The Pressure Relief System Design for Industrial Reactors

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.

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 Figure 1. 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: 𝑃=exp15.78−8798𝑇. (1) 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: 𝐺_vent=0.9⋅144⋅𝑑𝑃⋅𝑑𝑇32.2⋅𝑇778.16𝑐_𝑝vent^1/2, (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.

The analytical vent sizing equation for homogeneous vessel venting is [2–4] 𝐴_vent=𝑚_𝑜⋅𝑞_vent×𝐺_vent⋅𝑉𝑚_𝑜⋅144778.16⋅𝑇⋅𝑑𝑃𝑑𝑇^1/2+𝑐_𝑝vent⋅Δ𝑇_vent^1/2−1, (3) where Δ𝑇_vent=𝑇_𝑚−𝑇_𝑠,𝐷=4⋅𝐴_vent𝜋,𝑞_vent=12⋅𝑐_𝑝vent⋅𝑑𝑇𝑑𝑡_𝑠+𝑑𝑇𝑑𝑡_𝑚, (4) and all necessary physical and chemical data are presented as follows.

Physical-chemical parameters use for blow down system design: 𝜌_liq.mol⋅dm⁻³=1.37980.31598^{(1+(1−(𝑇/694.25))0.32768)},𝑐_𝑝,liqJ⋅Kmol⁻¹⋅K⁻¹𝑐=101.720+317.61⋅𝑇,_𝑝,𝑔J⋅Kmol⁻¹⋅K⁻¹=0.434⋅10⁵+2.445⋅10⁵⋅1152/𝑇sinh[1152/𝑇]²+1.512⋅10⁵⋅507/𝑇cosh[507/𝑇]²,𝜂_liq.[Pa⋅s]=exp−43.335+3881.7𝑇+4.3983ln(𝑇)+3.0548⋅10²⁴⋅𝑇⁻¹⁰,𝜂_vap.[Pa⋅s]=1.0094⋅10⁻⁷⋅𝑇^0.799,𝜆1+103.1/𝑇_vap.W⋅m⁻¹⋅K⁻¹=0.038846⋅𝑇^0.23921+985.81/𝑇+937.17/𝑇²,𝜆_liq.W⋅m⁻¹⋅K⁻¹=0.18837−0.0001⋅𝑇,Δ𝐻_evap.J⋅Kmol⁻¹=7.306⋅10⁷⋅𝑇1−694.25^0.4246,𝑃_vapour[Pa]=exp95.444−10.13𝑇−10.09⋅ln(𝑇)+6.7603⋅10⁻¹⁸⋅𝑇⁶. (5) Formation enthalpies (Perry and Green (2007) [7]): Δ𝐻^𝑓_NaOH=−469.15 kJ⋅mol⁻¹, Δ𝐻^𝑓_PhOH=−165.0 kJ⋅mol⁻¹ , Δ𝐻^𝑓_H2O=−285.83 kJ⋅mol⁻¹ , Δ𝐻^𝑓_NaPhO=−326 kJ⋅mol⁻¹, 𝜌_50wt.%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 𝑉_bd=2⋅𝑉_𝑜 (6) 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: 𝑃₁⋅𝑉₁𝑇₁=𝑃₂⋅𝑉₂𝑇₂ (7) 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 Table 1.(iii)Corresponding energetic balances for horizontal and vertical heat exchanger were written as 𝑄̇=𝑄̇_hot=𝑄̇_cold=𝑚̇_𝑐⋅𝑐_𝑝𝑐⋅𝑇_𝑐out−𝑇_𝑐in=𝑚̇_ℎ⋅𝑐_𝑝ℎ⋅𝑇_ℎout−𝑇_ℎin. (8) 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 𝑇LMTD=^ℎin−𝑇_𝑐out−𝑇_ℎout−𝑇_𝑐inln𝑇_ℎin−𝑇_𝑐out/𝑇_ℎout−𝑇_𝑐in. (9)(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], parameters𝑅_𝑡,𝑆,and𝐹_𝑡were obtained from the corresponding diagrams. Obtained parameters enabled calculation of mean temperature difference: 𝐷𝑇_𝑚=𝐹_𝑡⋅LMTD. (10)(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: 1𝐴=2⋅𝐴_min+𝐴_max. (11) 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: 𝑞𝐴=𝑈⋅𝐷𝑇_𝑚,𝑁_𝑡=𝐴𝜋⋅𝑑_out,𝑝⋅𝐿_𝑡=1.25⋅𝑑_𝑜,𝐷_𝑏=𝑑_𝑜⋅𝑁_𝑡𝐾₁^1/𝑛1,𝐷_𝑆=𝐷_𝑏𝐵+BDC,_𝑆=0.4⋅𝐷_𝑆,𝐴_𝑆=𝑝_𝑡−𝑑_out⋅𝐷_𝑆⋅𝐵_𝑆𝑝_𝑡,𝐺_shell=shellsideflowrate𝐴_𝑠,𝑑_𝑒=1.10𝑑_out⋅𝑝²_𝑡−0.917⋅𝑑²_out,𝑑Re=^𝑒⋅𝜇⋅𝜌𝜂,Pr=𝜇⋅𝑐_𝑝𝜆,ℎNu=^shell⋅𝑑_𝑒𝑘_𝑓,Nu=𝑗_ℎ⋅Re⋅Pr^1/3⋅𝜇𝜇_𝑤^0.14,Δ𝑃_shell=8⋅𝑗_𝑓⋅𝐷_𝑠𝑑_𝑒⋅𝐿𝑙_𝐵⋅𝜌⋅𝑢²_𝑠2⋅𝜇𝜇_𝑤^−0.14,𝑙_𝐵=0.7⋅𝐿4,𝑁_tpp=𝑁_𝑡,𝐺numberofpasses_tube=𝐺_in𝑁_tpp⋅𝜋⋅𝑑²_in,𝜈/4_tube=𝐺_tube𝜌_𝑖. (12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(i)The heat transfer coefficientℎ_tubefor turbulent flow was calculated by using next correlation: ℎ_tube𝐹=0.023⋅^𝑟𝑑_in⋅Re^0.8⋅Pr^0.33⋅𝑑1+ⁱⁿ𝐿. (30)(ii)The overall heat transfer factor was calculated on “outside pipes flow”: 𝑈_shell1=1×ℎ_shell+1ℎ_dirtout+𝑑_out𝑑_in⋅ℎ_dirtin+𝑑_out𝑑_in⋅ℎ_dirtin+𝑑_out⋅ln𝑑_out/𝑑_in2⋅𝜆_𝑤⁻¹. (31)(iii)The overall heat transfer factor was calculated on “inside pipes flow”: 𝑈_tube1=1×ℎ_tube+1ℎ_dirtin+𝑑_in𝑑_out⋅ℎ_dirtout+𝑑_in𝑑_out⋅ℎ_shell+𝑑_in⋅ln𝑑_out/𝑑_in2⋅𝜆_𝑤⁻¹. (32)(iv)The tube-side pressure drop was calculated: Δ𝑃_tube=1.5+𝑁_𝑡⋅2.5+8⋅𝑗_𝑓⋅𝐿𝑑_in+𝜇𝜇_𝑤^−𝑚⋅𝜌_𝑖⋅𝑣22. (33)Calculated parameters for horizontal and vertical condenser design 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: 𝑟_𝑐=𝑝_𝑠1𝑝_𝑠2, (34) where𝑝_𝑠1and𝑝_𝑠2were the saturated pressures calculated from (1). Obtained𝑟_𝑐was used for calculation of gas flow which needs to be neutralized and cooled down in vertical heat exchanger: 𝐺_HZT=𝑟_𝑐⋅𝐺_vent, (35) where𝐺_HZTand𝐺_ventare the reactor runaway flow and flow through horizontal condenser reported in Table 1. Calculated𝐺_HZTflow was used for vertical condenser design at the exact same procedure as for horizontal condenser. In the next step,𝐺_HZTflow needed to be cooled down and neutralized in absorption column using sodium hydroxide solution before it was left in the surrounding.𝐺_HZTflows 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 Table 1. During𝐺_HZTneutralization 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: 𝑄̇_total=𝑄̇_hotgas+𝑄̇_reaction, (36) where𝑄̇_reactwas calculated using reaction enthalpy as follows: 𝑄̇_reaction=Δ𝐻_reaction⋅𝐺_HZT (37) and the heat pro recycles using 𝑄̇_recycle=𝑄̇_total.Recycle (38)

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 PhCOOH+NaOH⟶PhCONa+H₂O (39)

Neutralization 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 𝐺Recycle=^HZT𝑉_scrubbertank. (40) 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)⋅(𝜌_𝐺/𝜌_𝐿)^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⋅𝜇^0.1)/(32.2⋅𝜌_𝑔⋅(𝜌_𝑔−𝜌_𝐿))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: 𝐺=𝐴⋅𝐺^∗,𝐴=𝜋⋅𝐷²4. (41)(42) 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: Δ𝑃_Fan=𝜌⋅𝑔⋅ℎ_chimney+𝜌⋅𝑣²2. (43)

Gas vapor velocity: ∅_𝑉Fan=𝜋⋅𝑑_Fan⋅𝜗_Fan⋅𝐴_Fan, (44) where 𝐴_Fan=𝜋⋅𝑑_Fan²4. (45)

Ventilator motor power: 𝑃_Fanmotor=∅_𝑉Fan⋅Δ𝑃_Fan. (46)

Axial fan velocity: 𝑣_𝑟=𝜋⋅𝑑_Fan⋅𝜗_Fan. (47)

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 speed𝑣_𝑟was 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: 𝐷𝑁_pipe=𝐺𝑓_pipe^1/2⋅𝜌𝑃^1/4, (48) 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.

Nomenclature

𝐿:	Length (m)
𝜆𝑤:	Thermal conductivity (W⋅m−1⋅K−1)
𝐶𝑝cold:	Specific heat of cold medium (J⋅g−1⋅K−1)
𝐶𝑝hot:	Specific heat of hot medium (J⋅g−1⋅K−1)
Cooling water:	Specific heat of cooling water (J⋅g−1⋅K−1)
𝑄̇:	Heat transfer (W)
𝑇hin:	Temperature of hot medium (°C)
𝑚𝑜:	Reactor charge (kg)
𝐴:	Area (m2)
𝑉:	Reactor volume (m3)
𝜌:	Density (kg·m−3)
𝑞̇𝑐:	Heat release per unit mass cold medium (W⋅kg−1)
𝑞ℎ̇:	Heat release per unit mass hot medium (W⋅kg−1)
𝐺:	Discharge mass flow rate per unit area (kg⋅m2⋅s−1)
𝑃:	Pressure (Pa)
𝑇:	Temperature (K)
𝑑𝑇/𝑑𝑡:	Self heat rate in runaway reactions (K⋅min−1)
𝑞:	Energy release rate (J⋅kg−1)
𝑄̇hot:	Heat of hot medium (W)
𝑄̇cold:	Heat of cold medium (W)
𝑇ℎ,out:	Temperature of hot medium (°C)
LMTD:	Log mean temperature difference (°C)
𝑅𝑡:	Parameter for temperature correction factor prediction
𝑆:	Parameter for temperature correction factor prediction
𝐹𝑡:	Temperature correction factor
𝑇𝑚:	Mean temperature difference (°C)
𝑈:	Overall heat transfer coefficient (W⋅m−2⋅K−1)
𝐴:	Average provisional area (m2)
𝑁𝑡:	Number of tubes
𝑑:	Pipe diameter (mm)
𝑟:	Ration
𝑚̇:	Mass flow (kg⋅s−1)
𝐹𝑡:	Friction factor
Δ𝑃shell:	Pressure drop on the shell side (Pa)
Δ𝑃tube:	Pressure drop on the tube side (Pa)
BDC:	Bundle diameter clearance (mm)
𝑤:	Wall thickness (mm)
𝑝𝑡:	Tube pitch (mm)
𝐷𝑏:	Bundle diameter (mm)
𝐷𝑠:	Shell diameter (mm)
𝐵𝑠:	Baffle spacing (mm)
𝐴𝑠:	Area for cross-flow (m2)
𝐺shell:	Shell-side mass velocity (kg⋅s−1⋅m−2)
𝑑𝑒:	Shell equivalent diameter (mm)
Re:	Reynolds number
Pr:	Prandtl number
Nu:	Nusselt number
ℎshell:	Shell-side heat transfer coefficient (W⋅m−2⋅K−1)
𝑗ℎat 45% of baffle cut:	Shell-side heat transfer factor
𝑙𝑏:	Baffle spacing (mm)
𝑁tpp:	Number of tubes per pass
𝐺tube:	Tube-side mass velocity (kg⋅m−2⋅s−1)
𝑉tube:	Tube-side velocity (m⋅s−1)
Retube:	Inside tube Reynolds number
Prtube:	Inside tube Prandtl number
Nutube:	Inside tube Nusselt number
𝑗𝑓at 45% of baffle cut:	Shell side friction factor
ℎtubeside:	Inside heat transfer coefficient (W⋅m−2⋅K−1)
𝑈i:	Overall heat transfer factor for inside tubes flow (W⋅m−2⋅K−1)
𝑈o:	Overall heat transfer factor for outside tubes flow (W⋅m−2⋅K−1)
𝑈:	Average heat transfer factor (W⋅m−2⋅K−1)
𝐹IMTP:	IMTP packing factor
𝐿NaOH:	NaOH flow (m3⋅h−1)
Recycle:	Recycle of NaOH in absorber (h−1)
𝑉scrubbertank:	Volume of scrubber tank (m3)
𝐴abs:	Absorber area (m2)
𝐷abs:	Absorber diameter (m)
Δ𝑃abs:	Pressure drop on absorber tray (Pa)
𝐹IMTP:	IMTP packing factor
IMTP packing:	Mark of absorber fill
𝑁𝐿:	Number of theoretical plates or trays
HETP:	Height equivalent to the theoretical plate (m)
𝐻abs:	Height of absorption column (m)
𝑉bd:	Blow down volume (m3)
𝑉max.:	Maximal tank volume (m3)
𝑞vent:	Heat release per unit mass at reaction runaway (W⋅s−1)
𝑐𝑝:	Specific heat of material (J⋅g−1⋅K−1)
(𝑑𝑇/𝑑𝑡)𝑚:	Self heat rate at temperature turn around or maximal over pressure defined by API RP 520 during run away (K⋅s−1)
(𝑑𝑇/𝑑𝑡)𝑠:	Self heat rate at set pressure of pressure relief devices (K⋅s−1)
𝑇𝑚:	Temperature of reactant correlates to the temperature turn around or maximum over pressure defined by API RP 520 during runaway (K)
𝑇𝑠:	Temperature of reactant correlates to set pressure of pressure relief devices (K)
Δ𝑇:	Temperature difference (K)
𝑡:	Time (s)
𝜂∶	Viscosity (Pa⋅s)
μ:	Kinematic viscosity (m2⋅s−1)
𝑉:	Volume (m3)
𝐷:	diameter (m)
𝜌:	Density (kg⋅m−3)
𝐹:	Flow reduction factor
𝑉:	Voltage (V)
𝐼:	Current (A)
𝑊:	Power supply (W)
GF:	Gas rate (m3⋅h−1)
LF:	Liquid rate (m3⋅h−1)
𝑐Li:	Liquid concentration (mol⋅m3)
𝑐gi:	Gas concentration (mol⋅m3)
𝑅:	liquid rate to tank volume (h−1)
∅𝑉Fan:	Ventilator fan flow (m3⋅h−1)
𝑣𝑟:	Axial fan velocity (m⋅s−1)
Δ𝑃Fan:	Ventilator fan pressure (Pa)
𝑃Fanmotor:	Ventilator motor power (W)
∅𝑉Fan:	Rotational velocity of ventilator fan (RPM)
𝐴Fan:	Ventilator area (m2)
𝑑Fan:	Fan diameter (mm)
ℎchimney:	Height of outflow chimney (mm)
𝑔:	Gravity acceleration (m2⋅s−1)
𝑣vapor:	Outflow vapor gas velocity (m⋅s−1)
𝑓pipe:	Flow reduction factor
Δ𝐻react:	Reaction enthalpy (J⋅mol−1)
𝐺∗:	Gas flux (kg⋅s−1⋅m−2)
DN:	Inner diameter (m)

Subscripts

bd:	Blow down
𝑚:	Pressure turnaround, mean
max.:	Maximal value
𝑠:	Relief set conditions, shell
𝑜:	Initial conditions
cin:	Cold in
cout:	Cold out
abs:	Absorber
vent:	Vent
BWG:	Birmingham Wire Gauge
dirt in/dirt out:	Coatings of dirt on the in and out site of pipe
𝑤:	Wall
tube:	Tube side
shell:	Shell side
𝑖,𝑗:	Refers to tube and shell side
𝑝:	Specific heat
1, 2, 3:	Refers to position
S1, S2:	Saturated pressure at position 1 and 2
HZT:	Horizontal
hin:	Hot in
hout:	Hot out
out:	Out site
⋅:	Unit in time
𝑒:	Equivalent
IMTP:	IMTP packing type
𝑐:	Cold
ℎ:	Hot
𝑡:	Pitch
𝑏:	Bundle
ℎ:	Heat
𝑓:	Friction
𝑖:	Inside
𝑜:	Outside
abs:	Absorber
𝐿:	Theoretical
𝜋:	3.14
𝑠:	Set point
PE:	Electrically grounded
U1, U2:	Electro phases
V1, V2:	Electro phases
W1, W2:	Electro phases
TK:	Electro phases
L1, L2, L3:	Electro phases
𝑁:	Neutral phase
S1:	Switch 1
0, 1, 2, 3, 4:	Position in switch and correction
F1:	Fusion 1
AC3:	Analog current
X1,X2:	Contactor
M:	Motor
3~,Λ/Δ:	Three phase electro motor
MAWP:	Maximal allowed working pressure.