1 / 20

Lecture 3 – Materials Balances

Lecture 3 – Materials Balances. Introduction to Environmental Engineering Lecture3 Dr. Jawad Al-rifai. The accounting of all mass in a chemical/Environmental process is referred to as a mass (or material) balance . ‘day to day’ operation of process for monitoring operating efficiency

tara
Download Presentation

Lecture 3 – Materials Balances

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Lecture 3 – Materials Balances Introduction to Environmental Engineering Lecture3 Dr. Jawad Al-rifai

  2. The accounting of all mass in a chemical/Environmental process is referred to as a mass (or material) balance. • ‘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

  3. Conservation of Mass – mass is neither created nor destroyed • Mass Flow – therefore mass flowing into a box will equal the flow coming out of a box • Black box – schematic representation X0 X1 0 1 M.B. with a Single Material

  4. One or more effluent • One or more Feed Source / influent X1 X1 X0 1 X0 0 X2 X2 2 [Accumulation]= [In]– [Out] +[Produced] – [Consumed] M.B. with a Splitting Single Material

  5. Steady state • The rate of input= rate of out put, mass rate of accumulation is Zero • Conservation: In many problems conservation is assumed • Material of concern is not consumed or produced • No chemical, biological or radioactive decay • Ex. Salt in Sewer & stream • M.B. equation • 0 = [In] – [Out] + 0 - 0 [In]= [Out] State of Mixing-Steady State

  6. 2. Reactions/ loss process dM/dt = [d(in)/dt – d(out)/dt]+ r r=-KVCn • K; reaction rate constant; S-1 or d-1 • C: Concentration of substances • n: reaction order • V: volume • - Indicate disappearance of substances The reaction rate is often complex function of T, P [Accumulation]= [In]– [Out] + [Produced] – [Consumed] State of Mixing- Reactions/ loss process

  7. General Rules for solving M.B. Problems • Draw the system as a diagram • Add the available information • Draw a dotted line around the component being balanced • Decide material to be balanced • Write the basic M.B. equation • If only one missing variable, solve • If more that one unknown, repeat the procedure Complex Processes with a Single Material

  8. A completely mixed lake receives two inflows: natural stream flow 0.1 m3/s, wastewater discharge 0.054 m3/s and has a constant volume of 2 x 106 m3. Given: • 1)the wastewater has 20 mg/L NH3-N • 2)stream has 1 mg/L NH3-N bacteria in the lake convert NH3 to NO3- by a process called nitrification. -rN = k*CN where k = a first-order rate constant = 0.03 day-1 and CN = concentration of ammonia-nitrogen mg/L FIND: lake and outflow NH3-N Assume steady-state, non-conservative mass balance: Ammonia is very toxic to fish, 1 mg/L NH4-N. Does the amount of natural nitrification in the lake allow wastewater discharge of 20 mg/L ammonia-N? Example

  9. QW*CNW + QN*CNN - QTCN - V*k*CN = 0 • where QW = wastewater flow, = 0.054 m3/s • CNW = wastewater ammonia-N = 20 mg/L • QN = stream flow = 0.1 m3/s • CNN =stream ammonia-N = 1 mg/L • QT = lake outflow = QW +QN = 0.154 m3/s • CN = lake and outflow ammonia-N = ? • V = lake volume = 2 x 106 m3 • t = 150 days Complex Processes with a Single Material

  10. find CN: by rearranging mass balance: QTCN + V*k*CN = QW*CNW + QN*CNN CN (QT + V*k) = QW*CNW + QN*CNN Divide everything by QT; CN (1 + V/ QT *k) = (QW*CNW + QN*CNN)/ QT CN =[ 1 / (1+ (V/QT)*k)]*[(QWCNW + QN*CNN)/QT]  CN = [ 1/(1+ (t)*k)]*[(QW*CNW + QN*CNN)/QT] CN = [1 /(l +(150d * 0.03d-1))]*[(0.054m3/s*20 mg/L + 0.1 m3/s*1 mg/l)/0.154m3/s] CN = 1.4 mg/L ammonia-nitrogen 1.4 mg/L ammonia-N > 1 mg/L standard. Complex Processes with a Single Material

  11. Aside:What is the detention time of water in the lake (Hydraulic Residence Time)? • Define detention time, t in the book: t = V/Q = volume/flow rate = time 2 x 106 m3/(0.1 m3/s + 0.054 m3/s)*(1 day/86,400 s) = 150 days Complex Processes with a Single Material

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

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

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

  15. 5000 kg DM water F1 Water 300 kg API 448 kg Impurity 5 kg Impurity 55 kg Water 2600 kg API 450 kg Filtration Water 7300 kg Impurity 50 kg API 2kg Paul Ashall, 2008

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

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

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

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

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

More Related