,~i
T
U Delft
Fabrieksvoorontwerp
Vakgroep Chemische Procestechnologie
Onderwerp
ABATEMENT OF NITROUS OXIDE EMISSIONS PRODUCED IN AN ADIPIC ACID PLANT
Auteurs
Saskia Cooman
Keshav Gorur
N aveen Menon
Hendrik Stok
Keywords
Telefoon
015-2624146
015-2618611
070-3270316
015-2159964
Nitrous oxide, adipic acid, selective catalytic reduction, monolith reactor, nitrogen oxides.
Datum opdracht
Datum verslag
7 February, 1996
17 April, 1996
The presence of nitrous oxide in the earth's stratosphere contributes to the destruction of the ozone layer and also to global warming. A major source of nitrous oxide emissions is in the adipic acid (AA) industry. Recently, agiobal initiative was set up
to greatly reduce nitrous oxide and nitrogen oxides emissions by the year 1998. For
this reason, a plant was designed to reduce the nitrous oxide content in adipic acid offgases to meet the above mentioned requirements. The basis of design was an
existing adipic acid plant in Ontario, Canada; the capacity of this plant was 100,000
tonnes/year.
The reduction of nitrous oxide was preceded by a selective catalytic reduction step of
nitrogen oxides in a monolith reactor. The catalyst used in the reduction of nitrogen oxides was vanadium pentoxide on y-alumina. An advantage of using a monolith reactor for this purpose is the extremely low pressure drop. The selective catalytic reduction of nitrogen oxides was placed upstream of the nitrous oxide reactor for
improved catalytic performance. The operating pressure and temperature of this
reactor were 2.2 bar and 330°C.
A multibed reactor with interstage cooling was designed to catalytically abate nitrous oxide. The catalyst used in this conversion was nickel oxide/co balt oxide on a zirconia
carrier. This reactor was designed to operate at pressures of 2.5-3 bar and
temperatures in the range of 400-700°C.
Abatement of nitrous oxide and nitrogen dioxide was shown to be effective using this
design; the convers ion of nitrous oxide and nitrogen dioxide were 99.4 mol% and 95.3
mol% respectively.
Finally, an economic analysis of this plant was conducted. This revealed that the
investment costs amounted to $ 8.66 million. Furthermore, the production costs
incurred (over a period of 15 years) were $ 49.74 million. It should be noted that this plant generates no profits, the products of this plant are predominantly nitrogen and oxygen and are sent back to the atmosphere.
We would like to express our sineere thanks to Ron Reimer of Dupont in Texas for his advice and assistance during the entire design stage of the nitrous oxide reactor. Our thanks also extend to Dr.ir. F. Kapteijn for letting us use his room for information retrieval and also to Joost Overeijnde for the overhead sheets.
Much appreciated thanks to Ir. Luteijn for supplying us with some relevant documentation and for the guidance provided.
1. INTRODUCTION ... 1
2. BASIC HYPOTHESIS ... 3
2.1 PLANT CAPACITY ... 3
2.2 BATIERY LIMIT ... 3
2.3 PROCESS ROUTE ... 4
2.4 ADDITIONAL SPECIFICA TIONS ... 4
2.4.1 Feed ... 4
2.4.2 Products ... 4
2.4.3 General reactor design considerations ... 5
2.4.4 Utilities ... 6
2.4.5 Location ... 6
2.5 CHEMICALS LIST ... 6
3. PROCESS STRUCTURE AND FLOWSHEET ... 8
3.1 PROCESS STRUCTURE ... 8
3.2 THERMODYNAMICS ... 9
4. EQUIPMENT DESIGN ... 10
4.1 NITROGEN OXIDE REDUCTION REACTOR (R5) ... 10
4.1.1 General design considerations ... 10
4.1.2 Derivation of the model equations ... 11
4.2 NITROUS OXIDE REDUCTION REACTOR (R7 + R9) ... 16
4.2.1 Derivation of the model equations ... 16
4.2.2 Modelling of the nitrous oxide reactor ... 18
4.3 HEAT EXCHANGER DESIGN ... .' ... 21
4.3.1 Overall heat transfer coefficient ... 22
4.3.2 Heat transfer area ... 22
4.3.3 Number of tubes ... 22
4.3.4 Pressure drop ... 22
4.3.5 Spiral cooler design ... 23
4.4 COMPRESSOR DESIGN ... 23
4.4.1 Theory ... 23
4.4.2 Compressor design values ... 24
S. PROCESS CONTROL ... 25
5.1 PROCESS CONTROL OF THE DIFFERENT SECTIONS ... 25
5.1.1 The feed section ... 26
5.1.2 The NO. reactor ... 26
5.1.3 The N20 reactor. ... 26
5.2 MISCELLANEOUS SAFETY MEASURES ... 27
6. PROCESS SAFETY ... 28
6.1 PROPERTIES OF SUBSTANCES ... 28
6.2 TOXICITY OF SUBSTANCES ... 29
6.3 THE DOW FIRE AND EXPLOSION INDEX ... 29
Contents
6.5 SAFETY FEATURES ... 30
6.5.1 Essential security aspects installed in the NOx reactor. ... 30
6.5.2 Important safety features of the N20 reactor ... 31
6.5.3 Miscellaneous safety features present in the plant.. ... 31
6.6 HAZARD AND OPERATIONAL STUDIES (HAZOP) ... 32
7. PROCESS ECONOMICS ... 35
7.1 THE PRODUCTION COSTS ... 35
7.1.1 The ca1culation of Kp' ... 37
7.1.2 The computation of KI' ... 37
7.1.3 The estimation of KL· ... 37
7.1.4 The ca1culation of KT ... 38
7.2 THE INVESTMENT COSTS ... 38
7.2.1 Taylor's method ... 39
7.2.2 Miller's method ... 39
7.2.2.1 Calculation ofthe PI investment .................................. 40
7.2.2.2 Computation of the Utilities 1nvestment ........................... 41
7.2.2.3 Estimation ofthe Storage & Handling 1nvestment ... 42
7.2.2.4 Evaluation ofthe Services lnvestment ... 42
7.2.2.5 The estimation of IF .......................... 43
7.3 ECONOMIC CRITERIA ... 44
7.3.1 Return on Investment.. ... 44
7.3.2 Pay-Out Time ... 45
7.3.3 Internal Rate of Return ... 45
7.4 THE ECONOMIC EV ALUATION ... 45
8. CONCLUSION AND RECOMMENDA TlONS ... 47
9. NOMENCLATURE ... 49
10. REFERENCES ... 52
LIST OF APPENDICES
ApPENDIX 1: Process flowsheet
ApPENDIX 2: Equipment lists and specifications
ApPENDIX 3: Mass and heat balance
ApPENDIX 4: Dow's Fire & Explosion index
ApPENDIX 5: Reactor model equations
ApPENDIX 6: Heat exchanger calculations
ApPENDIX 7: Stream summary
1
INTRODUCTION
Nitrous oxide in the earth's atmosphere is known to be increasing by approximately 0.2% on a yearly basis presumably by anthropogenic activity (Thiemens and Trogier, 1991). The presence of nitrous oxide in the earth's stratosphere contributes to the destruction of the ozone layer and also to global warming. A major source of nitrous oxide ernissions is in the adipic acid (AA) industry.
Adipic acid is among the top 50 synthetic chemicals produced in the United States, the current annual global production is estimated at 1.8 million metric tonnes. The main use of AA is in the manufacture of nylon 6,6 polyamide. Adipic acid manufacture results in the production of ca. 1 mole of nitrous oxide per mole of AA. It has been shown that emissions of nitrous oxide stemming from the AA industry make up 5-8% of the total, worldwide anthropogenic emissions (Reimer et al., 1994). Even though the nitrous oxide present in the atmosphere does not contribute greatly to ozone depletion, a major initiative between the world largest AA producers was set up in 1991. This initiative included setting up an inter-industry group for sharing information on technologies for nitrous oxide control. The major players in this group have decided to substantially reduce nitrous oxide emissions by 1996-1998.
Various methods have been investigated in the past for the decomposition of nitrous oxide. The principal methods are: (i) thermal destruction; (ii) thermal conversion to nitric oxide (for recovery as nitric acid); and (iii) catalytic reduction. The option chosen for this plant design was catalytic reduction to nitrogen and oxygen.
The design employed here is limited to the removal of nitrous oxide and nitrogen oxides from the off gases originating from a typical AA plant. The actual production of AA wil! not be considered. Furthermore, the removal of other volatiles, carbon monoxide or carbon dioxide will also not be discussed. These compounds are assumed to have been removed in an earlier conversion step in the AA plant.
For this design, the total adipic acid production is assumed to be 100,000 tonnes/year. Furthermore, the composition of the off gas stream has been taken to be sirnilar to the off gas composition in the DuPont plant in Victoria, Texas (Reimer et al., 1994). The composition of nitrous oxide in this stream is approximately 30% I. The required
be 95%. This will result in the emission of approximately 0.16% nitrous oxide and 0.38% nitrogen oxides (of which 6.3 x 10-3 % N02).
2
BASIC HYPOTHESIS
2.1
PLANT
CAPACITY
The capacity of the nitrous oxide decomposition plant is based on an adipic acid plant production of 100,000 tonnes/year. This correlates to handling approximately 76,000 tonnes off gas flow per year, based on a 350 days on-stream plant.
2.2 BATTERY
LIMIT
The battery limit for the process can be visualised in Fig. 1. The incoming streams are purely the off gas flow from the AA plant and the ammonia L1sed to catalytically reduce the nitrogen oxides.
orr gas stream
@l
ammonia stream
Catalytic removal of nitrous oxide
stack now
Fig. 1. Black box diagram for the process
The stream specifications around the battery limit have been outlined in Table 1.
Table 1. Batte~ limit stream sEecifications
Stream name Nu mber Mass flow Temperature Pressure Phase
(kg/h) (0C) (bar)
Off gas stream SI 9090.24 35 3.11 Gas
Ammonia stream S2 262.42 - 100 2.21 Gas
2.3 PROCESS ROUTE
The-process route chosen was considered to be the most economically feasible option. The route involves firstly catalytic convers ion of NOx into nitrogen and water and
subsequently selective catalytic reduction of N20 into nitrogen and oxygen. The
overall reaction equations for the two principal reactors are shown below. The first two reactions (1 and 2) are for the reduction of N02 and NO respectively, the last
reaction (3) is for the reduction of nitrous oxide. All the reactions are exothermic.
4 NO
+
O2+
4 NH3 ~ 4 N2+
6 H20 óHr=
-4.07 x 10 5J.mor1 NO (2) N20 ~ N2
+
Y2
O2 óHr=
-82 kj/mol (3) A simplified block scheme illustrating the process route can be found in Fig. 2. This schematic illustrates the main units in the conversion reaction.N20 SCR
Fig. 2. Overall process block scheme
2.4 ADDITIONAL SPECIFICATIONS 2.4.1 Feed
The ammonia used in the selective catalytic reduction of nitrogen oxides should be entered into the system as a gas, diluted with air. The dilution with air should be weIl with explosion limits (15-28% in air). The off gas composition contains ca. 57% N2 ,
30.34% N20, 6% CO2 , 3.9% O2 , 2% H20, 0.7% NOx, 0.03% CO and 0.03% ethylene
(volatiles). This has has been shown to be typical for an AA plant (Reimer et al.,
1994). The offgases from an AA plant are typically at a temperature of approximately 35-200°C and at a pressure of 1-5 barg (Riley and Richmond, 1993).
2.4.2 Products
carbon monoxide/dioxide and various volatiles in the system (mainly ethylene) are assumed to be already reduced to ~98%. The temperature of the stack flow should not be lower than approximately 150°C (at 2 bar)since there is a considerable amount of water vapour present which could condense during piping transport.
2.4.3 General reactor design considerations
For the selective catalytic reduction of nitrogen oxides, a low pressure drop across the reactor is desirabie, preferably in the order of 10 mbar. This can be achieved by designing a monolith reactor. The conversion of nitrogen oxide is high yet the pressure drop can be maintained low. The catalyst used in this reactor is vanadium pentoxide/ca1cinated y-alumina (Garcin et al., 1993). Some characteristics of this catalyst can be found below:
• carrier surface area
=
90-150 m2/g;• pore diameter = 1000 Á and 300 À with pore volumes of 0.25-0.7 cm3/g and 0.43-0.7 cm3/g;
• total pore volume
=
0.8-1.2 cm3/g;• ratio of active phase weight to total weight
=
0.5-20%.The optimal temperature to operate the reaction is at around 320°C. The ammonia slip at these temperatures is low. The N02INO ratio in most off gas streams is ca. 0.2-0.45. The optimal gas hourly space velocity (GHSV) of the reactor should be between 5,000 and 20,000 h-I. There is no substantial activity loss in the catalyst for the first
10 years.
In the nitrous oxide reduction reactor it is important to control the significant temperature rise as the decomposition reaction is highly exothermic. It is required here that the temperature be maintained below 800°C. The catalyst used in this reactor is NiO/CoO on a zirconia carrier (Anseth and Koch, 1993). Some details of the catalyst are outlined below:
• content of zirconia could vary between 20-90 wt%; • the ratio of NiO/CoO is usually 0.5-3: 1
• particle diameter is 0.125 inches.
The optimal temperature for conversion is around 400°C, however, the reaction temperature could go as high as 800°C without adverse effects to the catalyst. The formation NOx is predominant at temperatures higher than 800°C. The effectivity of
the catalyst is negatively influence with entrance concentrations of N02
>
100 ppm.For this reason the convers ion N02 should take place before the nitrous oxide
2.4.4 Utilities
The 'basic utilities needed in the plant are cooling water to control the significant temperature rise in the nitrous oxide decomposition reaction, The cooling water is converted to steam at low pressure. Furthermore, electricity wil! be needed for the operation of the various units including the trim-heater and the recycle gas compressor.
2.4.5 Location
The plant is designed for the DuPont AA plant in Maitland, Ontario in Canada. This existing facility produces close to 100,000 tonnes AA every year (Kirk-Othmer, Encyclopedia of Chemical Technology). Ammonia storage facilities nearby the NOx
reduction reactor is essential.
2.5 CHEMICALS LIST
The chemicals used in this process have been outlined in Table 2, along with some of their physical and chemical properties (Sax' s Dangerous Properties of lndustrial Materials and Handbook of Chemistry and Physics).
(gImol) (0C) (0C) (kglm3) vallIe rating
(~~m)
Nitrous oxide 10024-97-2 N20 44 -88 -90 1.90 2 a 1068 mg/m3 FI. 10.95 per kg
Nitric oxide 10102-43-9 NO 30 -152 -164 1.32 25 3 a 1068 mg/m3
Nitrogen dioxide 10102-44-0 N02 46 21 -11 2.02 2 3 b 200 ppm
Carbon monoxide 630-08-0 CO 28 -191 -205 1.23 25 3 b 5000 ppm FI. 181.3 per kg
Carbon dioxide 124-38-9 CO2 48 -56 -56 1.93 5000 I b 90000 ppm FI. 58,- per kg
Oxygen 7782-44-7 O2 32 -183 -219 1.40 3 c 106 ppm FI. 47.25 per m3
Nitrogen 7727-37-9 N2 28 -196 -2\0 1.23 FI. 4.40 per m3
Water (steam) 7732-18-5 H20 18 100 0 1.12 1
Ammonia 7664-41-7 NH3 17 -33 -78 0.75 25 3 b 30,000/5 M $ 0.315 per kg
Ethylene (VOC) 74-85-1 C6H4 28 -104 -169 0.61 3 b 950,000 ppm/ 5 M
iI LC50 (rat) - Lethal Concentration 50: A calculated concentration of a material in air, exposure to which for a specified length of time, is expected to cause the death of 50%
of an entire defined experimental animal population. It is determined from the exposure to the material of a significant number from that population.
b LCLo (inhalation-human) - Lethal Concentration Low: The lowest concentration of the material in air, other than LC50, which has been reported to have cost death in
hUll1ans or animals. The reported concentrations may be entered for periOtls of exposure which are less that 24 hours (acute), or greater than 24 hours (subacute and chronie).
C TeLo (inhalation-human) - Toxic Concentration Low: The lowest concentration of material in air to which lUlmans or animals have been exposed for any given period of
3
PROCESSSTRUCTUREAND
FLOWSHEET
3.1 PROCESS STRUCTURE
The process structure will be explained using the process flow sheet (PFS, see Appendix 1). The PFS was drawn using the technical drawing package, Autocad 12. In this process considerabie time was spent on the arrangement of heat exchangers. The overall process is exothermic, therefore heat integration is an important aspect in the design. Emphasis was placed on utilising the existing heat in the system and refraining from using extern al sources of heat.
The off gas stream coming from the AA plant enters the system at 3.11 bar and 35°C, the composition of this stream can be found in § 2.4.1. The feed is sent to the first feed/effluent heat exchanger (H2) and heated up to 186°C by the hot feed to the nitrous oxide reactor (R7). The incoming effluent stream enters the heat exchanger at 458°C and 3.2 bar (compressed from 375°C and 2.2 bar) and Ieaves H2 at precisely 400°C and 3 bar (at reaction temperature and pressure).
The feed is subsequently heated again by exchanging heat with the hot stack gas flow. The feed is heated in H3 to 290°C at 2.71 bar. The stack gases leave the system at 300°C and 2 bar. Since the feed is still not up to the reaction temperature, it is sent to a fired trim heater which serves to bring the feed to 320°C.
Compressed air (from Cl, at 2.21 bar) and gaseous ammonia are mixed in the correct quantities before being entered into the reaction system.
The hot feed is then mixed at the reactor entrance with the diluted gaseous ammonia feed. This mixing has the effect of reducing the reactor feed temperature to 314°C, which is optimal for the adiabatic operating reactor. The pressure of the feed is now at 2.21 bar.
The combined feed enters the deNOx reactor (R5) and the conversion of N02 is 95%. The reactor is operated adiabatically and the temperature at the exit is 343°C. Operating the reactor at higher temperature results in a lower overall conversion of
NO. The pressure drop in R5 is minima!, this is only about 10 mbar. The exit pressure is 2.2 bar.
The stream exiting the reactor R5 is mixed with part of the reactor effluent of the nitrous oxide reactor (R7
+
R9). This mixed stream (at 375°C and 2.2 bar) is sent to compressor C6 for compression to 3 bar, the temperature rises as a result of the compression to 458°C. The heat of this stream is exchanged with the cold feed and sent to the entrance of the reactor R7+
R9. The feed to this reactor is converted partially to nitrogen and oxygen in the first bed. The reaction is highly exothermic and is stopped when the temperature of the intermediate product reaches around 706°C. The pressure drop across the first reactor bed is 0.17 bar. This stream is then cooled by exchanging the heat with cold water in the spiral H8. The water leaves the exchanger 131°C. The intermediate product is cooled to 497°C and entered into the second packed bed of catalyst particles at 2.8 bar.The final conversion of nitrous oxide to nitrogen and oxygen takes place in the second bed (R9), the exit stream is at a temperature of 580°C and a pressure of 2.5 bar. This stream is cooled again by the heated water produced in H8, the water is heated further to steam at 380°C in this heat exchanger (H 10).
The cooled effluent of R7
+
R9 is split into two streams, the recycle stream makes up 57.3% of the reactor effluent. This split ratio is reqllired to ensure the incoming nitrous oxide mole fraction is 12%. This is required since sending a stream containing 30% nitrolls oxide will cause a much greater temperature increase in the reactor. The remaining porti on of the stream (make of 42.7%) is sent to stack.The stack gases have the following composition (all in mole percent): 0.16% nitrous oxide, 0.38% nitrogen oxides, 5.08% carbon oxides, 16.50% oxygen, 2.07% water vapour, 75.78% nitrogen, 0.03% volatiles and 0.01 % ammonia.
3.2 THERMODYNAMICS
The thermodynamic model chosen in the flow sheeting package was Soave-Redlich-Kwong (SRK). This model can be applied to near enough ideal gases and light non-polar hydrocarbons at medium pressures. The K-value option chosen was, again, Soave-Redlich-Kwong. The compressibilities and mixture fugacity coefficients for both vapour and liquid phases are derived from the SRK equation of state.
4
EQUIPMENT DESIGN
4.1 NITROGEN OXIDE REDUCTION REACTOR (RS)
4.1.1 General design considerations
A monolith reactor with square channels has been chosen for the decomposition of
NOx to N2 and H20 using NH3 as the reducing agent. The advantage here is the high
selectivity. Achieving a high selectivity requires reduced mass transfer limitations and
very precise process technology.
The following requirements are essential for an efficient reactor:
1. plug flow;
2. high active catalyst loading;
3. possibility of using a large variety of catalyst material;
4. low manufacturing cost;
5. low pressure drop;
6. easy stability and temperature control;
7. no extern al heat and mass transfer resistance from the bulk fluid to the catalyst
surface;
8. no internal heat and mass transfer limitations within the catalyst; 9. efficient radial heat and mass transfer;
10. possibility of efficient heat exchange with environment;
11. mechanical resistance and durability; 12. easy catalyst regeneration.
A monol1th reactor offers the advantages of points 5 and 8, a low pressure drop and no intern al heat and mass transfer limitations within the catalyst. The square channels offer the advantage of high insensitivity of the mass transfer coefficients to the
influence of kinetics. However, it must be noted that points 9, 10 and 12 are not
characteristic of the monolith reactor.
The decomposition of NO and N02 to N2 and H20 is efficient (XN02
=
0.95) at the• p
=
2.21 bar• T
=
300-340°C• GHSV
=
10,000-15000 h-1• 0.2
<
N02/NOx<
0.454.1.2 Derivation of the model equations
The design of the monolith reactor is based on the following balances (Villermaux and Schweich, 1994):
Mass balance over one square channel in the monolith reactor:
(4)
It is assumed that mass transfer is equivalent to the eonversion term in equation (4).
Furthermore, assurning the variation in volumetrie flow rate, eompared to the variation of concentration, with respect to the reactor length is smal!, equation (4) becomes:
, dC ,
u·d--=-(-r )·d-=-k ·(C-C.)·4d
dz II d .1
k ·d
for long channels Sh
=
3=
_ti - and Cs«
C so equation (5) beeomes DdC -4·Sh·D
- = ·C
dz u·d2
substitution of a residenee time't
=
L in (6) gives:u
dC -4·Sh·D
- = ·'t·C
dz L·d2
Heat balanee over one square channel:
(5)
(6)
(7)
o
=<j>pcTI
-<j>pcTI
+(-r )·(-Llli)·((2e+d)2 -d2)·dz (8)f! z P z+dz II
Furthermore, assurning the variation in volumetrie flow rate, eompared to the variation of temperature, with respect to the reactor leng th is smal!, with e
«
d, equation (8) simplifies to beeome:dT 4e
- = (-rJ . (-Llli) .
-dz dpcpu
b · · f L . su stltutlOn 0 't
= -
glves U dT 4e't -dz = (-ra)' (-DJi)·-d-. L-pc-IJ (10)The pressure drop can be derived from a force balance:
O=<j>(pu)I-<j>(pu)1 z z+ d+pd21-pd21 +'t !·4d·dz
Z z z+dz w. (11)
with 't w,f
= -
f·~
p(U)2
and assuming the variation in volumetrie flow rate, comparedto the variation of pressure, with respect to the reactor length is small, equation (11) becomes, after introduction of 't
=
L :u dp
=
dz (12)
Equations (7), (10) and (12) were solved in a software package (RRSTIFF) to simulate the monolith reactor. This meant that knowledge of the reaction kinetics was necesarry. The kinetics of the overall reaction was not found, however, some facts led to an estimation of the overall kinetics. These facts were:
• The reaction kinetics of the NO catalytic reduction with NH3 on vanadia oxide are
(Inomata and Miyamoto, 1979):
-rNO
=
23.8·exp(-116000/RT)·[NO] (molecules·(cm2cat'sr') (13)• The reaction kinetics of non catalytic N02 decomposition with NH3 are (Rosser
and Wise, 1960):
• The reaction rate of NO decomposition is faster than the reaction rate of N02
decomposition. Furthermore, the reaction rate of NOx decomposition is much
faster than that of NO decomposition alone (Tuenter et al., 1986) for a tungsten-vanadia-titania catalyst.
The high activity vanadia-oxide catalyst on a y-alumina carrier (Garein et al. 1993) has been chosen. The overall NOx decomposition reaction is a summation of the N02
and the NO decomposition reaetions.
(1)
(15)
These two reactions can be considered as parallel reactions as NH3 is used in both
reaetions. If the overall kinetics is based on NH3, the two kinetics equations (13) and (14) can be summed.
Taking the final kinetic "fact" into account (see above), rNO must be greater than -rN02 and -rNOx must be much greater than -rNO. Since equation (14) refers to a non-catalytic reaction, another pre-exponential factor was estimated, keeping the activation energy constant.
The overall reaction rate is based on NH3, so -rNH3 must be greater than the summation of equations (13) and corrected equation (14).
The final estimated overall reaction rate (af ter conversion into SI units) based on NH3
becomes:
_ 11600 _ 27500
-'Nl-/ =4.21018 ·(1.75810 16 • exp(--)· [NO]+4.577 10 16 ·exp(--)· [NO]· [NH3
D
J RT RT
(16)
The volumetrie flow rate does change significantly with respect to the reactor length. This variation is small compared to the variation of concentration, temperature and pressure with respect to the reactor length, but must be taken into account in the RRSTIFF calculation.
Assuming the gases to be ideal, the change in volumetrie flow rate is:
(17)
With 't = Land u = cp
u Ar' E -'"pace
equations (7), (10) and (12) can be solved in RRSTIFF until a conversion of N02 of 95% is reached by choosing a fixed reactor volume and adjusting the channel diameter.
The boundary conditions supplied to the reactor were:
• Z = 0 m; C NH3
=
1.404 X 10-1 mol/m3, T=
584.75 K, P=
2.21 X 105 Pa.Z
=
l.09 mThe reactor dimensions were calculated and the results can be seen in Table 3. A diagram illustrating the monolith reactor and its internals can be seen in Fig. 3.
Table 3. Monolith reactor dimensions
Property Value Property
V, 1 Emono d 5.345 Ecat e 0.7 Eca, L 1.09 dtatchannel D 1.08 Wcat GHSV 12,247 Wca,
Monolith reactor
/ / / \ \ \ \ \ / ---';', -'-'-'-'-'-'-'-'r, --'---\ , , , , , , , -'; ~ ~---~ - - -_ e:,ooE ~iiiiililii\1iiiiJliaj1 Value 0.5 0.2 0.3 9.6 671.4 1050---1170Fig. 3. Monolith reactor and its internals
The stream parameters, as indicated in the above diagram have been reported in the Table bel ow:
Table 4. Stream parameters of the monolith reactor
Parameter 1 2 <1>v l.7 1.8 P 2.21 x 105 2.2 X 105 T 585 614 Ftat 77.3 7704 FNo2 0.12 5.55 x 10-3 FNo 0040 0.33 P 1.53 1045
The variation of the ammonia concentration, temperature and pressure in the monolith reactor can be found in the following three figures.
1.60E-01 'M 1.40E-01
§
0 1.20E-01 .§. c 1.00E-01 .2 iä...
8.00E-02-
c 41 0 c 6.00E-02 0 0 Cl! 4.00E-02 '2 0 E 2.00E-02 E ~ O.OOE+DOO.OOE+DO 2.00E-01 4.00E-01 6.00E-01 8.00E-01 1.00E+D0 1.20E+D0
Reactor length (m)
Fig. 4 (a). Ammonia concentration profile in the monolith reactor
6.15E+D2 6.10E+D2 SZ 6.05E+D2 41 6.00E+D2
...
E Cl! Q; a. 5.95E+D2 E 41 5.90E+D2 I-5.85E+D2 5.80E+D2O.OOE+DO 2.00E-01 4.00E-01 6.00E-01 8.00E-01 1.00E+D0 1.20E+D0
Reactor Ie ngth (m)
2.21 E+D5 2.21 E+D5 2.21 E+D5 CQ ~ 2.20E+D5 Cl>
...
::I en 2.20E+D5 en Cl>...
a. 2.20E+D5 2.20E+D5 2.20E+D5O.OOE+ÛO 2.00E-01 4.00E-01 6.00E-01 a.00E-01 1.00E+D0 1.20E+D0
Reactor length (m)
Fig. 4 (c). Pressure profile in the monolith reactor
4.2 NITROUS OXIDE REDUCTION REACTOR (R7 + R9)
4.2.1 Derivation of the model equations
The design of the packed bed reactor for the selective reduction of nitrolls oxide was completed using the guidelines outlined in "Elements of Chemical Reaction Engineering," (Scott Fogler, 1992). In order to model the reactor, three basic equations relating the conversion, temperature and pressure to the catalyst weight are required. In the case of a tubular reactor without a catalyst bed, the independent variabie is the reactor volume. The catalyst weight (or volume) can also be easily translated into the height of the reactor.
The derivation of the relationship between the conversion and the catalyst weight is do ne by setting up an overall mass balance across an imaginary thin strip in the plug flow reactor (PFR). This leads to:
(18)
Rearranging this equation leads to the necessary model equation, relating the conversion to the weight of catalyst.
dX -r,vzo
(19) dW F,vzO.in
The derivation of the model equation relating the temperature to the catalyst weight can be done by first assuming a steady state PFR (without catalyst). The general heat balance could then be expressed as:
0=-FNzO
J
r.eic
p; dT-[M1~(TR)
J
/::.CpdTJ. FNzO . X~ ~
dX
Recalling that -rN20
=
FNzO dV leads to:(20)
(21)
The above equation can be rearranged to give the variation of temperature within a given reactor volume. However, before entering this equation as a model equation, two further assumption were made which enabled the equation to be simplified, these were:
• the value for (r.eCpi
+
Xöcp) was taken as the ave rage cp of the stream; and • the heat of reaction is a weak function of the reactor temperature.These two simplifications, coupled with the rearrangement of the above and recollection that W
=
(l-E)·AL·pc leads to:dT -rNzO ·(-M1r )
dW
P
g . (l - E) G> v • cl'(22)
The derivation of the pressure as a function of the catalyst weight was taken directly from Ergun's equation. This equation relates the pressure drop in a packed porous bed of particles to the reactor length. As shown above, it is possible to back correlate the reactor leng th to the catalyst weight. The finalised equation can be seen below.
dP
=
U" (150.(1-E)'TJ +1.75· ..u
.
]
dW A· d · 3 d
P
g
.1PB
p E p(23)
Some of the above terms are functions in themselves, these terms are classified as composed variables. For example, -rN20 is a function of the temperature and conversion as shown in the equation below.
(24)
The gas density, Pg, is a function of the pressure and temperature according to the following relationship (assuming ideal gas behaviour):
(25)
The superficial gas velocity, Us, is a function of the pressure and temperature according to: <I> v Po T u = . . -.I' A P T. o (26)
The volumetrie flow rate is a function of the superficial gas velocity (<I>v
=
A-us).By entering these three differential equations into a software package such as RRSTIFF, it is possible, given the boundary conditions, to model the nitrous oxide reactor.
The initial requirements for the calculation was that the pressure drop should not be too low, approximately 0.5 bar in tota!. The temperature of the outgoing streams should be lower than 700°C and the final conversion should equal 99%. This was achieved by designing the reactor with two beds, with interbed cooling.
4.2.2 Modelling of the nitrous oxide reactor
The reactor was modelled by entering the model equations described in the earlier section and supplying the equations with the relevant constants. A tab Ie summarising the constants for the two beds can be found below:
Table 5. Constants supplied to the model equations
Constant Top bed (R7) Bottom bed (R9)
FN20.in 22.96 5.143 E 0.39 0.39 -.0.Hr cp ko Ea R PN20 M <l>vo A Po Ta Pb dp T) 82050 1100 0.31 84700 8.3142 3.51 x 104 0.030195 4.4 3.25 3 x 105 673 915 3.175 x 10-3 3.21 X 10-5 82050 1100 0.31 84700 8.3142 6.13 X 103 0.030195 5.37 3.25 2.8 X 105 770 915 3.175 X 10-3 3.21 X 10-5
The two beds were calculated independently, the constants of the second bed were only deterrnined af ter the first bed was completely calculated. The data files of the two differential equations can be found in the appendices.
After entering in the constants, the only parameters that remained were the boundary conditions. These were:
• Bed 1 (R7): • Bed 2 (R9): W
=
0 kg, X=
0, T=
673 K, P=
3 x 105 Pa. W=
1250 kg. W=
0 kg, X=
0, T=
770 K, P=
2.8 x 105 Pa W=
1750 kg.Entering in these values into the program produced the bed model. The bed model has been summarised bel ow in a schematic drawing (Fig. 5), thï's drawing is supplemented by details of the bed dimensions (Tabie 7).
The important parameters to take note of here are the molar flow rates of nitrous oxide, the temperature and pressure. It should also be noted that the variation of volumetrie flow rate and density with respect to the temperature was taken into account before and after the coolers (see Table 6).
BED 1
2
steam
5
Fig. 5. Schematic representation of the nitrous oxide reactor
It should be recalled that the bed length was calculated according to this relationship: W
=
(l-E)·A-L·pc. The reactor was designed to operate with a pressure drop of 0.5 bar in total, this was achieved by varying the cross sectional area of the bed.Table 6. Stream properties - nitrous oxide reactor Property 1 2 3 4 5 F:tot 196.3 234.7 234.7 234.5 234.5 FN20 22.96 5.14 5.14 0.3341 0.3341 ~v 4.40 6.75 5.37 6.6 5.96 P 1.50 1.05 1.32 1.08 1.19 P 3 x 105 2.83 X 105 2.80 X 105 2.52 X 105 2.2 X 105 T 673 979 770 853 673 Y 0.117 0.022 0.022 0.0014 0.0014 PN20 3.5 x 104 6226 6133 359 313
The design values for the dimensions and overall characteristics of the beds were ca1culated at:
Table 7. Reactor dimensions Design parameter Bed area (m2) Bed diameter (m) Bed volume (m3) Bed height (m) GHSV (h-I) Bed 1 (R7) 3.25 2.0 1.4 0.43 19,008 Bed 2 (R9) 3.25 2.0 1.9 0.58 18,761
Three plots indicating the conversion, temperature and pressure profiles in the reactor are shown below.
l.00E+OO 9.00E-01 8.00E-01 c: 0 7.00E-01 ,ii êii 6.00E-01 > c: 0 5.00E-01 0 ïä c: 4.00E-01 .e ö ~ 3.00E-01
u:
2.00E-01 1.00E-01 O.OOE+OOO.OOE+OO 5.00E+Ü2 1.00E+Ü3 1.50E+03 2.00E+03 2.50E+Ü3 3.00&03
Catalyst weight (kg)
1.00E+03 ~.50&02 9.00E+02
g
e 8.50E+02 2 ~ 8.00E+02 lil c. E lil 7.50&02 I-7.00E+02 6.50E+02 0.00&00 3.00&05 2.95&05 2.90E+05 2.85E+05 ~ 2.80E+05 e:. lil 2.75E+05 :; UI UI 2.70&05 lil ct 2.65&05 2.60&05 2.55E+05 2.50&05 . 0.00&005.00E+02 1.00E+03 1.50E+03
Catalyst weight (kg)
2.00E+03
Fig. 6 (b). Temperature profile in the reactor
5.00E+02 1.00E+03 1.50E+03 2.00E+03
Catalyst weight (kg)
Fig. 6 (c). Pressure profile in the reactor
4.3 HEAT EXCHANGER DESIGN
2.50E+03 3.00E+03
2.50E+03 3.00E+03
The procedure used to design the heat exchangers in the system was Kern' s method,
as outlined in Coulson and Richardson (1991). The aIlocation of fluid to the sheIl or
tube side was determined by taking the foIlowing into consideration.
• Stream flow rates - the fluid with the lowest flow rate was allocated to the sheIl
side.
• Pressure drop - the fluid with the lowest allowable pressure drop was aIlocated
to the tube side.
There were other considerations (such as fluid viscosity, temperature, fouling and corrosion) but these were not looked at in great detail as the flow rates and pressure drops were the principaIly important parameters.
The important values in the design of a heat exchanger are: • the overall heat transfer coefficient (U);
• the heat transfer area (A); • the number of tubes (N);
• the pressure drop on the shell and tube side (.0.Ps and .0.Pt).
4.3.1 Overall heat transfer coefficient
The overall heat transfer coefficient is estimated and subsequently calculated in a later step. The values for typical applications can be found in Coulson and Richardson (1991).
4.3.2 Heat transfer area
The heat transfer area can be calculated given the heat transferred per unit time (Q, from the simulation package), the overall heat transfer coefficient (U, chosen) and the corrected temperature difference (.0.T m) according to the following equation.
Q=U ·A·.0.T m (27)
where the temperature difference is calculated via these relationships:
(28)
(29)
The value of Ft is a function of the shell and tube side fluid and the number of passes. This value can be looked up in standard graphs. The number of passes has a large influence on the pressure drop on the tube side.
4.3.3 Number of tubes
The number of tubes can be calculated by di vi ding the total area of heat transfer with the area per tube. The area per tube can be calculated af ter a specific tube size has been chosen. From here the bundie and shell diameter can also be calcuated.
4.3.4 Pressure drop
(30)
The pressure drop on the tube side can be determined by the following equation:
(31)
The viscosity correction function is usually negligible as it is very close to unity. If the
pressure drop on the tube si de is too high, the number of tube passes should be
decreased. If the pressure drop on the shell side is too high, then then baffle pitch
could be increased.
4.3.5 Spiral cooler design
The design of the spiral cooler was based upon equation (27). The cooler was initially designed as a shell and tube exchanger and later modified. The details of this design
can be seen in the appendix. The overall heat transfer coefficient was taken to be 250
W·m-2X-'. The area of heat transfer was calculated at 11.2 m2. The pressure drop
through the spiral was assumed to be in the order of 0.2 bar.
The details of the four heat exchangers have been summarised in Table 8.
Table 8. Characteristic values of the heat exchangers
Charateristic H2 H3 H8 79.6 60.6 250 122.7 297.6 11.2 404 981 2.03 x 10~ 2.04 x 104 1.86 x 10~ 2.41 x 104 4.4 COMPRESSOR DESIGN 4.4.1 Theory HIO 156.6 133.6 440 8.88 x 102 2.89 x 1O~
Two compressors are used in this process, the most important compressor being the recycle gas compressor used to recirculate the reactor effluent coming from the nitrous oxide reactor. The design of these compressors will be highlighted in this section. The required work for the polytropic compression can be calculated by this equation:
l
n-IJ
- w
=
Z· RT;n . _ n(P2
J
n -1M n-1
P
1(32)
The value of Z (compressibility factor) is a function of the reduced temperature (T R) and pressure (PR).
The value of n (the polytropic exponent) is a function of X (compressibility function dependent on the reduced pressure), Y (also a compressibility function dependent on the reduced pressure) and m (polytropic temperature exponent) according to the following equation:
n =
-Y-m·(1+X) (33)
Z·R
(1
J
where X = 0, Y = 1 (Coulson and Richardson, 1991) and m=-c-· E+X .TheI' f1
efficiency, Ep, is a function of the volumetrie flow rate and can be determined from Fig. 3.6 in Coulson and Richardson (Volume 6, 1991). Substitution of these values into the above equation produces a value for m. Using this value of m, use of equation (33) results in a value for n.
The polytropic work can now be calculated by use of equation (32). The actual work can be calculated by dividing the theoretical work by the efficiency of the compressor. The actual power of the compressor can be calculated by the equation shown below:
p=tf.. 'l'm .W (34)
4.4.2 Compressor design va lues
The design values of the two compressors have been summarised in Table 9.
Table 9. Design values of the two compressors
Unit Z TR PR X Y Ep m n W 'VaeIU"1 P
Cl 1.00 2.66 0.05 0 0.65 0.35 1.54 78700 121000 8306
5
PROCESS CONTROL
Safety is an aspect of prime importance in a chemical process. The safe operation of a chemical process is an essential requirement for the safety of the workers. Thus, the operating pressures, temperatures, quantites of chemicals, and so on, should always be within allowable limits.
There are three general classes of needs that a control system is called on to satisfy (Stephanopoulos, 1984)
• Suppressing the influence of external disturbances
• Ensuring the stability of a chemical process
• Optimising the performance of a chemical process.
5.1 PROCESS CONTROL OF THE DIFFERENT SECTIONS
Process conditions can be ensured to be within allowable limits by the application of controllers. In this plant, three types of controllers are used, namely, pressure controllers (PC), temperature controllers (TC) and flow controllers (FC). (please note:
Flow in this ehapter refers only to volumetrie flow rate).
Controllers having feedback configuration have been chosen above controllers having feed forward configuration. This is because feedback control is rather insensitive to the drawbacks of feed forward control. The drawbacks are that feed forward control: 1. requires identification of all possible disturbances and their direct measurements; 2. can not cope with unmeasured disturbances;
3. sensitive to process parameter variations; and 4. requires good knowledge of the process model.
Proportional integral derivative (PID) controllers have been used as feedback controllers in this process. This is because they anticipate what the error will be in the immediate future and apply control action which is proportional to the current rate of change in the error.
5.1.1 The feed section
It hàs been assumed that the offgas stream enters the plant at a constant pressure of 3.11 bar and a temperature of 35°C. It has also been assumed that ammonia is supplied to this plant at a pressure of 2.21 bar and a temperature of 25°C. A ratio controller serves to regulate the dilution of ammonia with air. Ammonia concentrations of 15 to 29% in the ammonia-air mixture could result in an explosion. Therefore, a deviation in the mixing ratio has to be avoided.
Air at atmospheric pressure is compressed to 2.21 bar; a bypass connected to a PC has been installed in order to counter pressure fluctuations. In case the outlet pressure of the compressor is too high, the PC increases the orifice in the back-flow valve, thus reducing the pressure at the compressor outlet by increasing flow through the recycle across the compressor. The opposite occurs in case of reduced pressure at the compressor outlet; the PC ensures decreased flow through the recycle across the compressor by decreasing the orifice in the back-flow valve. This in turn increases the compressor outlet pressure. The bypass valve has been designed as a Fail Open valve. In case of valve malfunctioning, this design prevents the pressure at the compressor outlet from reaching high values.
In case of a decrease in the temperature of the diluted ammonia stream (during winter conditions for example), the TC ensures that the trim heater heats up the incoming adipic acid offgas stream, thus compensating for the fall in temperature in the diluted ammonia stream.
5.1.2 The
NO
x reactorIn order to ensure that the offgas stream mixes in a good proportion with the diluted ammonia stream, a ratio controller has been installed before the in1et of this reactor. The flow controllers also keep the incoming pressure in check. As only gases are involved and no reaction has taken place yet, with other things remaining constant (such as pipe cross section etc.), pressure is directly proportional to the flow. If there is a pressure surge in the reactor, however, the PI caters for the issuing of an alarm. Should there be no action taken and the pressure still continue to rise in the reactor, the pressure relieve di sc slams open releasing the reactor contents.
For the reaction to proceed as desired, it is important that the reactor feed is at a temperature of around 311°C. Therefore, a TC has been installed after the mixing point of the adipic acid offgas stream and the diluted ammonia stream. Should the temperature fall short of the desired value, the TC triggers the trim heater on.
5.1.3 The N20 reactor
The flow to the N20 reactor is indirectly controlled by the PC placed across the
pressure. Therefore with other things remaining constant, pressure control before the reactor ensures flow control too.
Pressure on the other hand is directly controlled by the above mentioned Pc. The manner is the same as described under section 5.1.1. The valve involved here is a Fail Open valve too. Refer section 5.1.1 again for the working of the same. A PI has been installed at the reactor outlet and serves in the same lines as the PI installed af ter the NOx reactor (refer section 5.1.2). Even this reactor has been provided with a pressure
relief disco This breaks open in case no action is taken af ter issuing of the high pressure alarm and the pressure continu es to rise in the reactor.
The temperature of the incoming stream is brought to the required value by H2. Minor fluctuations in the temperature of the incoming stream do not have serious repercussions due to the fact th at the first bed operates in a broad temperature-range. It is of vital importance that the temperature in this reactor does not exceed 800°C at any instance due to the formation of NOx above this temperature. It is for this reason that
this reactor has been modulated as a double bed reactor with interim cooling. A TC has been placed after the first bed in order to regulate the flow of water (process fluid of H8) needed to maintain the temperature of the offgas flow at the outlet of H8 at 400°C.
Another TC has been placed af ter H8. The reaction is highly exothermic at high conversions. Shollid the stream enter the second bed with unabated temperature, there are chances of a runaway taking place there. Measures have to be taken therefore in case H8 malfunctions. In case of a malfunctioning of H8, this TC stimulates the opening of a bypass build across the second bed, shutting off the normal inlet valve of the second bed at the same time. As a consequence the reactants bypass the second bed in case their temperature is higher than the required inlet vallIe for the second bed. Finally, it is important that the temperature of the stream after this reactor be brought back to 400°C, because a part of this stream is recycled. HIO serves this purpose. Should there be temperature fluctuations at the reactor outlet or should Hl 0 malfunction, the TC placed af ter Hl 0 controls the flow of water (process fluid) to H8 so that the fluctuations or the malfunctions are countered.
5.2 MISCELLANEOUS SAFETY MEASURES
Af ter HiO, the outgoing stream is divided into the stack stream and a split stream which is recycled. A FC has been placed in the latter stream in order to regulate the recycle flow and it controls a Fail Open valve.
The N20 reactor has an intermediate risk factor (refer § 6.3). An extra precaution was therefore llndertaken to ensure continuous steady flow to this reactor. Apart from the compressor that serves to drive the recycle flow, a redundant compressor has also been installed parallel to it. These compressors are controlled by their respective PCs. Shollid the regular compressor fail for any reason, the auxilary compressor can be put to use.
6
PROCESS SAFETY
Safety is the most important aspect in the designing and working of a plant. In spite of all the control measures installed and the routine checks and maintenance carried out, the plant cannot be declared to be free of all risks. It is therefore imperative th at risks be thoroughly analysed and curtailed to a minimum. It is also a necessary part of most risk-insurance programs. In the designing of this plant, both technical and organisational steps have been taken to minimise risks. Steps taken higher up in the risk-chain result in a more effective risk management.
6.1 PROPERTIES OF SUBSTANCES
Ammonia gas by itself is difficult to ignite. However, a 15-29 vol% of ammonia in air is flammable. Therefore, during the dilution of ammonia with air, care should be taken to remain well clear of these concentrations. In this plant ammonia is diluted by excess air in such amounts that the ammonia concentration in the dilllted mixture is targeted at 9%. Ammonia itself has a self ignition temperature of 630°C at atmospheric pressure. As most of the ammonia is llsed up in the NOx reactor operating in the temperature range of 31 I-340°C, the risks of ammonia igniting itself are low.
Ethylene on the other hand is susceptible to decomposition or even to self-ignition when exposed to temperatures above 425°C at atmospheric pressures. It decomposes isslling an acid smoke and irritating fumes. The temperature at the outlet of the recycle compressor and the temperature in the N20 reactor do reach values above 425°C.
However, since the ethylene concentration is lower than 0.1 vol%, it poses low risks. The presence of carbon monoxide involves a dangerous fire and even an explosion hazard when exposed to heat or flame. It has a self-ignition temperature of 605°C at atmospheric pressure. The N20 reactor is the unit where the temperature exceeds
605°C. However, due to the presence of low carbon monoxide concentrations (also lower than 0.1 vol%), the risks offire or explosions are negligible.
Should there be a fire in any of the reactors, adequate provisions have been installed in the reactor that extinguish the fire quickly and thus prevent explosions (see further
All the above mentioned substances have been assigned a hazard rating of 3 (Sax' s Darrgerous Properties of Industrial Materiais). The hazard rating indicates the relative hazard for toxicity, fire and reactivity, with 3 denoting the worst hazard level (see Table 2). The properties of other components involved are such th at they pose low threats to the safety of the process as a whoie.
6.2 TOXICITY OF SUBSTANCES
From a theoretical point of view, nitrogen dioxide is by far the most toxic component in this plant as its LCLo value is the lowest at 200 ppm (refer section 2.5). It also has the lowest MAC value of 2 ppm. Carbon monoxide follows nitrogen dioxide in toxicity with a LCLo of 5000 ppm and a MAC value of 25 ppm which in its turn is closely followed by ammonia with a LCLo of 30,000 ppm and a MAC value of 25 ppm.
Practically, however, ammonia could be considered to exert a higher tOXIClty. Ammonia enters this plant in its pure form before being diluted with excess air. The other compounds mentioned above are in diluted forms at any instance in the reactor. Ammonia can be absorbed in the human system chiefly by inhalation or ingestion. It is an eye, mucous membrane and systernic irritant. Mutation data have also been reported over it. It is therefore of utmost importance that the personnel involved in the ammonia dilution unit are adequately protected against ammonia leakage.
6.3 THE DOW FIRE AND EXPLOSION INDEX
The Dow Fire and Explosion Index (DFEI) has been ca1culated based on two factors: 1. the Material Factor (MF); and
2. the Unit Hazard Factor (UHF).
(For details on the methad of calculation of the DFEI, refer to appendix 4).
For the NOx reactor, the DFEI value has been ca1culated to be 79.8. For the N20 reactor, ca1culations value the DFEI at 114.8. The Dow's Fire and Explosion Index Classification Guide attributes a certain degree of hazard to a particLllar DFEI-interval. The ratings are as follows :
Table 10. Tab1e imputing the DFEI values to the degree of hazard
DFEI DEGREE OF HAZARD
1-60 61-96 97-127 128-158 159-Light Moderate Intermediate Heavy Severe
From Table 10 it can be seen that the NOx reactor has a moderate degree of hazard while the N20 reactor exhibits an intermediate hazard degree. It is therefore
imperative, that apart from the safety features installed that pertain to the reactor feed (refer § 5.1.2 and 5.1.3), the reactors be installed with certain additional security features in order to ensure a good degree of safety in the plant. This is more so in the case of the N20 reactor.
6.4 THE DAMAGE FACTOR AND EXPLOSION RADIUS
The damage factor represents the overall effect of fire plus blast damage resulting from a fuel or reactive energy release caused by various contributing factors associated with the process unit.
The exposure radius represents the probable area of exposure that will be involved as a result of the combined effects of a Unit Hazard factor and of the specified Material factor.
For the NOx reactor, the damage factor has been determined as 0.72 and the exposure radius as 23.1 m. For the N20 reactor, the damage factor has been found to be 0.84 and the explosion radius to be 37.5 m.
Translated into the extent of damage: the conditions in the NOx reactor represent a
72% damage probability to 529 m2 of surrounding area and the conditions in the N20
reactor represent a 84% damage probability to 1406 m2 of surrounding area.
6.5 SAFETY FEATURES
The following section lists important safety features that the reactors have been equipped with and also the safety features present in the plant and its immediate surroundings that indirectly contribute to the safety of the reactors (Dow's Process Safety Guide).
6.5.1 Essential security aspects installed in the NOx reactor
1. Automatic water, foam and dry-powder spray quench systems and sprinkler
protection. Reactor protected by directional spray to the flame. Spray density
. -4 3 2 -4 3/ 2
varymg from 1.698
x
10 m /(s·m reactor) to 3.395x
10 m (s·m reactor).Congested or critical fire are as protected by area spray. The area sprays must be justified to flush flammables out of the area.
2. Combustible gas monitors. Combustible gas monitors placed in critical are as (i.e.,
areas of poor or marginal air circulation and/or possibility of appreciable release of flammables). Monitor to alarm and actuate deluge system and or to shut down
3. Remote operations. Special fac ili ties present for remote operations (valve operations for example) and observation of equipment.
4. Photoelectric smoke alarms. These have been installed as they are efficient aids in
cases where fires are preluded by smoke.
6.5.2 Important safety features of the N20 reactor
1. Fire proofing of structural supports. A minimum of 3 hrs fire resistance rating
applied. The application has been limited to principal structural supports.
2. Automatic water, foam and dry-powder spray quench systems and sprinkler protection (as described under § 6.5.1).
3. Sight glasses. These are reflex or equal sight glasses with normally closed block valves (spring or weight load closed).
4. Internal Explosion Protection. Explosion suppressor and equipment to contain or safely relieve an explosion have been included. These equipments do not include flame arrests as the latter cannot cope when the degree of hazard has been described as intermediate.
5. Combustible gas monitors (refer to § 6.5.1 for the working).
6. Remote operations (as described under § 6.5.1)
7. Blast wall. This have been included in the construction in order to curtail the
debris in case a blast occurs.
8. Physical separation in the reactor wall. A 0.1 m clear space separates the blast wall from the fire-proof outer wal!.
9. Photoelectric smoke alarms (refer § 6.5.1).
6.5.3 Miscellaneous safety features present in the plant
1. Adequate water supply for fire protection (this has been determined by multiplying the length of time the worst possible fire is expected to last (2 hrs) by its water
demand).
2. Ample access to area for emergency vehicles and exits for personnel evacuation.
3. lnsulation of dangerous hot surfaces.
4. Limitation of glass devices.
5. The fired heater has been protected against accidental explosion and the resultant
fire.
6. Process control rooms have been isolated by 1 hr fire walls from process con trol laboratories and electrical switch gears and transformers.
7. Snow loads have been included in the design due to severe winter conditions.
8. Snow removal and ice control equipments. Required due to the possibility of
reduced accessibility in and around the plant as a result of severe conditions in the
winter.
9. Communications, emergency telephones, radio, public address systems, paging systems and safe location and continuous manning of the communication centre.
Rapid and easy accessibility to emergency services (the ambulance ser~ice, the fire
6.6 HAZARD AND OPERA TIONAL STUDIES (HAZOP)
A HAZard and OPerational study (HAZOP) (Bibo and Lemkowitz, 1994) is a procedure for the systematic, critical, examination of the operability of a process. It indicates potential hazards that may arise from deviations from the intended design conditions.
The Dow's Fire & Explosion Index (refer § 6.3) indicated that the N20 reactor poses the highest risk in the offgas plant. A HAZOP analysis has been compiled for both the reactors. The aim of this analysis is to ensure that the feed enters the reactor at a specified flow rate, temperature and pressure and that the products exit the reactor at a specified conversion (of key component N02), temper at ure and pressure.
Table 11 shows the HAZOP analysis for the NOx reactor. Table 12 shows the same
T a e bi 11 HAZOP analysls I . t ort e h NO , reactor
Guide Deviation Possible causes Consequences Action Required
Word
Not, No flow (1) Malfunctioning upstream (a) No feed to reactor; (A) Institute good communications
No in the AA plant (compressor no reaction occurring, with maintenance personnel of AA
failure, line blockage, line thus. plant. fracture etc.)
(2) Line blockage in offgas (a). (B) Check lines and valves and
treatment plant or valve replace if necessary.
closed in error
(3) Upstream line fracture in (a) and (C) Shut down plant temporarily.
offgas plant or rupture in (b) Offgas with (D) Institute regular patrolling and
reactor casing. unreduced NO, and line inspection.
N20 levels or NH3
leaks into adjacent
(4) No NH3 available in NH3
atmosphere.
No reaction occurring (C) &
storage tank. in the NOx reactor (E) Ensure good communications with NH3 storage operator. More More flow (5) Val yes or FCs or PCs or (c) GHSV increases, (F) Increase routine checking of
compressors malfunctioning. leading to decrease in FC, PC and compressors. NO, conversion and
increase in pressure drop across reactor.
More P (5). (c) & (F) &
(6) Plugging in pipe. average Tincrease in (G) Check and clean pipes.
reactor.
More T (7) H2 or H3 malfunctioning. Optimal T-range for (H) Increase routine checks of heat
(8) TCs malfunctioning. reaction surpassed. exchangers or trim heaters or TCs.
(9) Trim heater turned on (I) Increase fire extinguishing
wrongfully. measures in pipes and reactors.
(10) Thermal expansion as a Situation of increased result of fire. risk arises.
Less Less flow (3) or (5). (b). (A) & (B) & (F).
(11) Throughput of AA plant (d)Less flow processed
decreases.
Less P (3) or (5) or (10). (b) or (d). (A) & (B) &(F).
Less T (8). Reaction rate (H) or
(12) Trim heater turned off decreases; NO, (1) Install a buffer heat exchanger
wrongfully. conversion decreases; for NHrair mixture to exchange
(13) Tof NHrair mixture too target abatement not heat with stack gas, whenever
low due to winter conditions. achieved. necessary.
Back Back flow (14) Reactor outlet blocked. (e) Pressure build up (K) Increase con trol of reactor inlet
in reactor and and outlet.
c\ogging (L) Inspeet pressure relief disc in
reactor.
(M) (C) if pressure guage issues
alarm.
Part of NH3 concentration in (5) Possibility of (N) (C) &
air in the possible explosion. increase checks on FCs and ratio
explosive range of controller of NHrair section.
a e analysls or t e z
T bi 12 HAZOP I · f hNO reactor
Guide Word Deviation Possible causes Consequences Action Required
Not, No No flow (2). (a). (B).
Upstream or recycle Offgas with (C) & (D).
line fracture or rupture unreduced NzO leaks in reactor casing. into adjacent
atmosphere.
More More flow (5). (b") GHSV increases (F).
leading to decrease in NzO conversion and increase in pressure drop across reactor.
More P (5) or (6). (b*) & (F) & (G).
average T increase in reactor.
MoreT (8). Optimal T-range for (H).
('") H8 or H20 reaction surpassed
malfunctioning. and danger of
runaway present. Situation of
( 10). increased risk arises. (1).
Less Less flow (3) or (5). (b) or (d). (B) & (F).
Less P (3) or (5). (b) or (d). (B) &(F).
Less T (8) or (I *). Reaction rate (H).
decreases, NlO
conversion decreases, target abatement not achieved.
Back Back flow ( 14). (e). (K) & (L) & (M) &
(N).
Part of Possibility of Possibility of a (1).
ethylene burning minor explosion.
