Adiabatic tank

The programs for the open tank (CONFLOl), for the closed isothermal tank (C0NFL02) and for the closed adiabatic tank (C0NFL03) are given below. 

For the adiabatic tank reactor, Q=0 Unless the reaction happens to be thermoneutral, Ti and Te will be different. 

Table 5-7 Conversion vs reactor volume for adiabatic tank reactor allyl chloride production...

ILLUSTRATIVE EXAMPLE 12.8 A 2 1 molar mixture of ethylene oxide (A) and water was fed to a 10 liter adiabatic tank flow reactor. The flow rate of the solution was 1000 L/h. The initial concentration of ethylene oxide was measured to be 38.3 gmol/L. The temperature at the reactor outlet was found to be 375°F. The heat of reaction, activation energy, average heat capacity and reaction velocity constant for this system are known ... 

Nitromethane [75-52-5] is produced in China. Presumably a modified Victor Meyer method is being employed. Nitromethane is transported in dmms or smaller containers. Two tank cars of nitromethane exploded in separate incidents in the 1950s. Both explosions occurred in the switching yard of a railroad station. In both cases, essentially adiabatic vapor compression of the nitromethane—air mixture in the gas space of the tank car resulted in the detonation of the Hquid nitromethane. Other nitroparaffins do not, however, detonate in this manner. 

Polymerization processes are characterized by extremes. Industrial products are mixtures with molecular weights of lO" to 10. In a particular polymerization of styrene the viscosity increased by a fac tor of lO " as conversion went from 0 to 60 percent. The adiabatic reaction temperature for complete polymerization of ethylene is 1,800 K (3,240 R). Heat transfer coefficients in stirred tanks with high viscosities can be as low as 25 W/(m °C) (16.2 Btu/[h fH °F]). Reaction times for butadiene-styrene rubbers are 8 to 12 h polyethylene molecules continue to grow lor 30 min whereas ethyl acrylate in 20% emulsion reacts in less than 1 min, so monomer must be added gradually to keep the temperature within hmits. Initiators of the chain reactions have concentration of 10" g mol/L so they are highly sensitive to poisons and impurities. 

If the tank has some headspace, as is usually the case, it is desirable to get a better estimate of the ac tual level, since tanks usually have some gas headspace even when filled with hquid. Two tank configurations are considered the gravity discharge tank (discharge is open to the atmosphere) and the pumped discharge tank. These calculations assume that the process is so rapid that an adiabatic model for the gas in the headspace is the correct choice. This is true when the drainage... 

Fig ure 6-22. Temperature versus conversion for a first order irreversible reaction in an adiabatic continuous flow stirred tank reactor. 

The boundary conditions (10.12-10.14) correspond to the flow in a micro-channel with a cooled inlet and adiabatic receiver (an adiabatic pipe or tank, which is established at the exit of the micro-channel). Note, that the boundary conditions of the problem can be formulated by another way, if the cooling system has another construction, for example, as follows x = 0, Tl = IL.o, x = L, Tg = Tg.oo, when the inlet and outlet are cooled x = 0, dT /dx = 0, x = L, Tg = Tg.oo in case of the adiabatic inlet and the cooled outlet, etc. 

Example 14.6 derives a rather remarkable result. Here is a way of gradually shutting down a CSTR while keeping a constant outlet composition. The derivation applies to an arbitrary SI a and can be extended to include multiple reactions and adiabatic reactions. It is been experimentally verified for a polymerization. It can be generalized to shut down a train of CSTRs in series. The reason it works is that the material in the tank always experiences the same mean residence time and residence time distribution as existed during the original steady state. Hence, it is called constant RTD control. It will cease to work in a real vessel when the liquid level drops below the agitator. 

Liquid flows from an upstream source, at pressure Pi, via a fixed position valve into the tank, at pressure P2. The liquid in the tank discharges via a second fixed position valve to a downstream pressure P3. The tank can be open to atmosphere, closed off to the atmosphere and can work either under isothermal or adiabatic temperature conditions. The detailed derivation of this problem is discussed both by Franks (1972) and Ramirez (1976, 1989). Data values in the problem are similar to those of Ramirez, who also made simulations of this problem. 

CONTINUOUS CLOSED ADIABATIC FLOW TANK PROBLEM OF RAMIREZ... 

There are a variety of limiting forms of equation 8.0.3 that are appropriate for use with different types of reactors and different modes of operation. For stirred tanks the reactor contents are uniform in temperature and composition throughout, and it is possible to write the energy balance over the entire reactor. In the case of a batch reactor, only the first two terms need be retained. For continuous flow systems operating at steady state, the accumulation term disappears. For adiabatic operation in the absence of shaft work effects the energy transfer term is omitted. For the case of semibatch operation it may be necessary to retain all four terms. For tubular flow reactors neither the composition nor the temperature need be independent of position, and the energy balance must be written on a differential element of reactor volume. The resultant differential equation must then be solved in conjunction with the differential equation describing the material balance on the differential element. 

ILLUSTRATION 10.3 ADIABATIC OPERATION OF CONTINUOUS STIRRED TANK REACTORS OPERATING IN SERIES... 

Hot aqueous solutions of the nitrate of above 50% concentration may decompose explosively under adiabatic conditions and under conditions of confinement (small, or no vents). A 9 m3 tank of 85% solution also containing 0.9% of nitric acid... 

Worst-case analysis based on the DSC data, namely, the test with the lowest onset temperature, resulted in a graph showing the relationship between initial temperature and time-to-maximum rate under adiabatic conditions. For an initial temperature of 170°C, it would take 2 hours to reach the maximum rate. Venting simulation tests were undertaken on a larger scale to detect safe venting requirements for the separator and for the MNB hold tank. Several vent sizes were tested. It was found that a 10-cm rupture disc with a burst pressure 1 bar above the operating pressure was adequate. 

Adiabatic compression test High pressure is applied rapidly to a liquid in a U-shaped metal tube. Bubbles of hot compressed gas are driven into the liquid and may cause explosive decomposition of the liquid. This test is intended to simulate water hammer and sloshing effects in transportation, such as humping of railway tank cars. It is very severe and gives worst-case results. 

A reversible reaction, At= B, takes place in a well-mixed tank reactor. This can be operated either batch-wise or continuously. It has a cooling jacket, which allows operation either isothermally or with a constant cooling water flowrate. Also without cooling it performs as an adiabatic reactor. In the simulation program the equilibrium constant can be set at a high value to give a first-order irreversible reaction. 

Nitrogen contained in a large tank at a pressure P = 200000 Pa and a temperature of 300 K flows steadily under adiabatic conditions into a second tank through a converging nozzle with a throat diameter of 15 mm. The pressure in the second tank and at the throat of the nozzle is P, = 140000 Pa. Calculate the mass flow rate, M, of nitrogen assuming frictionless flow and ideal gas behaviour. Also calculate the gas speed at the nozzle and establish that the flow is subsonic. The relative molecular mass of nitrogen is 28.02 and the ratio of the specific heat capacities y is 1.39. 

At the end of increment, P0 = 198000 Pa Expansion of the gas in the tank must be nearly adiabatic because heat transfer from the walls will be very slow. Furthermore, the gradual expansion will be almost frictionless and consequently isentropic expansion may be assumed. Thus... 

The pressure Pq before the pressure letdown valve is high enough to prevent any vaporization of feed at its temperature Tg and composition Xqj (mole fraction jth component). The forcing functions in this system are the feed temperature Tq, feed rate F, and feed composition x j. Adiabatic conditions (no heat losses) are assumed. The density of the liquid in the tank, is assumed to be a known function of temperature and composition. 

At the end of the washing step, the liquid is drained to the operative tank by the pump, and through the button filter, FB, then the gas-recovery step takes place to recover most of the vapour or gaseous CO2 from the chamber. As the gas-recovery can be assumed to be like an adiabatic process, the fluid in the chamber is pre-heated by an electric resistance heat exchanger, HE2, to contain the reduction in temperature. The CO2 vapour is compressed by the oil-free compressor CV and fed to HE1 to condense before being stored in the operative tank. Finally, the chamber is vented and the door can be opened to recover the cleaned garments. 

The reactor system may consist of a number of reactors which can be continuous stirred tank reactors, plug flow reactors, or any representation between the two above extremes, and they may operate isothermally, adiabatically or nonisothermally. The separation system depending on the reactor system effluent may involve only liquid separation, only vapor separation or both liquid and vapor separation schemes. The liquid separation scheme may include flash units, distillation columns or trains of distillation columns, extraction units, or crystallization units. If distillation is employed, then we may have simple sharp columns, nonsharp columns, or even single complex distillation columns and complex column sequences. Also, depending on the reactor effluent characteristics, extractive distillation, azeotropic distillation, or reactive distillation may be employed. The vapor separation scheme may involve absorption columns, adsorption units,... 

One of the simplest practical examples is the homogeneous nonisothermal and adiabatic continuous stirred tank reactor (CSTR), whose steady state is described by nonlinear transcendental equations and whose unsteady state is described by nonlinear ordinary differential equations. 

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