Safe operating temperature calculation

The Safe Operating Envelope has been defined as the range of process parameters (such as temperature, pressure, flow, level, composition) that are critical to safe operation. The Safe Operating Envelope upper/lower limits are listed in the Safe Operating Envelope tables maintained under Process Safety Information. Operating beyond these limits will cause a process upset. Key maximum and minimum Emits are set based on manufacturers specifications, theoretical calculations, or operating experience. 

There are two main approaches to determining whether the plant operating temperature is safe simulation and calculation. 

For higher CH4 content, more steam and CO2 at the anode reduce power and fuel utilization of the stack, thus a small (0/C)Ref ratio should be preferred for high system efficiency. On the other hand, the risk of carbon formation in the reformer increases with lower (0/C)Ref, so a compromise between system efficiency and a safe operation has to be found. Thermodynamic calculation indicated that for soot free operation an (0/C)Rer above 2.5 at temperature > 625 °C has to be assured. ... 

Calculate the combustion chamber volume. This requires that the actual volumetric flow rate of hot gas passing through the combustion chamber be known. If the auxiliary fuel is natural gas, the actual flow rate of hot gas will be equal to the flow rate of feed gas plus the flow rate of natural gas corrected to the combustion chamber temperature. The hot gas flow rate in the combustor must also include any air added to dilute the VOC concentration to a safe value, raise the oxygen content of the gas stream, or support combustion of fuel in the burner. The required combustion chamber volume is calculated from the total gas volumetric flow rate, at the actual operating temperature and pressure, and the required residence time. The calculated volume is customarily increased by 5% to allow for fluctuations in operating conditions (EPA Handbook, 1991). 

When the reactor is scaled up to 60 cm radius, however, the operating point is between the two curves. This means that the reaction can be safely run at 50°C in a well-agitated process vessel of 60 cm radius with the heat transfer coefficient as stated above becauseerating point is below the Semenov curve. In case the agitation is lost, however, the Frank-Kamenetskii curve becomes the better predictor of runaway temperatures, and because the operating point is above this curve, the estimate is that the reaction will run away. The calculation of the Frank-Kamentskii method is available in ASTME-1231 [166]. 

In heat removal line (2), the heat production line and the heat removal line have only one point of intersection (S3). Here, a critical situation exists. In practice, S3 is not a stable point for operation because a small temperature increase will lead to a runaway reaction. The point S3 is of interest, however, for the calculation of the maximum AT that can be used for safe cooling of a batch reactor. At S3, the reaction temperature T and the AT are at maximum values and the slopes of the two curves are equal. Equation (3-9) and Equation (3-10) are true and valid at point S3. Substituting the value of UAS(T — Tm)max... 

Another reason why calculations of the adiabatic reactor is important is for safety. Suppose we have a reactor operating in a stable fashion with cooling. What happens if the cooling is suddenly stopped The limit of this situation is the adiabatic reactor, and the engineer must always be aware of this mode because it is the worst-case scenario of any exothermic reactor. Note that if A Hr > 0, we must supply heat to maintain the reactor temperature, and loss of heat will cause the reactor to cool down and the rate will decrease safely. 

According to [6] the normal operation of a reactor is considered to be safe, if ATad < 50 K holds for the reaction occurring during normal operation and if there are no thermal instabilities of the reagents, the reacting mixture or the products in the temperature range [T, T + ATadl- However, this criterion should be used with care, since the time behaviour should be accounted for as well in assessing safety. Additionally, the uncertainties of the calculation should be observed [7]. 

Afigure-of-merit of thermal performance, thermal resistance is a measure of the ability of its mechanical structure (material, package, and external connections) to provide heat removal from the active region and is used to calculate the device temperature for a given mounting arrangement and operating conditions to ensure that a maximum safe temperature is not exceeded [Baxter 1977], It is defined as... 

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