**Journal of Environmental Protection
**Vol.5 No.11(2014), Article ID:48988,13 pages DOI:10.4236/jep.2014.511096

Ammonia Diffusion Phenomena through Nalophan^{TM} Bags Used for Olfactometric
Analyses

Selena Sironi^{1*}, Lidia Eusebio^{1}, Laura Capelli^{1},
Emanuela Boiardi^{1}, Renato Del Rosso^{1}, Jean-Michel Guillot^{2}

^{1}Department of Chemistry, Materials and Chemical Engineering “Giulio
Natta”, Politecnico di Milano, Milano, Italy

^{2}Laboratory of Industrial Environment Engineering, Ecole des Mines d’Alès,
Alès cedex, France

Email: ^{*}selena.sironi@polimi.it

Copyright © 2014 by authors and Scientific Research Publishing Inc.

This work is licensed under the Creative Commons Attribution International License (CC BY).

http://creativecommons.org/licenses/by/4.0/

Received 28 April 2014; revised 26 May 2014; accepted 21 June 2014

ABSTRACT

The ammonia loss through Nalophan^{TM} bags has been studied. Ammonia was
chosen as target compound in order to be representative of odorous molecules of
small dimensions. The losses observed for storage conditions and times as allowed
by the reference standard for dynamic olfactometry (EN 13725:2003) indicate that
odour concentration values due to the presence of small molecules may be significantly
underestimated if samples are not analysed immediately after sampling. The diffusion
coefficient of ammonia through the Nalophan^{TM} film was evaluated using
the Fick’s law, and it turned out to be equal to 2.38E−12 (m^{2}/s).
The results and their theoretical interpretation indicate that concentration losses
due to ammonia diffusion through the Nalophan^{TM} film can be decreased
by using large bags and filling them up to their maximum capacity.

**Keywords:**Sampling Bag, Diffusion, Odour Sampling, Ammonia, Nalophan

1. Introduction

Even though environmental odours are generally not harmful to health [1] , in the last 30 years odour pollution has become a serious environmental concern because it may be the cause of physiological stress to the population. For this reason, during the last years, several studies have been undertaken to assess how to control and monitor odour emissions [2] . For many years, researchers have tried to characterize odour using chemical and physical techniques like GC and GC-MS. Although dozens, sometimes hundreds of odorants can be identified, such identification mostly fails to predict odour as it is perceived by human nose [3] [4] . This is the reason why sensorial odour measurement using human observers has become the main tool to quantify odours. Dynamic olfactometry has therefore been consolidated as the best analysis method to quantify odour emissions in terms of odour concentration or odour emission rate [5] .

Because of the difficulties associated with the conduction of olfactometric analyses on site, samples are generally collected and then stored in suitable containers until they are analysed in an olfactometric laboratory [5] -[9] . The European Standard on dynamic olfactometry [10] fixes the general requirements relevant to the materials used for the realization of sampling equipment. According to the European Standard, the materials used for olfactometry shall be odourless, they shall be selected to minimize the physical or chemical interaction between sample components and sampling materials, have low permeability in order to minimize sample losses caused by diffusion and smooth surface.

The materials allowed for realizing sample containers (bags) and listed in point
6.3.1 of the actual standard are: tetrafluoroethylene hexafluoropropylene copolymer
(FEP); polyvinylfuoride (PVF, Tedlar^{TM}) and polyethyleneterephthalate
(PET, Nalophan™).

According to the European Standard these materials shall be tested for suitability, by verifying they can hold a mixture of odourants with minimal changes for periods of storage of 30 hours, which is the maximum storage time allowed by the European Standard.

Some authors have been studying the characteristics of the materials listed in the EN 13725 [10] with the aim to verify their suitability for olfactometric measurements. Previous studies have shown that FEP bags are quite inert but not very robust and rather expensive [11] . PVF bags are more robust but they have a background odour caused by the use of solvents during production [12] . These disadvantages are the reason why PET, which is relatively cheap and odourless, is actually the most widely used material for the realization of sample bags [11] [13] -[16] .

Many studies have been conducted in order to assess the diffusion of odorous molecules through polymeric films [11] [17] -[23] . In these studies, chemical analyses have been performed to quantify the losses of specific compounds over time and to compare the recovery efficiency of different materials [11] [17] -[19] [24] -[30] .

Despite of its inertia and cost effectiveness Nalophan^{TM} has been proved
to allow the diffusion of specific molecules, such as water, and its permeability
has been studied.

Both the nature of the polymer and nature of the diffusing molecule affect the diffusion rate through the material that is expressed by the diffusion coefficient D [31] .

Water can diffuse quickly through polymeric films because of its structure [14] . Also other molecules having a dimension similar
to water, such as ammonia (NH_{3}) and hydrogen sulphide (H_{2}S)
[14] [32]
[33] , which are typically found in emissions
from several operations such as solid waste and waste water treatment, can diffuse
easily.

The characteristics of the polymer itself affecting the diffusion processes are: the chemical nature of the polymer, its crystalline structure and orientation, the free volume, the molecular cohesion, the relative humidity, temperature, hydrogen bonding, polarity, solubility parameter, solvent size and shape [34] -[36] .

The experiments described in this paper have the aim to investigate the diffusion
phenomena through Nalophan^{TM}, which is one of the most widespread materials
used for the realization of sampling bags (EN 13725:2003), thereby calculating the
diffusion coefficient relevant to this material. Ammonia was chosen as target compound
for the study, which involved both an experimental part aiming to calculate the
specific D coefficient through Nalophan^{TM} as well as to evaluate the
influence of the surface/volume ratio on the diffusion kinetics. The described approach
is important in order to increase knowledge in this field, suggesting possible technical
expedients to reduce diffusion, and possibly improve regulatory issue.

2. Materials and Methods

2.1. Materials

The Nalophan^{TM} used to fabricate the bags employed for the experimental
tests consists in a one-layer foil of polyterephtalic ester copolymer with 20-µm
thickness supplied by Tilmmanns S.p.A.

The bags were obtained starting from a tubular film cut in different lengths.

One end was equipped with a clamp closure while the other end is provided with a Teflon inlet tube for sample collection (Figure 1).

The NH_{3} decay over time was evaluated using gas-chromatography (GC) for
the quantification of NH_{3} concentration inside the bag. The ammonia concentration
was measured using a HP Agilent 6890 gas chromatograph equipped with an Agilent
HP-5MS fused silica capillary column (CP 7591-PoraPlot Amines, length 25 m, internal
diameter 0.32 mm, film thickness 10 μm). The oven temperature follows a three steps
program: 100˚C for 12 minutes, from 100˚C to 200˚C with a rate of
8˚C/min, 200˚C for 5 minutes. The carrier gas was helium with a constant
flow of 3 mL/min (pressure of 1.21 atm and mean velocity of 53 cm/s). The gaseous
mixture inside the bags was analysed by a GC, equipped with a TCD detector, at specific
time intervals, in order to evaluate the variations of NH_{3} concentration
(ppm) over time.

A calibration curve was built to relate the area of the GC peak with the NH_{3}
concentration (ppm). Instrument calibration was performed analysing different standard
concentrations of NH_{3} in air ranging from 10,000 to 60,000 ppm. Standards
were obtained starting from different liquid mixtures of NH_{3} in water
and analysing the headspace obtained in a fixed volume of air where the liquid was
inserted and then kept at a controlled temperature.

All the tested samples were realized by filling the Nalophan^{TM} bags with
a gaseous mixture of ammonia in wet air, with an ammonia concentration of about
55,000 ppm_{V} and a relative humidity of 60%, which will be defined as
the “test mixture”. The test mixture was prepared using the headspace technique.
The liquid phase was realized at room temperature mixing 10.5 ml of a liquid solution
of NH_{3} at a concentration of 30% w/w and 50 ml of distilled water.

During storage, physical parameters like temperature and relative humidity were kept under control using a climatic chamber (Chamber GHUMY by Fratelli Galli, Milano, Italy).

2.2. Methods

All tests were conducted by measuring the NH_{3} concentration at different
time intervals after sample preparation. More in detail, NH_{3} was analysed,
every hour, from 0 to 26 h. Each measurement involved the withdrawal of 300 µl
of the test mixture by means of a syringe and the injection in the GC.

The diffusion of ammonia was first evaluated through a Nalophan^{TM} bag
having a capacity of about 6000 cm^{3} and a surface equal to 2580 cm^{2}.
This bag was filled with 6000 cm^{3} of the above defined test mixture and
then stored at a constant temperature of 23˚C and an external relative humidity
of 60%. The external relative humidity was set equal the internal relative humidity
in order to avoid water diffusion during storage and its potential influence on
ammonia diffusion. Based on the experimental data of residual NH_{3} concentration
inside the bag and on the Fick’s law, the diffusion coefficient D of ammonia through
Nalophan^{TM} was calculated. The measurements were repeated three times
each and the diffusion coefficient D is averaged over the 26 hours.

The role of the exchange surface (i.e., the bag surface area) on the NH_{3}
concentration decay inside the bag was evaluated by realizing bags having different
surface areas, i.e. 1900 cm^{2}, 2580 cm^{2}, 3520 cm^{2},
respectively. These bags have different capacities (3000, 6000 and 9000 cm^{3}),
but they were filled with the same amount (3000 cm^{3}) of the test mixture,
thus realizing bags with a different surface-to-volume ratio.

Table1 Experimental data relevant to NH_{3}
diffusion over time in a Nalophan^{TM} bag with V_{N} = 6000 cm^{3}
and S_{N} = 2580 cm^{2}

Figure 1. Nalophan^{TM} bags.

Also in this case, all tested samples were stored at a T of 23˚C and RH of 60% for the whole duration of the test. The ammonia concentrations over time were measured according to the above described test protocol.

3. Calculations

The diffusion phenomena through a polymeric film are described by the Fick’s law. According to it, the specific molar flow is defined as:

(1)

where:

• j is the specific molar flow (mol/m^{2}/s)

• D is the diffusion coefficient of the compound through the film (m^{2}/s)

• C is the concentration of the diffusing compound (mol/m^{3})

• x is the differential thickness of the film.

The film thickness can therefore be expressed as:

(2)

where z is the film thickness (m);

Referring to Figure 2, which schematizes the diffusion phenomenon through thethin filmwhich constitutes the sampling bag, we can define:

• S_{N} is the surface of the polymeric film (m^{2})

• z_{N} is the thickness of the film (m)

• C_{N} is the concentration in the inside volume (mol/m^{3})

• C_{N}_{+1} is the concentration outside the film (mol/m^{3}),
for a single bag it is generally considered negligible (C_{N}_{+1}
= 0)• j_{N} is the specific molar flow through the film (mol/m^{2}/s).

If the film thickness can be considered as negligible, then the accumulation term inside the material is negligible, as well.

With this assumption j_{N} is constant along the film (x).

By integrating Equation (1) in dx between 0 and Z_{N}, the specific molar
flow j_{N} can be expressed as:

(3)

j_{N} is relevant to an infinitesimal portion of the exchange surface dS.

Assuming that the internal molar concentration C_{N} is constant inside
the whole internal volume V_{N} and also the external concentration C_{N}_{+1}
is constant inside the external volume V_{N}_{+1}, then the global
flow J through the exchange surface S_{N} can be calculated by integrating
as follows:

Figure 2. Schematization of diffusion through the thin film of the bag.

(4)

(5)

Combining Equation (3) with Equation (5), the molar flow through the surface is expressed as:

(6)

If the external concentration C_{N}_{+1} is assumed to be equal
to zero (C_{N}_{+1} = 0), and if the volume V_{N}_{+1}
is taken equal to infinity (V_{N}_{+1} = ∞), which is the case if
the bag is placed in a neutral environment (where the presence of NH_{3}
may be considered negligible), then Equation (6) can be rewritten as:

(7)

According to this model, the concentration decay over time turns out to be a function
of the surface area (S_{N}), the volume of the sampled gas V_{N},
the film thickness (z_{N}), the time (t), the diffusion coefficient (D)
that depends on the characteristics of the material, and the concentration gradient
through the polymeric barrier (∆C).

The boundary conditions considered for the integration of Equation (7) are:

(8)

(9)

The integration of Equation (7) allows the calculation of the concentration trend over time:

(10)

(11)

4. Results and Discussion

Table 1 shows the NH_{3} concentration
values measured at different time intervals t_{i}. Each concentration value
reported in the table is the average of three replicate measurements.

The last column of Table 1 reports the diffusion
coefficient D_{ti} for each time interval t_{i} calculated according
to the following equation:

(12)

where t_{i} is the time interval and C_{ti} is the concentration
measured after t_{i}.

In order to give a better representation of the diffusion phenomena through the
polymeric film, as well as to make it possible to compare results obtained with
different bag filling volumes (V_{N}), it was decided to make all further
considerations about the bag contents considering the number of moles (n) instead
of the concentrations.

For this reason, the third column of Table 1 reports the number of millimoles (mmol) contained in the bag, and the fourth column represents the number of moles divided by the bag surface (n/S). This parameter allows highlighting the differences obtained with different bag surfaces.

The fifth column reports ∆n, which is the difference between the number of
moles at t_{0} = 0 h and the number of moles at time t (n_{0}-n)
and therefore represents the number of moles that have crossed the film.

∆n/t represents the number of moles passed through the film during the whole
time interval t, thus representing the direction coefficient of the line connecting
n_{0} and n, i.e. the average speed at which the moles have crossed the
film.

The diffusion coefficient of ammonia through Nalophan^{TM} is finally calculated
as the average of the different values of D_{ti} weighted on the corresponding
storage time t_{i}:

.

(13)

The resulting value for
is equal to 2.38 10^{−8} cm^{2}/s, with a standard deviation
equal to 3.70 10^{−11} cm^{2}/s.

The percent NH_{3} loss through the bag over time can be expressed as:

(14)

Figure 3 shows the experimental NH_{3}
losses over time during the test period of 26 h. These results were obtained using
a bag with a surface of 2580 cm^{2} filled with 6000 cm^{3} of the
test mixture (surface-to-volume ratio equal to 0.430 cm^{−1}). The
frequency of the measurements of the ammonia losses was focused on the first hours
of the storage time (1 - 7 h) in order to investigate the initial concentration
decrease trend, and close to the limit storage time (23 - 26 h) imposed by the European
norm, which is 30 h, in order to evaluate the cumulative losses.

The loss percentage of NH_{3} (%) after 26 h turns out to be equal to about
37%.

This trend is coherent with other data reported in scientific literature dealing
with the same subject. As an example, a study by Akdezin et al. [37]
also dealing with NH_{3} losses through polymeric films, reports losses
of about 25% after 48 h. This value is lower compared to the 37% found in this study
and reported in Figure 3. This may be due to the
fact that the starting NH_{3} concentration is much lower (ppb) than in
our case (thousands of ppm), thus resulting in a lower concentration gradient, which
is the driving force of the diffusion phenomenon.

A similar trend was observed in other studies by Beghi and Guillot [14] [33] , which investigate
H_{2}S diffusion through different Nalophan^{TM} film having a different
thickness. Also in this case, the reported H_{2}S losses through a 20 µm
thick Nalophan^{TM} film are lower, presumably due to the lower starting
concentration.

Figure 4 represents the experimental data relevant
to the number of NH_{3} moles over time (third column of Table 1). The experimental data show a good correspondence with the theoretical
trend derived from Equation (11), which can be alternatively expressed as:

(15)

The theoretical trend is shown in Figure 4 as a continuous line. This trend was obtained by inserting, in Equation (15), the averaged diffusion coefficient calculated as described above (Equations (12) and (13)).

Figure 3.
NH_{3} loss (%) over time from the Nalophan^{TM} bag with V_{N}
= 6000 cm^{3} and S_{N} = 2580 cm^{2}.

Figure 4.
Number of NH_{3} moles over time: experimental data (dots) vs. theoretical
trend (continuous line) in a Nalophan^{TM} bag with V_{N} = 6000
cm^{3} and S_{N} = 2580 cm^{2}.

As described by Equation (15), the variation of moles inside the bag depends on the surface-to-volume ratio (, hereafter defined simply as) of the bag.

In order to quantify this effect, different tests were performed using bags having
different surface-to-volume ratios. This was realized by fabricating bags having
different surface areas (i.e. S = 1900 cm^{2}, 2580 cm^{2}, 3520
cm^{2}, respectively) and therefore different capacities, and then filling
them with the same amount (V = 3000 cm^{3}) of the test mixture. Thus the
surface-to-volume ratio was changed: the surface area S_{N} was varied while
the gas volume V was kept constant.

Table 2 reports the number of NH_{3} moles
(n) and their percent variation over time (Δn%) for the three bags having different
surface-to-volume ratios, as described above. It is possible to observe that the
percent variation of ammonia in the considered 26 h interval increases with the
bag exchange surface S, passing from 51% for the bag with the smaller surface (S
= 1900 cm^{2}) to 72% for the bag with the larger surface (S = 3520 cm^{2}).

Table 2 also reports the ratio between NH_{3}
moles and the bag surface (n/S): this ratio allows better discriminating the experimental
data relevant to the bags having different surface-to-volume ratios, which are shown
in Figure 5. Figure 5
also illustrates the theoretical trends, calculated based on Equation (15), thereby
the surface S for each bag, and the average diffusion coefficient obtained from
the experimental data (2.38 10^{−8} cm^{2}/s).

A good correspondence is observed between experimental and theoretical trends.

Figure 6 illustrates the ammonia loss percentage
over time for bags filled with the same amount (V = 3000 cm^{3}) of the
test mixture, but with different surfaces. As already mentioned, diffusion is accentuated
in the bags with a larger exchange surface.

Figure 5.
n/S trends for the three bags having different surfaces (1900 cm^{2}, 2580
cm^{2} and 3520 cm^{2}, respectively) and different S/V ratios (0.63
cm^{−}^{1}, 0.86 cm^{−}^{1} and 1.17
cm^{−}^{1}, respectively).

Figure 6. NH_{3}
loss (%) over time from the three bags having different surfaces (1900 cm^{2},
2580 cm^{2} and 3520 cm^{2}, respectively) and different S/V ratios
(0.63 cm^{−}^{1}, 0.86 cm^{−}^{1} and
1.17 cm^{−}^{1}, respectively).

Table3 Number of moles and average permeation speed for different S/V ratios

The bag with a surface of 1900 cm^{2} and a surface-to-volume ratio of 0.633
cm^{−1} gives a NH_{3} loss percentage for prolonged storage
times (26 h) of about 50%. This loss increases to about 60% for the bag with a surface
of 2580 cm^{2} and a surface-to-volume ratio of 0.860 cm^{−1},
and to about 70% for the bag with a surface of 3520 cm^{2} and a surface-to-volume
ratio of 1.17 cm^{−1}. The experimental data are in agreement with
the theoretical trend expressed by Equation (15), which indicates that, at a given
time t, n/n_{0} is lower, i.e. the NH_{3} loss percentage is higher,
for higher surface-to-volume ratios S_{N}/V_{N}. The same assumption
is discussed in Sironi et al. [23] .

In order to evaluate the diffusion as a function of the surface-to-volume (S/V)
ratio, it is possible to use the data relevant to the different tests described
in this section (see Table 1 and Table 2) by fixing the time t, and then comparing the different values of
Δn (i.e. the total number of moles that have crossed the film at time t) or Δn/t
(i.e. the average speed at which the moles have crossed the film) obtained for the
bags having different S/V ratios. In order to make the results of the first test
(Table 1) comparable with those of the other tests
(Table 2), given the different number of initial
moles n_{0}, the data have to be normalized with respect to n_{0},
thus representing Δn/n_{0} or Δn/t/n_{0}. As an example, Table 3 reports the experimental values of Δn/n_{0}
and Δn/t/n_{0} measured after 7 hours for the different bags (having different
S/V). The last columns of Table 3 report the theoretical
data, as calculated based on Equation (15).

The values of Δn/t/n_{0}, i.e. the average permeation speed in the first
7 hours of storage normalized with respect to n_{0}, both measured experimentally
and calculated with Equation (15), are also represented in Figure 7.

The data reported in Table 3 and Figure 7 further prove how the surface-to-volume ratio affects diffusion: the average diffusion speed increases with the S/V ratio, thus resulting in higher percent losses after a given storage time t.

This means that, for a given material, to which corresponds a given diffusion coefficient D, one way to reduce diffusion over time is trying to reduce the surface-to-volume ratio.

For bags of given dimensions, i.e., same surface and same maximum capacity, the only way to minimize S/V is to fill the bags to their maximum capacity. Bags filled only partially will have a higher S/V and therefore higher losses over time.

Another way to reduce S/V is to realize bigger bags. As an example, if cylindrical bags having a slenderness ratio (i.e., h/D) equal to 2 are considered, the bag surface will be:

where d is the diameter of the bag. If the bag is filled completely with the sample gas, then gas volume V is equal to the bag volume:

Figure 7.
Average permeation speed (t = 7 h) normalized with respect to n_{0} for
different values of S/V.

The surface-to-volume ratio in this case depends on the diameter:

And will therefore be proportional to the inverse of the diameter, i.e. decrease with the bag capacity.

Moreover, for a given d, S/V may be reduced by reducing the slenderness ratio, giving that the lowest S/V ratio is obtained for a slenderness ratio equal to 1, i.e. h = D.

Similar considerations can be made considering other bag shapes, always giving that diffusion phenomena can be reduced by using bigger bags, filled to their maximum capacity.

5. Conclusions

This study allowed to evaluate and to quantify the phenomenon of ammonia diffusion
through Nalophan^{TM} films.

The experimental determinations allowed the calculation of the diffusion coefficient
of ammonia through Nalophan^{TM} according to the Fick’s law, which turned
out to be equal to 2.38 10^{−8} cm^{2}/s at a temperature
of 23˚C and a relative humidity of 60%.

The ammonia losses from the Nalophan^{TM} sampling bag always turned out
to be significant; for instance, in the case of a bag with a surface of 2580 cm^{2}
filled with 6000 cm^{3} of gas (i.e. a “test mixture” of ammonia in air,
at fixed temperature and relative humidity), the percent ammonia loss after 26 h
was 37%. This value is not negligible especially considering that the European Norm
EN 13725:2003 allows a maximum storage time of 30 hours, thus assuming that the
sampled mixture remains almost unaltered for 30 hours.

This study discusses the effect of the exchange surface on diffusion, by highlighting to which extent the surface-to-volume ratio affects the diffusion rate.

Diffusion was tested in bags with different S/V giving that the bag with a surface-to-volume
ratio of 0.633 cm^{−1} has a NH_{3} loss percentage after
26h of about 50%, while this loss increases to about 70% for the bag with a surface-to-volume
ratio of 1.17 cm^{−1}. The experimental data are in agreement with
the theoretical trend derived from the Fick’s law, which indicates that, at a given
time t, n/n_{0} is lower, i.e. the percent NH_{3} loss is higher,
for higher surface-to-volume ratios S_{N}/V_{N}. Of course, the
percentages of losses obtained during the presented experiments corresponds to a
range of ammonia concentration. These losses can present different values if ammonia
concentrations are in other ranges (higher or lower).

This means that, for a given material, to which corresponds a given diffusion coefficient
D, one way to reduce diffusion over time is trying to reduce the surface-to-volume
ratio. As a consequence, diffusion phenomena can be reduced by using bigger bags,
filled to their maximum capacity. For cylindrical bags obtained from tubular Nalophan^{TM},
S/V can be minimized by realizing bags with a slenderness ratio (h/D) equal to 1.

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NOTES

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