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Information

 

On this page we want to answer your questions in detail. We will concentrate on matters which could also be of interest to others.

  

Are there differences between silica gel and molecular sieve in the adsorber?

One experiment demonstrated the difference between silica gel orange-green and molecular sieves 4A and 13X when used in an adsorber. For the test, 0.5 kg each of the desiccant were tested at a mass flow of 46.6 g / h. This is calculated using a volume flow of 50 l / min, a temperature of 20 ° C and a relative humidity of 90%. Due to the lack of a moisture indicator, key figures for the color changes are determined for the molecular sieves.

The behavior of the breakthrough curve of molecular sieves differs from that of the silica gel. This has a value of approximately 0 for low loadings. Towards the maximum load, the breakthrough curve of the molecular sieves increases steeply up to the value 1. The steeper rise of the breakthrough curve suggests a more complete loading of the molecular sieve.

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What influence does relative humidity have on the adsorber?

At a constant temperature of 20 ° C, 0.5 kg of silica gel Orange was flowed through with a volume flow of 50 l / min. The relative humidity varies between 90%, 70%, 50% and 30%. The mass flow reduces with decreasing relative humidity.

Table: Set mass flows and their calculation

Volume flow [l/min]

relative humidity [%]

Absolute humidity [g/m³]

Mass flow

[g/h]

50

90

15,6

46,6

50

70

12,1

36,2

50

50

8,6

25,9

50

30

5,2

15,5

 

What influence does the diameter of an adsorber have?

The influence of the speed inside the adsorber was tested two diameters of 110mm and 150mm. For both adsorbers, the mass of the used silica gel orange is exactly 0.5 kg. This is flowed through at a temperature of 20 ° C, a relative humidity of 90% and a flow rate of 50l / min. The mass flow is thus 46.6g / h. The calculated velocities inside the adsorbers are listed in the table below. Increasing the adsorber diameter from 110 mm to 150 mm halves the speed inside the adsorber.

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What influence does the air flow have on the use of an adsorber?

0.5 kg each of the silica gel orange were perfused with 90% relative humidity and various air streams or mass flows. At a temperature of 20 ° C, the absolute humidity is calculated to be 15.6 g / m³. The calculated volume flow varies between 50l / min, 100l / min and 150l / min. According to equation 11 on page 26, the mass flow is directly dependent on the volume flow. The calculated volume and mass flows as well as the empty tube velocities are listed in the table.

An increase in the mass flow, ensures a higher adsorptive concentration in the bed, which accelerates the adsorption process.

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Are there differences between silica gel and molecular sieve in the adsorber?

One experiment demonstrated the difference between silica gel orange-green and molecular sieves 4A and 13X when used in an adsorber. For the test, 0.5 kg each of the desiccant were tested at a mass flow of 46.6 g / h. This is calculated using a volume flow of 50 l / min, a temperature of 20 ° C and a relative humidity of 90%. Due to the lack of a moisture indicator, key figures for the color changes are determined for the molecular sieves.
 
The behavior of the breakthrough curve of molecular sieves differs from that of the silica gel. This has a value of approximately 0 for low loadings. Towards the maximum load, the breakthrough curve of the molecular sieves increases steeply up to the value 1. The steeper rise of the breakthrough curve suggests a more complete loading of the molecular sieve.


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What influence does the amount of desiccant have on the adsorber?

An attempt was made to answer what effect the amount of desiccant on the function of the adsorber in use. Two adsorbers were filled with 0.25kg, 0.5kg, 1kg and 2kg silica gel orange and tested. The mass flow of 46.6 g / h is calculated from a temperature of 20 ° C, a relative humidity of 90% and a volume flow of 50 l / min.

The load curves show a temporary variation, so that the increase of the load curves decreases. The adsorbent masses of 1kg and 2kg kilograms keep the initial moisture to a minimum at low loads. In this area, the moisture is adsorbed almost completely.

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Which color indicators are used?

Many silica gels are provided by the manufacturer with a low level of moisture indica- tors. They indicate a loading condition based on a color change. The moisture indicators for silica gels are divided into two types:
·         pH-Indicators
·         Metal salts

The pH indicators work on the principle of neutralization. When dry, the indicator is surrounded by an acidic or alkaline environment. The milieu is generated by the indicator or an additionally applied acid or base. The entry of water sets the environment to a neutral pH. This change in pH results in a color change of the moisture indicator.
 

Valves in the adsorber

In light of the fact that many aeration dryers are not permanently flown through, the question arises as to whether the aeration dryers still adsorb moisture and how they can be protected from this unnecessary loading. This is especially interesting for ventilation dryers, which are not flowed through during most of the time of use. If there is no flow, moisture diffuses through the adsorbent bed. It comes to a loading and a color change, although the aeration dryer was only flowed through. To counter the presumption, GIEBEL worked on various solutions.

In the experiment described below, three identical aeration dryers are tested with closed connection openings. These are filled with the same amount of silica gel orange. They have different openings in the ground:

·         Holes (standard for cheap solutions)
·         Holes covered with a hydrophobic membrane
·         Valves with an opening pressure of 6mbar


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Basics about ventilation dryers

Aeration dryer, also called respiratory dryer, silica gel filter or adsorber, which protects against water and dirt. Main applications are the reward of hydraulic fluids, lubricants, fuels and electrical systems. These systems are entitled to a nice moisture and clean air. In systems with liquids, water that has been washed out can lead to mechanical parts or even failure of the system. Reasons for this are:

·         the promotion of fluid aging 
·         the deterioration of the lubricating effect
·         the corrosion of components
·         the formation of decomposition products.


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Basics of adsorption

The term adsorption is divided into gas phase and liquid phase adsorption. The eponymous difference lies in the aggregate state of the fluid phase. For aeration dryers, gas phase adsorption is of greater importance. This form of adsorption is characterized by a gas or a gas mixture as the fluid phase. This gas or gas mixture contains the adsorbing atoms or molecules.

This been with the control "Adsorptive" designation, see picture. The surface which adsorbs the adsorbent is the "adsorbent". This can be a liquid as well as a solid. For the dehumidification, a solid is used as the adsorbent.


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Gradient in the adsorber

As the load increases, the moisture indicator changes color. The gradient of a silica gel bed with moisture indicator is divided into three zones:

·        "Unloaded" is the zone with the original color of the moisture indicator.
·        "Loading" is the zone with the discolored moisture indicator.
·        "Color Change Zone" is the zone with a mixed color of the original and the loaded color.

The color change zone FWZ is the transition zone between the "unloaded" and the "loaded" zone, see figure. The color impression arises from a mixture of unloaded and loaded indicator.

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Use of hydrophobic PTFE membranes for water separation

Polytetrafluoroethylene or PTFE is a synthetic fluoropolymer of tetrafluoroethylene. This material is known for its chemical resistance, thermal stability and excellent hydrophobicity. PTFE is a simple polymer consisting of the two elements carbon and fluorine. The covalent compounds C-C and C-F are extremely strong.

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Function - Be-Defluxer better than Aerators

The use of a ventilation dryer is used to protect a hydraulic fluid, a gear oil, a fuel or other hygroscopic fluid. The sucked air is "pulled" by a desiccant. Due to the van der Waals forces of the contained adsorbent, the water molecules diffuse into the pores and adhere to the surface. This pro less is called adsorption. In contrast, the absorption describes acompound / mixture of two substances.

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Fundamentals for the regeneration of silica gel

Silica gels have an amorphous, disordered microstructure and thus a broad pore size distribution. Because of its polarity silica gels is a hydrophilic adsorbents, which is reflected in the affinity for dipolar molecules, such as water molecules. Because silica gel based on a physical adsorption, the water is bound by the not very strong van der Waals forces at the surface. The adsorbed water remains chemically unchanged and can be easily dissolved again. 

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Regeneration of silica gel in the normal household oven

After drying, the silica gel that is very hot and should be filled into a suitable container and sealed airtight as possible.
The cooling on the baking sheet under contact with the ambient air will loading the silica gel again and greatly limit the later use in the adsorber.
A prerequisite for the regeneration of a loaded silica gel in a domestic oven is that no oil or other substances are on the surface and the regeneration temperature is observed.

1. Preparations
Set a temperature of 120°C and circulating air to ventilate the silica bulk as well as possible.
The silica gel should be spread as flat as possible in a baking sheet. The flatter the bed, the better the flow through with hot air and the faster the regeneration will take place.

2. Process
Push the baking sheet with as much distance to each other into the oven.
The regeneration time until completely dry depends on the bed height of the bulk and is calculated as follows:

Silica gel amount on a sheet

Regeneration time

0,5 kg

appr. 90 min.

1,0 kg

appr. 180 min.

1,5 kg

appr. 270 min.

2,0 kg

appr. 360 min.


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Is silica gel with color indicator toxic and need labeling as hazardous?

It depends on the color indicator. According to European Union legislation pure silica gel, molecular sieve and alumina are not classified as hazardous. Furthermore, they are non-flammable and chemically stable. In case of appropriate handling and use no ecological problems are to be expected. If these adsorbents used in breather dryers, they are often coated with a color indicator.
 
Especially silica gel is available with various color indicators that indicate the water content above a certain weight. Some color indicators are classified as toxic and hazardous to health. Because of these color indicators some silica gels have to classify as hazardous.

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Is silica gel suitable for the aeration of ethanol tanks and is silica gel resistant to ethanol?

 
Yes, silica gel is reBasics of adsorbtionsistant and can be used for the ventilation of ethanol tanks. Silica gel is used among other things in the thin layer chromatography for the drying of ethanol. It is one of the polar adsorbents and be carried out at the 90% of all separations in the thin layer chromatography. It is a porous and amorphous silica dioxide which having a large internal surface. Separately, moderately polar analytes in organic solvents (e.g. ethanol) are solubled.

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GIEBEL FilTec GmbH

Carl-Zeiss-Str. 5 / 74626 Bretzfeld-Schwabbach
Phone: +49 7946 944401-0
E-Mail: info@giebel-adsorber.de

 

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GIEBEL FilTec GmbH
Phone: +49 7946 944401-0
E-Mail: info@giebel-adsorber.de