7 replies to this topic

[naverrak]

Hi, for this question I have done questions i) and some of ii) but I am stuck for the rest.

 

i)

let U=unknowns
     E=equations

 

stage 1:

no. of unknowns=5
no. of equations=4
U>E
 therefore this is not solvable

stage 2:

no. of unknowns=5
no. of equations=4

U>E

therefore this is not solvable

 

stage 3:

no. of unknowns =3

no. of equations =3
U=E

 

therefore stage 3 is solvable.

 

ii)

P3/D3=3
D3=10lb (given)

therefore P3=30lb

 

ratio A:B in P2 is 4:1

therefore P2 has composition 0.8A, 0.2B

 

Here is where I am stuck, I cannot solve for the other unknowns.

The final answers that were given are: D1=30lb, D2=30, P2=30, P3=30, E=10 and stream E consists of 59% A and 41% B.

 

Attached Files

•  cheme question.png   35.9KB   3 downloads

[breizh]

Naverrak ,

 

Should not be difficult . Just apply in= out for both ( global and individual A,B,C) , write the equations . As suggested many times by Art , you can use an excel sheet to perform the calculation . 

 

 

Breizh

[naverrak]

I'm still confused, for D1, applying in=out
0.5(100lb)=0.5(D1) => D1=100, but the answers says D1=30lb

[breizh]

No !

 

You need to perform the balance (total) >>> 1 equation + perform the balance on C solve 2 equations with 2 unknowns !

 

Breizh

[naverrak]

I am honestly still confused, could you show me the first step?.

[breizh]

Global

D1+D2+ D3+P3= F  >> D1+D2 =60 with D3=10 and P3=3*D3

 

 

balance on C  30= 0.27 *D1+0.73*D2

 

 

All the answers are correct !

 

Breizh
 

Edited by breizh, 01 September 2014 - 04:54 AM.

[naverrak]

Hi, to get E, I did
E+P2=D3+P3

If i use a mass balance:

E+(0.8)(30) = 10+(0.7)(30)
E=7

but this is obviously wrong since you said all the values were correct.

 

however if I do
E+P2=D3+P3
E+30=10+30
E=10.

 

How are the values different when the method used should get the same feed value for E (I think).

 

Also for the composition of E, I have to find P2 right?

 

using box 2 as a control volume:

 

mass balance A:

p1 has mass 100-30-10       (from D1 and E)

=60lb

 

P1=D2+P2

 

60A=0.17(30)+0.8(30)

A=0.485
therefore B=0.515

 

mass balance for Box 1:

F=D1+P1+E

using A as a component

0.5(100)=0.5(30)+0.485(60)+10A

A=0.59

therefore B=0.41

 

Did I use the correct method (even though answers were right)

Edited by naverrak, 01 September 2014 - 08:50 AM.

[breizh]

Naverrak  ,

 

You should follow what Art and I have explained to you , use an excelsheet like the one I uploaded.

To me this kind of homework is really great fun.

 

In red calculation , in black data (known)

 

With a table you can crosscheck easily your mass balance ( key point) .

 

Good luck !

 

Breizh

Edited by breizh, 02 September 2014 - 08:07 AM.