Heat exchanger jacketed/coiled vessels

In many chemical and processing applications, fluids need to be heated or cooled and a wide range of equipment may be utilized. Examples include double pipe and shell and tube heat exchangers, and stirred vessels fitted with cooling coils or jackets. Sometimes, heat is generated in the process, as in extrusion which is extensively carried out in the polymer and food industry. It may also be necessary to reduce the rate at which heat is lost from a vessel or to ensure that heat is removed at a sufficient rate in equipment such as screw conveyors. In most applications, it is the rate of heat transfer within process equipment which is of principal interest. However, with thermally sensitive materials (such as foodstuffs, fermentation froths, pharmaceutical formulations), the temperature profiles must be known and maximum permissible temperatures must not be exceeded. 

The solvent, as well as any other inerts, and the mass of the vessel are included in this summation. The heat exchange through a jacket or coils at temperature T, is... 

Cooling system failure could occur due to failure of pumps or controls supplying cooling media to the reactor vessel jacket, coils, or overhead reflux condensers. Piping to or from the condensers could become plugged or any of the heat exchange surfaces could become excessively fouled. 

If the rate of heat transfer to or from the broth is important, then the heat transfer area per unit volume of broth should be considered. As the surface area and the liquid volume will vary in proportion to the square and cube of the representative length of vessels, respectively, the heat transfer area of jacketed vessels may become insufficient with larger vessels. Thus, the use of internal coils, or perhaps an external heat exchanger, may become necessary with larger fermentors. 

There are various types of cooling water heat exchangers to be found in use today, and the permutations of these may number almost as many as the applications in which they are employed. Jackets on chemical reaction vessels are a specific type of heat exchanger, as are the coil bundles in evaporative condensers, plate and frame and shell and tube heat exchangers. 

Assumption 5.4 It is assumed that only the temperature measurements in the reactor and in the jacket are available, i.e., neither the concentrations nor the heat released by the reaction can be measured in real-time. As discussed in Sect. 2.7, the temperature can be usually controlled via the heat exchange between reactor and a heating/cooling fluid that circulates in a jacket surrounding the vessel or in a coil inside the vessel. In the following, it is assumed that the control system uses the inlet temperature of the fluid in the jacket as manipulated variable nevertheless, the control approaches developed in this chapter can be easily extended to different control strategies. 

Hydrogenation reactions are frequently run in fed-batch reactors. The chemical component to be hydrogenated is charged to the reactor vessel. The hydrogen is then fed into the vessel on pressure control. The temperature of the reactor is controlled by manipulating the flowrate of coolant to the jacket, coil, or external heat exchanger. Thus this system has two manipulated variables (the flowrate of hydrogen and the flowrate of coolant) and two controlled variables (pressure and temperature). 

When temperature regulation is dependent solely on external jackets, a disproportional increase in nitration vessel capacity as compared with jacket surface occurs when the size of the machine is enlarged. Thus, if the volume is increased from 400 to 800 gallons, the heat-exchange area increases as the square and the volume as the cube of the expanded unit. To overcome this fault, internal cooling coils or tubes are introduced, which have proved satisfactory when installed on the basis of sound calculations that include the several thermal factors entering into this unit process. 

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