Material Balance Involving Multiple Units Lecture 11 Material Balance Involving Multiple Units

Terminology process flowsheet, flowchart Block diagram a graphical representation of process (Fig. 11.2a, p.307) Block diagram units appear as simple boxes called “subsystems” (Fig. 11.2b, p.307)

Determination of the Number of Independent Material Balance in a Process Involving Multiple Units (Fig. 11.3a-f, pp.308-310) Mixer Splitter/Separator Overall material balance Independent material balance equations

Determination of the Number of Independent Material Balance in a Process Involving Multiple Units (E11.1, DMH) Examine Figure E11.1. No reaction takes. The system is open and steady-state. The arrows designate flows. The composition of each stream is as follows: (1) Pure A, (2) Pure B, (3) A and B, concentrations known: wA = 0.800, wB = 0.200, (4) pure C, (5) A, B and C, concentration known: wA = 0.571, wB = 0.143, wC = 0.286, (6) pure D, (7) A and D, concentrations known: wA = 0.714, wD = 0.286, (8) B and C, concentrations known: wB = 0.333, wC = 0.667 What is the maximum number of independent mass balances that can be generated to solve this problem? Write down the possible equations. Do they form a unique set?

Suggestion Frequently, the best way to start is to make material balances for the “overall process”, ignoring information about the “internal connections”.

Material Balances for Multiple Units in which No Reaction Occurs (E11.2, DMH) Acetone is used in the manufacture of many chemicals and also as a solvent. In its latter role, many restrictions are placed on the release of acetone vapor to the environment. You are asked to design an acetone recovery system having the flowsheet illustrated. All the concentrations shown of both the gases and liquids are specified in weight percent in this special case to make the calculations simpler. Calculate A, F, W, B and D per hour. G=1400 kg/hr.

Material Balances for Multiple Units in which No Reaction Occurs (E11.4, DMH) A sugar recovery process involving multiple serial units is shown in the given figure and the known data. You are asked to calculate the compositions of every flow stream, and the fraction of the sugar in the cane that is recovered.

Material Balances for a Process Involving Multiple Units and Reactions (E11.3, DMH) Combustion In the face of higher fuel costs and the uncertainty of the supply of a particular fuel, many companies operate two furnaces, one fired with natural gas and the other with fuel oil. In the RAMAD Corp., each furnace has its own supply of oxygen. The gas furnace uses air while the oil furnace uses an oxidation stream that analyzes: O2 20%; N2 76% and CO2 4%. The stack gases go up a common stack. See the figure. (Note that two outputs are shown from the common stack to point out that the stack gas analysis is on a dry basis but water vapor also exists. The fuel oil composition is given in mole fractions to save you the bother of converting mass fractions to mole fractions.) During one blizzard, all transportation to the RAMAD Corp. was cut off, and officials were worried about the dwindling reserves of fuel oil because the natural gas supply was being used at its maximum rate possible. At that time, the reserve of fuel oil was only 560 bbl. How many hours could the company operate before shutting down if no additional fuel oil was attainable? How many lb mol/hr of natural gas were being consumed? The minimum heating load for the company when translated into the stack gas output was 6205 lb mol/hr of dry stack gas. Analysis of the fuels and stack gas at that time were: Natural gas Fuel Oil (API gravity = 24.0 (mol%) Stack gas (Orsat analysis) CH4 96% C 50 N2 84.93% C2H2 2% H2 47 O2 4.13% CO2 2% S 3 CO2 10.84% SO2 0.10% Also, calculate the percent increase in toxic emissions of arsenic and mercury per hour caused by the combustion of fuel oil rather than natural gas.

Problems Lecture 11 P1, P2, P3 P11.8, P11.11, P11.14, P11.17