AN IMPROVED PICKLING BATH HEATING SYSTEM FOR
SULPHURIC ACID PICKLING LINE
FIELD OF INVENTION
The invention relates to an improved bath sulphuric acid pickling line
comprising heat exchanger and temperature controller of the regenerative
kind, heating system and in particular relates to a controlled heat
exchanger configured for use with hot rolled coils of steel in sulphuric acid
tank bath.
BACKGROUND ART
There is oxide layer of 5 to 20 microns on hot rolled coils called mill scale.
These scales must be removed before subsequent operation like cold
deformation, in order to prevent excessive wear on the rolls and to
avoid the surface defects of the finished product. This mill scale
is being removed by pickling i.e. electrochemical reductive
dissolution in mineral acids (HCI, H2S04, HN03 and HF). There is
sulphuric acid pickling line at Rourkela Steel Plant (RSP). The line was
commissioned in the year 1968 and there have been no major investment
in the line thereafter leading to poor equipment health.
Temperature of sulphuric acid pickling bath plays an important role in
pickling efficiency. The pickling rate considerably increases with an
increase in the pickling bath temperature. A temperature increase of 15°C
of the pickling bath doubles the pickling rate as shown in Fig.1. However,
pickling bath temperature should not be allowed to go beyond 95°C as
this will lead to formation of mono-hydrate of ferrous sulphate, which is
insoluble in acid under normal working conditions and it settle in tanks,
heat exchangers used for heating pickling bath, pipelines etc leading to
poor productivity and affecting the equipment health.
In the prior art, a Chinese specification CN102877081 (A) discloses pre-
pickling acid liquor circulation system and method for hot-rolled stainless
steel strip steel. The system comprises an acid mixing pickling circulation

tank, a sulfuric acid pickling circulation tank, a heat exchange device, and
a pickling bath; an outlet
of the acid mixing pickling circulation tank and an outlet of the sulfuric acid
pickling circulation tank are connected with a liquor inlet of an acid liquor
circulation pump; a liquor discharge port of the acid liquor circulation
pump is connected with an inlet of the heat exchange device; an outlet of
the heat exchange device is connected with an inlet of the acid mixing
pickling circulation tank, an inlet of the sulfuric acid pickling circulation
tank, and an acid inlet of the pickling bath; and an acid discharge port of
the pickling bath is connected with the inlet of the acid mixing pickling
circulation tank and the inlet of the sulfuric acid pickling circulation tank.
The circulation system is simple and practical; and by the circulation
system, investment cost is reduced, the pickling parameters can be
controlled continuously, the forming amount of sludge in a solution can be
reduced, and the pickling effect can be improved.
At Pickling Lines (PL) of Cold Rolling Mill (CRM) of Rourkela Steel Plant
(RSP) use H2SO4 acid as pickling media. The required pickling bath
temperature is maintained by indirect closed-loop pickling acid circulation
through graphite block heat exchanger. The indirect heating of the acid is
carried out by cross-flow of acid through the pores of graphite block and
steam through the shell of heat exchanger. The suction of the acid is
taken from the drain connections of the tank mounted at the side wall of
tank and pumped through heat exchanger for gaining the temperature
and finally poured back into the tank at other end as shown in Fig.2.
Graphite block heat exchangers are use at pickling line to maintain the
pickling bath temperature as shown in Fig.3. This type of heat exchanger
has less efficiency due to cross contamination of steam and acid, frequent
leakages, clogging of pores and maintenance prone. This results in lower
bath temperature leading to lower line speed and low productivity. Most
often line is stopped due to poor bath temperature. Uniform bath
temperature is not maintained due to absence of temperature control
system. This is the specific need in this field, which hitherto is not met.
The present invention meets this need and other associated needs. How

it is done will be clear from the description provided hereunder.
SUMMARY OF INVENTION
Accordingly, the primary object of the present invention is to provide an
improved bath heating system for sulphuric acid pickling line comprising
of: a tantalum shell & tube heat exchanger; an automatic temperature
control arrangement; and a flushing arrangement to clean the tantalum
heat exchanger.
Another object of the invention is to provide a heat exchanger which is
non-corrosive with sulphuric acid.
Yet another object of the invention is to provide an automatic heat control
arrangement configured to maintain the bath temperature of pickling tank
by controlling the steam flow rate to heat exchanger.
Yet another object of the invention is to provide a flushing arrangement
has also been installed to clean the tantalum heat exchanger with normal
water as and when required.
Yet another object of the invention is to provide a heat exchanger which
can be retrofitted and require less maintenance as it is fully welded metal
design which forms a monolithic piece of equipment with no potential
special parts requirement.
Yet another object of the invention is to provide a heat exchanger which
require significant reduction in steam consumption as compared with prior
art exchangers and which is versatile and easy to manufacture and use
less components.
How the foregoing objects are achieved will be clear from the following
description. In this context it is clarified that the description provided is
non-limiting and is only by way of explanation.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS

The nature and scope of the present invention will be better understood
from the accompanying drawings, which are by way of illustration of a
preferred embodiment and not by way of any sort of limitation. In the
accompanying drawings:-
Fig.1 .illustrates a graph showing the effect of bath temperature on pickling
efficiency;
Fig.2. illustrates the schematic of pickling lines at cold rolling mill;
Fig.3. illustrates the graphite block heat exchanger at Pickling line as prior
art
Fig.4. illustrates Iso-corrosion diagram for 5mpy of tantalum and other
metals in sulphuric acid;
Fig.5.illustrates a schematic diagram for automatic temperature control
system in accordance with the present invention;
Fig.6.illustrates tantalum shell & tube heat exchanger in operation in tank-
4, pickling line-2 in accordance with the present invention;
Fig.7. illustrates tantalum heat exchanger with flushing arrangement in
accordance with the present invention;
Fig.8.illustrates graph showing the comparison of temperature variations
inside the pickling tank comprising tantalum heat exchanger in
accordance with the present invention.
DETAILED DESCRIPTION
Having described the main features of the invention above, a more
detailed and non-limiting description of a preferred embodiment will be
given in the following paragraphs with reference to the accompanying
drawings.
Considering the drawbacks of prior art as discussed here before, the
invented system to take care of lower efficiency of heating system, the
bath heating system is modified by introducing an efficient heat
exchanger with automatic temperature control system. The system is
designed based on calculated heat load for pickling tanks. The heat load
calculation procedure was developed to design heating system for
different tanks of the pickling line.

Selection of suitable heat exchanger:
In the process of development, the available options for replacing the
existing graphite block heat exchanger were evaluated. Several trials
were conducted using direct steam injection through lead tube in the bath.
Direct steam injection in the baths lead to pickling bath dilution. This
affected the pickling efficiency by reducing the acid concentration.
Options of using immersion heat exchanger of Teflon PFA Tubing were
also evaluated. The immersion heat exchanger can be either installed
with tank wall or can be installed at bottom of the tank. Installing the heat
exchanger at the bottom of the tank was not found feasible due to large
amount of sludge present in the sulphuric acid pickling bath. Installing
with tank wall was ruled out due to possibility of damage of heat
exchanger by sidetracking of steel strip in the tank. This was possible only
when there is automatic side guide arrangement is installed in the line or
tank is modified to have pockets in the wall to accommodate the heat
exchanger.
Due to above reasons immersion type heat exchanger of reactive metal
were also ruled out. The choice left to replace graphite block heat
exchanger was shell and tube heat exchanger which can be easily
retrofitted with the existing system without any modification in pickling
tanks. In this type of heat exchangers, tubes (through which acid flows)
are made of reactive metals and shell is generally made of carbon steel.
Steam is circulated in the shell to heat the acid flowing through tubes. An
option of using different reactive metal was evaluated. Reactive metals
used for this type of heat exchangers are titanium, zirconium, niobium,
hostelry and tantalum. Fig.4. show iso-corrosion diagram of tantalum and
other metal in sulphuric acid. From the figure, it can be seen that tantalum
is the obvious choice for sulphuric acid application with practically nil
corrosion rate. The comparison of tantalum shell & tube heat exchanger
with graphite block heat exchanger is shown in Table -1. The table shows
the major design aspects of both heat exchangers. Hence, tantalum shell
& tube heat exchanger was selected for sulphuric acid pickling
applications. Tank#4 was chosen for installing the tantalum heat

exchanger, since maximum pickling take place in tank#4 due to high
concentration and fresh acid is added here. This installation is of its first
kind in any pickling line in India.

Total heat load taken to design heat exchanger was 1700 KW.
Table 2: Heat load calculation for designing of tantalum shell & tube heat
exchanger

Design of automatic temperature control system:
An automatic temperature control arrangement was designed to maintain
the bath temperature of tank-4 by controlling the steam flow rate to heat
exchanger. The system mainly comprises of pneumatically controlled
steam flow control valve with l/P converter, controller and RTED installed
in the tank. The schematic diagram of automatic temperature control
system is shown in Fig.5. The design of the system was based on steam
requirement of tantalum heat exchanger and available steam condition.
80NB steam control valve was selected. Microprocessor based
temperature PID single loop controller was selected for this application.
SS Valve body was selected for corrosive environment. A cover for
control valve was also designed to protect its positioned, l/P converter
and other accessories mounted on it from acid fumes.
Installation and commissioning of the complete system
The tantalum shell & tube was installed with the existing recirculation
system. This heat exchanger was installed near the existing graphite
block heat exchanger as shown in Fig.6. Provision for switching from
tantalum heat exchanger to graphite block heat exchanger has been kept
to use it during cleaning of tantalum heat exchanger. A flushing
arrangement has also been installed to clean the tantalum heat
exchanger with normal water as and when required as shown in Fig.7.
This prevents building of salts within heat exchanger tubes and thus
maintains the heating efficiency of heat exchanger. PVC with FRP lined
pipes are used for acid inlet and outlet pipes to heat exchanger.
Automatic temperature control system was also installed along with
tantalum heat exchanger. 80NB steam control valve was installed near
the heat exchanger. The controller was placed in delivery pulpit of pickling
line.
After installing the system, trail was conducted to ascertain the benefit of
the system. Temperature of the bath was monitored in every shift. The
temperature of the bath was set at 85°C. The temperature was recorded

in the log book of Pickling line. The variation in the temperature was
recorded. For ease of recording the data and monitoring the process, the
controller was installed in operation pulpit. The controller has a display,
where bath temperature is displayed through a RTD. Variation of
temperature on daily basis is depicted in Fig.8. It can be seen from the
graph that, there was wide variation in temperature before innovation.
With the installation of new system, temperature is maintained at set
value with ±2°C.
The system is running trouble free and it has helped in improving the
pickling line productivity and strip quality.
• The installed system has helped by:
i) Improvement in line productivity after introduction of the system by
10% [from 42.3 tons/hr (avg. Jan'11 - Oct'11) to 46.5 tons/hr (avg Nov'11
-May'13)].
ii) Reduction in operational delay by 17% [from 100 hr/month (avg
Jan'11 - Oct'11) to 83 hr/month (avg Nov'11 - May'13)] with improved
bath condition.
iii) Reduction in steam consumption by 50% due to efficient heating
system (from 3000kg/hrto 1500 kg/hr)
• The diameter of tube through which acid flows of tantalum heat
exchanger is ~19mm (about 4 times than graphite block pores). This
leads to higher flow rate during circulation of acid. With higher flow rate,
turbulence in pickling bath increases which helps in better pickling of strip.
• This heat exchanger require less maintenance as it is fully welded
metal design which forms a monolithic piece of equipment with no
potential special parts requirement. Fully welded metal design also resists
damages due to thermal and mechanical shock.
• With automatic temperature control arrangement, the bath temperature
is consistently maintained at 85±2 °C. Uniform bath temperature has
helped in better pickling quality leading to no under pickling and brighter
strip surface.

• Steam requirement for the installed Tantalum Shell & Tube heat
exchanger is 50% compared to previous installed graphite block heat
exchanger. Thus there is significant reduction in steam consumption.
Although the foregoing description of the present invention has been
shown and described with reference to particular embodiments and
applications thereof, it has been presented for purposes of illustration and
description and is not intended to be exhaustive or to limit the invention to
the particular embodiments and applications disclosed. It will be apparent
to those having ordinary skill in the art that a number of changes,
modifications, variations, or alterations to the invention as described
herein may be made, none of which depart from the spirit or scope of the
present when interpreted in accordance with the breadth to which they
are fairly, legally, and equitably entitled.

CLAIMS
1. An improved bath heating system for sulphuric acid pickling line
comprising of:
a tantalum shell & tube heat exchanger;
an automatic temperature control arrangement; and
a flushing arrangement to clean the tantalum heat exchanger.
2. An improved bath heating system as claimed in claim 1, wherein the
steam is circulated to heat the acid flowing through tantalum shell and
tube heat exchanger tubes.
3. An improved bath heating system as claimed in claim 1, wherein the
tantalum shell and tube heat exchanger is fully welded metal design
which forms a monolithic piece of equipment with no potential special
parts requirement and resists damages due to thermal and mechanical
shock.
4. An improved bath heating system as claimed in claim 1, wherein the
automatic temperature control arrangement comprises microprocessor
based temperature PID single loop controller.
5. An improved bath heating system as claimed in claim 1, wherein the
automatic temperature control arrangement further comprises of a
stainless steel steam control valve with cover.
6. An improved bath heating system as claimed in claim 1, wherein PVC
with FRP lined pipes are used for acid inlet and outlet pipes to heat
exchanger.
7. An improved bath heating system as claimed in claim 1, wherein the
automatic temperature control arrangement is placed at the delivery pulpit
of pickling line.
8. An improved bath heating system as claimed in claim 1, wherein the

automatic temperature controller has a display, where bath temperature is
displayed through a RTD.
9. An improved bath heating system as claimed in claim 1, wherein the
automatic temperature control arrangement is configured to consistently
maintain the bath temperature at 85±2°C
10. An improved bath heating system as claimed in claim 1, wherein
the flushing arrangement uses normal water to clean the tantalum heat
exchanger.

ABSTRACT

The present invention relates to an improved bath heating system for
sulphuric acid pickling line comprising of a tantalum shell & tube heat
exchanger, an automatic temperature control arrangement, and a flushing
arrangement to clean the tantalum heat exchanger.