The Silicon Tetrafluoride

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