7 replies to this topic

[chemeng2014]

Hi

 

Wondering if anyone can help with the following question. I have done the first part and believe it is 280kg/h (although would be gracious if this was confirmed)

 

Thanks in advance - sorry I couldnt catergorise it further in the topc title!

 

In the production of chlorosilanes, chloromethane gas (CH3Cl) is
produced as a by-product. This needs to be removed and treated to
prevent potential hazardous material entering the atmosphere. This is
achieved by continuously dissolving and reacting it with the
stoichiometric quantity of potassium hydroxide (KOH) solution in a
continuous scrubbing tower according to the reaction:
It is important that the reaction temperature is maintained below 22°C to
ensure maximum removal of the chloromethane. Cooling water at 10°C
is available and the maximum rise in water temperature should be 5°C.
Calculate:
(i) the amount of KOH solution required per hour
(ii) the adiabatic temperature of the reaction (no cooling is used)
(iii) the minimum amount of cooling water required to maintain the
temperature at 22°C, if no heat is lost to the surroundings.

CH3Cl(g) KOH(aq) KCl(aq) CH3OH(l) + → + ΔHr° = – 167.7 kJ mol–1

Temperature of chloromethane = 195°C
Molar heat capacity of chloromethane = 31.4 kJ kmol–1 K–1
Flow rate of chloromethane = 101 kg h–1
Concentration of KOH solution = 4% w/w KOH in water
Inlet temperature of KOH = 25 °C
Specific heat capacity of reaction mixture = 3.5 kJ kg–1 K–1
Specific heat capacity of water = 4.18 kJ kg–1 K–1
Relative atomic mass of K = 39, C = 12, H = 1, O = 16 and
Cl = 35.5.

[breizh]

Hi ,

1)Seems to be 2800 kg/h !

2) Should be 58 C .

 

 

For the rest we can help you as long you are showing your work.

 

Breizh

Edited by breizh, 06 January 2014 - 01:15 AM.

[chemeng2014]

I got 280kg/h. 101,000 (g)/(12+1+1+1+35.5) = 2000 mole

 

2000*(39+16+1)=112,000g

 

112,000/0.4=280,000g or 280kg/h

 

No idea how you get 58C??

[chemeng2014]

I have got figured specific heat capacity of Chloromethane to be 15.7 kJ/kg

 

15.7kJ/kg * 101kg * 170 DegC = 269,568kJ - Heat from chloromethane

 

Water mass = 168kg

 

168 * 4.18 = 702kJ - Heat from water

 

269,568kJ +702kJ / (381kg * 3.5kJ/kg/K) = 203 Deg C?

Edited by chemeng2014, 06 January 2014 - 03:26 AM.

[breizh]

it 's 0.04 not o.4 !

 

For heat balance , you need to apply : Enthalpy in + Generation = Enthalpy out ,

 

For temperature reference , take 25 C to cancel out the enthalpy of KOH solution

 

Note : pay attention to the units : KJ/Kmol/ K ;KJ/mol/ K ;KJ/kg/ K

 

Breizh

Edited by breizh, 06 January 2014 - 03:35 AM.

[chemeng2014]

it 's 0.04 not o.4 !

 

For heat balance , you need to apply : Enthalpy in + Generation = Enthalpy out ,

 

For temperature reference , take 25 C to cancel out the enthalpy of KOH solution

 

 

 

Breizh

Doh!

 

Can you ellaborate? Is my working totally wrong?

[chemeng2014]

And am i right in thinking the mix has 2000 moles?

[breizh]

1) mass balance : in = out : 101 +2800 = 2901 kg/h

 

2) enthalpy in : enthalpy CH3Cl +enthalpy KOH solution

enthalpy out : enthalpy mixture

 

pay attention that CP units are mixed and CP KOH solution is unknown the reason why you need to choose 25C as reference and liquid status .

HCH3Cl= 2000/1000*31.4 *(195- Tref)= 2*31.4*(170)

HKOH = 2800 *CpKOH sol *( 25-25) =0

Hmixture = 2901*3.5*(T-tref) = 2901*3.5*(T-25)

 

Generation ( exothermic reaction) = 2000*-DHr= 2000*167.7

 

>>> Solve : 2*31.4*170  +0 +2000 *167.7 = 2901*3.5*(T-25) ....〉T= 59 C

 

3) same methodology , taking account H water in and H water out and the fact that the exit  temp should be 22 C .

 

Qv = 18016 kg/h or 18 m3/h

 

Breizh

Edited by breizh, 06 January 2014 - 07:06 PM.