Quality Air Solutions

Heatless Desiccant Dryers Overview

2

What is Adsorption?Basic Definitions

Adsorption is a process that occurs when a gas or liquid solute accumulates on the surface of a solid or, more rarely, a liquid (adsorbent), forming a molecular or atomic film (the adsorbate).

It is different from absorption, in which a substance diffuses into a liquid or solid to form a solution.

The term sorption encompasses both processes, while desorption is the reverse process

3

What is Adsorption?Basic Definitions

4

Desiccant bead

Pores

Saturated air Air with decreased

moisture content

Heatless Desiccant DryerThe Working Principle - Desiccant

Water molecules are transported into the pores trough diffusion.

Molecules are accumulated on the pore surface due to: – Physical binding

– Chemical binding

– Capillary-condensation

5

Classic commercial adsorbents for water.Basic Definitions

Activated alumina

Molecular sieve (zeolites)

Silica gel

6

Classic commercial adsorbents for water.Basic Definitions

Activated Alumina:is manufactured from aluminum hydroxide by dehydroxydating it in a way that produces a highly porous material; this material can have a surface area significantly over 200 square metres/g.

The compound is used as a desiccant and as a filter of fluoride, arsenic and selenium in drinking water. It is made of aluminum oxide (alumina; Al2O3), the same chemical substance as sapphire and rubies (but without the impurities that give those gems their color).

7

Classic commercial adsorbents for water.Basic Definitions

Silica gel:is a granular, porous form of silica made synthetically from sodium silicate. Despite the name, silica gel is a solid.

Silica gel's high surface area (around 800 m²/g) allows it to adsorb water readily, making it useful as a desiccant (drying agent). Once saturated with water, the gel can be regenerated by heating to 150 °C (300 °F). Some types of silica gel will "pop" when exposed to enough water.

8

Classic commercial adsorbents for water.Basic Definitions

Molecular sieve:A molecular sieve is a material containing tiny pores of a precise and uniform size that is used as an adsorbent for gases and liquids.

Molecules small enough to pass through the pores are adsorbed while larger molecules are not. It is different from a common filter in that it operates on a molecular level. For instance, a water molecule may be small enough to pass through while larger molecules are not. Because of this, they often function as a desiccant. A molecular sieve can adsorb water up to 22% of its own weight.

Often they consist of aluminosilicate minerals, clays, porous glasses, microporous charcoals, zeolites, active carbons, or synthetic compounds that have open structures through which small molecules, such as nitrogen and water can diffuse.

9

Mass Transfer ZoneBasic definitions In an adsorber vessel, the transfer of water from the flow of air to the molecular sieves takes

place in the mass transfer zone (MTZ).

The mass transfer zone is that section of the adsorber bed in which the water load from the air flow is deposited on the drying medium, thereby reducing the level of humidity from the inlet concentration to that at the outlet.

The width of the mass transfer zone is primarily a function of flow velocity, inlet and outlet concentration of the drying medium and the type of drying medium.

The mass transfer zone migrates with increasing saturation from the inlet side to the outlet side of the adsorber vessel. Once the upper limit of the mass transfer zone reaches the end of the adsorber bed, the break-through of moisture commences.

10

Why Use Different Desiccant ?

What Desiccant Do We Use in Heatless Desiccant Dryers?

Heatless Desiccant DryerChoice Of Desiccant

Different desiccants have different levels of water attraction

The greater the attraction, the better pressure dew point is possible, but the more purge is needed to regenerate the desiccant.

Most desiccant cannot provide a -70°C PDP

Desiccant that can provide a -70°C PDP is more expensive

We don’t want the customer to pay for something that’s not needed

For PDP’s of -20 to -40°C PDP Activated Alumina

For PDP’s of -70°C PDP Molecular Sieve

We balance performance with capital investment and ongoing running cost through purge air

For smallest dryers Molecular Sieve is used as due to small amount of desiccant the cost of it is not critical

11

Classic commercial adsorbents for water.Basic Definitions

Equilibrium Adsorption of Water Vapor from Atmospheric Air at 25°C on

(A) Alumina (Granular);

(B) Alumina (Spherical);

(C) Silica Gel;

(D) 5A Zeolite;

(E) (E) Activated Carbon

12

Classic commercial adsorbents for water.Adsorbing capacity

The usable capacity of molecular sieves is largely unimpaired by a rise in temperature. Other drying media, on the other hand, show considerably higher capacity losses with rising operating temperature, as shown in diagram

Within the normal pressure range of the usual industrial compressed air systems, the operating pressure has little significance for the performance of molecular sieves

13

Classic commercial adsorbents for water.Desiccant aging

Adsorbent Aging is a gradual reduction of it’s adsorption capacity.

Two types of aging exist:

- Hydrothermal aging is an irreversible change of adsorbent structure caused by hydrothermal treatment during regeneration, resulting in reduced active area. Aging is therefore dependent on the number of regenerations and quality of the adsorbent. The rate of adsorption capacity reduction is more rapid at the beginning of the adsorbent’s life. With time, the rate stabilizes.

- Aging from contamination caused by coadsorption of undesired species and coke formation on the active surface of the adsorbent. This phenomenon is not completely reversible, and carbon deposits increase with each regeneration

14

What Is A Desiccant Dryer ?Operating Principal (heatless dryers)

Dryer consists of two “towers” of desiccant

Water vapor attracted more to desiccant than the compressed air

Air travels through one tower until desiccant saturated with water vapor

Meanwhile second tower totally regenerated by dry purge air

Outlet flow is therefore around 82% of inlet flow

Dryer consumes almost no electrical energy

15

Air enters the dryer through a series of filters, which are necessary to avoid that the

desiccant becomes poisoned from contamination - which prevents it working

The valve system directs the air into the “live”

vessel only, and is responsible for switching

the towers over when regeneration is complete

The wet compressed air enters the live tower from the bottom. The

desiccant removes the moisture from the air as

it travels upwards

Once the air reaches the top of the tower it is completely dry,

with a typical PDP of -40°F/-40°F

A small amount of the dried air, controlled via a small nozzle, is passed into the regenerating tower

The dryer air leaves the dryer through a filter, which removes any residual desiccant dust. The air is now ready for use in sensitive applications

The purge air enters the regenerating tower from the top. As it travels downwards it removes the water from the desiccant, and finally exists the tower through a silencer

The Towers Switch Over Every 3-6 Minutes Depending On The Model

Heatless Desiccant DryerThe Working Principle

16

Wet air from the compressor flows UP the tower. This means that gravity will encourage the water, already collected from the air, down the tower and away from the dry desiccant. More water exists at the bottom of the tower than at the top.

A small amount of the dried air, is feed through the purge line DOWNWARDS through the “wet” tower. Water is concentrated at the bottom of the tower due to the direction of the wet air.

The downward direction of the purge air means that the bulk of the water in the tower does not have to pass through the bulk of the desiccant. The moisture only has to travel a short distance before it’s ejected.

Heatless Desiccant DryerThe Working Principle - Flow Direction

17

sec.20 40 60 80 100 12030 50 70 90 11010

2

4

6

8

l/s

Desiccant Purging

1/2 CYCLE TIME

Equalisation

The Desiccant Purging part of the cycle is required to dry the desiccant material.

Dry purge air from the live tower removes the water from the desiccant as it passes down through the regenerating tower, where it finally exits via a silencer.

Directly after the regeneration is finished the exit valve is closed and the tower Equalises. This process is required so that when the towers switch over there is not a huge drop in pressure and a shock/pulse against the desiccant.

Actual Purge Consumption

Average Purge Consumption

Heatless Desiccant DryerThe 2 Phases Of The Purge Process

18

The Purge Process - Quantitative SummaryThe Purge Process - Quantitative Summary

Purge Air Used For Drying Desiccant = 18% - 20% of inlet capacity

Purge Air Used To Equalise The Beds = 9% - 10 %

Average Purge Air Used Over Complete Purge Cycle = 15%-18% of inlet capacity

+

=

Heatless Desiccant DryerPurge Loss Calculation - Phase 2 Re-Pressurization

19

No one wants to use more purge air than necessary, but if not enough is used the dryer performance will be unreliable, and in some cases will even cause the pressure dew point to “crash”. If this happens, the desiccant can be ruined.

The CD range of dryers uses a self cleaning, integrated nozzle to control the amount of

purge air used.

The purge on a CD dryer is carefully calculated, and accurately published, to ensure that the dryer provides the quoted performance reliably, even when operating conditions change.

Purge Nozzle

Heatless Desiccant DryerWhy Do We Quote 18% Purge Loss ?

This simple device is not only very reliable, but also maintenance free and self cleaning.

20

Demand

Air demand and purge consumption

0

20

40

60

80

100

120

0 200 400 600 800 1000 1200 1400

Minute of the day

l/s

41 l/s

25 l/s (55%)

8 l/s ( 18%)

102 l/s

25 l/s ( 25%)

18 l/s (18%)

CD 140 Std Purge Flow CD 140 With Purge Control

Heatless Desiccant DryerDewpoint Dependant Purge Control - What Is The Energy Saving ?

Energy is saved through the reduction of purge air.

Purge air is saved during the “waiting” time between the moment that the vessel becomes pressurized, and the moment that it once again starts drying the air

The time is dependant on 4 main parameters:

– Required pressure dew point

– Inlet temperature

– Inlet pressure

– Air demand

21

Heatless Desiccant DryerCut Costs Via The Purge Saver System

Cubicle for Timer Controlled version

For those dryers that do not have Elektronikon there is another way to save energy - using the Energy Management Contact (P4 contact) which is supplied as standard on all Atlas Copco Timer Controlled CD Dryers:

This contact allows the compressor and dryer to be electronically connected, meaning that when the compressor unloads the dry is paused. When the compressor starts again the dryer resumes operation where it left off.

Using this contact will reduce the purge air consumption proportionally with the load cycle of the compressor, significantly reducing running costs.

22

Oil Free Compressor RangeFAD for standard ZT1 units at 8.6 bar (at 50 Hz)

ZT37ZT30ZT22ZT

ZT37 - VSD

Variable speed

2 25 100 300 1050l/s

Complete CD Dryer Range

CD2-17

Compressors 2-350 kW (2.7-470 HP)

CD25-100 CD 110-300 CD 390-1050

Heatless Desiccant DryerA Broad & Deep Range Allows Us To Offer The Optimum Solution

Smallest - CD 2 = 2 l/s or 4.24 cfm

Largest - CD 1050 = 1050 l/s or 2226 cfm

Units available with different PDP’s, options and control systems

All variants available in 11 bar (159 psi) versions

CD2-300 available in 16 bar (232 psi) versions

All variants available 50/60 Hz and 230/110V

Various pressure approvals available - CE, ASME, DIR, SQL, UDT

23

Desiccant beds become saturated more quickly

Either pressure dew point suffers or Inlet flow must

be reduced.

Increase in purge air temperature means that the same amount of purge air can remove more water.

If you de-rate the dryer due to an increased inlet temperature, there is no extra wetness in the desiccant bed. However, the purge air flow remains constant (fixed nozzle) and is warmer, meaning that it has more water removal capacity. Theoretically this means that purge flow could be reduced.

What Does This Mean…...What Does This Mean…...

Heatless Desiccant DryerPerformance Recalculation - Increase Inlet Temperature (but nothing else)

If the inlet flow to the dryer is not reduced, the desiccant becomes “extra” wet, meaning that a -40°C PDP is not maintained. This “extra” wetness can be removed because the purge air is also warmer and therefore has greater water holding capacity

24

The capacity of a dryer is based on the actual flow at inlet, not FAD. Therefore the lower the inlet pressure the less FAD can be processed

The purge nozzle allows a fixed actual flow, regardless of pressure. This means that as pressure decreases the amount of purge available, once expanded to atmospheric pressure, is less.

If the FAD inlet flow is not decreased, the actual volume of air going through the dryer will increase. Therefore the speed of the air through the dryer also increases above the nominal speed which damages the desiccant. As there is more actual air flow through the dryer, the desiccant bed becomes “extra” wet.

Heatless Desiccant DryerPerformance Recalculation - Decrease Inlet Pressure (but nothing else)

What Does This Mean…...What Does This Mean…...

A lower pressure are dryer inlet also reduces the amount of purge air available, with the result that not even a normally wet bed can be regenerated. An extra wet bed will cause the dew point to crash even quicker. The dryer must be de-rated to account for this reduction in purge air.

25

When air passes through a regenerated desiccant bed,

the PDP is -40°C. This is fixed due to the properties

and amount of the desiccant in the vessel

The purge flow is fixed so that at nominal conditions it will remove a fixed amount of water from the desiccant bed.

If nothing else changes, increasing the PDP will not facilitate an increase in flow through the dryer. Although with a DPS version it will increase the “waiting time” between the vessel changeover.

In the event that there is also a higher inlet temperature, an increased PDP will mean a smaller correction factor.

Heatless Desiccant DryerPerformance Recalculation - Increase Required PDP (but nothing else)

What Does This Mean…...What Does This Mean…...

26

DODO

Heatless Desiccant DryerPerformance Recalculation - Using Correction Factors

Remember that everyone is governed by the same laws of physics and the same limitations. There cannot be huge variations in performance from one vendor to another.

Take into account that to a certain degree you can balance a high dryer inlet temperature with a worse pressure dew point. That is, the “de-rating” factor does not have to be so big, if a worse PDP can be tolerated.

Remember that a dryer must be de-rated if the actual inlet pressure is likely to fall below the nominal inlet pressure.

Use the sizing program

Allow the inlet flow, for any model, to exceed the nominal flow

Forget that a worse PDP does not allow an increase in inlet flow, except where the inlet flow is at a higher than nominal temperature

DON’TDON’T

27

Things To Remember About The Selection Program:

Heatless Desiccant DryerPerformance Recalculation - Using Correction Factors

Calculations are extremely accurate and based on actual test data

There is a big safety factor built into the model selection at high temperatures, so for inlet temperatures above 45°C check with Airpower on the sizing. Sometimes there may be possibility to reduce the model size

For inlet temperatures over 50°C molecular sieve desiccant has to be used, as activated alumina will “caramelize” at this temperature

28

Why is it that you cannot Why is it that you cannot up rate the dryer for inlet up rate the dryer for inlet

temperature below temperature below 35°C ?35°C ?

Heatless Desiccant DryerPerformance Recalculation - A Question

29

The Answer:

Heatless Desiccant DryerPerformance Recalculation - A Question

You cannot push more than the nominal flow through the dryer because the pressure drop increases dramatically.

Additionally, the speed of the air increases, meaning that the air is in contact with the desiccant for less time, which results in less effective drying and an increased PDP

The additional air speed also disrupts the desiccant bead and causes “channels” to be formed - resulting in less effective drying. The extra friction also reduces the desiccant lifetime.

Up-rating the dryer for a lower inlet temperature means that after 3 minutes the desiccant bed is saturated as normal. However, the purge air is also cooler than normal, meaning that it has less capacity to hold water. The purge flow (nozzle) is sized based on a purge air temp of 35°C, meaning that if the temperature of the air is only 30°C, it will not have the capacity to dry the desiccant bed.

30

We are committed to your

superior productivity through

interaction and innovation.

31