- 56 Views
- Uploaded on

Download Presentation
## PowerPoint Slideshow about ' Lecture 3 – Materials Balances' - vernon-reese

**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.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript

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

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- 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- 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 StatedM/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 processGeneral 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

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?

ExampleQW*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

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 MaterialAside: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 MaterialSequence of operations/steps repeated according to a cycle

- Batch cycle time
- Batch size

Paul Ashall, 2008

reactor

Separation &

purification

product

Fresh feed

(reactants, solvents,

reagents, catalysts etc)

Typical simple flowsheet arrangementwaste

Byproducts/coproducts

Paul Ashall, 2008

F1

Water 300 kg

API 448 kg

Impurity 5 kg

Impurity 55 kg

Water 2600 kg

API 450 kg

FiltrationWater 7300 kg

Impurity 50 kg

API 2kg

Paul Ashall, 2008

A + B

S + B

S

Mass balance – extraction/phase splitA – feed solvent; B – solute; S – extracting solvent

Paul Ashall, 2008

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 unitsPaul Ashall, 2008

Download Presentation

Connecting to Server..