A PROCESS OF MANUFACTURING NAPHTHALENE FOR HIGHER YIELD
FIELD OF INVENTION
The present application has a reference to a co-pending prior patent application
No. 949/KOL/2012 dated 18.08.2012 entitled "Improved system for naphthalene
crystallization in a tar distillation plant". In order to further elucidate the disclosed
methods as presently claimed we hereby incorporate by reference the said co-
pending patent application filed by the same applicants as the present patent
application includes under "Detailed Description" a process of manufacturing
naphthalene for higher yield.
The present invention relates to an improved process of manufacturing
naphthalene for higher yield through mechanical crystalliser route. More
particularly the improved process observes the cooling profile of charge mix in the
mechanical crystalliser and optimizes the cooling process to achieve higher
naphthalene yield. The developed process also helps in deciding the discharge
temp and thereby optimizes the crystallisation period and increase the availability
of crystalliser.
BACKGROUND AND PRIOR ART OF INVENTION
Naphthalene is an important by-product of coke oven plant which is obtained by
processing crude coal tar. Coal tar is dehydrated to the extent of 0.2-0.3% in two
stage evaporator (Stage I and Stage II). The vapours from stage II evaporator are
fed to the anthracene column. The lighter fraction from the anthracene column
enters the fractionating column at 220-230°C where naphthalene fraction is
separated at 210°C.
The naphthalene fraction is cooled in submerged cooler before it is collected in
fraction tanks. The temperature of submerged cooler is maintained to keep the
naphthalene fraction pumpable. The concentration of the naphthalene fraction is

around 70-80% and requires further treatment for achieving higher purity. Fig. 1
schematically depicts the conversion of naphthalene from crude coal tar.
Naphthalene fraction along-with Drained Naphthalene Oil (DNO) # and Press Oil##
constitute as Charge mix, is charged into mechanical drum crystallizer by means
of pumps. The charging temperature is around 70-80° C. The crystallizer has
paddles fitted on a rotating shaft along the axis of the drum. The paddles rotate at
slow rpm and help in homogenising the temperature of charge mix.
The freshly prepared charge mix in crystallizer is allowed to cool naturally till
Crystallization Temperature (CT) is reached. This usually takes around 14-16 hrs
and thereafter water is sprayed on the outer surface of the crystallization drum for
further cooling to produce naphthalene crystals. The crystallization process is
normally completed within 48 hours. The slurry from crystallizer is then discharged
into screw conveyor and the drained slurry comprising crystal of naphthalene and
naphthalene oil is centrifuged. In the centrifuge, crystal and oil are separated. The
removed oil is known as Drained Naphthalene Oil (DNO) and stored in a separate
tank. The naphthalene fraction, 90-94% pure, is fed into a press through a mixer.
This concentrated naphthalene fraction is hydraulically pressed to produce Hot
Pressed Naphthalene (HPN) cakes. The remaining oil squeezed out of the press
is known as Press Oil and stored in a press oil tank. The hot pressed naphthalene
cake thus produced is 97-98% pure.
Presently the yield of naphthalene has been declining mainly due to fixed
operation pattern, i.e. in spite of so many operation variables like naphthalene
content of charge mix, cooling water temperature and pressure, etc. The entire
process follows the fixed operation pattern like
1. Natural cooling of charge mix for 16 hrs.
2. Water spray on outer surface of crystalliser after 16 hr to 48 hr at constant
pressure
3. Fixed discharge period i.e. 48 hr
This eventually resulted in longer crystallization period and low availability of
crystallizer, poor yield, and more breakdowns due to improper cooling.

To overcome the above bottleneck, a process has been developed which optimize
the entire cooling process. This process helps to avoid hot discharge of charge
mix from crystallizers, break down of shaft due to improper cooling. Introduction of
such developed process has facilitated the plant operator to optimize the entire
crystallization process for enhancing the naphthalene productivity.
OBJECT OF THE INVENTION
As per the principle object of the present invention there is provided an improved
process of manufacturing naphthalene which includes modified cooling strategy of
charge mix in the mechanical crystalliser to achieve higher naphthalene yield.
Another object of the present invention is to increase the availability of crystalliser
to achieve higher yield.
Yet another object of the present invention is to provide an optimized
crystallization process.
Addition object of the present invention is to prevent the operators from exposure
of naphthalene vapours, and hot discharge consequences.
SUMMARY ON INVENTION
The Naphthalene content in coal tar varies from 8-10%, depending on coal
sources, coke oven operating temperature and the coking period, The higher the
coking temperature and rate of coking, higher the naphthalene content of the tar.
Coal tar is distilled in fractionating column and naphthalene fraction is separated
at 210°C with purity of 70-80% which requires further treatment for greater purity,
Bhilai steel plant uses mechanical crystallizer- centrifuge -hydraulic press to
recover naphthalene as hot pressed naphthalene with about 98% purity.
Therefore such as herein described there is disclosed a process for manufacturing
naphthalene directly from a mechanical crystallizer of a tar distillation plant
characterized by adopting controlled flexible crystallization period wherein the step

cooling is carried out in at least three different zones which includes slow cooling
zone for super saturation, stagnant zone for nuclei formation and accelerated
cooling zone for crystal growth and a fixed final discharge temperature.
Yield of naphthalene i.e., per kg of hot pressed naphthalene produced from
processing of 100 kg coal tar, depends upon the following factors:
• Tar distillation process parameters
• Concentration of naphthalene in the feed to crystallisers
• Final discharge temperature
• Cycle time of crystallization
• Temperature of cooling water
• Efficiency of centrifugation
To improve the efficiency of crystallisation process, monitoring of process
parameters e.g. charging level of crystalliser, feed liquid temperature etc, are
required. Therefore, the approach was kept as below:
• Detailed study of existing process parameter e.g. charging level, feed
temperature, cooling process
• Laboratory analysis of feed composition
• Optimisation of process parameters
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
F g. 1 illustrates a flow chart of conversion of Naphthalene from Crude Coal Tar in
accordance with the present invention;
Fg. 2 illustrates a flow chart of Naphthalene Crystallization Process at BSP in
accordance with the present invention;
Fg. 3 illustrates a schematic layout of detailed Instrumentation package for
Naphthalene Crystallizer in accordance with the present invention;
Fig. 4 illustrates cooling Profile of Crystallization Process in accordance with the
present invention;

Fig. 5 (a) and (b) illustrates cooling Profile of Crystallization Process for different
crystallizers in accordance with the present invention;
DETAILED DESCRIPTION
The existing crystallizers are designed for 70-72% naphthalene contents.
Therefore an optimum ratio of naphthalene fraction, DNO and press oil, depending
on their naphthalene concentration, is prepared, prior to charging. Conventional
lab based method used to estimate the naphthalene content in fractions, DNO
&Press OIL and they are reported in table (1).From analysis of this data it was
observed that there is fluctuation of naphthalene content in naphthalene fraction
coming from fractionating column as well as naphthalene concentration in DNO
and press oil.
The flow chart referred to in Fig 2 shows the naphthalene crystallization process at
BSP. There are 22 crystallizers, out of which 13 are in Tar Distillation Plant - I
(TDP-l) and 9 are in Tar Distillation Plant-ll (TDP- II). Here are 8 centrifuges, 5 in
TDP-l and 3 are in TDP- II. There are three hydraulic presses, 2 in TDP-l and 1
in TDP- II.
Fig. 3 represents the schematic layout of the instrumentation package developed
for naphthalene crystallizer. The non contact infrared radiation based temperature
sensors (IR Pyrometers) are used in measure temperature of the charge mix
(naphthalene fraction, DNO and Press oil) in the crystallizer, which has tendency
to solidify rapidly at room temperature.
So when this mixture is charged to crystallizer where the naphthalene content in
the charge mix varies widely. It result different cooling pattern so in order to
achieve higher naphthalene yield, different cooling strategy is required. It will also
affect crystallization period. Therefore the crystallization period should not be kept
fixed.
The crystallization period is optimized by adopting step cooling practice i.e.
change in cooling practice as per cooling profile.

Optimisation of process parameters
There are many process parameters which affect the naphthalene yield as
mentioned above. During crystallization the most important parameter is
naphthalene content of charge mix, which varies widely hence the nature of
cooling curve. It also affect naphthalene yield and crystallization period.
During trial complete cooling profile were drawn and the profile is divided in
general 3 zones as shown in fig. 4.
• The cooling curve is divided in three zones-
√ Slow cooling zone(80-65°C): Supersaturation of feed starts
√ Stagnant zone (65-58°C): Nuclei formation starts
√ Accelerated cooling zone(58-48°C): crystal growth becomes dominant
In each zone different phenomenon are predominant so a new cooling strategy i.e.
step cooling is devised to achieve higher naphthalene yield
A process for higher naphthalene yield requires step cooling of different zones:
√ Natural cooling by air for - Slow cooling zone
√ Slow forced cooling (water flow -1.5 kg/cm2) for - Stagnation zone
√ Fast forced cooling (water flow - 3 kg/cm2)for - Accelerated cooling Zone
This new pattern of cooling strategy helps to maximize the naphthalene yield. To
optimize the crystallization period discharge temperature fixing is essential.
In the developed process for higher naphthalene yield for Mechanical crystalliser
route major emphasis is given to
• Concentration of naphthalene in the feed to crystallisers
• Change in cooling pattern
• Final discharge temperature
• Cycle time of crystallization
• Temperature & pressure of cooling water

Concentration of naphthalene in the feed to crystallizers & Change in
cooling pattern
In general the crystallization process of naphthalene consists of two major events,
nucieation and crystal growth. In nucleation, solute molecules dispersed in the
solvent and start to gather into clusters of atoms on nanometre scale. These
clusters reach a critical size in order to become stable. Stability of the clusters
depends upon the operating conditions like temperature and super-saturation
state of solution. During the nucleation stage, atoms arrange in periodic manner
giving rise to crystal growth. Nucleation and crystal growth occur simultaneously
till super-saturation state exists. Once the super-saturation phase is exhausted,
the solid-liquid system reaches equilibrium and the crystallization process is said
to be complete unless otherwise operating conditions are modified. Forced cooling
by water spraying is introduced at this point to shift the equilibrium towards crystal
growth, so that super-saturation state is reached faster, as during natural cooling
the nucleation formation is dominant over crystal growth. Once the equilibrium
temperature is reached, there is no significant improvement in crystallization of
naphthalene. Therefore, at this point of time the naphthalene mixture should be
drained, making the crystallizers available for further production.
The crystallization temperature (stagnant temp zone) depends upon the
concentration of naphthalene in the charge mix. As the concentration of
naphthalene in crystalliser charge increases, the crystallization temperature
increase and vice versa. Therefore, with proper monitoring of temperature of the
crystallizing material, the crystallization period can be optimized as shown in fig. 4
crystallisation period reduced to 40 hrs from 48 hrs, change in cooling strategy
increase the naphthalene yield from 3.07 to 3.25% of tar processed.
As naphthalene content in naphthalene fraction coming from fractionating column,
DNO and press oil fluctuates widely shown in TABLE 1. It result different
behaviour of charge mix in the crys alliser, during trials several figure obtained for
cooling of charge mix.
On analysis of several curve Fig. 4 & 5 it was observed that these different stage
i.e. nuclei formation and crystal growth, which can be divided as per temperature

zone. From analysis of many cooling profile, these curve are divided in three
different zone as per temperatures of charge mix.
• The cooling curve is divided in three zones-
√ Slow cooling zone(80-65°C): Supersaturation of feed starts
√ Stagnant zone (65-58°C): Nuclei formation starts
√ Accelerated cooling zone(58-48°C): Crystal growth becomes dominant
The cooling curve of different crystalliser plotted in Fig. 4 & 5, it was observed that
nucleation formation is dominant over crystal growth in ambient cooling while in
the forced cooling, crystal growth becomes dominant. Therefore Cooling water
should be introduced once sufficient nuclei are formed
In each zone, different phenomenon is predominant so a new cooling strategy i.e.
step cooling is introduced to achieve higher naphthalene yield
√ Natural cooling by air for - Slow cooling zone
√ Slow forced cooling( water flow -1.5 kg/cm2)for-Stagnation zone
√ Fast forced cooling (water flow -3 kg/cm2)for - Accelerated cooling Zone
Therefore in order to achieve higher yield, the crystallization period (Discharging
time- Charging Time) should not be kept fixed. The crystallization period can be
optimized by adopting above mentioned step cooling practice. For dynamic and
more accurate control, online cooling profile is must and by using proper
instrumentation it is possible to get complete cooling profile of charge mix, then it
is possible to know the timing of different zone and by adopting above mentioned
step cooling strategy, higher naphthalene yield can be obtained and crystallisation
period can be optimized.
Final discharge temperature & Cycle time of crystallization
Final discharge temperature is fixed by optimization of three process parameters -
Cooling water temp, naphthalene yield & crystallization period.

The discharge temperature of the naphthalene slurry varies from 40 °C to 50 °C
depending upon the temperature of cooling water and ambient significantly. It was
found that cooling water temperature in TDP - II (Bhilai Steel Plant - Tar
distillation Plant - II) always remains greater than 35°C and hence bringing down
the temperature of naphthalene slurry below 45°C is very slow and time
consuming, with very little improvement in crystallization efficiency.
From trials it was found once temp of charge mix achieve 50°C, the rate of cooling
become very slow, further cooling is very time consuming and little improvement in
naphthalene yield. Hence, it was found that the optimum discharge temperature of
naphthalene slurry is 45°C + 2°C. This eventually, completes the crystallization
period within 35-40 hours compared to earlier average of 48 hours. Thus, the
availability of the crystallizers is also increased for further processing of
naphthalene.
Fixing of discharge temp also hep operators to avoid hot discharge from
crystalliser, earlier discharge period was fixed to 48 hr, irrespective of temperature
of charge mix so in some case there are hot discharges from crystalliser which
create problem in subsequent operations. Thus fixing of discharge temp helps
avoid hot discharge.
In general discharge temp can be fixed by following equation
td=tc+10
Where, td = Temperature of discharge, °C
tc= Temperature of cooling water
The developed process enables the operator to optimize the crystallization
process, which resulted in a reduction of crystallization period significantly and
improve the naphthalene yield from 3.07% to 3.7%.
Benefit:
1. Crystallization process time reduced from avg. 48hrs to 35-40hrs
2. Increased availability of crystallizer leads to yield improvement from 3.07%
to 3.7%
3. Less exposure of nathelene vapour to operator as it will avoid hot discharge of
charge mix.
4. Less exposure of operators to naphthalene vapours

5. Monetary benefit are as follows
a. ICAB (Incremental Annual benefit) (In lakhs) = 24.0
b. RCAB (Recurring Annual Benefit) (In lakhs) = 79.0

Numerous modifications may be made to the present invention, which still fall
within the intended scope hereof. Thus, it should be apparent that there has been
provided in accordance with the present invention an improved process of
manufacturing naphthalene for higher yield through mechanical crystalliser route
that fully satisfies the objectives and advantages set forth above. Although the
invention has been described in conjunction with specific embodiments thereof, it
is evident that many alternatives, modifications and variations will be apparent to
those skilled in the art. Accordingly, it is intended to embrace all such alternatives,
modifications and variations that fall within the spirit and broad scope of the
appended claims.

WE CLAIM:
1. A process for manufacturing crystallized naphthalene directly from a
mechanical crystallizer of a tar distillation plant characterized by adopting
controlled flexible crystallization period wherein the step cooling is carried out in at
least three different zones which includes slow cooling zone for super saturation,
stagnant zone for nuclei formation and accelerated cooling zone for crystal growth
and a fixed final discharge temperature.
2. A process for manufacturing crystallized naphthalene as claimed in claim 1,
wherein the slow cooling is carried out by natural cooling by air at temperatures
(80-65°C).
3. A process for manufacturing crystallized naphthalene as claimed in claim 1,
wherein the stagnant zone is applied with slow forced water cooling at
temperatures (65-58°C) and pressure (water flow -1.5 kg/cm2).
4. A process for manufacturing crystallized naphthalene as claimed in claim 1,
wherein the accelerated cooling is carried out by fast forced water cooling at
temperatures (58-48°C) and pressure (water flow -3 kg/cm2).
5. A process for manufacturing crystallized naphthalene as claimed in claim 1,
wherein in order to have accurate controlled cooling, complete cooling profile of
charge mix is obtained by using proper instrumentation to know the timing of
different zone and thereby adopting step cooling strategy for higher naphthalene
yield.
6. A process for manufacturing crystallized naphthalene as claimed in claim 1,
wherein the optimum discharge temperature of naphthalene slurry is 45°C ± 2°C.
7. A process for manufacturing crystallized naphthalene as claimed in claim 1,
wherein the discharge temp is equated as per following equation.
td=tc+10
Where, td = Temperature of discharge, °C; tc = Temperature of cooling water.

8. A process for manufacturing crystallized naphthalene as claimed in claim 1,
wherein the crystallization period is within 35-40 hours.
9. A process for manufacturing crystallized naphthalene as claimed in claim 1,
wherein the naphthalene crystallizer further comprises non contact Infra red
radiation based temperature sensors (IR Pyrometers) used to measure
temperatures of in the charge mix (naphthalene fraction, DNO and Press oil) ,
placed in the crystallizer.
10. A process for manufacturing crystallized naphthalene as claimed in claim 1,
wherein change in modified cool ng strategy increase the naphthalene yield from
3.07 to 3.25% of tar processed.

ABSTRACT

The present invention relates to an improved process of manufacturing
naphthalene for higher yield through mechanical crystalliser route. More
particularly the improved process observes the cooling profile of charge mix in the
mechanical crystalliser and optimizes the cooling process to achieve higher
naphthalene yield. The developed process also helps in deciding the discharge
temp and thereby optimizes the crystallisation period and increase the availability
of crystalliser.