Material Balances Chapter 6: Introduction to Material Balance Objectives:

Transcription

1 Material Balances Chapter 6: Introduction to Material Balance Objectives: 1. Understand the features of process, system, open, closed, steady-state, and unsteady-state systems. 2. Express in words what the material balance is for a process involving single or multiple components. 3. Understand how a chemical reaction affects the material balance. 4. Recognize a batch or semi-batch process and write the material balance. 44

2 Definitions Process - an operation or series of operations that causes a physical or chemical change thereby converting raw materials into products. Chemical Engineering Examples: reactors, mixers, separators, biological systems, etc. Balance - an accounting or inventory of mass and changes. System an arbitrary portion or whole of a process as specified by the engineer analyzing the problem. A B C Mix MX RP React Separate W P

3 Simplified Process Flow Diagram for Ethanol Production from Corn. What is the System Boundary? 46

4 System Classification Before writing a material balance (MB) you must first identify the type of system and process in question. Closed System: A system that does not have material crossing the system boundary. Open System: A system in which material crosses the system boundary. Steady-State System: Process variables (i.e., T, P, V, flow rates) do not change with time. Unsteady state System (Transient): Process variables change with time. Process Classification (After 50 min) Batch Process: A process in which material is neither added to nor removed from the process during its operation. Batch processes fall into the category of closed systems. Continuous process: A process in which material enters and/or exits continuously. Semi-Batch Process: any process that is neither batch nor continuous, materials enters the system but product is not removed during operation. 100 kg H 2 O/min 100 kg H 2 O/min 100 kg H 2 O/min 1000 kg H 2 O 1000 kg H 2 O 1500 kg H 2 O 1000 kg H2O System boundary 100 kg H 2 O/min 90 kg H 2 O/min 47

5 Test yourself: Process classification Classify the following processes as batch, continuous, or semibatch, and steady-state or unsteady-state (transient). 1. A balloon is filled with air at a steady rate of 2 g/min. 2. A bottle of milk is taken from the refrigerator and left on the kitchen table. 3. Carbon monoxide and steam are fed into a tubular reactor at a steady-rate and react to form carbon dioxide and hydrogen. Products and unused reactants are withdrawn at the other end. The reactor contains air when the process is started up. The temperature of the reactor is also constant, and the composition and flow rate of the entering reactant stream are also independent of time. Classify the process (a) initially and (b) after a long period of time has elapsed. 48

6 General Mass Balance A balance on a quantity in a system ( a single process unit, a collection of units, or an entire process) including reactions can be written in the following way: Accumulation = Input Output + Generation - Consumption Input streams to system Example: Mass Balance System over which mass balance is made System Boundary output streams from system Input = Output = Generation = Consumption = Accumulation = A country has a population of 10 million people in 1900 AD. Over the period from 1900 to 2000, 6 million people immigrated into the country, 2 million people emigrated from the country, 5 million people were born in the country and 3 million people died in the country. What is the population of the country in the year 2000 AD? 49

7 Material Balance Simplifications The following rules may be used to simplify the material balance equation: Accumulation = In Out + Generation Consumption If the system is at steady-state, set accumulation = 0 In Out + Generation Consumption = 0 If the material balance is for a single component, set generation = 0 and consumption = 0 Accumulation = In Out If the balanced substance is a nonreactive species, (neither a reactant nor a product), set generation = 0 and consumption = 0 Accumulation = In Out If the system is an unsteady-state process, the accumulation term over a time interval can be calculated as: Accumulation = Final material - Initial material in the system in the system 50

8 Reaction Balances CO 2, CO, O 2 CO 2, CO, O 2 2 CO + O 2 = 2 CO 2 A material balance can be written in terms of the following conserved quantities: Total Mass In Total (kg/hr) Out Total (kg/hr) Mass (or moles) of a chemical compound kg CO /hr kg O 2 /hr kg CO 2 /hr kg CO /hr kg O 2 /hr kg CO 2 /hr Mass (or moles) of an atomic species kg C /hr kg O /hr kg C /hr kg O /hr 51

9 Test yourself: Material Balance Verify if total mass balance is valid. Naphtha 250,000 ton /yr Steam Cracker CH 4, H 2 (39,500 T/yr) Ethylene (55,000 T/yr) C2= Reactions Fuel Gas Polyethylene (30,000 T/yr) Polystyrene (5,000 T/yr) Propylene PVC (40,000 T/yr) (45,000 T/yr) Acryonitrile (20,000 T/yr) C3= Reactions Dodecylbenzene (8,000 T/yr) Phenol / acetone (15,750 T/yr) Butene (30,700 T/yr) Synthetic rubber (10,000 T/yr) C4= Reactions Butenes (24,000 T/yr) Aromatics (68,000 T/yr) Aromatics Aromatics (48,000 T/yr) Reactions Fuel Oil (3,800 T/yr) Heavy Oil 52

10 Some Basic Process Unit Functions Splitter divides a single input into two or more outputs of the same composition (no reaction) Mixer combines two or more inputs (usually of different compositions) into a single output) (no reaction) Separator separates a single input into two or more outputs of different composition (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 Heat exchanger transfers heat from one input to a second input (no reaction) Pump changes the pressure of an input to that of the corresponding output (no reaction) Splitter Mixer Separator Reactor Heat exchanger Pump Actual process units can combine these different functions into a single piece of hardware, and are given different names 53

11 Distillation Column Reflux Condenser Bottoms Reboiler Inside the Column Separator Heat Exchanger + Splitter Heat Exchanger Internal trays (or packing) are used to enhance component contact Each tray accomplishes a fraction of the separation task by transferring the more volatile species to the gas phase and the less volatile species to the liquid phase Can perform material and energy balances on: an individual tray the column, bottoms reboiler, or top condenser the entire system white = vapour blue = liquid 54