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L10-11_Material_Balances - Material and Energy Balances...

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1 Material and Energy Balances 03/21/10 Mass Balance Calculations
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2 Material and Energy Balances 03/21/10 Objectives At the end of this chapter, you should be able to: Apply material balances for simple systems Formulate and solve material balance problems
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3 Material and Energy Balances 03/21/10 Mass Balance Calculations All material balance problems are variations on a single theme: Given values of some input and output stream variables, derive and solve equations for others. Deriving the balance equations from a description of a process and a collection of process data may be considerable difficult A procedure for reducing a description of a process to a set of equations that can be solved will be outlined in this lecture.
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4 Material and Energy Balances 03/21/10 Flowcharts The catalytic dehydrogenation of propane is carried out in a continuous packed bed reactor. One thousand kilograms per hour of pure propane is preheated to a temperature of 670 o C before it passes into the reactor. The reactor effluent gas, which includes propane, propylene, methane and hydrogen, is cooled from 800 o C to 110 o C and fed to an absorption tower, where the propane and propylene are dissolved in oil. The oil then goes to a stripping tower in which it is heated, releasing the dissolved gases; these gases are recompressed and sent to a distillation column in which the propane and propylene are separated. The propane stream is recycled back to join the feed to the reactor preheater. The product stream from the distillation column contains 98% propylene, and the recycle stream is 97% propane. The stripped oil is recycled to the absorption tower.
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5 Material and Energy Balances 03/21/10 Flowcharts The best way to organize the information convenient for calculations is to draw flowchart of the process A flowchart is drawn using boxes or other symbols to represent process units (reactor, mixers, separation units, etc.) and lines with arrows to represent inputs and outputs.
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6 Material and Energy Balances 03/21/10 Flowcharts - Example Suppose a gas containing N 2 and O 2 is combined with propane in a batch combustion chamber in which some of the O 2 and C 3 H 8 react to form CO 2 and H 2 O, and the product is then cooled, condensing the water. The flowchart of the two unit process might appear as shown. COMBUSTION CHAMBER CONDENSER 1000 mol O 2 3760 mol N 2 100 mol C 3 H 8 200 mol H 2 O 50 mol C 3 H 8 750 mol O 2 3760 mol N 2 150 mol CO 2 50 mol C 3 H 8 750 mol O 2 3760 mol N 2 150 mol CO 2 200 mol H 2 O
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7 Material and Energy Balances 03/21/10 Flowcharts A flowchart of a process can help get material balance calculations started and keep them moving To do this, the chart must be fully labeled when it is first drawn, with values of known process variables and symbols for unknown variables being written for each input and output stream.
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