The Integration of Process Safety into a Chemical Reaction

Engineering Course: the Review of the T-2 Incident

Ronald J. Willey, Northeastern University, Boston, MAH. Scott Fogler, University of Michigan, Ann Arbor, MIMichael B. Cutlip, University of Connecticut, Storrs, CT

Objectives/Outline

• Introduce you to a textbook example that will appear in the newest edition of H.Scott Fogler, “Essentials of Chemical Reaction Engineering”

• This example appears in Chapter 13 of the new book. “Unsteady-State Non isothermal Reactor Design”

• Methods used to model the reaction

• Demonstration of integration of Polymath to solve the problem

Text book problem statementExample 13-6 T2 Laboratories Explosion

†

Figure E13-6.1 Aerial photograph of T2 taken December 20, 2007.

Courtesy of Chemical Safety Board.

†This example was co-authored by Professors Ronald J. Willey, Northeastern University,

Michael B. Cutlip, University of Connecticut and H. Scott Fogler, University of Michigan.

Opening paragraph example statement

T2 Laboratories manufactured a fuel additive methylcyclopentadienyl manganese

tricarbonyl (MCMT) in a 2,450-gallon high-pressure batch reactor utilizing a three-

step batch process.

Step 1a. The liquid-phase metalation reaction between methylcyclopentadiene

(MCP) and sodium in a solvent of diethylene glycol dimethyl ether (diglyme) to

produce sodium methylcyclopentadiene and hydrogen gas:

+ Na

Na

+

1

2 H2

Hydrogen immediately comes out of the solution and is vented at the top in

the gas head space.

We included the previously unknown Reaction 2

diglyme decomposition 3 moles of gas and “tar” plus heat

3 moles of gas + tar +HEAT!

Reactor Details

• 2450 gallons [~9,000 dm3] batch reactor

• Working volume of 2,000 gallons

• Made of 3” steel

• Working pressure was 600 psig (4.13 MPa)

• Original design rating was 1200 psig (8.27 MPa)

• Relief was a 4” diameter rupture disc inserted into a 4” line.

• Rupture disc setting was 400 psig.

Figure 8 Reactor

Cross Section

Problem statement rendition

Cooling jacket water inlet

Cooling jacket steam outlet

Adiabatic Reaction Calorimetry used for kinetic constants estimates

reaction 1 exotherm

diglyme decomposition

Simplified Model

A = sodium,

B = methycyclopentadiene

S = Solvent (diglyme)

(1) A + B → C + 1/2 D (gas)

(2) S → 3 E (gas) + F

Initial temperature 422 K (300°F)

Reaction Kinetics

Reaction 1: Overall 2nd order, first order in Na and MCP

–r1A = k1CACB

k1A = 5.73 × 10−2 dm3 mol–1 hr–1

E1 = 128,000 J/mol K*

*Later modeling reduced this to 40,000 J/mol K

Reaction Kinetics Reaction 2

First order in solvent only:

–r2S = k2*CS

k2S = 9.41 ×10−16 hr–1

E2 = 800,000 J/mol K*

* Later information provided 80,000 J/mol K

Heat of Reactions

The heats of reaction are constant.

ΔHRx1A = −45,400 J/mol

ΔHRx2S = −3.2 × 105 J/mol

Assumptions provided in the example

• Liquid volume, V0 = 4,000 dm3

• Vapor space, VV = 5,000 dm3.

• Ideal gas law

• Venting given by this estimate (P-1)Cv,

• Cv1= 3,360 mol/hr•atm for the 1” vent line.

• Cv2 = 53,600 mol/hr•atm for the 4” vent line.

• The reactor fails when the pressure exceeds 45 atm or the temperature exceeds 600 K.

Problem Statement

Plot and analyze the reactor temperature and head space pressure as a function of time along with the reactant and product concentrations for the scenario where the reactor cooling fails to work (UA = 0). In problem P13-2(f) you will be asked to redo the problem when the cooling water comes on as expected whenever the reactor temperature exceeds 455 K.

(1) Reactor Mole Balances

Reactor (Assume Constant Volume Batch)

Liquid

1A

A

dCr

dt (E13-6.1)

dCS

dt r2S (E13-6.2)

(2) Head Space Mole Balances

Let Ng = H2 + CO

For H2 and CO in the head space.

From the text book example solution

(5) Energy Balance

Applying Equation (E13-18) to a batch system (Fi0 = 0)

jPj

aSRxSARxA

CN

TTUAHrHrV

dt

dT

22110 (E13-6.18)

Substituting for the rate laws and

N jCP j

dT

dtk1AHRx1ACACB k2SCS UA T Ta

1.26 107 J K (E13-6.19)

(6

From the text book example solution

(6) Numerical Solutions – “Tricks of the Trade”

A rapid change of temperature and pressure is expected as reaction (2) starts to run-away. This typically results in a stiff system of ordinary differential equations, which can become numerically unstable and generate incorrect results. This instability can be prevented by using a software switch that will set all derivates to zero when the reactor reaches the explosion temperature or pressure. This switch can have the following form in Polymath and can be multiplied by the right hand side of all the differential equations in this problem. Here the dynamics will be halted when the T become higher that 600 K or the pressure exceeds 45 atm.

SW1 = if (T>600 or P>45) then (0) else (1)

Polymath Code (first few lines)

# Reactor volumesV0= 4000 # dm3VH= 5000 # dm3

# Initial concentrationsCA(0)=4.3 #mol sodium/dm3CB(0)=5.1 # mol methylcyclopentadienel/dm3CS(0)=3 # mol of Diglyme/dm3

# Heat of reactionsDHRx1A=-45400 # J/mol NaDHRx2S=-3.2E5 # J/mol of Diglyme

# Sum of product of cp and mass of liquid-phase components in reactorcpSYS=1.26E7 #J/K

Screen capture of Polymath

Simulation shown in Text UA=0

Later work, with more information

Three scenarios compared

Questions?