CHE

• System


  Part of the universe that we are interested in studying
• Closed system


  • Can exchange energy (as heat or work) but not matter (mass)
• Open system


  • Can exchange both energy and matter with its surroundings
• Batch system


  • Feed is charged (fed) to a vessel at the beginning of the process and the vessel contents are removed sometime later
• Continuous system


  • The inputs and outputs flow continuously throughout the duration of the process
• Semi-batch system


  • Reactants are periodically added or products are periodically removed
• Steady state


  • Values of all the process variables do not change with time, except possibly for minor fluctuations about constant mean values
• Transient


  • If any of the process variables change with time
• General Balance Equation


  INPUT + GENERATION - OUTPUT - CONSUMPTION = ACCUMULATION
• Input


  Enters through system boundaries
• Generation


  Is produced within the system
• Output


  Exits through system boundaries
• Consumption


  Is used up within the system
• Accumulation


  Is built up within the system
• Differential


  Indicates what is happening with a system at an instant in time, each term of the balance equation is a rate
• Integral


  Describe what happens between two instants of time, each term of the equation is an amount of the balanced quantity
• If the balanced quantity is total mass, set generation = 0 and consumption = 0. Except in nuclear reactions, mass can neither be created nor destroyed.
• If the balanced substance is a nonreactive species (neither a reactant nor a product), set generation=0 and consumption=0
• If a system is at steady state, set accumulation 0, regardless of what is being balanced. By definition, in a steady state system nothing can change with time, including the amount of the balanced quantity
• For continuous processes at steady-state, the accumulation term in the general balance equation, equals zero
• For batch processes, the accumulation term in the general balance equation is = final - initial = generation - consumption
• Flowchart


  Used to summarize the layout of a process as well as to indicate known information about the state of the process streams
• Flowchart


  1. Write the values and units of all known stream variables at the locations of the streams on the chart

  2. Assign algebraic symbols to unknown stream variables and write these variable names and their associated units on the chart

  3. Having a consistent notation can aid understanding
• Splitter


  • Divides a single input into two or more outputs of the same composition (no reaction)
• Separator


  • Separates a single input into two or more outputs of different composition (no reaction)
• Mixer


  • Combines two or more inputs (usually of different compositions) into a single output (no reaction)
• Reactor


  • Carries out a chemical reaction that converts atomic or molecular species in the input to different atomic or molecular species in the output
• Pump


  • Changes the pressure of an input to that of the corresponding output (no reaction)
• Heat Exchanger


  • Transfers heat from one input to a second input (no reaction)
• In general material balance, there are 5 terms to consider: Input, Output, Generation, Consumption, Accumulation
• If in steady state, the accumulation term is set to be zero
• Scaling Up


  If the final stream quantities are larger than the original
• Scaling Down


  If the final stream quantities are smaller than the original
• Laboratory Scale


  • Done in test tubes to produce some grams of the product
• Pilot Scale


  • A small industrial machine that requires several men to operate it, possibly has its own building and can produce tens of kilograms to tins of product
• Bench Scale


  • Using a device that fits in a normal room, can produce up to kilograms of product
• Industrial Scale


  • Machinery may occupy a large area as seen in the actual industries
• Basis of Calculation


  An amount (mass or mole) or flow rate (mass or molar) of one stream or stream component in a process
• Guideline in Choosing a Basis

  1. If a stream amount or flow rate is given in a particular statement, it is usually most convenient to use this quantity as a basis of calculation

  2. If no stream amounts or flow rates given, assume one, preferably that of a stream with a known composition

  3. If mass fractions are known, choose total mass or mass flow rate of that stream as a basis (e.g. 100 kg or 100 kg/ h)

  4. If mole fractions are known, choose a total number of moles or a molar flow rate as basis (e.g. 100 mol)
• Degree of Freedom Analysis


  Method of analyzing systems to see whether they are over, under-specified, or if they are well defined