# The Pressure Relief System Design for Industrial Reactors

Authors: Iztok Hace
Journal: Journal of Industrial Engineering
 (2013)

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## Abstract

A quick and simple approach for reactor—emergency relief system design—for runaway chemical reactions is presented. A cookbook for system sizing with all main characteristic dimensions and parameters is shown on one realistic example from process industry. System design was done based on existing theories, standards, and correlations obtained from the literature, which were implemented for presented case. A simple and effective method for emergency relief system is shown, which may serve as an example for similar systems design. Obtained results may contribute to better understanding of blow down system frequently used in industrial plants, for increasing safety, decreasing explosion damage, and alleviating the ecological problems together with environmental pollution in case of industrial accidents.


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## Body

## 1. Introduction

In process industry, raw materials are converted into various commercial products using different techniques. One frequently used method is their conversion by exothermic chemical reactions which can lead to a reactor thermal runaway if the heat generation rate exceeds the heat removal rate during process [1]. Pressure build-up during the runaway is caused by an increasing vapor pressure of liquid components and by the production of noncondensable gases. Apart from the loss of reactor inventory due to an uncontrolled conversion process, a runaway reaction may lead to severely damaged equipment or even a physical explosion if pressure build-up inside the reactor exceeds the design pressure. The emergency relief system is composed of vent area, vent rupture membrane, safety relief valve, vent pipes, blow down tank, horizontal condenser, scrubber with absorber and vertical condenser, outflow chimney, corresponding pumps, fan, pipes, fitting, and supply system with electricity, cooling water, and neutralization medium. In case of reaction runaway, the vent rupture disc opens and the reactor mixture blows out into the vent pipes and flows into blow down tank. Due to short residence time of reactor mixture in the blow down tank, the volume changes and the pressure decreases at isothermal conditions, which results in the condensation of reaction mixtures. Remained two phases flow instantaneously blows into horizontal condenser, where it cools down, condensates, and flows into absorber with vertical condenser, where it is neutralized [2–4].In present study, a detailed design of emergency relief system is shown based on Design Method for Emergency Relief Systems (DIERSs). It incorporates the state-of-the-art knowledge obtained from mechanical, electrical, and process engineering based on a long year experiences in the process industries all over the world. All components were designed based on API RP 520 and API STD 526 standards [5, 6]. In case of problems, reaction runaway may appear due to loss of cooling water in the reactor, over- or miss-charged reactant, external fire, and loss of agitation or wrong reaction temperature in reactor. Such cases are difficult to predict, almost not possible to control, and can lead to explosion. Therefore, a proper and correct sized emergency relief system is appropriate method to prevent fatal accidences and environmental pollutions.The purpose of present study is to(i)
show the main mechanical, electrical, and process fundamentals for this system,(ii)
design emergency relief system with corresponding scrubber, absorption column, and corresponding connections.Designed emergency relief system is presented in Figure1. A real industrial example from process industry composed from twelve equal 30 m3 batch reactors equipped with pressure and temperature sensors, cooling and heating mechanism, and rupture discs was taken. In reactors, an exothermic phenol formaldehyde reaction takes place. For reactor cooling towers with capacity of 700 m3·h−1 of cooling water at 13°C/26°C, cooling temperature regime was used.Figure 1
Emergency relief system.Based on the reaction runaway the emergency relief system design was made in stages from plant vent system, blow down tank, and scrubber with corresponding heat exchangers and absorption column.


## 2. Pressure Relief Devices

Pressure relief devices design was made according to API RP 520 standard [5]. Detailed pressure relief system on the reactor is presented in Figure 3 and is composed of safety relief valve (SVR) and rupture disc (RD) with dimensions presented in Table 1. The pressure at which SVR piston opens depends on the physicochemical characteristics of reaction system. In present study, it is designed so that SVR opens when the reaction medium pressure is above 70 · 103 Pa. In case of runaway, the outflow medium from reactor blows into vent pipes connected to blow down tank. Very high tip velocities cause a phenomenon known as blow-off where the flame front is lifted and could eventually turn into a blow-out. Therefore, determination of the right pipe diameter is important as far as operation of the system is concerned. The vent size is a crucial step at emergency relief system design. In this study, an approach proposed by Fauske [1] and Leung et al. [3] was used. For pressure temperature relation, Antoine equation was used to calculate the vapor pressure at reaction runaway as follows:
(1)P=exp(15.78-8798T).
The two phase discharge flow rate per unit area was evaluated according to the homogeneous equilibrium model which can be approximated in the low quality region by relation proposed by Leung [2] as follows:
(2)Gvent=0.9·144·dPdT·(32.2778.16·Tcpvent)1/2,
wheredP/dTwas replaced by differential form of (1) and the two phase discharged flow per unit area was evaluated.Table 1
Calculated system dimensions and characteristics for vent area, horizontal and vertical heat exchanger, blow down and scrubber tank, absorption column, and outflow chimney.
(a)
Parameter
Unit
Value
Cooling water data
T
c
in
°C
13
Designed
T
c
out
°C
26
Designed
T
c
out
°C
24
Designed
Q
˙
m3·h−1
700
Designed
Power supply
I
A
130
Designed
V
V
380
Designed
W
kW
50
Designed
Material flow from reactor
q
˙
c
W
8.94
·
10
6
Calculated from (8)
q
˙
h
W
2.3
·
10
6
Calculated from (8)
(b)
Energy released data
Set condition
Peak condition
P
Pa
2.07
·
10
5
2.21
·
10
5
Obtained from literature Fauske (1988) [1], Leung (1986) [2]
T
K
394.6
396.7
Obtained from literature Fauske (1988) [1], Leung (1986) [2]
(
d
T
d
t
)
m, (dTdt)s
K·min−1
15
20
Obtained from literature Fauske (1988) [1], Leung (1986) [2]
c
p
J·kg−1·K−1
2930.8
Obtained from literature Fauske (1988) [1], Leung (1986) [2]
T
m
°C
124.3
Obtained from literature Fauske (1988) [1], Leung (1986) [2]
T
s
°C
121.2
Obtained from literature Fauske (1988) [1], Leung (1986) [2]
(c)
Parameter
Unit
Value
C
p
cold
J·g−1·K−1
4.183
Obtained from literature Perry and Green (2007) [7]
C
p
hot
J·g−1·K−1
2.9281
Obtained from literature Perry and Green (2007) [7]
Blow down tank
V
o
m3
30
Designed
V
bd
m3
60
Calculated from (6)
P
sat
Pa
1
·
10
5
Calculated from (1)
G
in
kg·s−1
346
Calculated from (2)
P
Pa
2.07
·
10
5
Obtained from literature Fauske (1988) [1], Leung (1986) [2]
P
1
Pa
1
·
10
5 Pa
Calculated from (7)
r
c
/
0.24
Calculated from (34)
G
HZT
kg·s−1
82
Calculated from (35)
W
m
25
·
10
-
3
Designed
MAWP
Pa
2.5
·
10
5
Designed
Safety relief valve
DN
m
0.65
Designed
P
safety
Pa
70
·
10
4
Designed
Vent area
q
vent
J·kg−1
846
Calculated from (6)
G
vent
kg·m−2·s−1
2124
Calculated from (2)
A
vent
m2
0.326
Calculated from (3)
D
vent
m
0.65
Calculated from (5)
Calculated data horizontal heat exchanger
L
mm
5000
Designed
h
DirtIN = hDirtOUT
W·m−2·K−1
698
<
h
<
998.7
Obtained from literature Perry and Green (2007) [7]
λ
w
W·m−1·K−1
16.2
Obtained from literature Perry and Green (2007) [7]
Q
˙
hot
W
585711
Calculated from (8)
Q
˙
cold
W
366125
Calculated from (8)
T
h
,
out
°C
80
Calculated from (8)
LMTD
°C
76.16
Calculated from (9)
R
t
/
6.83
Obtained from literature Richardson et al. (2009) [10]
S
/
0.0594
Obtained from literature Richardson et al. (2009) [10]
F
t
/
0.96
Obtained from literature Richardson et al. (2009) [10]
T
m
°C
73.11
Calculated from (10)
A
-
m2
54
Calculated from (11)
N
t
/
85
Calculated from (13)
Pipe dimensions for horizontal heat exchanger
d
out
mm
50.8
BWG 16 Perry and Green (2007) [7], INOX, AISI316
d
in
mm
46.732
Wall thickness
mm
2.034
p
t
mm
63.5
Calculated from (14)
D
b
mm
689.1
Calculated from (15)
BDC
mm
93
Obtained from diagrams Richardson et al. (2009) [10]
D
s
mm
782
Calculated from (16)
B
s
mm
313
Calculated from (17)
A
s
m2
0.04895
Calculated from (18)
G
shell
kg·s−1·m−2
1986
Calculated from (19)
d
e
mm
36.07
Calculated from (20)
R
e
shell
/
65127
Calculated from (21)
P
r
shell
/
7.48
Calculated from (22)
N
u
shell
/
323
Calculated from (23)
h
shell
W·m−2·K−1
5490
Calculated from (23) and (25)
l
b
mm
875
Calculated from (26)
Δ
P
shell
Pa
53800
Calculated from (25)
N
tpp
/
85
Calculated from (27)
G
tube
kg·m−2·s−1
7971
Calculated from (28)
v
tube
m·s−1
2596
Calculated from (29)
Retube
/
30390
Calculated from (21)
Prtube
/
0.575
Calculated from (22)
h
tube
W·m−2·K−1
0.044
Calculated from (30)
U
tube
W·m−2·K−1
0.022
Calculated from (32)
U
shell
W·m−2·K−1
1957
Calculated from (31)
Δ
P
tube
Pa
91500
Calculated from (33)
Calculated data vertical heat exchanger
Recycle
h−1
22.22
Calculated from (40)
Total heat vertical
kW
23553
Calculated from (36)
Heat per recycle
kW
145.4
Calculated from (38)
T
h
invert.
°C
80
Calculated from (8)
A
-
m2
5
Calculated from (11)
T
h
outvert.
°C
40
Calculated from (8)
T
c
invert
°C
15
Calculated from (8)
T
c
outvert
°C
24
Calculated from (8)
Pipe dimensions for vertical heat exchanger
L
mm
2500
Designed
d
out
mm
50.8
BWG 16 Perry and Green (2007) [7]
d
in
mm
46.732
Wall thickness
mm
2.034
p
t
mm
63.5
Calculated from (14)
D
b
mm
286.7
Calculated from (15)
BDC
mm
88
Obtained from diagrams Richardson et al. (2009) [10]
D
s
mm
388
Calculated from (16)
B
s
mm
155.2
Calculated from (17)
A
s
m2
0.012043
Calculated from (18)
G
s
kg·s−1·m−2
8072
Calculated from (19)
d
e
mm
36.07
Calculated from (20)
Reshell
/
264718
Calculated from (21)
Prshell
/
7.48
Calculated from (22)
Nushell
/
2547
Calculated from (23)
h
shell
W·m−2·K−1
43435
Calculated from (23) and (24)
l
b
mm
437.5
Calculated from (26)
Δ
P
shell
Pa
82000
Calculated from (25)
N
tpp
/
13
Calculated from (27)
G
tube
kg·m−2·s−1
6196
Calculated from (28)
v
tube
m·s−1
2018
Calculated from (29)
R
e
tube
/
23545
Calculated from (21)
P
r
tube
/
0.575
Calculated from (22)
Δ
P
tube
Pa
128000
Calculated from (33)
h
tube
W·m−2·K−1
0.036
Calculated from (30)
U
tube
W·m−2·K−1
0.01867
Calculated from (32)
U
shell
W·m−2·K−1
5617
Calculated from (31)
LMTD
°C
38.44
Calculated from (9)
R
t
/
4.44
Obtained from literature Richardson et al. (2009) [10]
S
/
0.1385
Obtained from literature Richardson et al. (2009) [10]
F
t
/
0.96
Obtained from literature Richardson et al. (2009) [10]
DT
m
°C
36.7
Calculated from (10)
A
-
m2
5
Calculated from (11)
N
t
/
13
Calculated from (13)
Absorber and scrubber
Scrubber
L
NaOH
kg·s−1
42.21
Calculated from reaction stoichiometry
Recycle
h−1
22.22
Calculated from (40)
V
scrubber
tank
m3
4.5
Calculated from (40)
w
m
10
·
10
-
3
Designed
MAWP
Pa
1.5
·
10
5
Designed
Absorber
(
L
/
G
)
·
(
ρ
g
/
ρ
L
)
1
/
2
/
0.023
Calculated ratio
G
*
2
·
F
·
μ
0.1
32.2
·
ρ
g
·
(
ρ
g
-
ρ
L
)
/
1.8
Obtained from correlations proposed in literature Seader et al. (2010) [11]
G
*
kg·m−2·s−1
49
Calculated from obtained factor
A
abs
m2
1.68
Calculated from (41)
D
abs
m
1.46
Calculated from (42)
Δ
P
abs
m
0.3048
Obtained from literature Seader et al. (2010) [11]
No foaming factorFnf
/
1
Obtained from literature Seader et al. (2010) [11]
F
IMTP
/
12
Obtained from literature Seader et al. (2010) [11]
IMTP packing
mm
88.9
Obtained from literature Seader et al. (2010) [11]
N
L
/
1.66
Obtained from literature Seader et al. (2010) [11]
HETP
m
1.83
Obtained from literature Seader et al. (2010) [11]
H
abs
m
3
Obtained from literature Seader et al. (2010) [11]
F
IMTP
12
Obtained from literature Seader et al. (2010) [11]
L
operation
m3/h·m2
59.9
Obtained from literature Seader et al. (2010) [11]
w
m
10
·
10
-
3
Designed
MAWP
Pa
1.0
·
10
5
Designed
Neutralization reaction in absorber and in scrubber
Δ
H
react
kJ·mol−1
22.32
Calculated from literature data Perry and Green (2007) [7]
Q
˙
reaction
kW
7.1
Calculated from (37)
Centrifugal pump
L
NaOH
m3·h−1
100
Calculated from reaction stochiometry
Δ
P
pump
Pa
400000
Designed
Ventilator fan design
Δ
P
Fan
Pa
500
Calculated from (43)
∅
V
Fan
m3·h−1
72000
Calculated from (44) and (45)
v
r
m·s−1
230
Calculated from (47)
ϑ
Fan
RPM
2920
Calculated from (44) and (45)
P
Fan motor
kW
34
Calculated from (46)
d
Fan
mm
1460
Calculated from (44) and (45)
Chimney design
h
chimney
mm
6000
Obtained from literature diagrams Bleier (1987) [12]
d
chimney
mm
500
Obtained from literature diagrams Bleier (1987) [12]
Pipe sizing
Pipes from blow down tank via horizontal heat exchanger to absorber
DNpipes
Blow down tank-horizontal heat exchanger
mm
400
Calculated from (48)
Horizontal heat exchanger-absorber
mm
400
Calculated from (48)
Pipes from scrubber via pump and vertical heat exchanger to absorber
DNpipes
Pipes from scrubber via pump
mm
125
Calculated from literature data Fauske (1986), Richardson et al. (2009) [3]
Cooling water pipes
DNpipes
Main cooling water pipes
mm
300
Calculated from literature data Fauske (1986), Richardson et al. (2009) [3]
Cooling water pipes after crossing
mm
200
Calculated from literature data Fauske (1986), Richardson et al. (2009) [3]The analytical vent sizing equation for homogeneous vessel venting is [2–4]
(3)Avent=mo·qvent×(Gvent·[(Vmo·144778.16·T·dPdT)1/2+(cpvent·ΔTvent)1/2(dPdT)1])-1,
where
(4)ΔTvent=Tm-Ts,D=4·Aventπ,qvent=12·cpvent·[(dTdt)s+(dTdt)m],
and all necessary physical and chemical data are presented as follows.Physical-chemical parameters use for blow down system design:(5)ρliq.[mol·dm-3]=1.37980.31598(1+(1-(T/694.25))0.32768),cp,liq[J·Kmol-1·K-1]=101.720+317.61·T,cp,g[J·Kmol-1·K-1]=0.434·105+2.445·105·[1152/Tsinh[1152/T]]2+1.512·105·[507/Tcosh[507/T]]2,ηliq.[Pa·s]=exp(-43.335+3881.7T+4.3983ln(T)+3.0548·1024·T-10),ηvap.[Pa·s]=1.0094·10-7·T0.7991+103.1/T,λvap.[W·m-1·K-1]=0.038846·T0.23921+985.81/T+937.17/T2,λliq.[W·m-1·K-1]=0.18837-0.0001·T,ΔHevap.[J·Kmol-1]=7.306·107·(1-T694.25)0.4246,Pvapour[Pa]=exp(95.444-10.13T-10.09·ln(T)+6.7603·10-18·T610.13T).
Formation enthalpies (Perry and Green (2007) [7]):ΔHNaOHf=-469.15 kJ·mol−1,ΔHPhOHf=-165.0 kJ·mol−1 ,ΔHH2Of=-285.83 kJ·mol−1 ,ΔHNaPhOf=-326 kJ·mol−1,ρ50wt.%NaOH=1529 kg·L−1.Reaction chemistry at runaway is very complex and depends on the type of used catalyst and other process conditions [1–4]. In present study, a “worst-case scenario” was used for the thermal runaway of reaction. Design bases of(dT/dt)mand(dT/dt)swere obtained from the literature [2–4, 8] and are presented in Table 1. Calculated vent area was compared to the nomogram method from study by Fauske [9] and yield a vent area of 0.326 m2 or 0.65 m vent area pipe diameter which is in good agreement with the calculated results. Pressure relief devices designs were made according to API RP 526 standard [5]. Detailed presentation of pressure relief devices are presented in Figure 3 with main dimensions shown in Table 1. In present design, it was suggested that RD is located between reactor and SRV. RD protects SRV piston from vapors and opens when tank pressure is above allowable reactor pressure. Maximal pressure which appears at reaction runaway was not known and was obtained from the literature reported data [9]. A design pressure of 2.21 · 105 Pa was used for system design with 20% safety factor. In present study, a vent membrane had a pressure relief from −0.75 bar to 0.75 bar. At pressures higher than 0.75 bar, the rupture disc opened and the material was discharged into the blow down tank.


## 3. Blow Down Tank Design

The purpose of the blow down tank is to capture the two phase material flow from reactor and to decrease the pressure of outflow material [9]. Blow down tank design was done based on API RP 520 standard [5]. Blow down tank volume was calculated from the reactor volume as
(6)Vbd=2·Vo
and is reported in Table 1. A factor 2 in (6) was used as engineering safety coefficient which decreases the pressure from reaction runaway by half. It may also be called additional safety margin. The pressure decrease in a blow down tank was calculated by applying the ideal gas behavior at constant temperatures as follows:
(7)P1·V1T1=P2·V2T2
and was1·105Pa. The maximal allowable working pressure (MAWP) for blow down tank was calculated according to API RP 520 standard [5] and was2.5·105Pa. MAWP was higher than peak pressure at2.21·105Pa. Due to volume change, the MAWP in blow down tank will not be reached during reaction runaway. In blow down tank, a solid part of two phase runaway material is captured.


## 4. Horizontal and Vertical Heat Exchanger Design

Various procedures for heat exchanger design may be found in the available literature [9, 10, 13]. Only brief calculation procedure is presented [5, 7, 10].(i)
Assume pipe diameter, length, inside, and outside fouling factor.(ii)
For the pipe construction, an INOX AISI 316 was used with corresponding characteristics presented in Table1.(iii)
Corresponding energetic balances for horizontal and vertical heat exchanger were written as(8)Q˙=Q˙hot=Q˙cold=m˙c·cpc·(Tcout-Tcin)=m˙h·cph·(Thout-Thin).
Equation (8) enables calculation of heat duty,q, and the hot medium outflow temperatures and results are presented in Table 1.(iv)
The Log Mean Temperature Difference (LMTD) for countercurrent flow was calculated by(9)LMTD=(Thin-Tcout)-(Thout-Tcin)ln((Thin-Tcout)/(Thout-Tcin)).(v)
Based on the experiences and literature data, divided flow shell and even tube passes were assumed for both condensers. Following the procedure proposed by Richardson et al. [10], parametersRt,S,andFtwere obtained from the corresponding diagrams. Obtained parameters enabled calculation of mean temperature difference:
(10)DTm=Ft·LMTD.(vi)
Overall heat transfer coefficients were assumed from the data reported in the literature [10] within their minimum and maximum values and were between 700 W·m2·K and 1000 W·m2·K. Predicted heat transfer coefficients enabled calculation of minimum and maximum provisional area which were taken as average for heat exchanger design:
(11)A-=12·(Amin+Amax).
Next, main parameters used for heat exchanger design were calculated using (12)–(18). Pressure drops and the heat transfer coefficients for heat exchanger were calculated using (19)–(25) for the shell and tube site, respectively.Equations used for horizontal and vertical heat exchanger design are as follows:(12)A=qU·DTm,(13)Nt=Aπ·dout·L,(14)pt=1.25·do,(15)Db=do·(NtK1)1/n1,(16)DS=Db+BDC,(17)BS=0.4·DS,(18)AS=(pt-dout)·DS·BSpt,(19)Gshell=shell  side  flowrateAs,(20)de=1.10dout·(pt2-0.917·dout2),(21)Re=de·μ·ρη,(22)Pr=μ·cpλ,(23)Nu=hshell·dekf,(24)Nu=jh·Re·Pr1/3·(μμw)0.14,(25)ΔPshell=8·jf·(Dsde)·(LlB)·(ρ·us22)·(μμw)-0.14,(26)lB=0.7·L4,(27)Ntpp=Ntnumber  of  passes,(28)Gtube=GinNtpp·(π·din2/4),(29)νtube=Gtubeρi.(i)
The heat transfer coefficienthtubefor turbulent flow was calculated by using next correlation:
(30)htube=0.023·Frdin·Re0.8·Pr0.33·(1+dinL).(ii)
The overall heat transfer factor was calculated on “outside pipes flow”:
(31)Ushell=1×(dout·ln(dout/din)2·λw1hshell+1hdirtout+doutdin·hdirtin+doutdin·hdirtin+dout·ln(dout/din)2·λw)-1.(iii)
The overall heat transfer factor was calculated on “inside pipes flow”:
(32)Utube=1×(din·ln(dout/din)2·λw1htube+1hdirtin+dindout·hdirtout+dindout·hshell+din·ln(dout/din)2·λw)-1.(iv)
The tube-side pressure drop was calculated:(33)ΔPtube=(1.5+Nt·[2.5+8·jf·Ldin+(μμw)-m])·ρi·v22.Calculated parameters for horizontal and vertical condenserdesign are reported in Table 1.


## 5. Vertical Heat Exchanger Design

For the vertical heat exchanger design, exactly the same procedure as proposed for the horizontal heat exchanger was used. Firstly, the amount of condensed vapor was calculated from the ratio of vapor pressures at reactor release in the blow down tank and after the volumetric expansion:(34)rc=ps1ps2,
whereps1andps2were the saturated pressures calculated from (1). Obtainedrcwas used for calculation of gas flow which needs to be neutralized and cooled down in vertical heat exchanger:
(35)GHZT=rc·Gvent,
whereGHZTandGventare the reactor runaway flow and flow through horizontal condenser reported in Table 1. CalculatedGHZTflow was used for vertical condenser design at the exact same procedure as for horizontal condenser. In the next step,GHZTflow needed to be cooled down and neutralized in absorption column using sodium hydroxide solution before it was left in the surrounding.GHZTflows from blow down reactor with temperature which is equal to the temperature of runaway flow due to short residence time. Similar predictions were obtained at design of other blow down processes [3, 8, 9].All calculated parameters are presented in Table1. DuringGHZTneutralization reaction, heat was formed. The heat which needs to be cooled down was calculated as the sum of vapor gas heat and the heat produced by neutralization reaction:
(36)Q˙total=Q˙hotgas+Q˙reaction,
whereQ˙reactwas calculated using reaction enthalpy as follows:
(37)Q˙reaction=ΔHreaction·GHZT
and the heat pro recycles using
(38)Q˙recycle=Q˙totalRecycle.


## 6. Scrubber Design

Since the reaction enthalpy for the neutralization reaction of phenol acid with sodium hydroxide cannot be found in the available literature it was calculated from formation enthalpies and was 22.32 kJ/mol [10]. Since the composition of the blow down gas is not known exactly, it was assumed that it is composed from phenolic acid [8]. The neutralization reaction of phenolic acid with sodium hydroxide was written as
(39)PhCOOH+NaOH⟶PhCONa+H2ONeutralization reaction gives the stoichiometric ratio of phenolic acid versus sodium hydroxide and enables the calculation of the amount of neutralization medium needed for neutralization. The necessary 50 wt.% neutralization liquid flow rate was calculated from the reaction and was 42.3 kg/s. To decrease the necessary volume of used sodium hydroxide, its recycling was assumed. The recycle flow was calculated using(40)Recycle=GHZTVscrubber  tank.
The recycle ratio gives scrubber volume of 4.5 m3 and enables the calculation of the liquid and the gas flow rates used for absorption column design. Scrubber design was performed according to API RP 520 standard. All main scrubber dimensions are presented in Table 1. MAWP of scrubber was designed to be 1.5 · 105 Pa.


## 7. Absorption Column Design

Before outflow gas from blow down system was left into surrounding air, it was neutralized by sodium hydroxide in counterflow absorption column. A lot of literature for absorption column design can be found in available literature [11, 14]. Due to high flow of gas and neutralization fluid the chemical reaction was the limiting step of this process. A generalized approach proposed in the literature was used for column design [7, 11, 14]. Firstly,(L/G)·(ρG/ρL)1/2factor was calculated. Pump characteristics limited the pressure drop in absorption column to 1′′ water column per feet of packing. Based on existing correlations, factor(G*2·F·μ0.1)/(32.2·ρg·(ρg-ρL))was obtained to be 1.8 and enabled to calculate the gas fluxG*. A ceramic IMTP packing number 70 was taken as packing material, with main parameters presented in Table 1. Calculated parameters enabled to design an absorption column area and diameter from the following equation:
(41)G=A·G*,(42)A=π·D24.
Large absorption column diameter is a consequence of large gas flow at reaction runaway which was designed to be up to 59.5 m3·h−1·m−2, which is lower than allowed maximal liquid flow rate of 122 m3·h−1·m−2 for IMTO packing number 70. Next, absorption column height was designed based on the procedure proposed by Seader et al. [11] and was obtained to be 3000 mm. All other column parameters are presented in Table 1 and enabled to design the absorption column pressure drop, which was 30 · 103 Pa.


## 8. Ventilator Fan Design

The vapor gas coming out from absorption column is mainly composed of different phenolic vapors which are heavier than air; therefore, a ventilator fan needs to be inserted into outflow chimney. The role of the fan is to suck the vapor gas coming out from absorption column and to blow it out via chimney into surrounding air. Equations used for the fan design are presented as follows [12].Design equations for ventilator fan design in outflow chimney.Pressure of ventilator:(43)ΔPFan=ρ·g·hchimney+ρ·v22.Gas vapor velocity:(44)∅VFan=π·dFan·ϑFan·AFan,
where
(45)AFan=π·dFan24.Ventilator motor power:(46)PFan  motor=∅VFan·ΔPFan.Axial fan velocity:(47)vr=π·dFan·ϑFan.The fan pressure and the fan capacity were designed based on capacity of 82.3 kg · s−1 of outcoming gas by (43)–(47). Radial velocity of ventilator fan speedvrwas calculated from (47) and clearly indicates that an axial ventilator must be applied. All main designed technical characteristics of the ventilator fan are shown in Table 1. A 34 kW ventilator fan motor with corresponding ATEX protection with II 2G EEx de II B T4 code and motor thermal protection should be used since the motor is located in Ex cone. According to ATEX directive, the electric board with all main connections should be located outside Ex cone which is presented in Figure 2. Chimney height was obtained from standards and was approximately 6000 mm. All other chimney dimensions are presented in Table 1 [4, 15].Figure 2
Electrical connections for pump and fan motor.Figure 3
Rupture disc and safety relief valve connection to reactor.


## 9. Blow Down and Cooling Water Pipes Design

Blow down equipment is connected using thick wall pipes made of INOX AISI 316 due to highly corrosive medium, high fluid velocities, and large pressures at reaction runaway. For the vent size pipes, diameter design at reaction runaway frequently used technique proposed by Bleier [12] was used:
(48)DNpipe=(Gfpipe)1/2·(ρP)1/4,
where all parameters are explained in the Nomenclature. The pipe system with 400 mm pipe diameter was proposed as connection between horizontal heat exchanger and absorption column at given conditions. All other data are presented in Table 1 and Figure 2. Connection pipe system between reactors and blow down tank should be as short as possible, straight with no elbows, no reductions to decrease the amount of condensing flow and to prevent the appearance of plugs which may lead to fatal accidents. Additionally, such system configuration ensures easy maintain and fast pipe cleaning. In parallel, the pipe pressure drops were calculated by method proposed by Crane [16] to ensure smooth and easy flow. The pipe length was designed to be as short as possible with minimal pressure drop which will result in low amount of condensed fluid. High fluid velocities, low friction factors, and minimal pressure drops are expected which agrees with results obtained from the literature [16]. For the cooling water pipes diameter and sodium hydroxide pipe diameter design, a method proposed by Mays [17] which is based on the steady state incompressible energy equation utilizing Hazel Williams friction loses and continuity equation was used. The results are presented in Table 1 and in Figure 2.


## 10. Pump Design

Neutralization process characteristics demand 100 m3·h−1 of 50 wt.% sodium hydroxide. For this, a corresponding pump with 5.5 kW electrical motor and capacity of 120 m3·h−1 at approximately 4·105 Pa is proposed. A corresponding electric schema with all relevant parts is presented in Figure 2. Since the pump is situated in Ex cone, a correct motor with motor II 2G EEx de II B T4 code and correct connection according to ATEX standard must be used [18]. Electric board with necessary connections should be located outside the Ex cone. A pressure sensor (PC) is proposed on the pipe from vertical heat exchanger to absorption column. PC has two tasks, to ensure that neutralization medium is pumped all the time from scrubber tank on the absorption column and to prevent the centrifugal pump running dry, which can result in pump failure and in the worst case in pump rotor damage.It needs to be mentioned that reaction mixture which appears at reaction runaway is very viscous, which may result in appearance of plug in designed pipes, equipment, and huge pressure drops. This problem will be avoided by decreasing the connection pipe length without reductions and low pipe elbow number. Horizontal heat exchanger which is located above the blow down tank decreases the fluid temperature which results in fluid condensation, that is why lower amount of gas need to be neutralized in absorption column. Less required gas for neutralization results in lower NaOH consumption which decreases operational costs of designed system. Additionally, horizontal heat exchanger produces huge pressure drop which results in lower gas velocities and higher gas/liquid mass transfer coefficients which increases the efficiency of absorption columns [14].


## 11. Conclusions

Designed results of this study demonstrate that proposed method can be used for emergency relief system design. Based on calculated data the following conclusions were made.Emergency relief system for exothermic reaction for reactor volume up to 30 m3 was designed. A 60 m3 blow down tank, with 700 m3·h−1 of 13°C/26°C cooling water, and two condensers—horizontal and vertical—with cooling area of 54 m2 and 5 m2 are proposed. An absorber column with diameter of 1.5 m and 3.0 m height, 4.5 m3 scrubber tank, and corresponding outflow chimney were designed. All other system parameters are presented in Table 1. It is hoped that the presented procedure will serve as an engineering tool for emergency relief system design in various process industries all over the world.



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*Source: 453509-2013-11-26.xml*