(177f) Mathematical Modeling of a Trickle Bed Bio-Desulfurizer of Hydro-Treated Diesel with Recycle for the Production of Ulsd (Ultra-Low Sulfur Diesel)

Mathematical
Modeling of a Trickle Bed Bio-desulfurizer of Hydrotreated Diesel for the
Production of ULSD (Ultra-low Sulfur Diesel)

M. Mukhopadhyay, R. Chowdhury* and P.
Bhattacharya

Chemical Engineering Department, Jadavpur
University

Kolkata-700032

India

*Author to whom correspondence may be addressed.

 

Abstract

Biodesulfurization
has been chosen as a complementary second stage desulfurization of
hydrodesulfurized diesel to produce near zero sulfur (NZS) diesel (15 ppm
sulfur). For such desulfurization, batch studies have been conducted in
Erlenmeyer flasks maintaining the concentration of diesel in the range of
0-100% in a diesel supplemented sulfur free aqueous medium. Hydrodesulfurized
diesel samples (purchased from Indian Oil Corporation, Kolkata) have the
specifications represented in the table1.

Table1:Characteristics
of diesel samples used:

Compound	I.B.P.	F.B.P.	Sp. Gravity (Basis:density of water=1000 kg/m3)	Sulfur	Aromatic
Diesel	140 oC	370 oC	0.8216	200 to 540 ppm	27.16 %(w/w)

The concentration
of biomass with time has been monitored using dry cell weight method. The
concentration of sulfur has been determined by ?Trace Sulfur in petroleum
distillate by Nickel reduction? (UOP 357-80) method and X-ray fluorescence
(XRF) (ASTM-D4294-03) method. From the growth curve it is observed that the
system follows uninhibited Monod type model within the range of substrate
concentration studied. The intrinsic kinetic parameters, namely, maximum
specific growth rate,
saturation constant Ks and yield coefficient Y_A/B have been
determined by a systematic and programmed investigation and are represented in
the following table.

Table2: Value of
the intrinsic kinetic parameters:

Saturation constant, Ks (mg/dm3)	Maximum specific growth Rate, (hr -1)	Yield coefficient, YA/B
71	0.0961	0.2

Based on the
knowledge gained from the batch fermentative process, experiments have been
conducted in a trickle bed reactor, using the same microbial strain immobilized
on a solid matrix to understand the reaction engineering behavior of sulfur
removal from diesel oil in continuous mode of operation. Biodesulfurization of
diesel oil has been conducted in a trickle bed reactor having a diameter of
0.066 m and a height of 0.6 m. Initial substrate concentration, liquid hourly
space velocity (LHSV) and recycle ratio have been chosen as process parameters.
Hydrotreated diesel fraction having different sulfur concentration, namely,
200, 330, 430, 540ppm has been taken as feed stock for degradation of residual
organo-sulfur compounds present in deeply hydrodesulfurized diesel using Rhodococcus sp.(no. 2891 NCIM, Pune).The
microorganisms have been immobilized on the packing material, namely pith balls
(dia0.012m) prior to desulfurization within the trickle bed reactor (TBR). Bacterial
medium containing Rhodococcus sp.
having the biomass concentration of 20g/dm³ has been circulated
through the packed bed until the bio-film thickness on the sphere becomes
0.1mm. Initial bed porosity has been determined to be 0.6. LHSV has been varied
in the range of 1.838 and 3.676 hr^-1.Since, the biodegradation has
been carried out with the aid of aerobic bacterial strain, the reactor is
continuously sparged with air at 480dm³/hr in the upward direction.
The reactor is run under atmospheric pressure. The substrate loading in the
reactor has been determined to be initially 1.46X10^-4 kg/m³/hr
for initial substrate concentration of 200 ppm at lowest inlet flow rate
of  25 dm³/hr and is varied
up to 7.84X10^-3 kg/m³/hr for 540 ppm of initial substrate
concentration in diesel at 0.5 dm³/hr inlet diesel flow rate. Within
the range of process parameter variation, substrate degradation profile with
the change in LHSV and recycle ratio have been observed. Under the present
process condition, sulfur reduction within the range of 84-98% has been
achieved.  It has been found that with
the decrease in initial substrate concentration, better substrate degradation
efficiency is achieved. Moreover, the substrate concentration decreases with
the axial length of the trickle bed reactor. Better removal efficiency results
with the increase in recycle ratio and lowering of LHSV. The value of sulfur
concentration in the reactor exit stream increases i.e. the ultimate sulfur
conversion decreases with the increase in the LHSV. At the lowest LHSV of
1.838hr^-1, the sulfur removal efficiency of 95 % has been achieved.
Similarly, the value of sulfur removal efficiency corresponding to the LHSV of
3.767 hr^-1 is 77.8 %. This is, however, expected as the increase in
LHSV implies the decrease in reactor residence time causing drop in the
ultimate conversion of the reactant.
The reduction in sulfur concentration increases as the recycle ratio
increases. At recycle ratio of 0.25 with lowest LHSV of 1.838 hr^-1 sulfur
removal efficiency is 95%. Again at recycle ratio of 2 with the same LHSV the
result shows 96.4% sulfur removal efficiency. This can be easily realized
because the higher the recycle ratio the more is the scope for the substrate to
come in contact with the immobilized microorganisms. Therefore, the chance of
substrate degradation reaction is more. A deterministic mathematical model for
the TBR has been developed using judicious assumptions to predict its
performance characteristics. The system equations based on differential mass
balance for organo-sulfur compounds of diesel & biomass used in the
mathematical model are as follows,

     (1)

Where A_L=
2∏(d/2+L_f).L_f
(2)

The equation for
the variation of biofilm thickness due to bacterial growth on the

surface of the
packing material is,

                                               (3)

The variation of
bed porosity within the reactor is represented by,

                             (4)

The boundary
conditions are as follows,

For t, Z=0,

 C_A= C_A0, C_B  = C_B0, L_f = L_f0

For t>0, Z =
0,

C_A= C_A0,
 C_B  =

Where,

A
Cross-sectional area of the reactor (m);

A_L
Bio-film area loss per unit packing sphere in contact (m);

C_A0
Initial biomass concentration (mg/dm³);

C_A
Biomass concentration at a particular time (mg/dm³);

C_Af
Final biomass concentration at the reactor outlet (mg/dm³);

C_B0
Initial substrate concentration (mg/dm³);

C_B
   Substrate concentration
at a particular time (mg/dm³);

C_Bnew
New inlet substrate concentration at a particular time during recycle
(mg/dm³);

C_Bout
Final substrate concentration at the reactor outlet (mg/dm³);

F_A
Volumetric flow rate of the liquid stream (dm³/hr);

Ks
Saturation constant;

L
Length of the reactor (m);

L_f            Bio-film thickness over each packing material (m);

N
Number of Pressure drop;

D
Diameter of each packing sphere (pith ball) (m);

R           Recycle ratio;

V
Volume of the TBR (m³);

V_L          Bio-film volume loss per unit packing
sphere in contact (m³);

Y_A/_B       Yield coefficient = Mass of biomass
produced /mass of substrate consumed;

Z
Axial position within the reactor;

Greek letters:

         Maximum specific growth rate of
biomass;

 ₀            Initial bed porosity;

 _f                 Porosity in bed with bio-film;

Abbreviations:

I.B.P.
Initial Boiling Point;

F.B.P.
Final Boiling Point;

LHSV      Liquid hourly space velocity = volumetric
feed flow rate / catalyst bed volume;

The equations
have been solved by 4^th order Runge-Kutta technique using a suitable
C-program. The simulated data obtained were represented graphically and the
corresponding experimental values have been superimposed on the same plot. From
the comparison of the simulated and experimental data it is evident that the
mathematical model can explain the reality satisfactorily.

Key words: Bio-desulfurization, diesel, uninhibited Monod model,
intrinsic kinetic parameters, immobilized, trickle bed reactor.