Expert Answer:Rotary Kiln System Health, Safety And Environmenta

  

Solved by verified expert:Paraphrasing of the word file submitted. A side note, the material used in the file submitted is mica, while my chosen material is silica sand (or quartz) so whenever you come across mica, replace it with either silica sand or quartz. Also, this assignment has turnitin enable, so make sure that as little plagiarism as possible is found.
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Health, Safety and Environmental Issues
Potentially the most dangerous threat to human health within the kiln system is the fluorine and hydrogen
fluoride which would be produced if insufficient quantities of sodium sulphate are added. These substances
are highly toxic, and corrosive, and are also capable of corroding equipment, and hydrogen fluoride also
contributes towards acidification of the environment if it is not cleaned from the stack gases. Consequently, it
is desired to react the fluorine with the bed material to form a solid before it reaches the kiln gas;
experimental tests will be carried out in order to determine an optimal amount of sodium sulphate which is
capable of maximising the amount of fluorine released which form solid compounds. It is believed that the
ratio of 1 kg of sodium sulphate for every 3.4 kg of micaceous feed would be sufficient, but this is based on
very tenuous assumptions, so laboratory tests are necessary.
.
Even with excess sodium sulphate, some fluorine near the surface will escape to the kiln gas stream before it
has a chance to react with the bed, and while it is in this gas stream, it is likely to react with water vapour to
form hydrogen fluoride. Being at very low concentrations (if enough sodium sulphate is present), this is not
much of a problem at low temperatures, but the corrosiveness of fluorine and hydrogen fluoride increases
dramatically with temperature, and so even this small concentration is likely to be capable of damaging
unprotected equipment. Consequently, piping and equipment such as fans which come into contact with kiln
gas at high temperatures are lined with a corrosion resistant metal (which is later chosen to be Inconel 600),
and a refractory brick which is resistant to fluorine induced silica depletion is chosen. Any hydrogen fluoride in
the kiln gas would be removed by reacting with limestone chips in the packed bed scrubber, to produce
calcium fluoride. The kiln system operates below atmospheric pressure, at 0.8 bar, to ensure that ambient air
leaks in, which is preferable to kiln gas leaking out. The exposure limits for fluorine and hydrogen fluoride are
given in Table 4 below. Another problematic health hazard would be dust inhalation. At particle sizes of below
around 4 µm, mineral dust can penetrate the lungs and cause fibrosis. Since mica has a fine particle size, this
therefore poses a health risk to personnel. Therefore, all units which convey solids, such as screw conveyors,
are enclosed to prevent dust escaping, and the kiln system operates at negative pressure, to prevent dust
laden gas from leaking out. Cyclones are used to remove relatively coarse dust particles from the kiln gas
stream, with the smaller particles being removed by a baghouse filter. Carbon monoxide is unlikely to be a
problem, as excess air is used during combustion, and nitrous oxide should be reduced to safe levels by using a
low NOx burner.
Long term exposure limit
(8 hr TWA reference period)
Fluorine
Hydrogen fluoride
Mica – total inhalable
Mica – respirable
ppm
1
1.8

mg/m3
1.6
1.5
10
0.8
Short term exposure limit
(15 minute reference period)
ppm
2
3

mg/m3
2.5
2.5

Table 4 – occupational exposure limits for the major hazardous substances in the kiln gas (HSE, 2007)
.
Overheating of the kiln, or rapid changes in temperature, also contribute a considerable safety hazard. The
refractory must be heated or cooled slowly to avoid thermal shock which can cause damage, and the shell
itself must be heated slowly so that differential thermal expansion between the shell and tyres doesn’t cause
damage to the shell. Consequently, startup and shutdown protocols are used which take this into account,
which are discussed later in the report, and cascade control is used to ensure steady and safe temperature
ranges across the kiln. Refractory damage due to corrosion or thermal shock can compromise the insulation
properties, which means that more heat can escape from the kiln interior and heat the shell. This reduces the
strength of the steel, and so can cause structural damage of the kiln, which could result in a loss of
containment. Consequently, the kiln shell is periodically monitored with a temperature gun to look for ‘hot
spots’ indicative of a refractory failure, so the kiln can be shut down and the refractory replaced before
irreversible damage is done to the shell.
5 – Process Control
.
5.1 – Piping and Instrumentation Diagram
Adequate process control is important to ensure that the product is manufactured economically, safely, and
with little damage to the environment; this involves the control of a wide range of factors, such as the
temperatures, pressures, and flow rates of several streams, and because of the interactions between these
variables, the effects of control can become complex. Basic control systems have been selected for the entire
lithium carbonate plant and are shown on the piping and instrumentation diagram (P&ID), but as the rotary
kiln is the focus of the report, the control systems in this region are discussed in detail. The complete control
system is shown at the end of the report on the P&ID as Figure A2. The nomenclature used in the P&ID will be
described in this section. The plant is divided into four regions on the P&ID, and the code for each piece of
equipment includes a number representing the region; the four regions are described below:
.
1 – Flotation region
2 – Magnetic separator region
3 – Rotary kiln region (the scope of this report)
4 – Leaching and precipitation region
.
The P&ID nomenclature for unit operations, valves, instrumentation, and piping will be described below.
Unit Operations
The generic labelling for a unit operation is ‘X-0000’. ‘X’ consists of one or two letters representing the
equipment, the first number represents the plant region as defined above, the second number represents the
system code, and the last two numbers is a two digit identifier number unique to the unit. The system codes
used are described as follows:
1 – Process liquid/slurry
2 – Process gas
3 – Process solids (powder or
cake)
4 – Fuel gas
5 – Heating/cooling utilities
6 – Air
.
The letters used to represent each type of unit operation are shown below:
BF – Baghouse filter
C – Cyclone
D – Dryer (spray or tube press)
F – Fan
FL – Flotation Cell
HE – Heat exchanger
M – Mill (hammer or pin)
MS – Magnetic separator)
P – Pump
PB – Packed Bed Scrubber
RC – Rotary cooler
RK – Rotary kiln
SC – Screw conveyor
SF – Screen/filter
T – Tank
.
An example of the use of this nomenclature is the kiln gas cyclone on the P&ID. It is a cyclone, so it is
represented by the letter ‘C’. It handles the process gas, so the first number is ‘2’, and is in the kiln area of the
plant, and so the second number is ‘3’. The last two numbers is a unique identifier number, which is 08 in this
case. The code for the kiln gas cyclone is therefore C-2308. In some cases, some pieces of equipment like fans
and pumps are standby units, which automatically come into operation if the main unit fails; this is
represented by a suffix of ‘-b’. For example, the code for the backup fan for the preheater cyclone is F-2315-b.
Valves
Valves have a five or six digit code on the P&ID, the generic form of which is ‘V0000X’. ‘V’ is constant for all
valves, and is simply used to show that the code is representing a type of valve. The first number represents
the region of the plant the valve is located in (1-4), and the last three numbers are a unique identifier code. In
the case of a generic block valve, that is the entirety of a code, but in the case of other types of valves, such as
a control valve, there is an additional letter at the end of the code (‘X’ in the generic code above) which
represents the type of valve; these types are shown below, along with a symbol representing each valve:
No suffix – Block valve
V – Vent
D – Drain
G – Globe valve
C – Control valve
P – Pressure relief valve
PC – Pressure control valve
X – Spectacle blind
K – Check valve / pendulum flap gate
B – Bleed (sampling point)
.
An example of this usage is the check valve after the primary fan used to keep the kiln system at 0.8 bar
pressure. It is in the kiln section of the plant, and so the first number is ‘3’, and it is a check valve, so its last
letter is ‘K’. The identifier code is ‘086’. The complete code for this valve is therefore V3086K.
Piping
The generic code for piping is ‘00”-X-000-00X-X’. The first two numbers is the pipe nominal diameter in inches,
the first letter is the commodity which the pipe is carrying. The first of the subsequent three numbers
represents the region of the plant (1-4), with the next two being the identifier code. In the ‘00X’ part of the
code, the first two numbers represent the piping specification (which for all pipes considered in this report, is
‘10S’) which represents the thickness, and the letter in the ‘00X’ part represents the construction material. The
final letter of the code represents the insulation specifications, with ‘N’ meaning no insulation, ‘H’ meaning
insulation for heat conservation, and ‘PP’ is insulation for personnel protection, which is used when there is a
reasonable chance that personnel could come into contact with hot surfaces. The commodities defined by the
first letter in the code are shown below:
A – Air
F – Fuel gas
G – Process gas
P – Powder
ST – Steam
W – Water
L – Process liquid/slurry
.
The materials of construction of the pipes are given a simplified code of A, B, C, or D, which are described
below
A – Mild steel
B – Grade 304 stainless steel
C – Grade 304 stainless steel with 1 mm Inconel 600 lining
D – Grade 310 stainless steel with 2 mm Inconel 600 lining
.
The gas outlet pipe immediately after the kiln will be used as an example. The nominal diameter of the pipe is
30”, and it transports process gas, so the first letter in the code is ‘G’. It is in the kiln section of the plant (‘3’),
and has an identifier code of 15. The piping specification is 10S, and the material of construction is Grade 310
stainless steel with 2 mm Inconel 600 lining (‘D’). Because the kiln gas pipe is unlikely to come into contact
with personnel during operation, heat conservation grade insulation (‘H’) will be used rather than personnel
protection grade. This results in a code of 30”-G-315-10SD-H. The piping was given codes only for pipes
carrying fluids in the rotary kiln section, as this was the scope of the piping calculations performed earlier in
this report.
Instrumentation
An example of an instrument symbol (in this case a pressure indicator) is shown below:
In the top hemisphere, the first letter represents the factor being measured (in this case, pressure), with the
subsequent letters representing the function of the measurement; in the above example, it is an indicator. In
the bottom hemisphere, the first number representing the region of the plant which the instrument is located
in, and the subsequent two numbers represent the control loop that the instrument is part of. The process
variables represented by the first letter of the top hemisphere of the instrument symbol are shown below:
T – Temperature
P – Pressure
F – Flow rate
M – Moisture
L – Level
dP – Differential pressure
.
The functions represented by the subsequent letters in the top hemisphere (which can be between one and
three letters) are shown below:
I – Indicator
T – Transmitter
C – Control
AHH – very high level alarm
ALL – very low level alarm
SD – Shut down
.
When ‘control’ is used as the second letter, this means that the variable being measured (the first letter) is the
control variable; in this case, there is also an implicit indicator used to measure the variable, and a transmitter
to convert this data into a form in which the control system can use, but these are not included in the P&ID for
the sake of brevity. When programmable logic controller (PLC) is used for complex control loops, several
variables can be used as inputs; in this case, all of the inputs which are control variables are designated as ‘C’,
whereas all other inputs which are used in the calculations, but are not being controlled by the PLC, are
designated as ‘T’. An example of this is shown below.
.
The above example is used for controlling the temperature of the combustion air and recirculated kiln gas
mixture entering the kiln (which is represented by the TC 346 balloon). It uses the flow rate of the natural gas
stream and the current temperature of the recirculated kiln gas stream (both represented by transmitters) to
determine the set point for the control variable. The PLC then produces an output signal which is used to
control the recirculation fan speed to control the flow rate (not shown on the above example). For a particular
variable being controlled, all other factors used in this particular control loop are given the same control loop
code, including the PLC, if one is used. In the case of indicators, even though they are not used in control, all
indicators on a given section of pipe (with no unit operations or pipe branches in between) are given the same
control loop number. If there are several instruments which measure the same variable on a pipe length (as
there are sometimes with pressure gauges, used to check that valves have closed during shutdown), they are
given suffixes, such as A or B, on the end of their control loop number to differentiate them.
.
5.2 – Rotary Kiln Process Control
There are many parameters which must be finely tuned to ensure efficient, productive, and safe operation of
the rotary kiln. The first control system encountered in the kiln section of the plant is control of the
stoichiometric ratio of micaceous feed and sodium sulphate. In order to dampen out variations in the flow
rates and moisture contents of the filter cake produced from the filter press at the end of the magnetic
separation section, a buffer hopper is installed (T-204) before the screw conveyor (SC-3205) which feeds the
mica into the kiln system. The flow rate into the solids mixer is controlled by the speed of this screw conveyor;
this must be kept roughly constant, in order to reduce variation in temperature and flow rate downstream.
The screw conveyor will attempt to transport mica at a rate that results in steady state in the buffer hopper;
two alarms have been installed on the hopper which warn when high or low levels are reached, in which case
the set point of the screw conveyor changes to counteract this. As the sodium sulphate must be added at a
specific ratio (3.4 kg of mica feed to 1 kg of sodium sulphate), the flow control of sodium sulphate powder into
the solid mixer takes into consideration the flow rate of the mica feed, and a PLC is used to adjust a control
valve to allow the desired amount of sodium sulphate through (control loop 301).
.
The temperature of the solid feed entering the kiln must be controlled at around 300°C. This is achieved in
control loop 305 by measuring the temperature of the solids in the underflow of the cyclone preheater C3302. This information is then used to change the motor speed of the fans on the preheater overflow, which
changes the fraction of kiln gas which is diverted to the preheater. Due to the residence time being in seconds
rather than minutes or hours, cascade control is unnecessary, and simple feedback control is sufficient.
Controlling the flow rate of the solids into the kiln is also important, as it is desired to keep the heat transfer
duties constant, and a large disturbance in flow rate can cause a thermal shock, damaging the refractory. This
is achieved in control loop 314 by varying the speed of screw conveyor SC-3304, which is fed by a hopper, T3303. As a steady state between the inlet and outlet of the hopper is needed, the set point for control loop
314 is chosen accordingly. If there are long term variations in the feed flow rates into the kiln section, then a
steady state will not be reached, and the level in the hopper will increase or decrease without limit. Once
these deviations reach a critical value, low or high level alarms are activated, and the set point of the screw
conveyor is changed in attempt to obtain a steady state.
The temperature control of the kiln is very complex, particularly because of the adjustable recirculation ratio
and preheated air addition. Measuring the temperature of the outlet gases and solids with thermocouples can
be used as an indication of the heat transfer rates within the kiln; using a heat transfer model similar to the
one used in this report, these two temperatures can be used to infer whether the solids have been above
850°C for more than an hour. The gas has a much shorter residence time than the solids, with the gas
residence time in the range of seconds, compared to a total of approximately 1.5 hours for the kiln. Therefore,
any disturbances in the temperature and flow rate of the feed or kiln gas will take a long time to have an effect
on the final temperature of the solids; a feedback control loop between the outlet solids temperature alone
and the firing rate of the burner is therefore impractical. Cascade control is therefore used as the outlet
temperature of the kiln gas is much quicker to respond to changes in inlet conditions for both the feed and the
gas. Measurements of the solids outlet temperature are used to calculate the set point for the kiln gas outlet
temperature control, which is the kiln gas temperature at which the control system expects the solids outlet
temperature to reach the desired value. The kiln gas temperature is then controlled to try to maintain this set
point, by varying the flow rate of natural gas which passes to the burner. If the solids outlet temperature still
deviates from its set point, the set point of the kiln gas is adjusted, and is attempted to be maintained by
varying the firing rate.
The natural gas flow rate is set by the desired outlet temperatures of the kiln gas and solids. This must be
combusted with 35% excess air, so the air flow rate must be controlled taking into account the natural gas flow
rate. It is also desired to utilise the heat retrieved from the rotary cooler by the cooling air, in order to reduce
the firing rate of the burner necessary for a given flame temperature and kiln gas heat content. There will be
significant variations in the preheated air temperature due to the dynamics of the solids cooling operation
upstream, along with differences in ambient temperatures, but it is desired to keep the temperature of the air
that is to form part of the precombustion gas mixture constant (at 300°C), to reduce variation in flame
temperature. The flow rate and temperature of this air stream is a product of the ambient air stream and the
preheated air stream, and because the temperature and flow rate of the preheated air stream is believed to
be around the required value for this mixed air stream, it will consist of predominantly the preheated air
stream. A PLC controller (control loop 371) is used to ensure that this mixed air stream has the optimum flow
rate and temperature for the amount of natural gas being burned.
If the temperature and flow rate of the mixed air stream is too great, then the PLC increases the opening in
control valve V3199C which diverts some of the preheated air away into the atmosphere. If this is insufficient
in reducing the flow rate, or if the temperature set point changes to be lower, then the globe valve on the
preheated air stream can be throttled to make it narrower, increasing the amount of preheated air diverted
away from the mixed air stream. If the temperature is too high and the flow rate insufficient, …
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