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Paul Ashall, 2008 Module 9001 Mass Balance. Paul Ashall, 2008 The accounting of all mass in a chemical/pharmaceutical process is referred to as a mass.

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Presentation on theme: "Paul Ashall, 2008 Module 9001 Mass Balance. Paul Ashall, 2008 The accounting of all mass in a chemical/pharmaceutical process is referred to as a mass."— Presentation transcript:

1 Paul Ashall, 2008 Module 9001 Mass Balance

2 Paul Ashall, 2008 The accounting of all mass in a chemical/pharmaceutical process is referred to as a mass (or material) balance.

3 Paul Ashall, 2008 Uses ‘day to day’ operation of process for monitoring operating efficiency Making calculations for design and development of a process i.e. quantities required, sizing equipment, number of items of equipment

4 Paul Ashall, 2008 Simple example – batch mixing process 200 kg of a 40% w/w methanol/water solution is mixed with 100 kg of a 70% w/w methanol/water solution in a batch mixer unit. What is the final quantity and composition?

5 Paul Ashall, 2008 continued Total initial mass = total final mass = 300 kg Initial methanol mass = final methanol mass 80 + 70 = final methanol mass = 150 kg Therefore final composition of batch is (150/300) x 100 = 50 % by wt.

6 Paul Ashall, 2008 Exercise 1000 kg of 8% by wt. sodium hydroxide (NaOH) solution is required. 20% sodium hydroxide solution in water and pure water are available. How much of each is required?

7 Paul Ashall, 2008 Batch processes Batch processes operate to a batch cycle and are non-steady state. Materials are added to a vessel in one operation and then process is carried out and batch cycle repeated. Integral balances are carried out on batch processes where balances are carried out on the initial and final states of the system.

8 Paul Ashall, 2008 Batch cycle Sequence of operations/steps repeated according to a cycle Batch cycle time Batch size

9 Paul Ashall, 2008 Simple batch reaction cycle 3 steps Start cycle t=0t, finish cycle Add reactants etcreaction Empty reactor Next cycle

10 Paul Ashall, 2008 Continuous processes These processes are continuous in nature and operate in steady state and balances are carried out over a fixed period of time. Materials enter and leave process continuously.

11 Paul Ashall, 2008 Law of conservation of mass When there is no net accumulation or depletion of mass in a system (steady state) then: Total mass entering system = total mass leaving system or total mass at start = total final mass

12 Paul Ashall, 2008 General mass balance equation Input + generation – output – consumption = accumulation Notes: 1. generation and consumption terms refer only to generation of products and consumption of reactants as a result of chemical reaction. If there is no chemical reaction then these terms are zero. 2. Apply to a system 3. Apply to total mass and component mass

13 Paul Ashall, 2008 Definitions System – arbritary part or whole of a system Steady state/non-steady state Accumulation/depletion of mass in system Basis for calculation of mass balance (unit of time, batch etc) Component or substance

14 Paul Ashall, 2008 Exercise 1000 kg of a 10 % by wt. sodium chloride solution is concentrated to 50 % in a batch evaporator. Calculate the product mass and the mass of water evaporated from the evaporator.

15 Paul Ashall, 2008 Mixing of streams F1 F2 F3 F4

16 Paul Ashall, 2008 Example Calculate E and x Fresh feed 1000kg, 15% by wt sodium hydrogen carbonate Recycle stream 300 kg, 10% satd. soln. evaporator feed E, composition x%

17 Paul Ashall, 2008 Flowsheets Streams Operations/equipment sequence Standard symbols

18 Paul Ashall, 2008 Flowsheets Process flow diagram PID

19 Paul Ashall, 2008 Typical simple flowsheet arrangement reactor Separation & purification Fresh feed (reactants, solvents, reagents, catalysts etc) product Recycle of unreacted material Byproducts/coproductswaste

20 Paul Ashall, 2008 Exercise A 1000 kg batch of a pharmaceutical powder containing 5 % by wt water is dried in a double cone drier. After drying 90 % of the water has been removed. Calculate the final batch composition and the weight of water removed.

21 Paul Ashall, 2008 Exercise – batch distillation 1000 kg of a 20% by wt mixture of acetone in water is separated by multistage batch distillation. The top product (distillate) contains 95% by wt. acetone and the still contains 2% acetone. Calculate the amount of distillate.

22 Paul Ashall, 2008 Use of molar quantities It is often useful to calculate a mass balance using molar quantities of materials and to express composition as mole fractions or mole %. Distillation is an example, where equilibrium data is often expressed in mole fractions.

23 Paul Ashall, 2008 Molar units A mole is the molecular weight of a substance expressed in grams To get the molecular weight of a substance you need its molecular formula and you can then add up the atomic weights of all the atoms in the molecule To convert from moles of a substance to grams multiply by the molecular weight To convert from grams to moles divide by the molecular weight. Mole fraction is moles divided by total moles Mole % is mole fraction multiplied by 100

24 Paul Ashall, 2008 Molar units Benzene is C 6 H 6. The molecular weight is (6x12) + (6x1) = 78 So 1 mole of benzene is 78 grams 1 kmol is 78 kg

25 Paul Ashall, 2008 Exercise – batch distillation 1000 kmol of an equimolar mixture of benzene and toluene is distilled in a multistage batch distillation unit. 90 % of the benzene is in the top product (distillate). The top product has a benzene mole fraction of 0.95. Calculate the quantities of top and bottom products and the composition of the bottom product.

26 Paul Ashall, 2008 Mass balance - crystalliser A crystalliser contains 1000 kg of a saturated solution of potassium chloride at 80 deg cent. It is required to crystallise 100 kg KCl from this solution. To what temperature must the solution be cooled?

27 Paul Ashall, 2008 T deg centSolubility gKCl/100 g water 8051.1 7048.3 6045.5 5042.6 40 3037 2034 1031 027.6

28 Paul Ashall, 2008 At 80 deg cent satd soln contains (51.1/151.1)x100 % KCl i.e. 33.8% by wt So in 1000 kg there is 338 kg KCl & 662 kg water. Crystallising 100 kg out of soln leaves a satd soln containing 238 kg KCl and 662kg water i.e. 238/6.62 g KCl/100g water which is 36 g KCl/100g. So temperature required is approx 27 deg cent from table.

29 Paul Ashall, 2008 Mass balance filtration/centrifuge feed suspension wash water/solvent solid waste water filtrate

30 Paul Ashall, 2008 Filtration F1 5000 kg DM water Impurity 55 kg Water 2600 kg API 450 kg Water 7300 kg Impurity 50 kg API 2kg Water 300 kg API 448 kg Impurity 5 kg

31 Paul Ashall, 2008 Mass balance - drier feed product water/evaporated solvent

32 Paul Ashall, 2008 Mass balance – extraction/phase split A + B S S + B A – feed solvent; B – solute; S – extracting solvent

33 Paul Ashall, 2008 Example (single stage extraction; immiscible solvents) E1 feed solvent raffinate extract

34 Paul Ashall, 2008 F = 195 kg; x f = 0.11 kg API/kgwater S = 596 kg chloroform y = 1.72x, where y is kgAPI/kg chloroform in extract and x is kg API/kg water in raffinate. Total balance 195 + 596 = E + R API balance 19.5 = 175.5x 1 + 596y 1 19.5 = 175.5x 1 + 596.1.72x 1 x 1 = 0.0162 and y 1 = 0.029 R is 175.5 kg water + 2.84 kg API and E is 596 kg chloroform + 17.28 kg API Note: chloroform and water are essentially immiscible

35 Paul Ashall, 2008 Mass balance – absorption unit feed gas stream feed solvent waste solvent stream exit gas stream

36 Paul Ashall, 2008 Mass balances – multiple units Overall balance Unit balances Component balances

37 Paul Ashall, 2008 Multiple units E – evaporator; C – crystalliser; F – filter unit F1 – fresh feed; W2 – evaporated water; P3 – solid product; R4 – recycle of saturated solution from filter unit R4 ECF F1 W2 P3

38 Paul Ashall, 2008 Mass balance procedures Process description Flowsheet Label Assign algebraic symbols to unknowns (compositions, concentrations, quantities) Select basis Write mass balance equations (overall, total, component, unit) Solve equations for unknowns

39 Paul Ashall, 2008 Exercise A mass balance and tracking of usage of a solvent used in an API production process is required for a Pollution Emission Register (PER). Discuss and outline in general terms how you would do this. Ref. www.epa.ie

40 Paul Ashall, 2008 Definitions Stoichiometric quantities Limiting reactant Excess reactant Conversion Yield Selectivity Extent of reaction

41 Paul Ashall, 2008 Stoichiometry Refers to quantities of reactants and products in a balanced chemical reaction. aA + bB cC + dD i.e. a moles of A react with b moles of B to give c moles of C and d moles of D. a,b,c,d are stoichiometric quantities

42 Paul Ashall, 2008 Reactor mass balances

43 Paul Ashall, 2008 Example – aspirin synthesis reaction

44 Paul Ashall, 2008 Limiting reactant/excess reactant In practice a reactant may be used in excess of the stoichiometric quantity for various reasons. In this case the other reactant is limiting i.e. it will limit the yield of product(s)

45 Paul Ashall, 2008 continued A reactant is in excess if it is present in a quantity greater than its stoichiometric proportion. % excess = [(moles supplied – stoichiometric moles)/stoichiometric moles] x 100

46 Paul Ashall, 2008 Example – aspirin synthesis

47 Paul Ashall, 2008 Conversion Fractional conversion = amount reactant consumed/amount reactant supplied % conversion = fractional conversion x 100 Note: conversion may apply to single pass reactor conversion or overall process conversion

48 Paul Ashall, 2008 Yield Yield = (moles product/moles limiting reactant supplied) x s.f. x 100 Where s.f. is the stoichiometric factor = stoichiometric moles reactant required per mole product

49 Paul Ashall, 2008 Example – aspirin synthesis

50 Paul Ashall, 2008 Selectivity Selectivity = (moles product/moles reactant converted) x s.f. x100 OR Selectivity = moles desired product/moles byproduct

51 Paul Ashall, 2008 Extent Extent of reaction = (moles of component leaving reactor – moles of component entering reactor)/stoichiometric coefficient of component Note: the stoichiometric coefficient of a component in a chemical reaction is the no. of moles in the balanced chemical equation ( -ve for reactants and +ve for products)

52 Paul Ashall, 2008 Examples A B i.e. stoichiometric coefficients a = 1; b = 1 100 kmol fresh feed A; 90 % single pass conversion in reactor; unreacted A is separated and recycled and therefore overall process conversion is 100% reactor separation F R P

53 Paul Ashall, 2008 Discussion - Synthesis of 3,3 dimethylindoline

54 Paul Ashall, 2008 Discussion - Aspirin synthesis

55 Paul Ashall, 2008 References Elementary Principles of Chemical Processes, R. M. Felder and R. W. Rousseau, 3 rd edition, John Wiley, 2000


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