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Process Design of Fermenter (R2003)

7.2 Process Design of Fermenter (R2003)

Summary of R2003

Pressure: 1 atm.

Temperature: 30C.

Diameter: 1.7 m

Height: 5.16 m

Impeller Diameter/speed:1.03m/180rpm

Volume: 16 m3

This is fermenter in which microbes (microorganism A and microorganism B)

convert substrate glucose to 2-keto-L-gluconic acid. Microorganism A is stain of Acetobacter

which converts sorbitol into L-sorbosre and microorganism B is Acinetobacter which

converts L-sorbose formed into 2KLGA. Reactors for this fermentation process are Stirred

Tank Reactor operated in fed-batch mode. It gives uniform mixing which is required for

maintaining concentration and temperature uniform in the reactor. We are using air for

sustenance of microorganisms and oxygen required for oxidation of Sorbitol.

We assume that Microorganism B is same as A and they follow same growth kinetics

of Acetobacter. So at any time we can assume that there will be twice the concentration of

microorganism A. The parameters to be determined in the specification of reaction vessel

include:

1) Reactor Volume

2) Vessel geometry

3) Vessel agitation

4) Sparger design

5) Vessel Heat transfer consideration.

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Fed-Batch Operation

Bailey (1977) and Stanbury (1984) have discussed the fundamentals of the

Fermentation Technology. The basic advantage of fed-batch process is that the concentration

on the fed nutrients in the culture liquid in the bioreactor can be controlled by feed rate. In

this system the limiting nutrient is added in the reactor in reactor at fixed flow rate after

certain fixed time. In this reaction Sorbitol is growth limiting reactant which is fed to the main

fermenter after 6.5 hr of first inoculation. First inoculum contains 576 Kg of Sorbitol with

necessary nutrients and water. 647 Kg of sorbitol is then fed with nutrients and water for 10

hr at constant flow rate.

Microorganism follows Monod Kinetics and specific growth rate is given as;

][

][max

SKs

S

+=

For Acetobacter (Asenjo et al 1995) max =0.086 hr-1 and Ks is Monod constant and

its value is 0.001M.

The biomass at any time t is given as,

xt = x0 + Y ( So S )

So is original substrate concentration.

S is substrate concentration remained.

The final biomass concentration when S=0 may be described as xmax provided that x0

is small as compared to xmax.

xmax = Y So; at this time medium feed is started such that the dilution rate is

less than max , at this point it can be assumed that substrates will be consumed as

soon as it enters the culture, thus

Y

XSF R =.

F is flow rate of medium feed.

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X is total biomass in culture given as x.V. Thus the total biomass increases with the

time, but cell concentration remains virtually constant dx/dt =0.

Therefore = D (Dilution rate =F/V). As time progresses the dilution rate will

decrease as the volume increases and dilution rate can be given as

FtV

FD

+=

0

when D < < max and Ks

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Substituting rest of the values we can calculate the volume required to increase the

biomass upto x.

V=6.44 m3.

This is the additional volume so, total volume is

Vt = 6.44 + 5.35 m3.

Vt = 12 m3

Vessel geometry

We assume height (H) to diameter (T) ratio of reactor equal to 1:1.

Therefore diameter of vessel = 2.48 m.

We provide torispherical dished ends hence volume provided by this is

33 135.024

xTxT +

= 16 m3.

Total Volume of reactor = 16 m3

Vessel agitation

Minimum speed of agitation to suspend microbial mass at end of 6.5 hr is

85.0

13.045.02.01.0)/.(.

D

XgdpsNm L

=

Normally impeller used is 60% of the tank diameter.

D= 1.03 m

= viscosity = 0.04 Pa.s

l = 1300 Kg/m3.

dp = diameter of bacterial = 10 m

s = 2 x (D/T) 1.33 = 1.0138.

= kinematic viscosity = / l .

Nm =0.99 RPS

We have to keep agitation speed more than Nm to keep biomass in suspension.

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Considering this reaction to be mass transfer controlling,

Oxygen Transfer Rate (OTR) for the fermentation broths varies from 0.4 to 0.6

mg/L/hr.

Assuming OTR = 1.857 x 10-5 Kmol / m3.s

smKmolCCKOTR La35

/10875.1)( ==

Where,

CrCC = 2.1

CCr = critical dissolved oxygen concentration for the culture

= 0.022 m mol / L = 0.022 g mol/m3

C = 1.2 0.022 10-3

= 2.64 10-5 K mol /m3

C* = H PO2

Where C* = equilibrium concentration in liquid

H = 7.3 10-4 K mol / m3.atm= Henrys constant for the system at 30 0C

P O2 = partial pressure of oxygen in the air

= 0.21 atm

C* = 7.3 10 4 0.21

= 1.533 10-4 K mol/ m3

KLa = OTR / (C*-C)

=55 10)64.233.15/(10875.1

= 0.147 S-1

Volumetric flow rate of air from mass balance is

Qg = 0.172 m3/s.

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Cross sectional area = /4.D2 = 2.32 m2

Vg = 7.41 cm/s.

Hoshino et al (1992) studied the process for given system in 3 liter Fermenter

with 800 rpm as impeller speed. Shuler et al (2002) has given the criteria for the scale

up of fermenters, in which the most effective method for scale up is constant Power to

volume ratio, in which the N3D2 must be same in both the vessels.

Assuming Height to Diameter ratio on lab scale equals to 3.

Dl=0.108 m; Nl= 800 rpm

Main Fermenter is of 1.03m diameter and assuming Power to Volume ratio remains

constant, so for scale up

2323 xDNxDN ll =

Speed of impeller on plants scale = 180 rpm

Power Consumption

This is based on assumption that reaction volume is constant and equal to 10 m3

.

Power consumed in absence of gas53. DNNpPo L=

We use pitched blade turbine with Np=3.

D= 1.03 m l = 1300 Kg/m3.

Po= 122 kW.

Power in presence of gas is given by

45.0

56.0

32 ...

=

Q

DNPKP og

Q = 0.172 m3/s

K is constant = 1

Pg = 100 kW

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We have to calculate the optimum impeller speed such that the mass transfer

coefficient KLa can be achieved, these calculations where done in Microsoft Excel

sheet by taking volume of liquid as 10 m3

Algorithm:

1. Assume impeller speed (N) more than Nm.

2. Calculate power in absence of gassing as53. DNNpPo L=

3. Calculate power in presence of air

2.0

66.0

4225.0

..

.1.0

=

gVW

DN

Q

VN

P

P

LG

L

O

G

Where, W= D/8

4. Estimate the mass transfer coefficient as5.0

4.0

026.0 GL

Gl V

V

Pak

=

5. Check whether this Kla matches with the KLa . If no go back to step one and

change the agitation speed till the values match.

The optimum value of impeller speed is 156 rpm

Thus by comparing the values of impeller speed the one which is calculated

from the scale up method satisfies all the constraint of mass transfer as well as solid

suspension.

Sparger Design

The volumetric flow rate of air through the hole of the sparger is given by,

ororor NVDV =24

Where,

V = volumetric flow rate of air = 0.172 m3/Sec

Dor = diameter of the orifice

Vor = superficial gas velocity through the orifice

Nor = number of orifices

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For effective distribution, Re > 10,000

000,10/Re = gorororVD

g = 1.294 Kg/m3

g = 0.9 10-5 Pa.s

Hence, we take

DorVor > 0.0695

Hence assume DorVor = 0.07 m2/Sec

Substituting this in the equation of volumetric flow rate, we get,

DorNor=x

x

07.0

4172.0m

Assume hole diameter = 6 mm

Nor = 3.128/0.006 = 523 holes

Design of air sterilization

Air is sterilized by filtration of microorganisms with a glass fiber filter.

Volumetric flow rate of air at 300C = 0.172 m3/sec

The filtration efficiency of the filter depends upon the length of the filter and

superficial velocity of the air.

4.09.0075.0)1/(1log(

= VsLn

Where, L = length of the filter

N = overall collection efficiency = 99.5%

= Packed density of glass fiber = 82.5 Kg/m3

Vs = superficial velocity of the air

Air at 300C is assumed to contain 104 particles of microorganisms

1 is size per m3

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L = 19.5 cm

Assuming Diameter of filter = 0.5 m

Heat Transfer Area

From energy balance calculation the heat released is 39 kW.

We need chilling water to maintain temperature at 30C in reactor.

Water inlet temperature is 10C and outlet temperature is 25C

TLMTD =

)1030(

)2530(

)1030()2530(

In= 10.8 C

U.A. TLMTD = 39 kW

Value of U is taken as 500 W/ m2 K

A=7 m2.

Design of Heat transfer elements

Limpet Coil

A limpet coil is preferred over a jacket as this gives higher values of heat transfer

coefficients, because the cooling water velocity in jacket would be really small.

We provide heat transfer coils upto 1.5m.

Assuming diameter of limpet coil =100mm.

N is number of turns of coil.

N. . Dvessel dcoil = 7 m2

N = 10.

Distance between two successive turns is

10)100(

1500=

+x

x = 50 mm

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