S.D. Vlaev D. Georgiev

Bulgarian academy of Sciences Institute of Chemical Engineering, 1113 Sofia

Bourgas University "Prof. Dr. A. Zlatarov" Chemical Engineering Department, 8040 Bourgas

COST Action Flowing Matter 2015 event in Sofia, June 27-28, 2015

Wall shear on particle in backswept periodical flow:

gassed versus no gas conditions

SCOPE

● Recalling the aims of COST Action MP1305 FLOWING MATTER to improve the understanding of complex flows, this presentation considers the impact of near-wall forces around a target spherical particle uncovering wall parameters (e.g. shear) generated in non-Newtonian circulation flow. ● The flow is a highly non-uniform one due to the impeller induced generic wide-spectra velocity variation in stirred tanks, as well as due to the non-linearity of shear stress vs. shear deformation rate in moving complex fluids.

Driving force of the study

The 100-fold striking difference between fluid inner shear rate of 50-80s-1 and wall shear rate on immersed objects approaching 5-10ks-1

inner fluid shear deformation rate shear rate fluid-particle

The increased shear impact on intact biomass deserves analysis

Main topics

# Introduction of shear flow analysis for bioreactor qualification# Description of the exp. part

Background and validation examples

# Description of the math. part

Background Details Validation Results

# Case Study Results and Discussion

# Conclusions on shear and gas presence

Introduction

Understandingshear flow analysis in terms of bioreactor qualifications

Rapid deformations occur in many industrial systems, including cell and mycelia cultures in bioreactors for production of proteins, antibiotics, a.s.o.

Therefore much work has been devoted to proper analysis of cell fragmentation and process strategies in relation to rotational speed.

● Mixing intensity is increasing cell death - the specific cell death constant (kd) is related to the mean specific dissipation rate in the bioreactor : kd~(εT/ν3

)0.75 (εT=P/ρV) (Croughan etal. 1987)

● Gas bubbles have been reported to increase the shear stress around floating micro-objects (Amanullah et al…, 2003).

At low particle concentrations, the transport of momentum by the fluid is important, while at lower rates and higher concentrations the sliding forces become important.

Understandingshear flow analysis for bioreactor qualifications

Most often the bioreactor is a mixing vessel. There are four regions in danger for cell

fragmentation: (1) sparger, (2) impeller discharge, (3) bubble rise through

vessel bulk, (4) bubble bursting at the air-biofluid interface In the case of this study, the object is a sphere mounted in

the discharge stream. In this region, liquid jets (up to 5m/s)

and gas cavities are produced that may increase shear stress up

to 100 to 300 N/m2 (Chalmers and Bavarian, 1991).

Objective

Referring to diluted colloidal cultures, in this study the impact of flow assessed as maximum velocity gradient at the wall of a particle idealized sphere is examined.

Aims

Aims of the study●To uncover the shear conditions near particle surface

corresponding to maximum relative velocity imposed by specific mixing impeller discharge flow.

●In view of responding to practical engineering interest, to compare these conditions with reference critical values and assess the flow properties of a relevant bioreactor for operation.

Limitations

Considering cases of viscosity less than 50 mPa.s: Specific impeller considered. Large high-shear zones - larger sizes Circulation of the content through the high shear

zone Analysis based on finite-volume discretization

and time-averaging Gas flow rate 1vvm as a general industrial

practice

EXPERIMENTAL

● Previous comparison of radial flow and backswept flow (BSF) in single phase flow showed mild operating conditions in favor of the latter.

● Accordingly, the backswept circulation has been the preferred one to examine shear stress on particles in gas presence and impeller with modified curved blades was employed to generate the backswept circulation.

ExperimentalFocus on the equipment

The backswept impeller employed

Experimental

● The reference practical range of S for mixing of cell culture ~ 10 -104 s-1 was realized by rotational speed N (Paul, EL, Atiemo-Obeng, VA, Kresta, SM (eds.), Handook of Industrial Mixing, Wiley, New Jersey, 2004).

Focus on the physical model

● The significance of fluid friction property for fragmentation analysis has been recognized and included in the model; the prototype fluids' rheology is described in Table 1.(Reynolds numbers for rotational flow correspond to under-developed turbulence).

Table 1

Fluids of various non-Newtonian flow properties were used:

(K-consistency index, m-flow index):

Idea about flow circulationvector presentation

The case of backswept flow (BSF) is a small scale radial flow

Methods to determine

Experimental method used - the contact electrodiffusion probe

a microsensor inserted onto the particle surface (of 1mm

diameter) and Fe-electrolyte. Mathematical method used – Computational flow modeling CFD Principles

ss

s

Electrodifusion method for model validation

The external flow velocity gradient was evaluated by a contact electrochemical probe (sensor 1mm, Ae,Le). The local diffusion limited current Id, of ferric cyanide ion reduction (Deff), at the probe interface was measured, as related to solid-liquid mass transfer (ks) that occured in the boundary layer.

520

2

33477.1

eeff

d

z dCD

I

Fns

Exp. method validation results

System Re Cf Exp. Cf Ref.

Water, RTWater, NSXanthan gum RTXanthan gum NS

30001000367470

0.240.360,690,34

0.27 *0.4-.5 **0.5-.8 ***0.5-.8 ***

* Shames (1997), ** Clift et al. (1989) *** Chhabra and Richardson (2001)

Measurement vs. reference friction drag coefficients

CFD characterization

FLUENT13 Unstructured mesh for complex shape wit approx. 1 mln cells tetrahedral mesh for the volumes immediately surrounding the impeller blades and the disk. hexahedral mesh for the tank.The MRF approach and SKE turbulence model coupled with Eu-Eu formulation of 2 phase flow Conformal grid interface between the inner rotating frame and and the outer cylindrical vessel Convergence improved by a segregation implicit solution approach and SIMPLEC pressure-velocity coupling Up to 50 000 iterations within 500 timesteps 0.01s

Mesh refinement degrees

down to <0.05 mm linear dimension

Basic equations

The hydrodynamic stress was determined from shear rate and the constitutive equation of the fluid (a non-Newtonian power law one).

Modeling validation studies

computed shear rate in water:

10 500 s-1– 11 100 s-1

Measured shear rate =10 700 s-1.

Validation by performance parameters

Experimental power number Po~1 ± 0.2

CFD values obtained in water at N=600 rpm – Po=1.20; in glycerol 56 mPa.s at N=900 rpm – Po=1.21; in xanthan gum (n=0.5, K=1 mPa.sn), N=600 rpm Po=1.14; in xanthan gum (n=0.23, K=2.2 mPa.sn). N=900rpm Po=1.19

Comparison of performance parameters

 

No.

 

Fluid properties/Re

Dimensionless

/ N mesrd

shear rate/ N

predicted by CFD

 

1

Impeller blades

water Re>104

Acc. to [3]:

600-1200

 

660 (Re

1.1.104

 

2

3

4

Near-particle

in water

in CMC* 0.02_n= 0.78

in XG**

[this study]:

910-1000

600-800

550-900

 

900-4000

300-800

300-800

s s

Validation experiments in CFDComparison of exp. versus CFD-predicted values of shear

RESULTS

Assuming that cell fragmentation is proportional to the relative speed of approach, the wall shear rate was the representative parameter of interest and answers of three basic issues have been sought:

(1)What level of particle wall shear rate (Sw) is generated by the impeller-imposed flow?

(2)What and at what extent is the effect of gas presence?

(3)Within the practical range of rotational velocity, could the flow produce wall shear stress (w) values critical for processing of animal cells or mycelia cultures?

Answers of three basic issues have been sought:

Presentation of results

(1) The vessel dimension by planes

(2) The body dimension by radial coordinates 3-D solid body diagrams

The flow field vessel dimension and body dimension were analysed

Fig. 1 The vessel dimension

2-D contours of velocity magnitude (in m/s) of plane x=y at N=750 rpm and VG=0 corresponding to various flow behavior, e.g. n=0.78 (K=0.02Pa.sn), n=0.34 (K=0.55Pa.sn), and n=0.34 (K=4.6 Pa.sn), respectively.

Increasing deviation from Newtonian flow from left to right (shear rate in s-1) decreases flow mobility

RESULTSComparison liquid velocity at low consistency

750 rpm 0.02/0.78

ungassed gassed

m/s

Liq

Fig. 2

Low velocity region extended

RESULTS

Comparison liquid velocity at high consistency (0.1mPas)

Liq Gas

m/s

Liq

Fig. 3

no gas gas presence

Low velocity region unchanged

RESULTSNo gas vs. gassed

no gas vs. gassed

gas velelocity

liq velocity

Velocity, 750rpm_n=0.78, K=0.02Pa.s

Velocity, 750rpm_n=0.78, K=0.1Pa.s

Fig. 4 V-Contours magnified

RESULTS

750 rpm_0.1_No Gas

750 rpm_0.1_with Gas

Pressure field near particle

Fig. 5

No Gas

Aeration

Pressure drop increases at particle rear

Fig. 6 Fluid shear

2-D contour plots of fluid shear deformation rate (in s-

1) The bed dimension for BS Flow!

Increasing deviation from Newtonian flow from left to right (shear rate in s-1)decreases mobility of flow.

RESULTS

RESULTS FLUID SHEAR NEAR PARTICLE

np gas 750rpm_n=0.78, K=0.02Pa.s gas

np gas 750rpm_n=0.78, K=0.1Pa.s gas presence

Fig. 8

decrease

unchanged

The effect of gas presence

RESULTS

Comparison Vector plots NO GAS vs. GASSING 

Fig. 9

The vector plots related to volume-averaged single phase flow indicate separation conditions (2MV_750rpm_n=0.78_K=0.1Pa.s) likely to enhance flow separation at the particle rear.

Discussion

Referring to the results at vessel dimension:

● Low velocity zones at particle rear.

● Accompanied by low pressure zones formed.

● Separation is likely to occur.

● In gas presence, velocity redistribution is observed.

● Gas pockets in low-pressure zones are formed that are

likely to shield part of the particle surface.

APPLICATION FLOWING MATTER

Wall shear at particles in colloidal dispersions

Fig. 10 Particle Side View

The particle dimension for BSF! The effect of gas

Shear depression due to gas-decele-rated liquid is observed in both cases

The high stress zone is seen by the side stream

RESULTS

Fig. 11 Particle Rear View

In gas presence, decrease of the low shear zone size observed in the diluted dispersion

The ‘lowest stress’ zone coincides with the low pressure gas-domi- nated zone at the particle rear

The particle dimension for BSF! The effect of gas

RESULTS

Shear decrease determined

The effect of gas presence on shear

The effect at low consistency

Fig. 10A

Fig. 12a

RESULTS

Shear unchanged or

shear increase determined

The effect of gas presence on shear

The effect at high consistency

Fig. 10B

Fig. 12b

APPLICATION FLOWING MATTERTracing cell demege in mycelia and cell culture bioprocessing

Max shear on particle 2-fold decrease

Fig. 13 The effect of consistency!

Discussion

Referring to the effects seen at body dimension :  Evidence for areas of critical performance -

# The high stress zone is expected by the side stream, while the lowest stress zone in gas presence coincides with the low pressure gas filled zone at the particle rear. This is valid strongly for the case of low consistency (K=0.02 Pasn).   # The case of high consistency (0.1Pa.sn) shows a slight decrease in shear (something like 1200 to 1160 s-1, due to only the density effect of the gas-liquid mixture. In cases due to variation of gas spread, the shear in gas presence may increase.

Area-averaged parameters

While the results so far explain the effect, the magnitude of the effect itself is the important result for practice:  

What are the ranges of shear stress and

How do they conform to reported criteria for cell fragmentation? The results are seen in Table 2

Table 2 Average wall shear stress and the effect of gassing

Shear stress should not exceed 2-3 N/m2 for animal cultures and 80 N/m2 for mycelia (Ludwig et al., 1994 )

area-averaged values

At low and high K, opposite response is revealed

In order the bioprocessing vessel specified to be acceptable for operation, shear deformation should not exceed shear stress 2-3 N/m2 for animal cultures (Ludwig et al., 1994) and 80 N/m2 for mycelia.

Referring to the data of Table 2 related to the impeller discharge area, the presence of gas at consistency rise increases shear up to 30 %; yet

The backswept impeller-induced circulation is well within the limits for proper bioprocess growth in mycelia cultures

However, even at low mixing intensity the critical values of shear stress for animal cells are exceeded.

DISCUSSION

# The study presents a CFD-based assessment of important flow parameter - an image of shear imposed on particles immersed in complex (non-Newtonian) fluid with engineering application in fermentation and cell culture agitated bioreactors.

# The study reveals the maximum impact of flow at the wall of a particle in colloidal dispersion circulated by means of impeller in presence of gas.

# Referring to knowledge of critical values reported in the literature, the flow condition results allow to classify practically occurring operational regimes as optional in terms of potentials for cell damage.

# Referring to the critical values reported, evidence is given for areas of critical performance in case of primary circulation that imply cell fragmentation in practical cases of cell culture bioprocessing.

CONCLUSIONS

References

Hanratty, T.J., Campbell, J.A. Measurement of wall shear stress. In: Goldstein, R.J. (Ed.), Fluid Mechanics Measurements. Hemisphere, Washington, 1987.

Ludwig, A., Kretzmer, G. and Schügerl, K., Determination of a "critical shear stress level" applied to adherent animal cells. Enzyme Microb. Technol. 14 (1992) 209-913.

Acknowledgement

COST Action MP1305 FLOWING MATTER support in delivering this study to specialized audience is acknowledged.

Thank you!

APPLICATION FLOWING MATTER

Tracing cell demage in mycelia and cell culture bioprocessing

Visualized flow impact on shear-sensitive cells =wall shear at particles in colloidal dispersions