Self heating

Charcoal may also be screened (filtration of contaminants and separation into size classes), crushed into powder, or made into briquettes wherein the charcoal is compressed into compact, high carbon-density masses with or without a binder. Wet charcoal presents a particular combustion hazard because moisture catalyzes oxidation leading to combustion and self-heating. 

Activated, see Terminology, Activated, p.231 Carbon paper, see Fibres and Fibrous Products, p.90 

Castor beans, 9 Castor flake, 9 Castor meal, 9 Castor pomace, 9 

The castor plant Ricinus communis) is a coarse herb of the spurge family Euphorbiaceae. When processed, the seed syn. castor bean yields half its weight in a triglyceride oil that is, uniquely, up to 85% ricinoleic acid. The oil is used as an ingredient or precursor in the manufacture of paints, varnishes, resins, synthetic fibres, lubricants, polishes, cosmetics, and many other materials. 

The cake remaining after mashing, crushing, or solvent extraction of the oil is known variously as castor pomace, castor flake, castor meal, or poonac. It contains ricin, a highly toxic protein ricinine, a toxic alkaloid and CB-IA, an extremely potent allergen. The cake with its high potassium, fibre, and protein concentration is used as a fertilizer and as animal feed after detoxification. 

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Under typieal operating eonditions, in the absenee of self-heating from the reaetion, the equilibrium for this step lies in favour of the prodiiet This speeies undergoes a series of intramoleeular hydrogen-abstraetion and fiirther 02-addition steps before fragmentation of the earbon ehain. This final step prodiiees tlnee radieal... 

Since efficient composting invafiably involves self-heating as biodegradation proceeds, this also offers an option for extending the bioremediation season into the winter months in cold climates. A potential drawback of composting is that it usually increases the volume of contaminated material, but if fully successful the finished compost can be returned to the site as a positive contribution to soil quaUty. 

Storage and Handling. Plutonium can be stored safely in dry air. Because of self-heating, storage accompanied by heat removal is advisable. The metal can be machined in moisture-free air containing at least 70 vol % Ar or He. Casting and foundry operations that requite melting of the metal must be carried out in vacuum or inert atmospheres and special containers. 

Like many other combustible Hquids, self-heating of ethyleneamines may occur by slow oxidation in absorbent or high-surface-area media, eg, dumped filter cake, thermal insulation, spill absorbents, and metal wine mesh (such as that used in vapor mist eliminators). In some cases, this may lead to spontaneous combustion either smoldering or a flame may be observed. These media should be washed with water to remove the ethyleneamines, or thoroughly wet prior to disposal in accordance with local and Eederal regulations. 

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. 

Certain materials which are generally considered to be stable at ordinary temperatures can inflame even in tlie absence of normal ignition sources. Such spontaneous combustion results from exotliermic autoxidation when the heat liberated exceeds that dissipated by the system. Materials prone to self-heating are listed in Table 6.7. In most cases, such fires involve relatively large, enclosed or thermally-insulated masses, and spontaneous combustion usually occurs after prolonged storage. 

Fibrous materials are subject to self-heating when impregnated with the following vegetable/animal oils... 

Other materials subject to self-heating (depending upon composition, method of drying, temperature, moisture content)... 

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. ... 

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