Accurately Calculate Nitrogen Requirement for Pressure Purging Filed in Technical Papers December 4th, 2011
Olaleye S Adio – MEng CEng MIChemE GECL – Oil & Gas Consultant
[email protected] The start-up of process plants containing hydrocarbon feed streams are usually preceded by creating an inert atmosphere within the system. One of the options of creating this inert atmosphere is via pressure purging, using nitrogen. Pressure purging is based on using nitrogen to inert a system to low oxygen concentration below which a flammable atmosphere is not sustainable. To achieve this, nitrogen is used to raise the pressure of the system (e.g vessel, heat exchanger, piping etc) from initial condition, which is usually atmospheric, to a chosen value (x barg). With availability of pressure control from the nitrogen header into the system, the pressure of the system does not have to reach the nitrogen system pressure. After pressurizing (to x barg), the system is vented back to the initial (atmospheric) condition. This pressure-venting cycle is repeated until the required inert condition is achieved within the system. The following steps are used to determine number of pressure-venting cycles and nitrogen requirement for pressure purging to achieved required inert O2 concentration 1. Choose or calculate inert O2 concentration required (vol %). 2. Calculate number of cycles (pressure-venting) based on initial & inert (final) oxygen concentration, initial system pressure and chosen nitrogen pressure. 3. Calculate quantity of nitrogen per cycle to determine total nitrogen required.
Step 1. Choose or calculate inert O2 concentration required (vol%) The first stage is to determine what level of O2 is acceptable within the system for the hydrocarbons being introduced. As a rule of thumb 9 vol% Oxygen is below the Minimum Oxygen Concentration (MOC) vol% required for complete combustion of hydrocarbons, whilst 6 vol% is still sufficient for incomplete combustion. As such most systems are assumed to be safe at approximately 4 – 5 vol% O2 . If the required inert O2 concentration is not known, then the MOC for the hydrocarbon stream must be calculated. A value below the MOC is then selected as the inert O2 concentration. The minimum oxygen concentration required by a gas can be calculated from the equation:
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Accurately Calculate Nitrogen Requirement for Pressure Purging Flammability data for some more common gases is provided below Flammability Limits of Some Common Hydrocarbons
Compound
LFL UFL %vol in Air %vol in Air
Compound
LFL UFL %vol in Air %vol in Air
Benzene
1.2
7.8
Ethane
3
12
Butane, n-Butane
1.6
8.4
Ethanol
3
19
n-Butanol
1.4
11.2
Ethylbenzene
1.0
7.1
Carbon Monoxide
12
75
Ethylene
2.7
36
Cyclobutane
1.8
11.1
Gasoline
1.4
7.6
Cyclohexane
1.3
7.7
n-Heptane
1.05
6.7
Cyclohexanol
1
9
n-Hexane
1.1
7.5
Cyclohexanone
1
9.5
Isobutane
1.8
9.6
Cyclopentane
1.5
9.5
Kerosene – Jet Fuel
0.7
4.9
Cyclopropane
2.4
10.4
Methane (Natural Gas)
4. 5
15
Decane
0.8
5.4
Naphthalene
0.9
5.9
Diesel
0.6
7.5
Octane
1
7
Diethanolamine
2
13
iso-Octane
0.79
5.94
Diethylamine
1.8
10.1
Pentane
1.5
7.8
Toluene
1.2
6.75
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Accurately Calculate Nitrogen Requirement for Pressure Purging Example 2 – Calculate the MOC for a HC mixture; 60 vol% CH4, 30 vol% C2H6 and 10 vol% C3H8. Assume complete combustion – 0.6CH4 + 0.3C2H6 + 0.1C3H8 + 2.75O2 –> 1.5CO2 + 2.5H2O Note: Volume fractions ratio of 6:3:1 is equal to mole fractions ratio of 6:3:1 on ideal gas molar volume basis Methane LFL – 4.5%vol in Air, Ethane LFL – 3%vol in Air, Propane LFL – 2.15%vol in Air
Step 2. Calculate number of cycles required to achieve chosen oxygen concentration It can be proved analytically that the number of cycles (n) is related to the initial and final oxygen mole fractions (yo and yn) and the initial (LOW) and HIGH pressure used for purging by the equation
Note: The above relationship assumes pure nitrogen is used for inerting. It should also be noted that the number of cycles is independent of the vessel volume. As such a 2m3 vessel will require the same number of cycles as a 50m3 vessel. Example 3 – Calculate the number of cycles required to inert an atmospheric vessel down to 2%vol O2, using a Nitrogen pressure of 2.27barg An atmospheric vessel will be at PL = 1.01325bara and yo = 0.21; based on Air of 79% N2 and 21% O2. with yn = 0.02 and PH = 3.283bara,
From a design point of view, nitrogen pressure (e.g 2.27barg) can always be chosen to ensure that number of cycle is a whole number, such as 2, 3, 4 etc. If nitrogen pressure is constrained and number of cycle is not a whole number then cycles should be rounded up to the next whole number, such as a 1.4 cycles should be designed as 2 cycles.
Step 3. Calculate quantity of nitrogen required to achieve chosen oxygen concentration To determine the total amount of nitrogen required, the nitrogen per cycle needs to be calculated. Total nitrogen is then calculated as the number of cycles multiplied by nitrogen/cycle.
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Accurately Calculate Nitrogen Requirement for Pressure Purging Example 4 – Calculate the total quantity of nitrogen in Nm3/hr required to achieve the inert atmosphere in example 3 for a vessel and associated piping of 15m3 at 25˚C and Nitrogen temperature of 37˚C One approach that has been used to calculate the quantity of nitrogen is to determine the quantity per cycle as
However, this approach is only a good estimation when the system and nitrogen temperature are equal but not equal to zero degrees Celsius (given that Normal gas volume is defined at 1.01325bara & 0˚C) and is even less accurate when the nitrogen temperature is not equal to the temperature of the system to be inerted. A more accurate approach will be to apply a material & energy balance to determine the number of moles of nitrogen required to achieve the required pressure in the vessel. To determine the quantity of nitrogen more accurately, use the following steps
Step 3.1 Calculate the number of kmols in the vessel and piping system at initial conditions Using PV = nRT
Step 3.2 Iteratively determine the mixture temperature and number of moles in the vessel at high pressure of 3.283 bara This step involves calculating the number of kmols in the vessel at the high pressure end of the cycle. However, as the equation is dependent on temperature in the system at the high pressure and this final temperature is not known due to difference in Nitrogen and system pressure, the calculation becomes iterative. The basis of the iteration is to 3.2.1 Guess a value for final temperature 3.2.2 Calculate number of kmols in vessel based on high pressure and (guessed) final temperature 3.2.3 Determine number of kmols N2 introduced into system (kmols in 3.2.2 – initial kmols)
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Accurately Calculate Nitrogen Requirement for Pressure Purging 3.2.4 Check energy balance “heat gained by initial kmols in system (0.61314 kmols) = heat lost by kmols of N2 introduced”. 3.2.5 If energy balance in 3.2.4 is satisfied, then correct value of temperature and N2 kmols is obtained. If not, repeat steps 3.2.1 to 3.2.4 with new value of temperature. 3.2.6 Calculate Nm3 of N2 using Normal molar gas volume of 22.4 Nm3/kmol. The calculation can be set up in excel and solved for temperature using goal seek. An iteration table is also showed below for the case being considered here:
Data: N2 : Cp = 1.04 kJ/kg & RMM = 28kg/kmol, Air: Cp= 1.006 kJ/kg K & RMM = 28.96 kg/kmol Guess T (˚C) kmols @ kmols of N2 3.283bara
Heat gained Heat lost by N2 by Air (kJ) (kJ)
Energy balance (kJ)
31.00
1.9474
1.3343
107.18
233.13
125.95
34.00
1.9284
1.3153
160.77
114.90
-45.86
32.50
1.9379
1.3247
133.97
173.59
39.62
33.25
1.9331
1.3200
147.37
144.14
-3.23
33.19
1.9335
1.3204
146.30
146.49
0.19
Using the last line of data at temperature of 33.19˚C each calculation step from 3.2.2 to 3.2.6 is detailed below
Compare the 59.16Nm3 to the nitrogen value of 68.10Nm3 determined from the first approximate method in this example, there is a conservative 15% over estimation. This approach thus provides an accurate way of determining the nitrogen requirement for pressure purging.
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