INDUSTRIAL STOICHIOMETRY II

3rd Semester,

B.Sc. Chemical Engineering

Session 2011

Delivered by:

Mr. Usman Ali

Department of Chemical Engineering

University of Engineering & Technology, Lahore

Units and Dimensions

Base Units

Multiple Prefixes

Derived Units

Reference for Practice

Loebel Arnold B. “Chemical Problem-Solving by Dimensional Analysis” 2nd Ed. Houghton Miffin Company. Boston Publisher.

Strategy for Analyzing/Solving Problems

Read the problem carefully.

Draw a sketch/figure of the process.

Label flow streams with symbols.

Put all the known information/values of different parameters(temperature, pressure, composition, chemical reactions etc.) on the figure/respective streams.

Select a basis.

Make a list of unknown values/parameters or mention them on the respective streams.

Write down the independent equations.

Solve the equations.

Check the answer (if possible).

Example

• A limestone analysis is

CaCO3 = 92.89%

MgCO3 = 5.41%

Insoluble = 1.70%

Calculate; 1. lbs of CaO can be made from 5 ton of limestone

2. lbs of CO2 can be recovered per lb of limestone

CaCO3 CaO + CO2

MgCO3 MgO + CO2

Draw a sketch/figure of the process

Heating

Label the flow streams

Heating R

P1

P2

Put all the known data

Heating R

P1

P2

Select a Basis: • 100 lbs of limestone

Heating R

P1

P2

Specify the unknown/s

Heating R

P1

P2

lbs of CaO from 5 ton of limestone lbs of CO2 per lb of limestone

Part 1

• 1 lbmol CaCO3 = 100 lbs of CaCO3

• 100 lbs of CaCO3 = 56 lbs of CaO

• 92.89 lbs………… = (56/100) x 92.89 = 52 lbs of CaO

• 92.89 lbs of CaCO3 = 52 lbs of CaO

• (2240x5x.9289)lbs ………..= (52/92.89)x(.9289x2240x5) = 5824 lbs of CaO

1 ton = 2240 lbs

Part 2

100 lbs of CaCO3 = 44 lbs of CO2

92.89 lbs………… = (44/100) x 92.89 = 41 lbs of CO2

84 lbs of MgCO3 = 44 lbs of CO2

5.41 lbs of MgCO3 = (44/84)x5.41 = 2.8 lbs of CO2

Total CO2 produced per 100 lbs of limestone = 41+2.8=43.8lb

Total CO2 produced per 1 lb of limestone = 43.8/100 = 0.44lbs

Process

Input

Output

Process Unit

Process Unit Input Output

Process Classification

Batch

Semi-Batch Continuous

Activity

• A balloon is filled with air at steady rate of 2 g/min.

• A bottle of milk is taken from refrigerator and left on the kitchen table.

• Water is boiled in an open flask.

• 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 Products and unused reactants are withdrawn at the other end.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 constant and the composition and the flow rate of the entering reactant stream are also independent of the time. Classify the process (a) initially and (b) after a long period of time has elapsed.

Activity

• Each year 50,000 people move into a city, 75,000 people move out, 22,000 are born and 19000 die. Write a balance on the population of the city.

One thousand kilograms per hour of a mixture of benzene (B) and toluene (T) that contains 50% benzene by mass are separated by distillation into two fractions. The mass flow rate of benzene in the top stream is 450kg B/h and that of toluene in the bottom stream is 475kg T/h. The operation is at steady state. Write balances on benzene and toluene to calculate the unknown component flow rates in the output streams.(F&R)

Material Balances on Continuous Steady State Processes

Material Balances on Continuous Steady State Processes

Batch Process

Two methanol-water mixtures are contained in separate flasks. The first mixture contains 40 wt% methanol, and the second contains 70 wt% methanol. If 200g of first mixture are combined with the 150g of the second, what are the mass and composition of the product? (F&R)

Activity

To prepare a solution of 50% sulphuric acid , a dilute waste acid containing 28% H2SO4 is fortified with a purchased acid containing 96% H2SO4. How many kilograms of the purchased acid must be bought for each 100kg of dilute acid.(Himmelblau)

Sludge is wet solids that result from the processing in municipal sewage systems. The sludge has to be dried before it can be composted or otherwise handled. If a sludge containing 70% water and 30% solids is passed through a drier, and the resulting product contains 25% water, how much water is evaporated per ton of sludge sent to dryer. (Himmelblau)…

Activity

Activity

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 670oC before it passes into the reactor. The reactor effluent gas, which includes propane, propylene, methane and hydrogen, is cooled from 800oC to 110oC 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 tower 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 recycle stream is 97% propane. The stripped oil is recycled to the absorption tower.(F&R)

Problem Bookkeeping

Counting the number of unknown variables on the flowsheet/chart, and then counting the number of independent equations relating them.

Material Balance

An Energy Balance

Process Specification

Physical Properties and Laws

Physical Constraints

Degree of Freedom

Degree of freedom = total number of stream variables

- total number of independent balance equations

-total number of specified independent stream variables

- total number of subsidiary relations

Activity

Examine figure E3.4a, which presents a simple flow sheet for a single unit. Only the value of D is known. What is the minimum number of other measurements that must be made to determine all other stream and composition values? (Himmelblau)

Activity

A feedstock available at the rate of 1000mol/h and consisting of

20% propane

30% isobutane

20% isopentane

30% n-pentane

Is to be separated into two fractions by distillation. The distillate is to contain all of the propane fed to the unit and 80% of the isopentane fed to unit and is to consist of 40% isobutane. The bottom stream is to contain all the n-pentane fed to the unit. (Rekalitis)

Activity

In a new pigment plant that is to produce 4000lb/h dry TiO2 product, an intermediate stream consisting of TiO2 precipitate suspended in an aqueous salt solution, is to be purified of salt so that the final product contains, on water free basis, at most 100 parts per million of salt. The slat removal is to be accomplished by washing the precipitate with water. If any pigment stream contains 40% TiO2, 20% salt and the rest water ( all mass %) and if the washed pigment is, upon settling, projected to consist of about 50% TiO2 solids. What will be the composition of the waste water stream be? (Rekalitis)

A labelled flowchart of continuous steady state two unit distillation process is shown below. Each stream contains two components, A and B, in different proportions. Three streams whose flow rates and compositions are not known are labelled 1,2 and 3. calculate the unknown flow rates and compositions of streams 1,2 and 3. (F&R)

Activity

A mixture containing 50% acetone and 50% water by weight is to be separated into its components. The separation process consists of extraction of the acetone from the water into solvent that dissolves acetone but is nearly immiscible with water, followed by distillation to separate the acetone from the solvent. The description that follows introduces some of the terms commonly used in reference to liquid extraction processes.

Activity

The acetone(solute)-water(feed solvent) mixture is first contacted with the MIBK(wash solvent) in a mixer that provides good contact between two liquid phases. Most of the acetone in the feed transfers from the aqueous(water) phase to the organic (MIBK) phase in this step. The mixture passes into the settling tank, where the phases separate and are separately withdrawn. The phase rich in the feed solvent(water) is refereed to as the raffinate, and the phase rich in the wash solvent(MIBK) is the extract.

The raffinate passes to second extraction stage, where it is contacted with the second stream of the pure MIBK, leading to the transfer of more acetone. The extracts from the two mixer-settler stages are combined and fed to the distillation column. The vapour product(overhead) from the column is rich in the acetone and is the product of the process. The bottoms stream is rich in the MIBK, and in real process it is further treated and recycled back to the extractors.

In a pilot plant study, for every 100kg of acetone-water fed to the first extractor, 100kg of the MIBK are fed to the first extractor and 75kg are fed to the second extractor. The extract from the first extractor is found to contain 27.5% acetone. The second stage raffinate has a mass of 43.1kg and consists of 5.3% acetone, 1.6% MIBK, and 93.1% water, and the second stage extract contains 9% acetone, 88% MIBK, and 3 % water. The overhead from the distillation column contains 2% MIBK, 1% water and the balance acetone.

For an assumed basis of 100kg acetone-water feed, calculate the masses and the compositions (component percentages by mass) of the stage 1 raffinate and the extract, the stage 2 extract, the combined extract, and the distillation overhead and the bottoms products. (F&R)

The flowchart of a steady state process to recover crystalline potassium chromate(K2CrO4) from an aqueous solution of this salt is shown.

Forty five hundred kilograms per hour of a solution that is 1/3K2CrO4 by mass is joined by a recycle stream containing 36.36% K2CrO4, and the combined stream is fed to the evaporator. The concentrated stream that leaves the evaporator in which it is cooled(causing crystals of K2CrO4 to come out of solution) and then filtered.

Activity

The filter cake consists of K2CrO4 crystals and a solution that contains 36.36% K2CrO4 by mass; the crystals account for 95% of the total mass of the filter cake. The solution that passes through the filter, also 36.36% K2CrO4, is recycle stream.

Calculate the weight of water removed in the evaporator, the rate of production of crystalline K2CrO4 , the ratio (kg recycle)/(kg fresh feed), and the feed rates that the evaporator and the crystallizer must be designed to handled. (F&R)

Suppose that the filtrate was discarded instead of being recycled. Calculate the production rate of crystals. What are benefits and costs of recycling?

Activity

Activity

Seawater containing 3.50 wt % salt passes through a series of 10 evaporators. Roughly equal quantities of water are vaporized in each of the 10 units and then condensed and combined to obtain a product stream of fresh water. The brine leaving each evaporator but the tenth is fed to the next evaporator. The brine leaving the tenth evaporator contains 5.00 wt% salt.

a) Draw a flowchart of the process showing the first, fourth, and the tenth evaporators. Label all the streams entering and leaving these three evaporators.

b) Write in the order the set of the equations you would solve to determine the fractional yield of fresh water from the process(kg H2O recovered/kg H2O in process feed) and the weight per cent of salt I n the solution leaving the fourth evaporator. Each equation you write should contain no more than one previously undetermined variable. In each equation, circle the variable for which you would solve. Do not do the calculations.

c) Solve the equations derived in part (b) fort the two specified quantities.

An equimolar liquid mixture of benzene and toluene is separated into two product streams by distillation. A process flowchart and a somewhat oversimplified description of what happens in the process follow:

Activity

Inside the column a liquid stream flows downward and a vapor stream rises. At each point in the column some of the liquid vaporizes and some of the vapor condenses. The vapor leaving the top of the column, which contains 97 mole % benzene, is completely condensed and split into two equal fractions: one is taken as the overhead product stream, and the other(the reflux) is recycled to the top of the column. The overhead product stream contains 89.2% of the benzene fed to the column. The liquid leaving the bottom of the column is fed to a partial reboiler in which 45% of it is vaporized. The vapour generated in the reboiler(the boilup) is recycled to become the rising vapor stream in the column, and the residual reboiler liquid is taken off as the bottom product stream. The composition of the streams leaving the reboiler are governed by the relation, where yB and the xB are the mole faction of the benzene in the liquid and the vapor stream.

(a) Take a basis of 100 mol fed to the column. Draw and completely label a flowchart, and for each of four systems(overall, column, condenser, reboiler), do the degree-of-freedom analysis and identify a system with which the process analysis might appropriately begin.

(b) Write in order the equations you would solve to determine all unknown variables on the flowchart, circling the variables for which you would solve in each equation.

(c) Calculate the molar amounts of the overhead and the bottom products, the mole fraction of the benzene in the bottom product , and the percentage recovery of the toluene in the bottom product(100xmoles of toluene in bottoms/moles toluene in feed)

In an absorption tower, a gas is contacted with a liquid under conditions such that one or more species in the gas dissolve in the liquid. A stripping tower also involves a gas contacting a liquid, but under conditions such that one or more components of the feed liquid come out of the solution and exit in the gas leaving the tower.

A process consisting of an absorber and a striper is used to separate the components of a gas containing 30 mole % CO2 and the balance CH4. A stream of this gas is fed to the bottom of the absorber. A liquid containing 0.5 mole % dissolved CO2 and the balance methanol is recycled from the bottom of the stripper and fed to the top of the absorber. The product gas leaving the top of the absorber contains 1mole % of CO2 and the essentially all of the CH4 fed to the unit. The CO2-rich liquid solvent the bottom of the absorber is fed to the top of the stripper and a stream of nitrogen gas is fed to the bottom.

Activity

Ninety precept of the CO2 in the liquid feed to the stripper come out of the solution in the column, and the nitrogen/CO2 stream leaving the column passes out to the atmosphere through stack. The liquid stream leaving the stripping tower is 0.5% CO2 solution recycled to the absorber. The absorber operate at temperature Ta and pressure Pa and the stripper operates at Ts and Ps. Methanol may be assumed to be non-volatile- that is none enters the vapor phase in either column-and the nitrogen may be assumed insoluble in the methanol.

(a)In your own words, explain the overall objective of the two unit process and the functions of the absorber and the stripper in the process.

(b) The streams fed to the top of the each tower have something in common, as do the streams fed to the bottoms of each tower. What are these commonalities and what is the probable reason for them.

(c)Taking a basis of 100mol/h of the gas fed to the absorber, draw and label a flowchart of the process. For the stripper outlet gas, label the component molar flow rates rather than the total flow rate and the mole fractions. Do the degree of freedom analysis and write in order the equations you would solve to determine all unknown stream variables except nitrogen flow rate entering and leaving the stripper.

(d) Calculate the fractional CO2 removal in the absorber(moles absorbed/ mole in feed) and the molar flow rate and the composition of the liquid feed to the stripper.

(e) Calculate the molar flow rate of the gas to the absorber required to produce the absorber product gas flow rate of 1000kg/h.

(f) Would you guess that Ts would be higher or lower than Ta. Explain? What about the relationship of Ps to Pa?

(g) What properties of the methanol would you guess make it the solvent of the choice for this process?

Material Balance Involving Chemical Reactions

Activity

Consider the reaction,

C4H8 + 6O2 --------> 4CO2 + 4H2O

1. Is the stoichiometric equation balanced?

2. What is the stoichiometric co-efficient of CO2?

3. What is the stoichiometric ratio of H2O to O2?

4. How many lb-moles of O2 react to form 400 lb-moles of CO2?

5. One hundred g-moles/min of C4H8 are fed into a reactor, and 50% reacts. At what rate is water formed?

Activity

Acrylonitrile is produce in the reaction of propylene, ammonia, and oxygen;

C3H6 + NH3 + 1.5 O2-----> C3H3N + 3H2O

The feed contains 10 mol % propylene, 12% ammonia, and 78% air. A fractional conversion of 30% limiting reactant is achieved. Taking 100 mol of feed as a basis, determine which reactant is limiting, the percentage by which each of the other reactants is in excess, and the molar amounts of all product gas constituents for a 30% conversion of the limiting reactant.

Activity

Activity

Activity

Methane is burned with oxygen to yield carbon dioxide and water. The feed contains 20 mole % CH4, 60 % O2, 20% CO2, and the 90% conversion of the limiting reactant is achieved. Calculate the molar composition of the product stream using

(1) Balances on molecular species

(2) Atomic balances

(3) The extent of reaction.

Activity

Activity Methanol may be produced by the reaction of carbon dioxide and hydrogen.

CO2 + 3H2 --------> CH3OH + H20

The fresh feed to the process contains hydrogen and carbon dioxide in a stoiciometric proportion , and 0.5% inert(I). The reactor effluent passes to a condenser, which removes essentially all of the methanol and the water formed, and none of the reactant or inerts. The latter substances are recycled to the reactor. To avoid build-up of the inerts in the system, a purge stream is withdrawn from the recycle. The feed to the reactor contains 2% inerts, and the single-pass conversion is 60%. Calculate the molar flow rates of the fresh feed, the total feed to the reactor, and the purge stream for a methanol production rate of 1000mol/h.