Question

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.

Data: 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.

Answer 1

The reaction is CH3Cl + KOH---->CH3OH+KCl

Flow rate of CH3Cl = 101 kg/hr, molar flow rate= mass/Molar mass = 101/(12+3+15.5)= 2 kg moles/hr

As per the reaction, moles of KOH required= moles of KOH= 2 kg moles/hr

molar mass of KOH= 56, mass of KOH= 2*56=112 kg/hr

Since weight % of KOH is only 4%, mass of KOH required= 112*100/4= 2800 kg/hr of 4 % (W/W) KOH solution

Heat gained by KOH= heat lost by Chlorometane

.2800*3.5*(25-T)= 2*31.4*(197-T), T is the equilibrium temperature ( also known as adiabatic temperature)

245000-9800T= 12372-62.8T

24500-12372= T(9800-62.8) , T =1.24 deg.c

When temperature needs to be maintained at 22 deg.c, heat need to be removed = 2*31.4*(197-22)=10990 KJoule/hr

Cooling water ( minimum required)= 10990/(4.18*5)= 526 kg/hr