous Non-Modular algorithm:
- All relationships for the process are written out together and solved simultaneously
- Constraints can be added as equations to the set defining the problem
- Simultaneous Modular algorithm:
- Combines modularizing equations related to specific equipment with simultaneous equation-solving technique
4.2 Network Decomposition Algorithms
- Network decomposition involves:
- Partitioning: finding units that must be solved as a group (partitions)
- Precedence ordering: defining the order for solving the partitions
- Example of network decomposition algorithm:
- Start with a list of units
- Remove units with no outputs outside the list
- Repeat until no more units can be removed
- The remaining units are the partitions
- The order of solving the partitions is the precedence order
4.3 Tearing Algorithms
- Tearing involves:
- Identifying loops in the flowsheet
- Breaking the loops by introducing tear streams
- Solving the resulting sub-problems
- Example of tearing algorithm:
- Apply the Barkley and Motard algorithm (1973)
- Identify the fewest tear streams needed to break the loops
- Solve the resulting sub-problems
5. Simulation Software Application
- The process simulation involves:
- Data introduction
- Thermodynamic models
- Selection of equipment parameters
- Data introduction involves:
- Selecting chemical components
- Inputting topology of flowsheet
- Selecting feed stream properties
- Selecting units and equipment parameters
- Selecting output display options
- Selecting convergence criteria and running the simulation
5.1 Data Introduction
- Selecting chemical components:
- Identify all components in the process
- Add user-defined components to the simulator's database
- Inputting topology of flowsheet:
- Use a graphical interface to draw the flowsheet
- Define the connectivity of the streams and units
- Selecting feed stream properties:
- Define the composition, temperature, and pressure of each feed stream
- Ensure that the number of specifications is equal to the degrees of freedom
- Selecting units and equipment parameters:
- Define the parameters for each unit operation (e.g. heat exchanger, reactor, etc.)
5.2 Thermodynamic Models
- Thermodynamic models are used to predict:
- Phase equilibria
- Physical properties (density, viscosity, etc.)
- Enthalpies of phase change
- Types of thermodynamic models:
- Equations of state (e.g. SRK, PR)
- Activity coefficient models (e.g. Wilson, NRTL, UNIQUAC)
5.3 Selection of Equipment Parameters
- Selection of equipment parameters involves:
- Specifying the design variables for each unit operation
- Defining the performance relationships for each unit
- Examples of equipment parameters:
- Heat exchangers: outlet temperature and pressure
- Pumps: outlet pressure
- Compressors: outlet pressure
- Reactors: conversion and selectivity
- Distillation columns: number of theoretical plates, reflux ratio, etc.
This quiz covers the steps involved in simulation process in chemical engineering, including selecting chemical components, thermodynamic models, and equipment parameters. It also discusses the importance of convergence criteria and output display options. Test your knowledge of chemical engineering simulation processes!
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