The silicon tetrafluoride (STF) reacts with 25% caustic solution to form sodium fluoride, sodium silicate and water (see reaction below).
SiF4 + 6NaOH = 4NaF + Na2SiO3 + 3H2O
Assuming 200 pounds of STF are left per tube at a temperature of 298 K and 1 atm.
Table 1. Silicon tetrafluoride
STF left per tube
200 lb
90.7 kg
90702.9 g
871.48 mol
The current cleaning method uses the following mixture that is refreshed a total of 3 times per trailer.
1000 gallons of water
220 gallons of 25% caustic
AIMM is projecting 2000 gallons of water for their cleaning process (400 gallons per tube).
Table 2. Amount of water per tube
Current Proposed
600 gal 400 gal
2271.0 L 1514.0 L
2271000 kg 1514000 kg
Table 3. Water Properties
Water Properties @ 25°C
Density 1000 kg/L
Heat Capacity, Cp 4.19 kJ/kg*°C
Table 3. Caustic solution
25% Caustic
M.W. = 40 g/mol
132.00 gal
499.62 L
1064.19 kg
1064190.60 g
6651.67 mol
For this analysis the standard enthalpy of reaction was used to calculate the amount of heat produced by this reaction. The enthalpy of reaction is the change that occurs in a system when one mole of matter is transformed by a chemical reaction under standard conditions.
∆H_reaction=∑▒〖nH_products-∑▒〖mH〗_reactants 〗
Table 4. Enthalpy of formation and molecular weight
Enthalpy of Formation Molecular Weight
SiF4 -1615 kJ/mol 104.08 g/mol
NaOH -469.15 kJ/mol 40.00 g/mol
NaF -569 kJ/mol 41.99 g/mol
Na2SiO3 -1561.43 kJ/mol 122.06 g/mol
H2O -285.83 kJ/mol 18.01 g/mol
∑▒〖nH_(Products )= 〗 4 (mol NaF)/(mol SiF_4 )*-569 kJ/(mol NaF)+1 (mol 〖Na〗_2 SiO_3)/(mol SiF_4 )*-1561.43 ( kJ)/(mol 〖Na〗_2 SiO_3 )+3 (mol H_2 O)/(mol SiF_4 )*(-285.83 kJ)/mol
∑▒〖nH_(Products )=〗-4694.92 kJ/(mol SiF_4