‘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
Module 9001 Mass Balance Paul Ashall, 2008 The accounting of all mass in a chemical/pharmaceutical process is referred to as a mass (or material) balance 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 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? 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 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? 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 Paul Ashall, 2008 Batch cycle • Sequence of operations/steps repeated according to a cycle • Batch cycle time • Batch size Paul Ashall, 2008 Simple batch reaction cycle steps Add reactants etc reaction Empty reactor Next cycle Start cycle t=0 t, finish cycle 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 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 Paul Ashall, 2008 Reactor mass balances Paul Ashall, 2008 Example – aspirin synthesis reaction 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) 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 Paul Ashall, 2008 Example – aspirin synthesis 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 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 Paul Ashall, 2008 Example – aspirin synthesis Paul Ashall, 2008 Selectivity Selectivity = (moles product/moles reactant converted) x s.f x100 OR Selectivity = moles desired product/moles byproduct 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) Paul Ashall, 2008 Examples A B i.e stoichiometric coefficients a = 1; b = 100 kmol fresh feed A; 90 % single pass conversion in reactor; unreacted A is separated and recycled and therefore overall process conversion is 100% R F reactor separation Paul Ashall, 2008 P Discussion - Synthesis of 3,3 dimethylindoline Paul Ashall, 2008 Discussion - Aspirin synthesis Paul Ashall, 2008 References • Elementary Principles of Chemical Processes, R M Felder and R W Rousseau, 3rd edition, John Wiley, 2000 Paul Ashall, 2008 ... in nature and operate in steady state and balances are carried out over a fixed period of time Materials enter and leave process continuously Paul Ashall, 2008 Law of conservation of mass When... 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 Paul Ashall, 2008 General mass balance equation... acetone Calculate the amount of distillate 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