Brian J. Motil and Henry K. Nahra

Glenn Research Center, Cleveland, Ohio

Vemuri Balakotaiah

University of Houston, Houston, Texas

Hydrodynamics of Packed Bed Reactor

in Low Gravity

NASA/TM—2005-213806

August 2005

2005–01–3035
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Brian J. Motil and Henry K. Nahra

Glenn Research Center, Cleveland, Ohio

Vemuri Balakotaiah

University of Houston, Houston, Texas

Hydrodynamics of Packed Bed Reactor

in Low Gravity

NASA/TM—2005-213806

August 2005

National Aeronautics and

Space Administration

Glenn Research Center

Prepared for the

35th International Conference on Environmental Systems (ICES)

cosponsored by the SAE, AIAA, AIChE, ASME, and the

International ICES Committee

Rome, Italy, July 11–14, 2005

2005–01–3035
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NASA/TM—2005-213806 1

Hydrodynamics of Packed Bed Reactor in Low Gravity 

Brian J. Motil and Henry K. Nahra 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Vemuri Balakotaiah 

University of Houston 

Houston, Texas 77002 

Abstract 

Packed bed reactors are well known for their vast and 

diverse applications in the chemical industry; from gas 

absorption, to stripping, to catalytic conversion. Use of this 

type of reactor in terrestrial applications has been rather 

extensive because of its simplicity and relative ease of 

operation. Developing similar reactors for use in microgravity 

is critical to many space-based advanced life support systems. 

However, the hydrodynamics of two-phase flow packed bed 

reactors in this new environment and the effects of one 

physiochemical process on another has not been adequately 

assessed. Surface tension or capillary forces play a much 

greater role which results in a shifting in flow regime 

transitions and pressure drop. Results from low gravity 

experiments related to flow regimes and two-phase pressure 

drop models are presented in this paper along with a 

description of plans for a flight experiment on the International 

Space Station (ISS). Understanding the packed bed 

hydrodynamics and its effects on mass transfer processes in 

microgravity is crucial for the design of packed bed chemical 

or biological reactors to be used for water reclamation and 

other life support processes involving water purification. 

Introduction 

The typical operation of a packed bed reactor (PBR) 

involves the simultaneous liquid and gas flow that on ground 

can be co-current or counter-current. The packing can be of 

different shape and materials depending on the application. For 

chemical reactions, the packing usually serves as a catalyst. 

For biological reactions, the packing can serve as a structural 

support for biofilm growth. For physical processes such as 

absorption or leaching, the packing provides for gas-liquid 

phase interaction and extended contact in order to achieve the 

highest mass transfer rates. 

The results of a recent reduced gravity aircraft experiment 

will be compared in this paper to a previously published 

microgravity model. The model predicts flow regime 

transitions and pressure drop for spherical packing. The model 

was initially validated with a small scale rectangular column 

and glass beads. The latest set of flights involved a realisticscale cylindrical bed with a common packing material 

(aluminum oxide spheres). The results will be compared in a 

preliminary fashion in order to expand the validation of the 

model to a different bed geometry and packing material. A 

critical validation still remains for flows that require longer 

time scales (> 20 seconds) than are available on the aircraft. 

Testing the PBRE within a long duration microgravity 

environment is the next critical step to validating the 

hydrodynamic models and this will be addressed by a planned 

ISS flight experiment. The fully validated models will enable 

designers to predict reactor bed performance in space (transit 

missions) directly or establish a “0-g boundary condition” to 

develop the predictive framework for planetary operations 

(moon or Mars). 

Experiments on NASA’s Low Gravity 

Facility 

Experimental System 

A simplified flow schematic of the experimental apparatus is 

illustrated in figure 1. The experiment was designed to fly on 

NASA's low gravity aircraft, which is capable of flying 

parabolic trajectories to create a reduced-gravity environment 

for approximately 20 seconds. Acceleration levels were 

recorded in three mutually orthogonal axes (one coinciding 

with the direction of flow) and typically averaged less than 

0.01 m/s2

 during the 20-second “low-g” period. Gas was 

provided to the test section by a compressed air cylinder and 

was controlled by regulating the pressure upstream of an 

orifice. Two different sized orifices were selected to maintain 

choked flow conditions for all flow rates and were verified 

with a mass flow meter before each test day. Liquid flow was 

maintained by applying air pressure to a piston, which in turn 

forced the liquid through a meter and into the test section. A 

pressure-driven piston, rather than a pump, is necessary during 

low gravity to ensure steady liquid flow. The two phases were 

mixed prior to the test section and then introduced into the 

column through a coarse mesh screen. To maintain a steady 

pressure at the outlet of the test section, a two-phase 
NASA/TM—2005-213806 2

Figure 1.—Schematic of the aircraft flow loop. 

separator/collector tank was used to retain the liquid phase 

while venting the gas (air) to the cabin via a back-pressure 

regulator. Average pressures within the packed bed test section 

were held between 100 to 200 kPa. Between trajectories, the 

liquid was pumped back to the supply tank.

First campaign.—Two test sections were flown in two 

separate campaigns on board of the two phase flow rig. In the 

first campaign, a clear rectangular polycarbonate column with 

a cross section of 2.54 cm by 5.08 cm and 60 cm long was 

used for the test section. The fluid viscosity was varied 

between 1-20 cP, the surface tension from 60 to 72 dynes/cm, 

and the liquid density from 1 to 1.2 g/cm3

. Spherical glass 

packing was used in the column. Two packing diameters of 

2 and 5 mm were tested in the first campaign. Five flushmounted differential pressure transducers were spaced at even 

intervals with the first location approximately 4 cm from the 

inlet port and the subsequent pressure transducers spaced at 

13 cm intervals along the column. In addition, absolute 

pressure transducers were located at the first and last positions. 

To observe pulse formation and characteristics adequately, the 

data from the differential pressure transducers were acquired at 

1000 Hz. Flow rates, temperatures, and absolute pressures 

were recorded at 1 Hz. Visual observations were recorded 

using a high-speed SVHS video system at 500 frames per 

second. The column was randomly packed with spherical glass 

beads of identical diameter by slowly dropping the beads into 

the top of the bed. Handling and shaking the bed was kept to a 

minimum prior to the aircraft flights, but the vibration and 

alternating 0 to 1.8-g environment created a more tightly 

packed bed. The void fraction was measured by comparing the 

dry weight to the weight of the bed filled with water (recorded 

after the flight). Typically, the beads settled after only several 

parabolas leaving a gap of about 2 to 3 cm on the top of the 

column. The average void fraction was found to be 0.345. As 

expected, this was slightly lower than the normal range of 

0.36 to 0.43 reported in Kaviany (1995), but is consistent with 

the findings of Tosun (1984). 

Second campaign.—In the second campaign, the geometry 

of the packed bed changed in addition to changing the packing 

materials. The column geometry in the second flight was 

cylindrical, with 7.62 cm (3 in.) diameter and 91.4 cm (36 in.) 

length. The first pressure transducer was 12.7 cm (5 in.) from 

the column inlet and the subsequent transducers were separated 

19.1 cm (7.5 in.) from each other. The materials of the tested 

packing were glass and alumina (aluminum oxide), with the 

packing having the same diameter of 3 mm. The liquid 

properties were constant since only water was used in the 

column. Bed settling due to acceleration changes was nearly 

similar for the circular column. The data acquisition system 

was similar to the first flight as well as the high-speed camera. 

Table 1 compares the two low gravity experiments. Figure 2 

shows the hardware flown on NASA’s low gravity platform. 

TABLE 1.—RANGE OF PARAMETERS BETWEEN THE 

TWO EXPERIMENTS CONDUCTED ON THE 

NASA’S LOW GRAVITY PLATFORM. 

First 

campaign 

Second 

campaign 

Parameter Range 

 Liquid Density ρL 1000 to 1200 kg/m3

 1000 kg/m3

 Gas Density ρG 0.95 to 2.3 kg/m3

 0.95 to 1.4 kg/m3

 Liquid Viscosity μL 0.001 to 0.02 kg/m s 0.001 kg/m s 

 Gas Viscosity μG 1.8×10–5 kg/m s 1.8×10–5 kg/m s 

 Liquid Surface Tension σL 0.060 to .072 kg/s2 0.070 kg/s2 

 Packing Diameter dP 0.002 and 0.005 m 0.0035 m 

 Packing Materials Glass 

(spherical beads) 

Aluminum oxide 

(semi-spherical) 

 Bed Porosity ε 0.34 0.35 
NASA/TM—2005-213806 3

Figure 2.—Photograph of the two-phase aircraft rig. 

Hydrodynamic Model 

Pressure Drop 

Pressure drop is an important parameter in designing a gasliquid reactor bed. Not only does it affect the energy required 

to operate the bed, but it is often used to correlate the mass 

transfer from either the gas to liquid or the liquid to solid 

phases (Reiss, 1967; Gianetto et al., 1978). 

In the literature, pressure drop models based on 

superimposing the kinetic and viscous energy losses (Erguntype equations) have worked reasonably well for gas-liquid 

flows by restricting use to the gas-continuous trickle flow 

regimes and neglecting the effects of gravity and interactions 

between the gas and liquid phases. In the absence of gravity, 

the surface tension forces become increasingly dominant as the 

liquid flow rates are decreased. Furthermore, neglecting 

gravity effects are justified even for the liquid-continuous 

bubbly and pulse regimes. Motil et al., 2003 extended this 

approach for microgravity by superimposing an additional 

“dynamic interaction” term resulting in: 

( )

3

2

2

1 3 * 2 * *

1

1 150 0.8 Re 1.75

Re Re

 

 

P

TP

L LS

L

GS

LS LS

Dynamics Interaction

P d f Z U

Su

−∆ ε = ρ − ε

⎡ ⎤

⎢ ⎥ ⎛ ⎞

=+ + ⎜ ⎟ ⎜ ⎟ ⎝ ⎠

⎣ ⎦

14444244443

 

 (1) 

where 

( ) *2 2 *

1 , 

Re

L L p

L

LS L LS

Su d

Su

We

ρ σ = = μ

The dynamic phase interaction term is indicated in 

equation (1). In the limit of zero interfacial tension between 

fluids (σ ≈ 0) , it reduces to the single phase Ergun equation 

(dynamic interaction term goes to zero) and in the limit of no 

gas flow ( ) Re 0 *

GS = , it again reduces to the single phase 

Ergun equation. In the inertia dominated limit ( ) Re 1 *

LS >> , 

the friction factor becomes independent of the interfacial 

tension and viscous terms and approaches the same asymptotic 

solution as the inertia dominated single phase solution. 

Sample plots of equation (1) are shown in figure 3 for two 

different Suratman numbers. The data points were generated 

from the first aircraft flight campaign. The Suratman number 

was varied by changing the packing size and liquid viscosity. 

Agreement is good for all flow ranges including pulse and 

bubbly flow. The scatter is small compared to other existing 

correlations for normal gravity two-phase pressure drop. 

1

2

4

6

8 10

2

4

6

8 100

2

4

6

8 1000

f

TP

1 10 100 1000 Re*LS

Single Phase Ergun Equation

Re*GS = 33

Re*GS = 65

Re*GS = 99

Re*GS = 131

Re*GS = 180

Re*GS = 267

SuL = 900

1

2

4

6

8 10

2

4

6

8 100

2

4

6

8 1000

f

TP

1 10 100 1000 Re*LS

Single Phase Ergun Equation

Re*GS = 13

Re*GS = 26

Re*GS = 40

Re*GS = 53

Re*GS = 72

Re*GS = 106

SuL = 146,000

Figure 3.—Experimental validation of 

pressure drop in microgravity. 
NASA/TM—2005-213806 4

In these experiments, pressure drop increases as the square 

root of the gas Reynolds number (or essentially the gas flow 

rate). This relationship holds reasonably well over the wide 

range of liquid velocities and viscosities. Figure 4 is a 

qualitative illustration of the effects of gas and liquid flow 

rates on the pressure drop. The lowest curve represents the 

Ergun equation for the single (liquid) phase where the viscous 

regime obeys the Darcy relationship ∆P U~ LS . As the liquid 

flow is increased there is a “transition region” and then, at very 

high liquid flow rates, the Burke-Plummer (inertia regime) 

relationship 2 ~ LS ∆P U dominates. The effect of increasing the 

gas flow increases the pressure drop by the dynamic 

interaction relationship 1/2 1/3 ~ GS LS ∆ + PU U . This term 

dominates the pressure drop at the lower liquid flow rates, but 

also becomes negligible along with the viscous term as the 

liquid flow rate is increased into the liquid inertia regime. 

Preliminary Scaling 

As mentioned earlier, one of the objectives of the second 

flight campaign was to validate the pressure drop model using 

a more realistic packing (semi-spherical alumina) and a bed 

scaled to the size of an expected biological or chemical reactor 

unit. The model showed excellent agreement with the 

experimental two-phase friction factor as illustrated in the 

parity plot in figure 5. 

Future Plans for a Flight Experiment 

Justification 

NASA considers packed bed technology as a unit operation 

for several advanced life support systems. Examples of packed 

bed reactors include the Volatile Removal Assembly (VRA), 

the Biological Water Processor (BWP), and the Sabatier 

Reactor. NASA is currently developing air and water systems 

with reactor beds for ground and flight-based testing without a 

fundamental understanding of how the reduced gravity 

environment affects the performance and reliability of the 

reactors. This is especially critical when gas and liquid flows 

are involved. The expected outcome of this research effort is to 

develop a set of guidelines and tools to enable engineers to 

reliably design and operate fixed packed bed reactors for 

microgravity as well as the lunar and Martian environments. 

The flight experiment will provide critical hydrodynamic 

information for a project with a broader scope, which includes 

reduced gravity aircraft and ground-based (1-g) experiments. 

The main objective of the overall project is to develop and 

validate macroscopic equations that can be used in partial and 

microgravity conditions to accurately predict flow pattern 

transitions; pressure drops; and chemical and biological 

Increasing Gas Flow Rate

No Gas Flow

Viscous Regime

Transition

Inertia Regime

Liquid Velocity

Pressure Drop

Region

P ~ ULS + (U1/2

GS) U1/3LS

P ~ U2

LS

Figure 4.—Qualitative effect of gas 

flow rate on pressure drop. 

140

120

100

80

60

40

20

Experimental

20 40 60 80 100 120 140

Calculated

Dimensionless Two-Phase Friction Factors (f

TP)

Figure 5.—Comparison plot for 

second flight campaign. 

transport rates in gas-liquid flows through randomly packed 

beds. The hydrodynamic investigations discussed in this 

document will focus on the transitions between flow regimes 

(i.e., bubbly-to-pulse flow) and the associated pressure 

gradients for each flow regime over the range of relevant test 

parameters (e.g., liquid flow rates, gas flow rates, and particle 

sizes). These design tools will provide important information 

for specific water reclamation and air revitalization 

technologies in NASA’s Advanced Life Support 

(ALS) program. 
NASA/TM—2005-213806 5

There are several approaches to predicting the reaction rates 

(mass transfer) of a fixed PBR, but all are based on liquid (or 

gas) holdup within the reactor. Initial microgravity tests 

conducted in the reduced gravity aircraft and drop tower 

clearly show that the liquid holdup is significantly influenced 

by gravity (Motil et al., 2003). Furthermore, most of these 

models have only been verified using the trickle flow regime 

(not found in microgravity). Basic hydrodynamic models 

developed specifically for reduced gravity have been partially 

validated within the constraints of the aircraft and ground 

facilities. However, typical flow ranges used for real water 

recovery systems require up to 2 to 3 minutes to fully develop. 

Testing on the ISS is required for complete validation of the 

semi-empirical models. 

Science requirements in brief.—A Packed Bed Reactor 

Experiment (PBRE) is planned for flight on ISS in 2008. The 

experiment consists of several packed beds. Two of the beds 

are designated as the Fundamental Hydrodynamics Study 

(FHS) reactors. The purpose of these reactors will be to 

produce a full understanding of the packed bed hydrodynamics 

embodied by the generation of design guidelines and 

correlations for designing and sizing a packed column for a 

particular application. Two beds will be tested with wetting 

and non-wetting packing materials. Next, a hydrodynamic 

simulation of the Volatile Removal Assembly (VRA) packed 

bed will be tested under standard temperature and pressure. 

The actual VRA operates at 130 C and 450 kPa. Finally, three 

beds will simulate the hydrodynamics for a Biological Water 

Processor (BWP). These beds will use a material such as 

BioBaleTM with varying degrees of porosity to simulate 

biological growth. Increases in pressure drop and the 

distribution and retention of the gas phase will be carefully 

monitored. 

TABLE 2.—CANDIDATE PACKED BEDS 

AND THEIR CHARACTERISTICS 

Bed ID 

in (cm) 

Length 

in (cm) 

Packing 

Size 

in (cm) 

Packing 

Material 

FHS-W 2.5 

(6.4) 24 (70) 0.079 

(0.2) 

Glass 

(spheres) 

FHSNW 2.5 

(6.4) 24 (70) 0.079 

(0.2) 

Teflon 

(spheres) 

VRA 1.4 

(3.6) 

20 

(51) 

0.065 

(0.165) 

Glass 

(spheres) 

BWP–1 2.0 

(5.1) 

5 

(12.7) NA 

Bio BaleTM – 

0% simulated 

growth 

BWP–2 2.0 

(5.1) 

5 

(12.7) NA 

Bio BaleTM – 

50% simulated 

growth 

BWP–3 2.0 

(5.1) 

5 

(12.7) NA 

Bio BaleTM – 

80% simulated 

growth 

Tolerances on: 

x = ± 0.1 inches (0.254 cm) 

x.x = ± 0.03 inches (0.076 cm) 

x.xxx = ± 0.001 inches (0.00254 cm) 

Flow rate requirements.—The flow rates required for the 

PBRE vary depending on the particular bed considered. For 

the FHS, since we planning to study the hydrodynamics over a 

wide range of flow rates, the variations required in mass flows 

are the greatest. For VRA on the contrary, the flow rates are 

very small as adjusted for ambient temperature and pressure. 

Table 3 shows the range of the gas flow rates for the different 

beds, and Table 4 shows the liquid flow rates. 

TABLE 3.—MASS FLOW RATE REQUIREMENTS 

OF THE DISPERSED PHASE FOR THE PBRE 

Bed Minimum

(kg/hr) 

Maximum

(kg/hr) 

Maximum 

increments between 

set points 

(kg/hr) 

Control 

(± kg/hr) 

FHS 0.05 5 

0.05 for < 1 kg/hr 

0.1 for > 1 kg/hr 

0.05 for < 1 kg/hr 

0.1 for > 1 kg/hr 

VRA[1] 0.0006 0.003 0.0003 0.0003 

BWP 0.001 0.5 

0.001 for < 0.01 

kg/hr 

0.01 for 0.01 to 0.1 

kg/hr 

0.1 for > 0.1 kg/hr 

0.001 for < 0.01 

kg/hr 

0.01 for 0.01 to 0.1 

kg/hr 

0.1 for > 0.1 kg/hr 

TABLE 4.—VOLUMETRIC FLOW RATE REQUIREMENTS 

OF THE LIQUID PHASE FOR THE PBRE 

Bed 

Minimum 

flow rate 

(liters/hr) 

Maximum 

flow rate 

(liters/hr) 

Control 

(± liter/hr) 

FHS 10 200 1 

VRA 4 10 0.5 

BWP 1 20 0.5 

The requirements also address the accuracy of 

measurements, the thermal and pressure control and the 

microgravity environment. Moreover, the phase distribution 

will be measured using tomography techniques throughout the 

column as well as liquid film thickness measurements which 

will be achieved using tiny impedance probes embedded into 

the packing pellets. 

In the PBRE, the packed bed hydrodynamics will be 

evaluated without the drawbacks of the NASA’s low gravity 

platform limitations. The pressure drop data outcomes will 

further improve the development of the pressure drop model 

because the low gravity time limitation is not an issue any 

longer and thereby, steady flows will be more readily 
NASA/TM—2005-213806 6

achievable. Moreover, the hydrodynamics results will further 

define the flow map regimes and flow transitions. The BWP 

and VRA results will further impact the performance 

characteristics of the corresponding packed beds. Overall, the 

PBRE results will provide to designers design correlation for 

pressure drop and flow regimes that can be used to design 

packed beds for advanced life support. 

Conclusion 

We addressed in this paper the latest development in packed 

bed hydrodynamics and the findings related to the 

hydrodynamics of packed bed and the future plans for 

developing the current findings into more accurate correlations 

for packed bed designers for long duration space missions. We 

summarized in this paper the low gravity platform findings 

connected with two-phase friction factor, although flow maps 

and transition predictions have been addressed in great details 

by Motil et al. 2003. Then, we presented the plans for a Packed 

Bed Flight Experiment PBRE with the objective to generate 

hydrodynamics design correlations and performance data 

for the design and sizing of future low or microgravity 

packed beds. 

References 

Gianetto, A.; Baldi, G.; Specchia, V.; Sicardi, S., “Hydrodynamics 

and Solid-Liquid Contacting Effectiveness in Trickle-Bed 

Reactors,” AIChE J., 24 [6] (1978) 1087–1104. 

Kaviany, M., Principles of Heat Transfer in Porous Media, 2nd 

edition, Springer-Verlag, New York, 1995. 

Motil, B.J., Balakotaiah, V., and Kamotani, Y., “Gas-Liquid TwoPhase Flows Through Packed Beds in Microgravity,” AIChE J., 49 

[3] (2003) 557–565. 

Reiss, L.P.; “Cocurrent Gas-Liquid Contacting in Packed Columns,” 

Ind. Eng. Chem. Process Des. Dev., 6 [4] (1967) 486–499. 

Tosun, G., “A Study of Cocurrent Downflow of Nonfoaming GasLiquid Systems in a Packed Bed. 1. Flow Regimes: Search for a 

Generalized Flow Map 2. Pressure Drop: Search for a Correlation,” 

Ind. Eng. Chem. Process Des. Dev., 23 (1984) 29–39. 
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NASA TM—2005–213806

2005–01–3035

E–15145

WBS–22–101–53–09

12

Hydrodynamics of Packed Bed Reactor in Low Gravity

Brian J. Motil, Henry K. Nahra, and Vemuri Balakotaiah

Packed bed; Bioreactor; Hydrodynamics

Unclassified -Unlimited

Subject Categories: 29 and 34

Prepared for the 35th International Conference on Environmental Systems (ICES) cosponsored by the SAE,

AIAA, AIChE, ASME, and the International ICES Committee, Rome, Italy, July 11–14, 2005. Brian J. Motil and

Henry K. Nahra, NASA Glenn Research Center; and Vemuri Balakotaiah, University of Houston, Downtown,

1 Main Street, Houston, Texas 77002–1014. Responsible person, Brian J. Motil, organization code RUF,

216–433–6617.

Packed bed reactors are well known for their vast and diverse applications in the chemical industry; from gas

absorption, to stripping, to catalytic conversion. Use of this type of reactor in terrestrial applications has been rather

extensive because of its simplicity and relative ease of operation. Developing similar reactors for use in microgravity

is critical to many space-based advanced life support systems. However, the hydrodynamics of two-phase flow packed

bed reactors in this new environment and the effects of one physiochemical process on another has not been adequately

assessed. Surface tension or capillary forces play a much greater role which results in a shifting in flow regime

transitions and pressure drop. Results from low gravity experiments related to flow regimes and two-phase pressure

drop models are presented in this paper along with a description of plans for a flight experiment on the International

Space Station (ISS). Understanding the packed bed hydrodynamics and its effects on mass transfer processes in

microgravity is crucial for the design of packed bed chemical or biological reactors to be used for water reclamation

and other life support processes involving water purification.