Circulator, isothermal

In the Godrej-Lurgi process, olefins are produced by dehydration of fatty alcohols on alumina in a continuous vapor-phase process. The reaction is carried out in a specially designed isothermal multitube reactor at a temperature of approximately 300°C and a pressure of 5—10 kPa (0.05—0.10 atm). As the reaction is endothermic, temperature is maintained by circulating externally heated molten salt solution around the reactor tubes. The reaction is sensitive to temperature fluctuations and gradients, hence the need to maintain an isothermal reaction regime. 

To evaluate the flow pattern efficiency, a knowledge of the actual hydrodynamic behavior of the process gas circulating in the centrifuge is necessary. Primarily because of the lack of such knowledge, the flow pattern efficiency has been evaluated for a number of different assumed isothermal centrifuge velocity profiles. 

The thermos phon circulation rate can be as high as 10 to 15 times the coolant evaporation rate. This, in turn, eliminates any significant temperature difference, and the jacket is maintained under isothermal conditions. In this case, the constant wall temperature assumption is satisfied. During starting of the thermosiphon, the bottom can be 20-30°C hotter, and the start of circulation can be established by observing that the difference between the top and bottom jacket temperature is diminishing. Figure 2.2.5 (Berty 1983) shows the vapor pressure-temperature relationship for three coolants water, tetralin, and Dowtherm A. 

Mass-transfer deposits can lead to blockages in non-isothermal circulating systems, cis in the case of liquid-metal corrosion. In fused salts, the effect can be reduced by keeping contamination of the melt by metal ions to a minimum e.g. by eliminating oxidising impurities or by maintaining reducing conditions over the melt . 

Polymerization. Isothermal polymerizations were conducted in 1/8" molds controlled at the stated isothermal temperature. Below lOO C circulating water was the temperature control medium whereas circulating air was used above 100 C. Reactants were... 

Temperature influences skin permeability in both physical and physiological ways. For instance, activation energies for diffusion of small nonelectrolytes across the stratum corneum have been shown to lie between 8 and 15 kcal/mole [4,32]. Thus thermal activation alone can double the rate skin permeability when there is a 10°C change in the surface temperature of the skin [33], Additionally, blood perfusion through the skin in terms of amount and closeness of approach to the skin s surface is regulated by its temperature and also by an individual s need to maintain the body s 37° C isothermal state. Since clearance of percuta-neously absorbed drug to the systemic circulation is sensitive to blood flow, a fluctuation in blood flow might be expected to alter the uptake of chemicals. No clear-cut evidence exists that this is so, however, which seems to teach us that even the reduced blood flow of chilled skin is adequate to efficiently clear compounds from the underside of the epidermis. 

The introduction of forced air circulation in an oven allowed the control up to 300 - 400°C and the air bath is the main type of column oven available today for laboratory gas chromatographs. Several types of temperature controllers are used to control the temperature of these ovens. They basically differ in terms of cost, accuracy, and flexibility. They are usually advertised as isothermal or temperature-programming controllers but all, in general, use the following basic types of control ... 

STRs in the most recent fine chemical hydrogenations [26]. The L/S slurry is circulated back at high flow in a loop connected to a Venturi. The local underpressure in the neck causes gas to be sucked in the intense turbulence achieves a very large interfacial area between tiny bubbles and the slurry. An external heat exchanger on the loop enables an almost unlimited heat removal, convenient for extremely high exothermic reactions, and isothermal operations. On the other hand, JLRs are restricted to a batch mode and can only accommodate catalysts compatible with the pump (low hard ness, low attrition). 

Decomposition of N,0 on a catalyst and isothermal oxidation of CO on platinum catalyst in circulating reactor Hugo (37) X ... 

The experimental setup closely resembles that used by Beeck et al. A mixture of C2H4 and H2 is circulated rapidly over the catalyst by a magnetically driven glass turbine of the type described by these authors (circulation rate 1500 liters/hr.). The highly exothermic character of the reaction makes it difficult to perform the experiments isothermally, and only after many fruitless efforts a method was found that appeared reasonably satisfactory. 

Performing a reaction under isothermal conditions is somewhat more complex. It requires two temperature probes, one for the measurement of the reaction mass temperature and a second for the jacket temperature. Depending on the internal reactor temperature, the jacket temperature is adjustable. The simplest method is to use a single heat carrier circuit to act either on the flow rate of cooling water or on the steam valve. With a secondary heat carrier circulation loop, the temperature controller acts directly on the heating and cooling valves by using a conventional... 

Figure 9.10 Isothermal control with a secondary heat carrier circulation loop.

As the plastic material is introduced into the reactor, it melts onto the sand particles and coats them. The vigorous cyclic movement of the particles can prevent agglomeration problems. Temperature is measured by means of three thermocouples placed at different radial positions in the reactor and provided with free vertical movement. Bed isothermic-ity is noteworthy, which is attained due to the vigorous particle circulation. 

S3] A process for the oligomerization of ethylene for the production of linear a-olefins had to be developed in stirred tank reactor assuming isothermal conditions and was executed in the kinetic regime. The latter was assured by increasing the rotational speed of the impeller until the rate of reaction did not increase further. The autoclave reactor was heated by an external blanket and supplied with cooling water circulation through an internal coil. 

Figure 20 shows temperature profiles for three different ways of controlling the cooling stream in a partial oxidation reaction. If the coolant is circulated so fast that its temperature in the reactor scarcely changes, then its flow direction is irrelevant and a temperature profile with a pronounced temperature maximum becomes established this is typical for strongly exothermic reactions (Fig. 20A). If the coolant is circulated in cocurrent and its velocity is chosen so that it becomes noticeably hotter over its path, an almost isothermal temperature behavior can be achieved (Fig. 20B). This is because the reactive gas at the inlet is in contact with the coldest coolant and the cooling temperature rises in step with the consumption of the reactants, so that the reaction rate remains virtually constant over a fairly long section [33-35, 40],...

New perspectives arising from isothermal oxidation. The next chapter of this book describes the greatly altered perspective of the fuel cell industry, when Grove s ideas are updated. The second chapter describes the detail of Regenesys, or ESS-RGN. This system has changed hands, as noted above, and information is available from http //www.vrbpower.com/. (The initials VRB stand for Vanadium Redox Battery, a low-power alternative to Regenesys.) The new 2005 VRB Power Systems shorthand is ESS-VRB for 2.5 to 10 MW and ESS-RGN for 10 to 100 MW. In Chapter 2 the reader will be acquainted with ESS-RGN, one of the two VRB fuel cell systems (incompressible liquid based) which can be termed complete . The redox battery uses small pumps as circulators. 

Grove did not realise the irreversibility of his diffusion-based fuel cells and electrolysers, without circulators. Barclay (2002) shows that with circulators an isothermal device is possible which is in precise equilibrium at zero current. Drawing a very small current represents perfect fuel cell action. Supplying a very small current represents perfect electrolyser action, a matched pair, from one device. A solution involving two different devices is not possible, since that would preclude reversibility, as supposed by Grove. 

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