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https://dx.doi.org/10.20961/mekanika.v22i2.71250
Revised 20 August 2023; received in revised version 28 August 2023; Accepted 14 September 2023
Available Online 30 September 2023
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Mekanika: Majalah Ilmiah Mekanika
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Volume 22 (2) 2023
Adsorption Characteristics of Silica Gel-Water Pairs in Personal
Protection Equipment
Shazia Hanif1, Suryadijaya Adiputra2, Indri Yaningsih2*, Eko Prasetya Budiana2
1 Department of Agricultural Engineering, Muhammad Nawaz Sharif University of Agriculture, Multan, Pakistan
2 Department of Mechanical Engineering, Faculty of Engineering, Universitas Sebelas Maret, Surakarta, Indonesia
*Corresponding Author’s email address: indriyaningsih@staff.uns.ac.id
Keywords:
Adsorption
Silica gel
Water vapor
GAB
LDF
Abstract
The utilization of RD-type silica gel material as an adsorbent within the Personal
Protective Equipment (PPE) layer underwent a comprehensive analysis aimed at
elucidating its unique adsorption characteristics through the application of MATLAB
programming. This study aims to determine the characteristics of silica gel RD to water
vapor in terms of adsorption capacity and rate. A layer modeling approach was employed
to simulate the Personal Protective Equipment (PPE), which comprised four distinct
layers: the surrounding environment air, the fabric layer, the RD-type silica gel layer,
and the air gap separating the silica gel from the skin surface. The simulation
encompassed environmental conditions set at 27℃, while the human body's temperature
was maintained at 35℃. This study uses a simulation method using GAB (Guggenheim–
Anderson–de Boer) modeling calculations to determine isothermal characteristics and
LDF (Linear Driving Force) modeling to determine kinetic characteristics with an
adsorbent temperature of 26.84℃. The simulation results show that the isothermal
characteristics of silica gel RD at a relative humidity of 60% or a relative pressure of 0.6
have an absorption capacity of 0.38 kg/kg. Moreover, the kinetic characteristics of silica
gel RD have an absorption rate of 0.38 kg/kg of water vapor with a time of 980 s until a
significant reduction in the absorption value occurs.
1 Introduction
Field workers, especially health workers, must use Personal Protective Equipment (PPE) when
working or performing their duties. PPE serves to maintain the safety and security of workers in a hazardous
work environment. According to WHO, in the current era of the coronavirus disease 2019 (COVID-19)
pandemic, especially for health workers are required to use PPE in the form of medical masks, goggles,
face masks, gloves, and especially hazmat clothes to prevent the spread of COVID-19. Certain ergonomic
requirements must be met when using PPE to reduce discomfort caused by physiological or mental stress
factors of health workers. Researchers have developed and innovated international standards for
determining the protection factors for various types of PPE [1].
PPE improvisation is needed to prevent heat stress. Heat stress is a condition where the body can no
longer maintain the body temperature. In tropical climates, which have an environmental temperature of
25-34°C to prevent heat stress, PPE, which can limit the evaporation of excess heat, is needed [2]. PPE is
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generally made of thick, multi-layered, and heavy material. This material causes thermal circulation to be
hampered because it reduces the rate of heat exchange and the limited water vapor permeability of the
clothing layer. The result is an increase in temperature inside the PPE, which can become a significant
issue, especially in hot conditions or when working in hazardous environments. It not only diminishes user
comfort but can also be harmful to their health [2].
The equipment that uses PPE must have guaranteed safety and quality, but comfort must still be
considered by ensuring that these materials remain comfortable to use on the human body. To achieve
thermal comfort and maintain the body temperature at 37°C, the layers of PPE material need to be combined
with other adsorbent materials, such as silica gel [3]. This addition plays a crucial role in enhancing the
overall user experience. Within the realm of PPE, maintaining the appropriate moisture level is the key to
user comfort and health. Silica gel functions as an effective moisture-absorbing agent, capturing moisture
that may accumulate within PPE during use. Silica gel has characteristics that are very suitable for
adsorption, namely providing a cooling effect for water with a low heat source or below 85°C [4].
Over a wide range of pressures, the isotherm characteristics of silica gel in relation to water vapor
can be observed. At low pressures, silica gel has a low adsorption capacity for water vapor. RD-type silica
gel can achieve 95% regeneration at 90°C, and at 140°C, it can desorb 95% in 4 minutes and has a high
cooling capacity [5-7]. Research becomes highly significant as it can provide valuable insights into the
isotherm and kinetic properties of RD-type silica gel when used as an adsorbent in Personal Protective
Equipment (PPE). Furthermore, the objective of this research is to assess the impact of using silica gel in
PPE materials to improve user comfort.
Silica gel-water, also known as silica gel, is a popular material in personal protective equipment (PPE)
due to its desiccation capability, low humidity, protection against rust and corrosion, and ability to reduce
mold and mildew growth [8]. It can be regenerated easily, making it a cost-effective solution for PPE.
Aqueous silica gel is compatible with various PPE materials, making it a versatile choice for various types
of personal protection [9-10]. It is non-toxic but should not be consumed, making it safe for use in PPE and
work environments [11]. However, proper usage and careful manufacturer instructions are essential to
maintain its effectiveness [12-13].
2 Experimental Methods
2.1 Heat Transfer
Figure 1 illustrates the layers used in the modeling process for this study. The layer modeling
comprises the skin surface, an air gap, silica gel, fabric, and the surrounding environment. The analysis of
heat and mass transfer resulting from the introduction of silica gel to APD (Air-Purifying Respirator) was
conducted utilizing the Nusselt number (Nu) and Linear Driving Force (LDF) functions to compute the
adsorption rate. Additionally, the Guggenheim–Anderson–de Boer (GAB) model was employed to
calculate the adsorption rate. The analysis of heat and mass transfer encompasses the properties of air,
fabric, silica gel, and the skin.
Figure 1. Layer Modelling
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The following equation calculates heat transfer and mass transfer in layers. According to the law of
energy balance, heat transfer is calculated by taking the thermal resistance for convection and conduction
into account. The mass transfer involves the mass resistance of the silica gel and cloth, as shown in Eqs.
(1), (2), (3), and (4) [14-15].
(1)
(2)
(3)
(4)
2.2 Isotherm Modelling
The isothermal adsorption characteristics were obtained using GAB modeling to get the equilibrium
adsorption uptake (q) value as in Eq. (5), Eq. (6), to (8), which are used to find the constant variable
related to heat [16].
(5)
(6)
(7)
(8)
Where is the pressure, is the saturation pressure, and is the specific gas constant for water
vapor; , , , , , and are variable constants related to the heat; , are the water vapor
sorption heats.
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2.3 Kinetic Modelling
The linear driving force (LDF) model is the most commonly used model to calculate adsorption
kinetic. The adsorption rate (dw/dt) can be calculated in terms of the adsorbent/adsorbate pair by Eq.(9) to
(13) [17].
The Arrhenius equation in Eq. (14) [18] can be used to obtain the parameter, namely the diffusion
coefficient on the surface. According to the journal references, Eq. (15) can be used calculate , the total
mass transfer coefficient given in Eq. (15), where the value of is 15.
(9)
(10)
(11)
(12)
(13)
(14)
(15)
2.4 Properties
Table 1. shows the values of the variables listed in equations 3.1 to 3.32 and used to solve these
equations.
Table 1. Properties for Equation (1) to (15)
Properties
Symbol
Value
Unit
Relative humidity
Skin temperature
Air temperature
Air surface area
Layer thickness
, 1
Layer length
Humidity ratio of air
Silica gel mass transfer coefficient
Silica gel area
Silica gel temperature
Humidity ratio of silica gel
Thermal conductivity of silica gel
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Cont.
Properties
Symbol
Value
Unit
Heat of adsorption
Fabric temperature
Fabric surface area
Overall mass transfer coefficient
GAB isotherm model parameters
Functions related to heat adsorption
Functions related to heat adsorption
Constant gas
Relative pressure
Geometry parameter
Surface diffusivity
Pre-exponential coefficient
Activation energy
Radius of adsorbent particles
3 Result and discussion
3.1 Isothermal Adsorption
This study conducted a simulation to collect data in the form of isothermal adsorption characteristics
in the silica gel adsorption process with water vapor. The data obtained from the simulation are utilized to
determine the equilibrium absorption performance of the adsorption process on silica gel adsorbents against
water vapor. Isothermal adsorption simulations were carried out using the GAB modeling equation. The
temperature used in the simulation was 26.84℃, with a predetermined relative pressure range of 0.1 to 0.9.
In this simulation, this temperature condition was employed to represent various potential scenarios,
and different relative pressure levels was used to understand how adsorption occurs at varying relative
humidity levels. The results of this simulation are crucial for understanding how silica gel behaves when
interacting with water vapor under various conditions, which can be valuable in practical applications and
gaining a deeper understanding of the adsorption process.
Figure 2. compares the equilibrium adsorption uptake () value or the equilibrium absorption
performance value of the adsorption process with a predetermined relative pressure. Figure 2 shows a
significant increase in the value of (), often with an increase in the relative pressure value. These
simulation results align with the theory of isothermal adsorption, where it is evident that the more
significant the relative pressure value, the greater the equilibrium adsorption uptake. In this context, this
finding confirms the fundamental principles associated with isothermal adsorption, implying that with an
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increase in relative pressure, the adsorbent material's ability, in this case, silica gel, to attract and retain
water vapor molecules also increases.
Figure 2. Adsorption Uptake Isotherm
The isothermal characteristic of the adsorption process reveals the remarkable capabilities of the RD-
type silica gel adsorbent. Its ability to absorb water vapor under varying relative pressure and humidity
conditions is demonstrated. At a relatively low relative pressure of 0.1, equivalent to a 10% relative
humidity, the adsorbent efficiently absorbs 0.0439 kg of water vapor per kilogram of its mass, showcasing
its effectiveness even in low humidity conditions. However, as the relative pressure steadily rises to 0.9,
corresponding to a high 90% relative humidity, the adsorbent's performance escalates significantly, with a
water vapor absorption capacity of 0.6158 kg/kg, highlighting its efficiency in high humidity environments.
In this study, a relative humidity of 60%, equivalent to a relative pressure of 0.6, was selected for
testing. The RD-type silica gel adsorbent demonstrated its ability to absorb 0.3867 kg of water vapor per
kilogram of its mass under these conditions, underscoring its versatility and effectiveness in various
practical applications, particularly in environmental humidity control systems. Understanding these
isothermal characteristics provides valuable insights for engineering and industrial applications, where
silica gel adsorbents play a crucial role in moisture management.
3.2 Kinetic Adsorption
The adsorption kinetic simulation was conducted utilizing the LDF modeling equation. The
simulation employed a temperature of 26.84°C, with a predefined time range spanning from 0 to 3000
seconds and a constant gas input (R) of 8.134 j/mol. The equilibrium adsorption uptake value, which serves
as the reference point, was established at a relative pressure of 0.6 or under ambient conditions with a
relative humidity of 60%. This comprehensive approach enabled a detailed analysis of the adsorption
kinetics of the RD-type silica gel adsorbent under specific environmental conditions, providing valuable
insights into its performance over time and improving our understanding of its practical applications in
various settings.
Figure 3. compares the instant adsorption value () or the instant absorption performance value of
the adsorption process with a predetermined time. Figure 3 depicts an increase in value as time is added to
the achievement value. The simulation results are in accordance with the adsorption kinetic, in which the
adsorption value increases rapidly with time until it reaches the points.
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Figure 3. Adsorption Kinetic
The kinetic characteristic of the adsorption process highlights the RD-type silica gel adsorbent’s
ability to absorb water vapor quickly. In this study, the adsorbent was exposed to conditions of 26.84°C
temperature and 60% relative humidity, with an equilibrium absorption value of 0.3876 Kg/Kg. Under these
specific conditions, the RD-type silica gel adsorbent demonstrated a swift absorption rate, initially
absorbing 0.38 Kg/Kg of water vapor within a mere 980 seconds. However, as the absorption process
continued, there was a noticeable decrease in the rate of increase in absorption, indicating a gradual
saturation of the adsorbent. This finding underscores the adsorbent's effectiveness in rapidly regulating
humidity levels, particularly in environments with conditions similar to those in the study. It provides
valuable insights into its practical applications for moisture management.
3.3 Heat Transfer
In the modeling layer used in this research, there are four layers consisting of air, cloth, silica gel, and
the air gap between the silica gel and the surface of the clothes. Each of these layers has heat and mass
transfer phenomena, which are calculated according to the conditions and limits determined using Eq. (1)
– (4). Variations used in layer modeling are thickness variations, namely at 0.3 mm and 1 mm.
Figure 4.5 depicts the heat transfer rate () for thickness variations of 0.3 mm and 1 mm in the four
layers. The heat transfer rate curve is known from the convection and conduction heat transfer rates obtained
from previous calculations wherein the phenomenon of convection heat transfer occurs in the first and
fourth layers. In the second and third layers, a conduction heat transfer phenomenon occurs.
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Figure 4. Heat Transfer Rate Comparison
Figure 4 shows the heat transfer rate value obtained in the four layers. In Figure 4. for a thickness
variation of 0.3 mm, it can be seen that there is a decrease in the value from the first layer to the fourth
layer, with each value sequentially in the first to fourth layers being 448, 435.2, 434.13 and 425, 32. For a
thickness variation of 1 mm, it can be seen that there is a decrease in the value from the first layer with a
value of 448 and the second layer with a value of 441.8. The third layer has a value of 441.6, and the fourth
is 435.51, while the second and third values of the heat transfer rate do not change. It is because, in solids,
less heat is transferred. The decrease in the temperature value from the first to the fourth layer causes the
value curve to decrease along with the temperature value in the layer. The reduction in the value is also due
to convection heat being channeled more efficiently than by conduction. Based on the comparison of
variations of 0.3 mm and 1 mm, the greater the thickness of the layer, the higher the value.
4 Conclusion
The research findings revealed key characteristics of RD-type silica gel as an adsorbent in both
isothermal and kinetic adsorption processes conducted at a temperature of 26.84℃. In terms of its
isothermal behavior, silica gel exhibits an impressive adsorption capacity that ranges from 0.0828 kg/kg at
a relative pressure of 0.1 to a remarkable 0.6158 kg/kg at a relative pressure of 0.9. Its performance at a
relative humidity of 60% or a relative pressure of 0.6 is particularly noteworthy, with silica gel type RD
demonstrating a significant adsorption capacity of 0.3867 kg/kg. The kinetic characteristics of silica gel
type RD were derived based on the equilibrium absorption value observed at 60% relative humidity, which
stands at 0.3867 kg/kg. Under these specific conditions, the RD-type silica gel adsorbent displays rapid
water vapor absorption, initially taking up 0.38 kg/kg within a short span of 980 seconds. However, as the
adsorption process continued, there was a noticeable decline in the rate of absorption increase, indicating a
gradual saturation of the adsorbent. These findings underscore the remarkable adsorption capabilities of
RD-type silica gel, particularly in regulating humidity levels in environments resembling those examined
in the study. This research is a pivotal step in advancing the performance of PPE by shedding light on the
adsorption properties of RD-type silica gel. It not only benefits the design and functionality of protective
equipment, but it also plays a vital role in ensuring the safety, comfort, and well-being of those who rely
on it.
Layer
Q-value
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