Self-heating rate

Liquid ethylene oxide under adiabatic conditions requires about 200°C before a self-heating rate of 0.02°C/min is observed (190,191). However, in the presence of contaminants such as acids and bases, or reactants possessing a labile hydrogen atom, the self-heating temperature can be much lower (190). In large containers, mnaway reaction can occur from ambient temperature, and destmctive explosions may occur (268,269). 

The second context is the process reac tor. There is a potential for a runaway if the net heat gain of the system exceeds its total heat loss capabihty. A self-heating rate of 3°C/day is not unusual for a monomer storage tank in the early stages of a runaway. This corresponds to 0.00208°C/min, 10 percent of the ARC s detection limit. ARC data for the stored chemical would not show an exotherm until the self-heating rate was 0.02°C/min. Therefore, onset temperature information from ARC testing must be used with considerable caution. 

Accelerating Rate Calorimeter (ARC) The ARC can provide extremely useful and valuable data. This equipment determines the self-heating rate of a chemical under near-adiabatic conditions. It usu-aUy gives a conservative estimate of the conditions for and consequences of a runaway reaction. Pressure and rate data from the ARC may sometimes be used for pressure vessel emergency relief design. Activation energy, heat of reaction, and approximate reaction order can usually be determined. For multiphase reactions, agitation can be provided. 

Fig. 2. RSST results on various resoles. The three bulk-charged resoles are at approximately 58% solids, 50% solids, and 40% solids. The programmed formaldehyde has no water charged except that contained in the 50% formaldehyde. The 50 and 58% solids resins reach self-heat rates of nearly 600°C/min. The 40% solids resin does not exceed 10 C/min. (Chart courtesy of Borden Chemical and Bill Burleigh.)...

Fig. 3. RSST results on some typical production novolacs. The solids on these materials are 74, 70, and 62%, respectively. Note that the reaction does not become initiated significantly below 70 C and that the high solids system is capable of self-heating rates as high as 5500°C/min under these conditions. (Chart courtesy of Borden Chemical and Bill Burleigh.)...

There are two philosophies regarding how to best slow a runaway reaction. One view holds that simple water deluge is the best method as it provides immediate cooling and dilution. The anti position is that the batch should be deluged and neutralized simultaneously. Kumpinsky reports that minimum self-heat rates occur between pH 4 and 7 [78]. Since neutralization involves production of additional heat, because the pH of a runaway batch is rarely known, and since the phenolic reactions are catalyzed by acid, base, and salt it seems likely that simple deluge is the surest method. 

The (t)-faetor does not aeeount for the heat loss to the environment. It is used to adjust the self-heating rates as well as the observed adiabatie temperature rise. 

In the ARC (Figure 12-9), the sample of approximately 5 g or 4 ml is placed in a one-inch diameter metal sphere (bomb) and situated in a heated oven under adiabatic conditions. Tliese conditions are achieved by heating the chamber surrounding the bomb to the same temperature as the bomb. The thermocouple attached to the sample bomb is used to measure the sample temperature. A heat-wait-search mode of operation is used to detect an exotherm. If the temperature of the bomb increases due to an exotherm, the temperature of the surrounding chamber increases accordingly. The rate of temperature increase (selfheat rate) and bomb pressure are also tracked. Adiabatic conditions of the sample and the bomb are both maintained for self-heat rates up to 10°C/min. If the self-heat rate exceeds a predetermined value ( 0.02°C/min), an exotherm is registered. Figure 12-10 shows the temperature versus time curve of a reaction sample in the ARC test. 

Figure 12-11. Self-heat rate analysis. ARC data are shown along with a fitted model obtained by assuming the following kinetic parameters reaction order = 1, activation energy = 31.08 kcal/mol, and frequency factor = 2.31 El 2 min ...

The PHI-TEC or VSP bench scale apparatus can be employed to determine information about the self-heat rate and vapor disengagement when this is not readily available. Additionally, the VSP equipment can be used for flashing flow characteristics using a special bottom vented test cell. Here, the flowrate, Gq (kg/sm ), is measured... 

G = mass vent eapaeity per unit area, kg/m s hfg = latent heat, J/kg q = self-heat rate, W/kg V = total vessel volume, m ... 

An 800-gal reaetor eontaining a styrene mixture with a speeifie heat of 0.6 eal/gm °C has a 10-in. rupture disk and a vent line with equivalent length = 400. The vessel MAWP is 100 psig and the rupture disk set pressure is 20 psig. The styrene mixture had a self-heat rate of 60°C/min at 170°C as it is tempered in a DIERS venting test. Determine the allowable reaetor mixture eharge to limit the overpressure to 10% over the set pressure. 

Using Eauske s nomograph from Eigure 12-35 at a self-heat rate of 6.3 K/min and a set pressure of 217.5 psia, the corresponding vent size area per 1,000 kg of reactants = 0.0008 m. ... 

SELF HEAT RATE AT MAXIMUM TEMPERATURE, K/s MASS FLUX PER UNIT AREA, kg/m 2.s ... 

Accelerating Rate Calorimetry. This is a heat-wait-search technique (see Fig. 5.4-62). A sample is heated by a pre-selected temperature step of, typically, 5 C, and then the temperature of the sample is recorded for some time. If the self-heating rate is less than the calorimeter detectability (typically 0.02 "C) the ARC will proceed automatically to the next step. If the change of the sample temj)erature is greater than 0.02 °C, the sample is no longer heated from outside and an adiabatic process starts. The adiabatic run is continued until the process has been completed. ARC is usually carried out at elevated pressure. 

The more reactive ethylene oxide clearly shows much higher self-heating rates and hazard potential than its higher homologue, though more detailed work is needed to quantify the differences [3],... 

The data obtained from the calorimeters include maximum self-heat rate, maximum pressure rate, reaction onset temperature, and temperature and pressure as a function of time. 

A laboratory test has shown that the reaction will not result in a two-phase relief. Thus a vapor relief system must be designed. Furthermore, calorimeter tests indicate that the maximum self-heat rate is 40°C/min. The physical properties of the material are also reported ... 

If not, what are the important characteristics of the thermal decomposition if it should occur, such as time to onset, enthalpy of decomposition, self-heat rate, rate of pressure rise, rate of temperature rise, moles of gas/mole of substance and... 

FIGURE 2.26. ARC Plot of Self-Heat Rate as a Function of Temperature. 

Figure 2.26 represents an example of an ARC plot of the logarithm of the self-heat rate versus the reciprocal temperature. This graph shows the temperature at which a sample or mixture starts to decompose or react measurably, and the rate at which the sample or mixture liberates heat as a function of temperature. In the ARC experiment represented in Figure 2.26, exothermic decomposition or reaction is first observed at 80°C with a self-heat rate of 0.025°C/min. The maximum temperature reached is 142°C with a maximum self-heat rate of 6°C/min. The data must be corrected for the thermal inertia () of the system. 

The subscript "s" refers to experimental values. The plot of the self-heat rate as a function of the reciprocal temperature (at the start of the reaction) may result in a straight line with a slope of Ea/R which is the zero-order line. 

Although in determining the onset temperature and the global kinetics in the initial stage of the reaction the, corrections are of lesser importance, these corrections have a dramatic effect on the maximum self-heat rate and the maximum temperature. Care must be taken in the interpretation of such data from an experiment with a phi-factor greater than 1. For direct simulations of plant situations, a phi-factor of 1.0 to 1.05 is used [89],... 

De Haven [127] gives an overview of the results of accelerating rate calorimeter (ARC) experiments. The ARC was described in Section 2.3.2.3. As mentioned in the previous description, care must be taken in scale-up of results from experiments with relatively high phi-factors. For direct simulation of plant operating conditions, a phi-factor of 1.0 to 1.05 is required. As stated in [127], a decrease in the phi-factor from 2.0 to 1.0 increases the adiabatic temperature rise by a factor of 2, but the maximum self-heat rate increases by a factor of 20. Later in Chapter 3 (Section 3.3.4.6), an example of scale-up of ARC results is given. 

Special Studies High Sensitivity Calorimetry A- DESIREEV UNDESIRED dT/dt ATadiab Kinetics, EA, A Sample size 1- 50 ml, pW/g sensitivity Shelf life studies by accelerated aging Combine with low adiabatic to confirm solids low self-heating rate studies... 

This test can be used to give early detection of the initial exothermicity. It is possible to estimate thermokinetic parameters (e.g., the activation energy and the adiabatic self-heat rate) and to estimate how the initial temperature for self-sustaining reactions will vary with the quantity of material present. 

Adiabatic calorimetry Chemical testing technique that determines the self-heating rate and pressure data of a chemical under near-adiabatic conditions. ( Adiabatic refers to any change in which there is no gain or loss of heat.) This measurement technique conservatively estimates the conditions for, and consequences of, a runaway reaction. 

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