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BROWN INDUSTRIES

ADVANCED REACTIVE SCREENING TOOL

Department of Chemical Engineering

University of Michigan

Ann Arbor, MI. 48109

August 17, 2005

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ADVANCED REACTIVE SCREENING TOOL

This manual only summarizes some aspects of the ARSST. Students need to read the

other manuals and literature in the lab and office for detailed information.

I — EQUIPMENT DESCRIPTION

The ARSST (Figure 1) is an easy to use and cost effective calorimeter that can

quickly and safely identify potential chemical reactivity hazards. It can also yield critical

experimental data on the kinetic characteristics of runaway reactions that can be scaled

up to full-scale process conditions and can be used directly to estimate the size of a relief

device that would be required to protect the reactor against the over-pressure that would

result from a runaway reaction.

The ARSST consists of a containment cell, stirrer and computerized control

module. The stainless steel containment cell has an internal volume of 350 ml.

Figure 1: Advanced Reactive Screening Tool System.

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The containment cell

The containment cell contains the 10 ml test cell, the heaters, thermocouples, and

inlet and outlet ports.

Figure 3: Containment Cell Diagram.

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The test cell

Figure 2: Test Cell Diagram.

II — RUNAWAY SYSTEMS

There are three possible conditions (Figure 3) that can be encountered with a

runaway reaction which can result in an overpressure situation:

1. A vapor (or tempered) condition, in which the rise in pressure is due only to the

increase in vapor pressure of the liquid in the reactor as the temperature increases

from the heat evolved from the exothermal reaction.

2. A gassy condition, in which the pressure rise is due to the evolution of a non-

condensable gas by the reaction (e.g. a decomposition reaction).

3. A hybrid condition, in which the pressure increase results from both the increase

in vapor pressure as well as evolution of a non-condensable gas.

The nature of the temperature versus time relation gives an indication of which

type of condition exists in the reactor

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Figure 3: Runaway Reaction Classes.

Observation of the pressure before and after the run, as well as the pressure and

temperature rise rates, tells whether the reaction is tempered (vapor), hybrid, or gassy:

1. If the pressure at the end of the run (after cool-down) is the same as at the start,

then no non-condensable gas has been evolved and the system is tempered (vapor)

2. If the final pressure is greater than the initial pressure, but the pressure and

temperature both level off at the same time (same temperature), then the system is

gassy.

3. If the final pressure is greater than the initial pressure, but the pressure and

temperature level off at different times (temperatures), then it is a hybrid system.

For a vapor system, if the total mass lost from the liquid is not more than 1% of

the initial mass, and/or there are no pressure spikes during the run, the reaction is not

likely to result in a serious overpressure condition (this can be confirmed if the vapor

pressure of the liquid as a function of temperature is known). If the mass loss is

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significant, and/or a pressure spike occurs, a relief vent must be sized properly and

installed on the reactor. This requires knowledge of the vapor pressure versus

temperature properties of the liquid, which can be obtained from either an independent

knowledge of the liquid vapor pressure versus temperature, or it can be measured in a

separate test run with pressures up to the relief set pressure.

If the system is gassy, a relief vent can be sized based on the PMAAP and the

maximum pressure rise rate during the run. If the resulting required vent area is

excessively large, the possibility of a hybrid system can be determined by running a

second test up to the set pressure. If the maximum temperature reached during this

second test is less than that reached during the initial test, vapor (i.e. a tempered system)

is present. If this occurs, both the pressure rise rate and the temperature rise rate at the set

pressure are required to size the vent (see equations to follow later). If the maximum

temperature from the second test is not less than that from the first test, the system is

gassy and the vent can be sized using only the maximum pressure rise rate at the PMAAP

(see equations to follow.)

If the initial screening indicates the presence of a hybrid system, a second test

should be run for pressures up to the set pressure. As before, if the maximum temperature

reached during this second run is less than that reached in the first run, vapor (tempering)

is present and the vent must be sized for a hybrid system using both the pressure rise rate

and the temperature rise rate at the set pressure (and the corresponding temperature). If

the maximum temperature reached in the second run is not less than that reached in the

first run, no vapor (tempering) is present and the vent can be sized for a gassy system

using only the maximum pressure rise rate at the PMAAP.

III — VENT SIZING THEORY FOR RUNAWAY REACTIONS

All vessels subject to potential over-pressure conditions must be fitted with a

safety relief vent (either a safety relief valve, which re-closes when the vessel pressure

falls to a safe level, thus containing most of the vessel contents, or a rupture disk which

can vent the entire vessel contents). An excessive pressure condition arises when energy

in the form of pressure, or any form that can be converted to pressure, is input into the

vessel at a rate faster than it is removed. In the case of a runaway reaction, the source of

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the energy input is the exothermic heat of reaction. For reactions taking place in the

liquid (condensed) phase, this heat is absorbed by the liquid, increasing the vapor

pressure and hence the pressure in the vessel. Such reactions are called “tempered” or

“vapor” systems, since the heat of reaction increases the superheat and vapor pressure of

the liquid. Another source of pressure from a reaction results when the products of the

reaction are gases, and more moles of gas are generated than are consumed by the

reaction. Such reactions are called “gassy”.

The required vent size is determined by an energy balance on the reactor, with the

criterion that the size of the vent must be large enough to expel energy from the vessel (in

the vented stream) at a rate equal to the maximum rate at which it is generated. For a

tempered (vapor pressure) system, the energy balance on the vaporizing liquid is

TVcTmcmvv

&&& !=="

The left side of the equation is the latent heat removed from the vessel by the fluid

that is relieved at the rate m& through the vent, and the right side is the rate of heat

generated by the reaction. Assuming single-phase flow (i.e. complete disengagement of

the vapor from the liquid before entering the vent) and choked vapor flow through the

vent, the mass flux through the vent is given by application of Bernoulli’s equation to an

isentropic nozzle:

vvkdPCK

A

mG !==

&

where

61.01k

2kC

1k

1k

k!"

#

$%&

'

+=

(

+

Here k is the isentropic exponent for the gas/vapor (or the ratio cp/cv for an ideal

gas) and Kd is the vent discharge coefficient. If m is eliminated from Eqns (1) and (2), the

result can be solved for the ratio of the vent area A to the volume of the reacting mass, V:

wkd

v

M

RT

PCK

Tc

V

A

!

"=

&

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For a gassy system, the pressure rise rate results from generation of a gaseous

product from the reaction. The mass flow rate corresponding to this pressure rise rate is

given by:

PRT

VMm

w && =

Eliminating m from Eqns (2) and (5) gives

RT

M

P

P

CK

1

V

Aw

kd

&

=

This equation must be modified for application to the ARSST, however, since the

reactant volume V is that in the test cell, whereas the pressure is measured in the

containment volume. Thus the reactant mass must be scaled up from that in the test cell

to what it would be in the containment volume, i.e.:

RT

M

P

P

m

v

CK

1

V

Aw

tkd

&!=

where t m is the test sample mass, v is the ARSST containment volume and ρ is

the density of the reacting mixture. For a hybrid system, the required vent areas for both

the vapor and gas release are added (again assuming single phase gas/vapor choked flow

through the vent).

The equations that determine the required vent area (A) for a given volume (V) of

reaction mass are derived from an energy balance on the reactor. The energy released

(generated) by the reaction must be removed by mass flow from the reactor at a rate at

least equal to the rate at which it is generated to prevent over-pressuring the reactor.

The energy balance on the reacting mass determines the required rate of mass

removal from the reactor. This is coupled with the mass flux capacity of the vent as

determined from an energy balance on the vent (assumed it to be an isentropic nozzle), to

determine the area of the vent which permits the required flow rate for the given driving

force. The driving force for flow in the vent depends upon whether or not the velocity

reaches the speed of sound (i.e. critical (choked) or sub-critical flow). Critical (choked)

flow is the most common condition encountered in typical relief scenarios.

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For a tempered (vapor) system, the driving force is the saturation vapor pressure

in the reactor (Ps) at the relief pressure/temperature. If the flow is critical (choked), the

vent flow rate is independent of the exit or back pressure (Pb), but for sub-critical flow

the back pressure does affect the flow rate. The subcritical form of the equation assumes

a constant density fluid, which applies to gas/vapor flow only at relatively low pressures.)

Tempered, critical flow:

1 2

sv

D s w v

R TA 1 c T

V 0 6 1C P M

/

.

! "#= $ %

& ' (

&

Tempered, subcritical (low pressure) flow:

( )

1 2

sv

D s b s wv

RTA c T

V c P 2 1 P P M

/

/

! "#= $ %

& '( )

&

where

A = vent area (m2)

V = reactant volume (m3)

ρ = reactant density (kg/m3)

cv = liquid specific heat (J/(kg K))

T& = self-heat rate (K/s)

λ = latent heat (J/kg)

Ps = relief set pressure (Pa)

R = gas constant (8314 Pa m3/(K kmol))

Ts = relief set temperature (K)

Pb = backpressure (Pa)

Mwv = molecular weight of vapor (kg/kmol)

CD = discharge coefficient (-)

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For a gassy system, the reaction rate and required mass flow rate are directly

proportional to the rate of gas release, as measured by the pressure rise rate. The relief

area is determined by this mass flow rate and the mass flux capacity of an isentropic

(ideal gas) nozzle. For critical (choked) flow, the driving force is the relieving pressure

only, independent of the exit back pressure. The MAAP is the maximum allowable

pressure above the relief set pressure, and is that pressure at which maximum flow

through the vent occurs (this is typically 110% of the relief set pressure). It is noted that

sub-critical gassy flow is rarely encountered.

1 2

wg

D t

MA v P

V 0 61C m P RT

/

.

! "#= $ %

& '

&

where

P& = maximum rate of pressure rise (Pa/s)

mt = test sample mass (kg)

P = maximum allowable accumulated pressure, MAAP (Pa)

Mwg = molecular weight of gas (kg/kmol)

v = ARSST containment volume (3.5 x 10-4m3)

This equation includes v, the ARSST containment volume, because the pressure is

measured in this volume and the maximum pressure rise rate is used to calculate the

corresponding rate of mass release by the reaction. If a containment volume other than

350 cc is used, this number must be modified accordingly.

For a hybrid system the energy released by the reaction is removed by both the

vapor generated by the evaporating or flashing liquid (which is proportional to the

temperature rise rate), as well as the rate of non-condensable gas evolution (which is

proportional to the pressure rise rate).

For critical (choked) flow, the vent driving force is independent of the exit back

pressure, but for sub-critical flow the back pressure is important. The form of the

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subcritical equation shown applies only at relatively low pressures at which the gas

density can be considered constant.

Tempered, critical flow:

1 21 2

wgsv

D s wv t s s

MRTA 1 c T v P

V 0 61C P M m P RT

//

.

! "# $# $% %& '= + ( )( )*& '+ , + ,- .

& &

Tempered, subcritical (low pressure) flow:

( )

1 21 21 2

wgsv

D s wv t s s b s

MRTA 1 cT v P 1

V C P M mP RT 2 1 P P

///

/

! "! "# $# $% %& '= + & '( )( )* +& ', - , - . /. /

& &

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V — VENT SIZING SOFTWARE PACKAGE (VSSP)

VSSP is loaded on the same computer running ARSST. You will need to use the

software during your lab section if there is extra time or sign up for an extra time with the

GSI. The following parameters must be entered into the program.

• Set pressure

• Backpressure

• Vessel Volume and charge

• Liquid Specific Heat (0.5 BTU/lb-F)

• Liquid Density (49.938 lb/ft3)

• Vapor Density (1 lb/ft3)

• Latent Heat (if you assume a Clausius-Clapeyron relationship, use 0)

• Vessel CSA (only needed for bubbly & churn-turbulent flow)

• Surface Tension (only needed for bubbly & churn-turbulent flow)

• 2 P-T points—use 5.4 psia, 122 F and 30 psia, 221 F (assumed from Raoult’s law)

• Type of relief behavior (use homogeneous-vessel venting to ensure your answer is

consistent with the answer obtain from hand calculations

• Cause of emergency relief (runaway reaction or constant heating)

• 2 dT/dt points from your experiment data—one point should be at the onset

temperature for the runaway reaction and one point should be at the tempering

temperature for the system

• Overpressure percentages

IV — ARSST SYSTEM PROCEDURE

1. Check the equipment following the vessel checkout procedures section in the lab

manual.

2. Prepare 10 ml of a 3:1 mole ratio of water (or methanol) and acetic anhydride. Keep

the mixture chilled to minimize reactivity.

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3. Prepare a test cell according to the test cell assembly lab manual.

4. Place the test cell in the ARSST vessel. Complete all necessary connections

(thermocouple, dual bottom heater) as described in the lab manual.

5. Place the fill-tube assembly securely on the vessel (may need to grease the O-ring to

make this easier), ensuring that the feed port goes into the neck of the test cell.

6. Screw on the vessel lid tightly.

7. Using a syringe, add 10 ml of sample to the RSST using the feed port on the fill tube

assembly. BE SURE TO CLOSE THE FEED PORT VALVE WHEN COMPLETE!

8. Connect the vessel to the ARSST control box. Power on the ARSST control box.

Open the ARSST software package on the PC.

9. Connect the vessel to the nitrogen supply and pressure it to the desired pressure (15 or

300 psig).

10. Close the nitrogen feed valve on the vessel (the vessel should be completely sealed

now), and disconnect nitrogen supply.

11. Place the ARSST vessel on the stir plate. Turn on stirring.

12. Enter the necessary parameters into the ARSST program.

13. Begin the test. Shut down the test when the temperature inside the test cell levels off.

14. Obtain data files containing your test data.

15. Disconnect the vessel from the RSST control box, and relieve vessel pressure IN THE

HOOD by opening the valve on the fill tube assembly.

16. Disassemble and clean the RSST, and dispose of all chemicals properly.