Gas Sparging
1
Department Editor: Scott Jenkins S parging refers to the process of inject- ing a gas through a diffuser into a liquid phase. It is used in both physical and chemical process applications. In physical process applications, the gas is used to either dissolve gas into liquid (such as aeration and carbonation) or to remove contaminants from the liquid phase (such as oxygen stripping, volatile organic compound stripping and water stripping). In chemical process applications, sparging is used to dissolve reactant gases into a liquid phase for further reaction (such as in hydrogenation, oxidation, fermentation and ozonation reactions). The following informa- tion discusses some of the considerations required for setting up a sparging operation and selecting sparging equipment. Spargers (diffusers) come in various sizes, configurations and materials of construction. They are chosen based on the design and operating conditions of the process. Metal spargers are used in high-temperature, corrosive or oxidizing conditions, whereas ceramic spargers are sufficient for mild conditions. The type and configuration of the sparger used depend on factors such as whether a process is continuous or batch, gas flowrate, tank size, mechanical agita- tion, operating pressure and temperature. Mass-transfer rate The main purpose of a sparger is to increase the gas-to-liquid mass-transfer ef- ficiency, which is the ratio of the amount of active gas component (that is dissolved in liquid) to the gas injected. A low efficiency will result in an increased gas-injection rate and therefore increased cost to achieve the desired results. The liquid-phase mass- transfer resistance primarily controls the gas-to-liquid mass-transfer efficiency. The gas-to-liquid mass-transfer rate per unit volume is given by K L a(C* – C). In this equation, K L is the liquid-phase mass- transfer coefficient that is dependent on the diffusivity, liquid viscosity, temperature, and mixing; a is the interfacial area of gas bubbles in contact with liquid; C* is the saturated concentration of the gas in liquid; and C is the concentration in bulk liquid. As the interfacial area a is increased by sparg- ing small gas bubbles with high surface-to- volume ratio, the gas mass-transfer rate is improved. The mass-transfer driving force (C* – C) also has a big impact on the gas dissolution rate as the high-purity gas is used instead of the lower-purity gas. For example, the saturated concentration of oxygen in water from pure oxygen is five times higher than that from air, resulting in a large increase in the oxygen dissolution rate with pure oxygen. Sparging equipment selection There are several important factors to consider when selecting a sparger. The gas exit velocity at the sparger surface is an important design criterion for sparger selection. The actual gas volumetric flowrate for exit velocity is calculated using the pressure (P) that is the sum of tank headspace pressure (P Headspace ), liquid head pressure at the sparger (P Liquid ), and pressure drop across the sparger element (ΔP). The minimum sparger surface area is based on the gas exit-velocity limit for the process. The exit velocity limit is lowest for the static sparging operation when there is no mechanical agitation of the liquid phase. For agitated tank sparging and dynamic sparging, where liquid has high forced velocity along the sparger surface, the gas exit-velocity limits are significantly higher, re- quiring smaller spargers for the same gas flow. The exit-velocity limit for agitated tank sparging and dynamic sparging depends on the impeller speed and liquid veloc- ity, respectively. Agitation effects Apart from using a properly designed sparger, it is important to focus on the mix- ing of gas and liquid. In chemical process applications, the reactor vessel is often closed so that the unreacted high-purity gases, such as hydrogen or oxygen, are not vented through the system. In these applica- tions, specially designed mixing impellers are used, depending on the operating conditions of the reactor. Typically, one im- peller turbine is located above the sparger to shear and disperse the gas bubbles. Agitation at the liquid surface may also be required to entrain the headspace gas into the liquid phase. Contaminant removal In physical process applications where a contaminant is removed from the liquid phase, it is important to estimate the gas required for the process, as well as other components volatilized from the liquid phase. Sometimes downstream processing equipment is required to recover the con- taminant from the gas phase before venting the sparging gas to the atmosphere. Biodiesel example Many parameters have to be taken into consideration when estimating the gas flowrate and vent-gas composition in any sparging application. For example, when a gas is used to remove moisture from biodiesel, the key variables to estimate the gas requirement are temperature, pressure, moisture in influent gas, biodiesel composition, initial and final moisture concentration in biodiesel, batch size and batch time. Figure 1 shows the effect of the operating temperature and pressure on the normal- ized gas flowrate to reduce the moisture from 2,000 ppm to 500 ppm in a batch process using pure nitrogen gas (~99.99% purity). The gas- and liquid-phase composi- tions can be estimated using the vapor- liquid equilibrium calculations. Figure 2 shows the calculated moisture concentration in biodiesel at 10-psi and 14.7-psi operating pressures for processes at 200˚F with 1-h batch time. The moisture concentration in both processes reduces from 2,000 ppm initial concentration to 500 ppm final concentration after sparg- ing, but the process with 10 psi pressure requires 37% lower gas flow. This reduces the nitrogen consumption, as well as the biodiesel vented with the gas. Editor’s note: Content for this edition of “Facts at Your Fingertips” was contributed by Air Products (Allentown, Pa.; www.airproducts.com). Gas Sparging Normalized nitrogen flowrate 120 100 80 60 40 20 0 200 °F, 14.7 psi 200 °F, 10 psi 200 °F, 7.5 psi 150 °F, 10 psi Water, ppm 2,500 2,000 1,500 1,000 500 14.7 psi 10 psi 0 10 20 30 Time, min 40 50 60 FIGURE 1. Normalized nitrogen gas lowrate for moisture removal from biodiesel FIGURE 2. Moisture concentration in biodiesel with nitrogen gas sparging at 200˚F
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Transcript of Gas Sparging
Department Editor: Scott Jenkins
Sparging refers to the process of inject-ing a gas through a diffuser into a liquid phase. It is used in both physical
and chemical process applications. In physical process applications, the gas is used to either dissolve gas into liquid (such as aeration and carbonation) or to remove contaminants from the liquid phase (such as oxygen stripping, volatile organic compound stripping and water stripping). In chemical process applications, sparging is used to dissolve reactant gases into a liquid phase for further reaction (such as in hydrogenation, oxidation, fermentation and ozonation reactions). The following informa-tion discusses some of the considerations required for setting up a sparging operation and selecting sparging equipment.
Spargers (diffusers) come in various sizes, configurations and materials of construction. They are chosen based on the design and operating conditions of the process. Metal spargers are used in high-temperature, corrosive or oxidizing conditions, whereas ceramic spargers are sufficient for mild conditions. The type and configuration of the sparger used depend on factors such as whether a process is continuous or batch, gas flowrate, tank size, mechanical agita-tion, operating pressure and temperature.
Mass-transfer rateThe main purpose of a sparger is to increase the gas-to-liquid mass-transfer ef-ficiency, which is the ratio of the amount of active gas component (that is dissolved in liquid) to the gas injected. A low efficiency will result in an increased gas-injection rate and therefore increased cost to achieve the desired results. The liquid-phase mass-transfer resistance primarily controls the gas-to-liquid mass-transfer efficiency.
The gas-to-liquid mass-transfer rate per unit volume is given by KLa(C* – C). In this equation, KL is the liquid-phase mass-transfer coefficient that is dependent on the diffusivity, liquid viscosity, temperature, and mixing; a is the interfacial area of gas bubbles in contact with liquid; C* is the saturated concentration of the gas in liquid; and C is the concentration in bulk liquid. As the interfacial area a is increased by sparg-ing small gas bubbles with high surface-to-volume ratio, the gas mass-transfer rate is improved. The mass-transfer driving force (C* – C) also has a big impact on the gas dissolution rate as the high-purity gas is used instead of the lower-purity gas. For example, the saturated concentration of oxygen in water from pure oxygen is five times higher than that from air, resulting in a large increase in the oxygen dissolution rate with pure oxygen.
Sparging equipment selectionThere are several important factors to consider when selecting a sparger. The gas exit velocity at the sparger surface is an
important design criterion for sparger selection. The actual gas volumetric flowrate for exit velocity is calculated using the pressure (P) that is the sum of tank headspace pressure (PHeadspace), liquid head pressure at the sparger (PLiquid), and pressure drop across the sparger element (∆P). The minimum sparger surface area is based on the gas exit-velocity limit for the process. The exit velocity limit is lowest for the static sparging operation when there is no mechanical agitation of the liquid phase. For agitated tank sparging and dynamic sparging, where liquid has high forced velocity along the sparger surface, the gas exit-velocity limits are significantly higher, re-quiring smaller spargers for the same gas flow. The exit-velocity limit for agitated tank sparging and dynamic sparging depends on the impeller speed and liquid veloc-ity, respectively.
Agitation effectsApart from using a properly designed sparger, it is important to focus on the mix-ing of gas and liquid. In chemical process applications, the reactor vessel is often closed so that the unreacted high-purity gases, such as hydrogen or oxygen, are not vented through the system. In these applica-tions, specially designed mixing impellers are used, depending on the operating conditions of the reactor. Typically, one im-peller turbine is located above the sparger to shear and disperse the gas bubbles. Agitation at the liquid surface may also be required to entrain the headspace gas into the liquid phase.
Contaminant removalIn physical process applications where a contaminant is removed from the liquid phase, it is important to estimate the gas required for the process, as well as other components volatilized from the liquid phase. Sometimes downstream processing equipment is required to recover the con-taminant from the gas phase before venting the sparging gas to the atmosphere.
Biodiesel exampleMany parameters have to be taken into consideration when estimating the gas
flowrate and vent-gas composition in any sparging application. For example, when a gas is used to remove moisture from biodiesel, the key variables to estimate the gas requirement are temperature, pressure, moisture in influent gas, biodiesel composition, initial and final moisture concentration in biodiesel, batch size and batch time.
Figure 1 shows the effect of the operating temperature and pressure on the normal-ized gas flowrate to reduce the moisture from 2,000 ppm to 500 ppm in a batch process using pure nitrogen gas (~99.99% purity). The gas- and liquid-phase composi-tions can be estimated using the vapor-liquid equilibrium calculations.
Figure 2 shows the calculated moisture concentration in biodiesel at 10-psi and 14.7-psi operating pressures for processes at 200˚F with 1-h batch time. The moisture concentration in both processes reduces from 2,000 ppm initial concentration to 500 ppm final concentration after sparg-ing, but the process with 10 psi pressure requires 37% lower gas flow. This reduces the nitrogen consumption, as well as the biodiesel vented with the gas.
Editor’s note: Content for this edition of “Facts at Your Fingertips” was contributed by Air Products (Allentown, Pa.; www.airproducts.com).
Gas Sparging
No
rma
lize
d n
itro
ge
n f
low
rate
120
100
80
60
40
20
0200 °F,
14.7 psi
200 °F,
10 psi
200 °F,
7.5 psi
150 °F,
10 psi
Wa
ter,
pp
m
2,500
2,000
1,500
1,000
500
14.7 psi
10 psi
0 10 20 30Time, min
40 50 60
FIGURE 1. Normalized nitrogen gas lowrate
for moisture removal from biodiesel
FIGURE 2. Moisture concentration in biodiesel
with nitrogen gas sparging at 200˚F
