Separation, energy requirement evaporation

Suppose we wish to evaporate one mole of water, as expressed in equation (7). One mole contains the Avogadro number of molecules (6.02 X 1023) and has a weight of 18.0 grams. Using a calorimeter, as you did in Experiment 5, you could measure the quantity of heat required to evaporate one mole of water. It is 10 kilocalories per mole. This value is called the molar heat of vaporization of water. This is the energy required to separate 6.02 X 1023 molecules of water from one another, as pictured in Figure 5-1. 

So far only the energy requirement for a process in the form of work has been considered. Freezing, vapor compression, and reverse osmosis processes are examples of processes that require a work input. There are, however, other important processes, such as multiple-effect evaporation and flash evaporation, for which the energy input is in the form of heat. How does one relate the energy requirement of these processes to the minimum work of separation One method is to convert the heat requirement to a work equivalent by means of the Carnot cycle. If T is the absolute temperature of the heat source and T0 the heat-sink temperature, then one can use the familiar relation... 

Other than the main Bunsen reaction, the other three steps in this section involve separation of gases and H2O from the flowing chemical stream. O2, which forms as a result of water splitting, is removed from the recycled stream to avoid formation of complexes in the rest of the section. SO2 is washed/separated from the Bunsen reactor output to prevent any side reaction downstream. The last step in Section I involves the extraction of H2O from the HI product stream before it is sent to Section III. This will reduce the energy requirement, as H2O evaporation processes impose a high heat demand based on the current flow sheet. Hence, any reduction in H2O content in the Bunsen reaction products will be beneficial to the overall cycle efficiency. 

The free energy in a crystal is a minimum with respect to the arrangement of molecules and ions within it. A useful but approximate indication of the forces between components in a crystal is given by the energy needed to evaporate crystals into their separate molecules or ions. Typical values are the following for the ionic crystals lithium fluoride and sodium chloride, the energies required to break up the crystal into component anions... 

The single largest variable cost factor in making a separation by evaporation is the cost of energy. If crude oil is the ultimate source of energy, the cost of over 126.67 per m ( 20 per barrel) is equivalent to more than 3.33 for 1 million kJ. Water has a latent heat of 480 kJ/kg at 760 mm of mercury, absolute, so the energy required to evaporate 1 kg of water exceeds... 

Membrane Process for Recovery of Alkanesulfonates. Many attempts have been made over the years to reduce the wastewater load—which represents a loss of product—by a number of different methods. These include evaporation, extraction, reverse osmosis, and ultrafiltration. All of these processes have the disadvantage of high equipment cost and high energy requirements, and the space-time yield is low. The first breakthrough came with the development of new types of membrane with a definite separating efficiency and a large surface area, so-called spiral-wound modules. 

Thermal energy is needed for endothermic reactions. This type of energy is also important for some separation and purification processes, e.g. distillation and evaporation. The heat properties of liquids are therefore of importance for these processes. The energy required to heat a liquid is usually much less than the energy needed for vaporization (see Table 8.6). 

Solubility parameter 5 is a measure of the energy required to separate the molecules of a liquid, and is given by Eqn. 3, where AU is the change in internal energy on evaporation, A He the enthalpy of evaporation, R the gas constant, T the absolute temperature and Vm the molar volume ... 

This largely explains why SWRO has supplanted distillation/evaporation for seawater desalination. In situations where waste heat is available, thermal desalination may still be economically attractive. It should be noted that, despite an eightfold reduction in energy required for SWRO separation since the 1970s, additional SWRO energy reductions wiU be harder to achieve because of proximity of current SWRO systems to the theoretical minimum [11,58]. 

Make efficient use of the available energy. This may take several forms. Evaporator performance often is rated on the basis of steam economy-pounds of solvent evaporated per pound of steam used. Heat is required to raise the feed temperature from its initial value to that of the boiling liquid, to provide the energy required to separate liquid solvent from the feed, and to vaporize the solvent. 

Section 6.3.3.3 studies RO in bulk flow parallel to the force configuration and describes various membrane transport considerations and flux expressions. Practical RO membranes are employed in devices with bulk feed flow perpendicular to the force configuration, as illustrated in Section 7.2.I.2. A simplified solution for a spiral-wound RO membrane is developed analytical expressions for the water flux as well as for salt rejection are obtained and illustrated through example problem solving. A total of sbt worked example problems have been provided up to Chapter 7. Chapter 9 (Figure 9.1.5) shows a RO cascade in a tapered configuration. Section 10.1.2 calculates the minimum energy required in reverse osmosis based desalination and compares it with that in evaporation. Section 11.2 covers the sequence of separation steps in a water treatment process for both desalination and ultrapure water production. The very important role played by RO in such plants is clearly illustrated. 

First we will illustrate the minimum energy required to separate a small amount of mixture for the following processes evaporation of water from a saline solution recovery of water by reverse osmosis separation of an ideal binary gas mixture by membrane permeation. Then we will consider the definition of net work consumption for thermally driven processes. Next we will consider a variety of separation processes vis-k-vis their minimum energy requirement for separation. 

Note that this energy requirement to recover 1 liter of pure water is essentially identical to that calculated using the evaporation process in Section 10.1.1. This is as it should be since, by equation (10.1.1), the minimum energy required merely depends on the initial and final values of the Gibbs free energy, which is a state function. It does not matter how one goes from Gt to therefore the separation... 

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