Distillation columns risk assessment: when the regular

HAZOP evaluation is not enough

Dalva Janine RITA

Carlos MARENCO

Ivan MANTOVANI

Fabiana. TEDESCHI

Cláudio TAKASE

Rhodia Poliamida e Especialidades Ltda

Paulínia, São Paulo, Brasil

dalva.rita@br.rhodia.com; carlos.marenco@br.rhodia.com;

ivan.montovani@br.rhodia.com; fabiana.tedeschi@br.rhodia.com;

claudio.takase-External@@br.rhodia.com

SUMMARY

The temperature increase is one of the process parameters deviations

evaluated during HAZOP analyzes in a distillation system. One of the causes of

this deviation can be the failure of the cooling system resulting in the reduction

of condenser capacity which might cause the emission of volatile organic

compounds to the atmosphere. In the presence of an ignition source, a fire with

serious consequences on the health of the people might occur, and also

important material losses. This risk can be reduced to an acceptable level by

installing safety systems (such as steam valve closure). Nevertheless, in some

distillation systems, the regular instrumental safety chain is not enough to

guarantee a safe unit shut down. After the cooling system failure and the steam

valve closure, the volatilization of the most volatile components due to the heat

content that remains in the system might occur. This subsequent event might

lead to a release of product to the atmosphere. The objective of this work is to

alert about this possible scenario and propose an approach for the calculation

of the amount of volatiles released to the atmosphere, using simplified heat

balances.

1- INTRODUCTION

The HAZOP method (HAZard OPerability study) was developed by ICI in the early

70s. In the 1980s risk studies gradually came into use in petrochemicals, oil, chemicals,

rail transport, automobiles and other industries. This methodology has been used to assess

the safety of new projects or existing units and their modifications. The purpose of this

risk analysis is to identify potential accident scenarios that can occur at a facility and to

reduce the corresponding risks to acceptable levels. The risk analysis is performed by a

multi-disciplinary group.

At Rhodia the HAZOP methodology principles were implemented in the beginning of

80s. The risk analysis is mandatory and performed at each 5 years for some facilities and

at each 3 years to others1.

2. ASSESSING AND REDUCING THE RISK IN DISTILLATION

COLUMNS

In the case of distillation systems, considering column and peripherals, different

scenarios are evaluated during the HAZOP study. In steady state operation, normally, the

HAZOP group analyses the deviations on temperature, pressure, level, flow rate, etc., and

the possible consequences of those failures (such as human and environmental losses). A

common cause of the temperature or pressure deviation is the total loss of cooling

capacity. In this scenario, the loss of condensation capacity leads to the loss of reflux

flow causing, by consequence, the variation of the temperature and pressure profiles of

the distillation column. This scenario culminates in the unwanted event, a process

accident: the emission of volatile products to the atmosphere via column vent system and

its possible ignition. In addition, overpressure in the column above of the Maximum

Allowed Working Pressure (MAWP) may occur, and by consequence causing the rupture

of the column and/or its peripherals.

Some recent studies presented the use of dynamic simulation for safety analysis in

distillation systems 2, 3

. The dynamic simulation was used to simulate the consequences of

operational failures including reduction or total loss of cooling capacity and it is possible

to observe the dynamic response (such as the pressure increase) and also to evaluate the

safety systems installed.

The reduction of the risk associated with the illustrated scenario is normally related to

the installation of active safeguards, such as pressure relief devices (PSV) and

instrumental safety chain. The Figure (1) shows a classic distillation system, with TISH

safety chain type - (high temperature) installed in the column vent actuating by closing

the on-off steam valve. The logic of the safety chain is such that an increase of the vent

temperature above the stipulated value in steady state conditions, leads to the closure of

the steam to the reboiler. Other safety chains might also act on other points of the facility

preventing unwanted chain events.

In HAZOP studies carried out at Rhodia 4, the reduction of the scenario risk is

associated with the Instrumented Safety Systems reliability, expressed by the SIL (Safety

Integrity Level).

FEED

CONDENSER

COLUMN

TIC

TISH

REBOILER

STEAM

HEAVIES

LIGHTSREFLUX

VENT/

ATMOSPHERE

COOLING

WATER

CONDENSATE

UV

PSV

Figure 1 – Safety barriers in a distillation system

3. BEYOND REGULAR HAZOP ANALYSIS

In some distillation systems it is necessary to envisage beyond the horizon of

conventional HAZOP analysis and the installed safety barriers.

Let is consider the system showed in the Figure (1). After further analysis, one

possible scenario has been identified for distillation systems where the difference of

volatility of the compounds is high. In this case the safety barriers must take into account

the intrinsic dynamic effect of the heat accumulation in the system.

In this scenario, after losing cooling capacity and the safety barriers taking action

(TISH, closing the steam to the column), the most volatile compound might be released

to the atmosphere due to great difference of temperatures between the top and the bottom

of the column. The necessary energy for the volatilization comes, basically, from the

metal of the column, the metal of the reboiler and other possible peripherals, and amount

of heat cumulated in the liquid at the bottom of the column. In this situation the inertia of

the system is observed and this phenomenon might occur from 5 to 30 minutes after the

safety barrier actuation. The amount of volatiles released to the atmosphere depends on

the column and reboiler dimensions (amount of metal), difference of volatility of the

compounds, difference of temperature between the top and the bottom, the holdup of the

column, the inertia of the steam valves and the capacity of the condensers.

4. METHODOLOGY APPROACH

The approach is based on the First Law of Thermodynamics and heat transfer

equations and can easily be applied to the distillation columns operating in different

conditions. Based on the results it is possible to evaluate the existing safety systems.

The cumulated heat available for the evaporation of the most volatile component of

the system takes into account different sources, as shown in the Equations (1) (4):

1) Heat content due to the steam in the reboiler

STEAMSTEAMmQ λ11 = (1)

2) Heat content due to the steam flow after the security system actuation (the inertia of

the steam valves)

STEAMSTEAMmQ λ22 = (2)

3) Heat content due to difference of temperature and composition between top and

bottom of the column

( )Tfbottompbottom TCmQbottom /3 ∆=

(3)

4) Heat content due to the metallic parts (column and reboiler tubes)

( )Tfmetalmetalpmetal TCmQ /4 ∆=

(4)

The equilibrium temperature or final temperature of the system (Tf) is the boiling

temperature of the most volatile compound at the pressure of the vent system.

Considering the Equations of (1) (4), the amount of product that can be evaporated

will be given by:

p

i

calc

Q

mλ

=∑4

1 (5)

mcalc is the maximum amount that can be evaporated in the system, but the amount

effectively evaporated must take into account the holdup (of volatile product) of the

column

If calcvol mholdup ≥+ , so:

calcevap mm = (6)

If calcvol mholdup <+ , so:

volevap holdupm += (7)

The amount of volatiles that effectively will be emitted by the vent system is

calculated considering the remaining condensation capacity, as some water remains in the

condenser. The reached temperature is Tf.

The equation (8) represents the amount of heat removed due to presence of water and

metal in the condenser, with temperature lower than Tf.

( ) )/(/ TfmetalpmetalTfCWpwatercond TCmTCmQmetalcondwater

∆+∆= (8)

The amount of condensed volatiles will be given by:

( )p

condcond

Qm

λ= (9)

Finally, the amount of volatiles released to the atmosphere will be:

condevapemitted mmm −= (10)

If the remaining condensation capacity is higher than the amount of evaporated

product, there will not be emission of product to the atmosphere. Otherwise, it will be

necessary the implementation of extra safety barriers to prevent the emission of product

to the atmosphere via the vent system.

The equations (1) to (10) represent a simple method to determine if a product

emission will occur in distillation columns operating with very distinct temperatures

between the top and the bottom.

It is important to mention that if the system in question have many high volatility

components it is necessary to increase the calculation accuracy of the equation (1) to (10)

in order to consider these other volatiles present in the mixture, invalidating the

simplification indicated in (7).

5. CONCLUSIONS AND FUTURE WORK

In HAZOP studies, the case of distillation columns is evaluated considering that the

risk of any unwanted event is reduced by the installation of safety systems, and especially

in the case of loss of cooling capacity, the usual safety barrier is the steam valve closure.

However, for some distillation systems, where the difference of volatility of the

components is high, the simple safety chain suggested might not be enough to guarantee a

safe unit shut down. The dynamic effect of cumulated heat in the system might provide

energy enough causing the re-vaporization of the most volatile compound. In this case the

risk of releasing volatiles to the atmosphere remains.

By using simplified heat balances and construction and operation data of the column

and peripherals, it is possible to evaluate the amount of volatile compounds that can be

released to the atmosphere.

This simplified approach can be used in different distillation systems and it is an alert

to prevent organic emissions to the atmosphere. In addition, it is possible to assess, case

by case, the risk level. In continuation to the study, a second approach will be carried out

using dynamic simulation to quantify the rate of emission of volatiles to the atmosphere.

6. NOMENCLATURE

λsteam enthalpy of vaporization (steam)

λp enthalpy of vaporization (most volatile compound)

( )jiT /∆ difference of temperature between i and j.

holdup+vol holdup of the most volatile compound

CW cooling water

Cpi calorific capacity of the i mwater amount of water in the condenser mbottom amount of product in the bottom of the column mmetal amount of metal (column and peripherals)

mmetalcond amount of metal (condenser tubes)

mcalc possible evaporated amount of volatile compound

mevap possible evaporated amount of volatile compound, considering the holdup

of the most volatile compound

memitted amount of volatile compound released via vent system

msteam1 amount of steam in the reboiler

msteam2 amount of steam released from steam valves after the actuation of the

safety system, due to the inertia of the valves

Q1 heat transferred due to the steam in the reboiler

Q2 heat transferred due to the steam flow after the security system actuation

(the inertia of the steam valves)

Q3 heat transferred due to the temperature and composition differences

between top and bottom of the column Q4 heat transferred due to the metallic parts (column and reboiler tubes)

Qcond amount of heat removed due to presence of water and metal in condenser

Tf equilibrium (final) temperature

6. REFERENCES

1. Rhodia Responsible Care Function, Procedure-Process Safety Risk Analysis; Rhodia S.A., 2007.

2. N.Ramzan, F. Compart, W.Witt, Application of Extended HAZOP and Event-Tree Analysis for

investigating Operational Failures and Safety Optimization of Distillation Column Unit; Process Safety

Progress vol. 26, (3), 2007.

3. S. Werner, W. Fred and Compart, Assessing Safety in Distillation Column Using Dynamic Simulation

and Failure Mode and Effect Analysis (FMEA), Journal of Applied Sciences, vol. 7, (15), 2007.

4. Rhodia Responsible Care Function, Process Safety Guide-Assessing and Reducing Risks, Rhodia S.A,

2008.