Critical Considerations in Process Safety

Scale-Up

Introduction

From kilo lab to pilot plant: Understanding the magnitude of the thermal and pressure risk

In the process of developing industrial chemical processes, the first steps involve the characterization of the chemical reactions, desired and secondary, and the potential risks associated with them: decomposition reactions, chemical runaways. The characterization aims to be able to address these risks and put mitigation measures that decrease the likelihood of such events. However, there are cases where the complete elimination of risks is unattainable. In that case, it is fundamental to gather as much information as possible on the magnitude of the thermal runaway risks using adiabatic calorimetry.

Adiabatic calorimetry, the analysis of heat production when there is no energy exchange between the systems and its surroundings, is a powerful tool when addressing chemical process safety.

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From pilot plant to manufacture: Simulating plant conditions to inform safety control factors

Before the final scale-up to manufacturing, it is necessary to fully understand what safety control factors are required to mitigate the worst-case scenarios. The ability to fully simulate thermal runaway hazards under manufacturing conditions in the lab is a valuable way of understanding the process and ensuring the appropriate safety measures are planned and implemented prior to manufacturing.

Using an adiabatic calorimeter that utilizes low thermal mass (i.e., low Phi factor) test cells, such as the Phi-TEC II, means that very little of the heat produced during the reaction and runaway is consumed in warming up the test cell. This enables the conditions of a large-scale plant reaction to be emulated, with the maximum temperature attained (T_end) and the rate of pressure increase representative of what would be expected to occur during a plant-scale incident. The kinetic data obtained can be used to calculate the adiabatic temperature rise of the runaway (ΔTad,d) and the time to maximum rate of decomposition (TMR_d). Unlike adiabatic calorimeters with a high Phi factor, the reaction rate is not suppressed by the high thermal mass of the test cell. So little to no data manipulation is required to compensate for heat losses to the test cell.

Comparing Reaction Scenarios

The Phi-TEC II also enables different reaction scenarios to be simulated and compared. In the example introduced earlier, the worst-case scenario that would release the most energy is the reaction of equimolar amounts of the alcohol and anhydride.

Temperature

Comparing the temperature results, it can be seen from the plot in Figure 11a that halving the amount of anhydride reduces the maximum temperature from over 200⁰C to 170⁰C and the time taken to reach that point. This change is further highlighted by considering the plot of the rate of temperature against temperature (the plot in Figure 11b), where the maximum rate reached drops from 4000⁰C/min to just over 1600⁰C/min when the amount of anhydride is halved.

These reactions were carried out using a Phi-TEC II on a low Phi-factor test cell and under adiabatic conditions. The parameters describing the exothermic event (MTSR, TMR, ΔTad, T_end) are highly representative of what would be expected to be observed in a plant-scale incident.

Figure 11a: Temperature data from different alcohol/anhydride ratios at 30 ⁰C — Fig.11a – Temperature data from different alcohol/anhydride ratios at 30°C

Figure 11b: Temperature data from different alcohol/anhydride ratios at 30 ⁰C — Fig.11b – Rate of Temperature rise data resulting from different alcohol/anhydride ratios at 30°C

Pressure

As Figure 12 illustrates, a similar effect can be seen with the pressure data: when the level of anhydride is reduced, the maximum pressure reached is reduced, and the rate of pressure increase is slowed. In this example, the lower pressure rise directly results from the lower temperature.

Safety Considerations

The data illustrates that for the worst-case scenario, it barely takes 3 minutes (after the chemicals come together) for the reaction to runaway to completion, and in the process:

reaching a temperature of approximately 200⁰C
a pressure of 16 bar
with the rate of temperature increase peaking at 4000⁰C/min.

In contrast, by reducing the anhydride amount to a quarter that of the alcohol, the incident takes 12 minutes to reach the worst conditions, which are now reduced to:

a maximum temperature of 125⁰C
a maximum pressure of 8 bar
a peak rate of temperature change of 150⁰C/min.

This is because the higher proportional levels of alcohol act as a thermal diluent, leading to the lower final temperature, lower final pressure, and a lower rate of increase in temperature and pressure observed.

Subsequently, this information enables the correct combination of reactor type and protection features to be selected, which could include the following:

Emergency & evaporative cooling
Quenching
Controlled depressurization
Vent sizing

Thus, directly comparing the impact of different operating scenarios enables the necessary safety controls to be selected upfront for the manufacturing plant.

Solutions

The Phi-TEC II is a more advanced type of adiabatic calorimeter, which supports using low Phi factor test cells. This capability means that very little of the heat produced during a reaction or thermal runaway is consumed in warming the test cell. As a result, the runaway rate is not tempered.

The measured rate of pressure increase and final temperature (T_end), along with the calculated Time to Maximum Rate (TMR) and adiabatic temperature rise (ΔTad), are representative of what would be expected to occur during a manufacturing scale incident. Thus, the Phi-TEC II enables the hazards to be thoroughly evaluated and explored, facilitating their mitigation before scale-up.