C. Material Balance basis will w. Whenever we apply the principle of conservation of mass to define a material balance,we nt to specify the basis for it. Generally, the basis is either the quantity of total mass or the mass of a particular component or conserved species for which the material balances will be defined. Or, for continuous processes, it might be the mass flow rate of a component or Quite often the basis will be set by the specification of the problem to be solved. For instance, if we are told that a tank contains 5,000 pounds of a particular mixture about which certain questions are to be answered, then a natural basis for the problem would the 5,000 pounds of the mixture. Or, if we are looking at a continuous process to make 10,000 Kg/hr of ethanol reasonable choice for a basis would be this production rate Some problems, however, do not have a naturally defined basis so we must choose one For instance, if we are asked what is the mass ratio of Naoh to H2SO+ required to produce neutral solution of NaCl in water, we would have to specify a basis for doing the calculations We might choose 98.08 Kg of H2S04(1.0 Kg-mol) as a basis. Or we could chose 1.0 lb of H2SO4. Either is acceptable. One basis may make the calculations simpler than another, but in this day of personal computers the choice is less critical than it might have been years ago Whatever the choice of basis, it is mandatory that all material balances are defined to be consistent with it D. Material balances We are now in a position to define material balances for some simple systems. (Note Material balances are sometimes referred to as mass balances. There are three basic situations for which we will want to do this a discrete process in which one or more steps are carried out over a finite but definite period of time An example of such a process is the dissolving of a specified quantity of salt in a quantity of water contained in a tank. We are only interested in the concentration in weight of the salt water after it is completely dissolved and not how long it takes for the salt to dissolve A continuous process operating in the steady state interna By definition, continuous process operating in the steady state undergoes no changes in its state variables such as temperatures, pressures, compositions, and liquid levels. In addition, all the flow rates of all streams entering and leaving each item of equipment are constant What this means from the standpoint of material balances is that there is no change in any of the holdups in the system-7- C. Material Balance Basis Whenever we apply the principle of conservation of mass to define a material balance, we will want to specify the basis for it. Generally, the basis is either the quantity of total mass or the mass of a particular component or conserved species for which the material balances will be defined. Or, for continuous processes, it might be the mass flow rate of a component or conserved species. Quite often the basis will be set by the specification of the problem to be solved. For instance, if we are told that a tank contains 5,000 pounds of a particular mixture about which certain questions are to be answered, then a natural basis for the problem would the 5,000 pounds of the mixture. Or, if we are looking at a continuous process to make 10,000 Kg/hr of ethanol, a reasonable choice for a basis would be this production rate. Some problems, however, do not have a naturally defined basis so we must choose one. For instance, if we are asked what is the mass ratio of NaOH to H2SO4 required to produce a neutral solution of NaCl in water, we would have to specify a basis for doing the calculations. We might choose 98.08 Kg of H2SO4 (1.0 Kg-mol) as a basis. Or we could chose 1.0 lb of H2SO4. Either is acceptable. One basis may make the calculations simpler than another, but in this day of personal computers the choice is less critical than it might have been years ago. Whatever the choice of basis, it is mandatory that all material balances are defined to be consistent with it. D. Material Balances We are now in a position to define material balances for some simple systems. (Note: Material balances are sometimes referred to as mass balances.) There are three basic situations for which we will want to do this: 1) A discrete process in which one or more steps are carried out over a finite but indefinite period of time. An example of such a process is the dissolving of a specified quantity of salt in a quantity of water contained in a tank. We are only interested in the concentration in weight % of the salt in the water after it is completely dissolved and not how long it takes for the salt to dissolve. 2) A continuous process operating in the steady state. By definition, continuous process operating in the steady state undergoes no changes in its internal state variables such as temperatures, pressures, compositions, and liquid levels. In addition, all the flow rates of all streams entering and leaving each item of equipment are constant. What this means from the standpoint of material balances is that there is no change in any of the holdups in the system