Industrial Utility Efficiency    

Regenerative Desiccant Compressed Air Dryers

Compressed air is dried to prevent condensation and corrosion which can disrupt manufacturing processes and contaminate products. Water is the primary promotor of chemical reactions and physical erosion in compressed air systems.1 A myriad of desiccant dryer designs have been devised to provide “commercially dry” air, air having a dew point of -40°F or less, to prevent corrosion.2  Desiccant dryers use solid adsorbents in granule form to reduce the moisture content of compressed air.

Adsorbents are miraculous microporous granules with a plethora of nanopore cavities throughout which are too small to be seen even with the aid of an optical microscope.  Larger pores in the 500 to 2,000 angstrom range, macropores, provide access to the nanopores deep within the adsorbent particle. The molecular forces inside the confined spaces of the nanopores are extraordinary exhibiting unusual effects on molecules entering the cavities.3 Natural zeolites, adsorbents formed in acidic lakes near prehistoric volcanic eruptions, may have been the birthplace of our first organic life forms. The catalytic nature of the adsorbent surfaces may have promoted chemical reactions resulting in the formation of the initial ammoniates and amino acids.4

Recent research at the Oak Ridge National Laboratory has revealed that the adsorption process produces a physical change-of-state. Water molecules were found to dislocate upon adsorption and the hydrogen and oxygen atoms reform in a continuous association inside the nanopores.  The adsorbed state is neither gaseous, liquid, or solid, but rather a “quantum tunneling” state discovered by neutron scattering experiments.5 

Synthetic adsorbents have been developed for industrial services to dry and purify air.  These include activated alumina, silica gel, and molecular sieves.  Two adsorbent filled vessels are installed in a desiccant dryer, one for drying the compressed air and one for regeneration.  The vessels are switched from on-stream drying service to off-stream regeneration alternately. 

Alumina

Activated Alumina

Silica

Silica Gel

Sieve

Molecular Sieve

Figure No. 1 Synthetic Desiccants  (Photos courtesy of Porocel)

 

The adsorption process is reversible and the adsorbed water can be desorbed by applying energy into the moisture laden desiccant. The energy source is most often an electric immersion heater, but other energy sources can be more cost effective such as high pressure steam, gas fired heater, solar heating, subterranean geothermal heating, hot air from the compressor discharge, microwave energy, or even the retention of the heat released during the adsorption process.  The distinguishing feature between the various types of desiccant dryers is the method of adsorbent regeneration used to provide continuous service.

The effectiveness of the regeneration process and the dew point achieved are functions of the type of desiccant, regeneration temperature, the purge intake humidity and the operating temperature of the desiccant bed.  The operating temperature augmented by the heat of adsorption is typically 25°F higher than the compressed air inlet temperature.  The regeneration air temperature is usually 400°F or lower, sufficient to desorb moisture from the wet desiccant without degrading the adsorbent. Hydrothermal degradation affects all adsorbents. Activated alumina crystallizes over 500°F in humid conditions becoming nonporous and inert, silica gel loses the hydroxides on its internal surfaces at 740°F in the presence of water vapor becoming inactive, and molecular sieves recrystallize into different less adsorptive forms of zeolite above 840°F.

Desiccant dryers are divided into two classes, pressure-swing regenerative dryers and externally heated regenerative dryers.

Regenerative Desiccant Dryers

Pressure-Swing Dryers Externally Heated Dryers
Heatless Heated Compressed Air Purge
High Pressure Heatless Atm. Blower Wet Air Purge
Heat Assisted Zero Purge Loss
Internally Heated Zero Switch-Over Spike
Figure No. 2    Classification of Regenerative Desiccant Dryers

 

Pressure-Swing Regenerative Dryers

The simplest compressed air desiccant dryer is the heatless dryer.  It is composed of two or more desiccant filled vessels operating side by side, one vessel on-stream drying compressed air at line pressure and the other vessel or vessels off-stream depressurized undergoing regeneration.  More than two vessels are often used in very large systems to reduce the purge consumption in the regeneration process.

 

 HeatlessDryers

Figure No. 3 Heatless Compressed Air Dryer  (Courtesy of Aircel LLC)


Energy is released during the adsorption process as required by the Gibbs phase rule, and the temperature of the adsorbent is raised along with the temperature of the compressed air passing through the interstitial voids between the granules. The heatless dryer operates on a short NEMA cycle to retain the heat of adsorption within the desiccant bed.  The short cycle, five to ten minutes, prevents the emergence of the heat front at the dryer outlet before the vessels are switched over.  The heat retained within the desiccant bed during the drying phase of the process is used during the regeneration phase to desorb the moisture.  The retention of the heat of adsorption is assisted by installing a dense medium with a high volumetric heat capacity at the outlet end of the desiccant bed.  Clinoptilolite, a dense natural zeolite, and tabular alumina have been found to be very effective in retaining the heat within the desiccant bed6.  Steel balls have also been applied for this function.7

Heatless dryers consume about 15% of the dry product air as purge when operating at 100°F and 100 psig to regenerate the desiccant and continue the drying process.  The purge consumption can be significantly reduced by the introduction of a small amount of heat directly into the purge exhaust end of the desiccant vessel.  The heat assisted pressure swing dryer benefits from the elevated water vapor partial pressure in the most contaminated region of the desiccant bed. The higher partial pressure results in a reduction in the quantity of purge required to convey the desorbed moisture from the desiccant vessel. The purge consumption can be reduced to 10% or less even when operating at 100°F and 100 psig with a low wattage heater installed near the purge exhaust end of each desiccant bed.8

Another method used to reduce the purge consumption is to vent the purge exhaust to vacuum.  Reducing the pressure within the regenerating desiccant bed lowers the purge rate required to convey the desorbed moisture out of the vessel.  Vacuum systems are costly, and this method is best considered when a vacuum utility service is available at the installation site.

Heat of Compression Desiccant Dryers Basic Principles

High Pressure Pressure-Swing Dryers

At elevated pressures, 500 psig and higher, the air is sufficiently dense to dissipate the heat of adsorption before the end of the drying process.  No attempt is made to retain the heat in high pressure heatless dryers and they are operated on a longer cycle time, typically thirty minutes to an hour NEMA cycle time.

HighPressureDryer

Figure No. 4 High Pressure Pressure-Swing Dryer  (Courtesy of Aircel LLC)


The required purge rate is decreased as the operating pressure is raised.  High pressure heatless dryers typically operate on 5% dry purge consumption.  The unheated purge air provides the energy needed to regenerate the desiccant bed and a large temperature depression results. The purge exhaust temperature may be as much as 100°F lower than the compressed air inlet temperature.9

 

Internally Heated Regenerative Dryers

Internally heated dryers with axial heating elements inserted into the desiccant beds or band heaters clamped onto the shells of the desiccant containment vessels have been applied successfully to regenerate desiccant beds. Desiccant is a thermally refractive media, and to be effective, the heating surfaces must be close, no more than four inches apart.  Even with this close spacing, the surface temperatures can reach 1000 °F during regeneration and desiccant granules in close proximity to the heating surfaces are gradually deactivated.  Internally heated dryers require about 6% dry purge air to distribute the heat throughout the desiccant bed, convey the desorbed moisture out of the vessel, and to prevent water vapor condensation.  Lower purge rates produce higher vapor pressures in the desiccant bed resulting in condensation on the cool vessel walls. 

The dry air purge and high regeneration temperature can provide very low outlet dew points, less than -100 °F pressure dew point in some cases.  The heat of adsorption is not retained and the dryer operates on a long NEMA cycle time, typically eight hours, four hours drying the compressed air and four hours regenerating at atmospheric pressure.  The desiccant must be changed-out frequently because of the hydrothermal destruction of the adsorbent at elevated temperatures in moist environments.

 

Externally Heated Regenerative Dryers

Externally heated regenerated desiccant dryers are built with a heater installed outside the desiccant vessel to indirectly heat the wet adsorbent.  Purge air is required to convey the thermal energy between the heater and the bed of desiccant.  Regeneration can be accomplished either at line pressure or at atmospheric pressure, but is most effective at low pressure.  When desorption of moisture is complete, unheated purge is required to flow through the desiccant bed to provide cooling. 

The purge flow rate required to accomplish regeneration is determine by an overall heat balance:

        Qp = {Mw [(cp)w (T2 – T1) + Ha] + [Md (cp)d + Mv (cp)v](T3 – T1) + ql} / [ρo (cp)p th (T3 – T2)]

The purge exhaust temperature, T2, can be determined by a mass balance or approximated by the Bud Ginder chart 10:

Regen. Pressure   [psig]

 Ginder Chart

Regeneration Temperature [°F]

310°F

350°F

400°F

450°F

500°F

600°F

150

179

188

197

205

212

223

120

172

180

189

197

203

214

100

167

174

183

190

197

207

80

160

167

176

183

189

198

60

152

160

167

173

179

188

10

117

123

128

133

136

145

5

110

116

121

127

130

136

0

102

107

112

115

120

126

Figure No. 5 Purge Exhaust Temperature as a Function of Regeneration Pressure and Temperature

 

The thermal energy required to heat the purge to the regeneration temperature is found by a heat balance based on the purge exhaust temperature: 

            qh = [Qp ρo (cp)p th (T3 – T2)] + ql

The amount of cooling air required to reduce the desiccant bed temperature to the adsorption bed operating temperature is approximately 1.25 pounds of air per pound of desiccant.  Dry purge air is too costly to provide complete cooling, and most often the dry purge dryers are cooled to about 50%. The fraction of bed cooling provided is determined by a heat balance:

           Fraction of Bed Cooling ≈ [Qc x ρo x tc / (1.25 x Md)]  

 

Externally Heated with Dry Compressed Air

The simplest externally heated compressed air dryer is regenerated with dry air purge both during heating and bed cooling phases of the process.

Dry Purge Air Dryer

Figure No. 6   Externally Heated Dry Purge Air Dryer  (Courtesy of Aircel LLC)


The externally heated dry purge air dryers operate on a long NEMA cycle, typically eight hours. 

The water laden vessel is depressurized for regeneration. The required purge is dependent on the inlet flow rate, temperature and pressure. Higher temperatures and lower pressures increase the water loading on the desiccant by Dalton’s law, and the purge and heat required to accomplish regeneration are increased.10   Purge consumption is reduced approximately 25% by deenergizing the heater early when the heat content in the desiccant bed is sufficient to complete the regeneration process. Continued purge air flow will convey the heat into the unregenerated portion of the bed and the regeneration process will be completed when the cooling front overtakes the heat front at the purge exhaust end of the vessel.  This method results in complete thermal regeneration and total cooling of the desiccant bed.  

Using dry air purge, the residual moisture in the desiccant bed is minimized and the externally heated desiccant dryers with dry purge air can achieve very low pressure dew points, often below -100°F.

The dry purge consumption can be further reduced by including an air injector with a pressure recovering venturi.  The air injector operating on 7% of the dry product air can boost the pressure of ambient air to provide a combine purge air flow rate of 15%.  When the desiccant bed is fully regenerated but hot, the heater is deactivated and the ambient air inlet is closed.  The dry purge air continues to flow through the vessel to partially cool the desiccant bed.

Hot, wet compressed air can be used for desiccant regeneration in an externally heated dryer rather than dry process air to reduce operating costs. The air compressor discharge temperature elevated by the heat of compression to approximately 300°F can be applied directly to the moisture laden desiccant.  The moist compressor discharge dew point is high, typically around 140°F, and to achieve an outlet dew point of -40°F, the air must be heated to 500°F or more.  Additional valves, heat exchangers and an auxiliary immersion heater are required to attain the benefits from the heat of compression dryer.  Compressed air dryers that rely on the heat of compression can be designed for split flow or full flow, and they can be designed as either two vessel systems or as rotating drum systems with partitions to separate the drying, heating, and cooling sections.

 

Externally Heated Dryers with Atmospheric Blower and Wet Air Regeneration

The externally heated dryer with an atmospheric pressure blower can regenerate the desiccant bed providing -40°F dew point “commercially dry” air while operating on an eight hour NEMA cycle with a 400°F regeneration temperature.  Ambient air, though moist at room temperature, is relatively dry once it has been heated to 400°F.  Ambient air at 100°F saturated with water vapor when heated to 400°F has a relative humidity of 0.4% which is quite suitable for desiccant bed regeneration.  An atmospheric pressure blower is installed to provide the regeneration air.  An inlet damper or bypass control valve is used on the blower to maintain the required purge flow rate.  The purge flow is heated in passing through the blower by the frictional heat losses and by the heat of compression.  Typically these result in a temperature elevation of 20 °F to 60 °F.  The purge air is then heated to the regeneration temperature, normally 400 °F, by an immersion heater.  The regeneration temperature is maintained by adjusting the blower intake control valve, decreasing the regeneration air flow rate to increase the temperature, or increasing the flow rate to lower the temperature.  Once the flow rate is set, the control valve will require only occasional adjustment.  The blower adjustment can also be accomplished with a variable frequency drive (VFD) to alter the rotational speed of the motor.

Dryer with Purge
Figure No. 7 Externally Heated Dryer with Atmospheric Blower Purge (Courtesy of Aircel LLC)

 

After heating the desiccant, the vessels are partially cooled either with dry purge air, 2% average of the dryer design flow rate, or with moist ambient air.  When ambient air is used for cooling, the flow direction through the dryer must be reversed to keep the outlet end of the desiccant bed dry and the desiccant bed must be enlarged to account for the adsorption of atmospheric moisture entering the dryer with the cooling air.

Zero purge loss externally heat regenerated desiccant dryers use ambient air for thermal regeneration followed by closed loop bed cooling with dried ambient air.  The regeneration blower is used to circulate the dry air. The flow direction is reversed for closed loop cooling to maintain dry desiccant at the outlet end of the desiccant bed, and an air cooler is installed at the blower inlet to remove the heat conveyed out of the desiccant beds.  The blower intake is left open as there is no flow through the inlet line with closed loop cooling.

Closed loop cooling provides complete desiccant bed cooling and as a result the moisture spikes and temperature elevations that normally occur during the bed switch-over process are minimized.  These can be eliminated altogether by installing a second regeneration air cooler at the blower discharge to remove the heat developed in the blower housing.  Switch-over spikes will not occur when the bed is completely cooled to the dryer operating temperature.

Regenerative desiccant air dryers can be designed and manufactured to meet very stringent service conditions.  In the process of selecting the most appropriate dryer, the design requirements must be thoroughly evaluated to insure that the product will be sufficient and dryer sizing must be confirmed by calculations to assure the adequacy of the design.

 

By Donald White, Chief Engineer, Aircel, email: don.white@airceldryers.com, tel: 865-268-1011, www.Airceldryers.com.

To read more Compressed Air Dryer Technology articles, please visit www.airbestpractices.com/technology/air-treatment.

 

Notations

(cp)p = specific heat of purge air, btu/lb-°F

(cp)d = specific heat of desiccant, btu/lb-°F

(cp)v = specific heat of vessel, btu/lb-°F

(cp)w = specific heat of water, btu/lb-°F

Ha = heat of adsorption, btu/lb of water

Md = mass of desiccant, lbs per vessel

Mv = mass of desiccant vessel, lbs

Mw = mass of adsorbed water per drying period, lbs

qh = total heat required for regeneration, btu

ql = heat losses to atmosphere, btu

Qc = cooling air flow rate, scfm

Qp = purge heating air flow rate, scfm

tc = regeneration cooling time, minutes

th = regeneration heating time, minutes

T1 = temperature of desiccant bed initially, °F

T2 = purge exhaust temperature, °F

T3 = regeneration air inlet temperature, °F

ρo = standard air density, lb/Std.cu.ft.

Literature Cited

  1. White, D., “Why dry compressed air? The harmful effects of moisture”, Engineer’s Digest, p. 21, January (1985).
  2. Chemical Engineers’ Handbook, J.H. Perry, editor, McGraw-Hill Book Co., p.877 (1950)
  3. D.H. White, Jr., “Adsorption Technology – Art or Science”, Pall Corp., Presentation at Badger Engineering, Jan. 10 (1985).
  4. A.G Cairns-Smith, “The First Organisms”, Science Week, pp. 90-100, (1982).
  5. Kolesnikov, A.I., G.F. Reiter, N. Choudhury, T.R. Prisk, E. Mamontov, A. Podlesnyak, G. Ehlers, A.G. Seel, D.J. Wesolowski, and L.M. Anovitz, “Quantum Tunneling of Water in Beryl: A New State of the Water Molecule”, Physical Review Letters 116, 167802 (2016), Pub. April 22, (2016).
  6. D.H. White, W.P. Weber and B.G. McGill, “Sorption Systems with Naturally Occurring Zeolites, and Methods”, U.S. Patent No. US 7,717985 B2, May 18, 2010.
  7. D.M. Ruthven, S. Farooq, and K.S. Knaebel, Pressure Swing Adsorption, VCH Pub., p. 213, (1994).
  8. White, D. H., and P. G. Barkley, “The Design of Pressure Swing Adsorption Systems,” Chemical Engineering Progress, p. 30, (1989).
  9. R.T. Yang, Gas Separation by Adsorption Processes, Butterworths Pub., p.251 (1987).
  10. P.D. Marsh, B. McGill, D. White, Jr., “Heat Reactivated Desiccant Compressed Air Dryers”, Donaldson Co., Inc., pp. 6 and 7, May 20 (2005).