Available work heat streams

In the energy balance, work and heat are counted the same. In the availability balance, work and heat are not counted the same. All work input increases the availability of material flowing through the process. Only a portion of heat transferred into a system is available to increase the availability of flowing streams. The heat is degraded by a coefficient equal to 1 - (Jg / ) This coefficient is precisely the Carnot... 

A chemical plant has saturated steam available at 2,700 kPa, but bejcause of a process < has little use for steam at this pressure. Rather, steam at 1,000 kPa is liequired. Also availa saturated exhaust steam at 275 kPa. The suggestion is that the 275-kPa steam be compresse 1,000 kPa, obtaining the necessary work from expansion of the 2,700-kPa steam to 1,000 kPa. two streams at 1,000 kPa would then be mixed. Determine the rates at which steam at each i pressure must be supplied to provide enough steam at 1,000 kPa so that upon condensatio saturated liquid heat in the amount of 300 kJ s-1 is released,... 

The number of unknown variables for a single unit is the sum of the unknown component amounts or flow rates for ail inlet and outlet streams, plus all unknown stream temperatures and pressures, plus the rates of energy transfer as heat and work. The equations available to determine these unknowns include material balances for each independent species, an energy balance, phase and chemical equilibrium relations, and additional specified relationships among the process variables. 

While there are obvious advantages to solvent-free reactions, there may be small savings if organic solvents are used for work-up [57] or if purification of spent aqueous streams is costly. A disadvantage of solvent-free reactions is that controlling reaction temperatures can be difficult, as the heat capacity of a solvent is not available to help dissipate the heat of reaction. For efficient conversions,... 

Clearly, to obtain the maximum useful work all pro-ce.sses should occur reversibly, and all heat transferred to the surroundings should leave the system at Tp. since heat available at any other temperature could be used with a Carnot or similar engine to obtain additional work. For a similar reason, the feed and exit streams in an open process should also be at the ambient conditions. 

Ortho-xylene may be oxidized directly by air in vapor phase over vanadium pentoxide catalysts under conditions resembling those used in oxidation of naphthalene to phthalic anhydride. The stability of the cyclic anhydride structure of phthalic anhydride at the temperatures required and in the presence of oxidizing conditions is, of course, the distinctive feature. Since the oxidation of o-xylene to phthalic anhydride requires the theoretical interaction of only six atoms of oxygen relative to the nine required by naphthalene, the amount of heat generated per unit of product is less, and the volume of diluent gases in the product stream may be lower. Because of decreased formation of quinones and color bodies, product purification should be easier. Very little is available by way of information relative to commercial operating conditions. Some laboratory results of early work showed a maximum conversion to total acids of 18.2 per cent when commercial xylene was oxidized in vapor phase over unfused vanadium oxide catalyst. Recent work with o-xylene showed a conversion of 42.7 per cent to phthalic anhydride over unfused vanadium oxide catalyst and conversions up to 61.7 per cent to phthalic anhydride plus fi.6 per cent to maleic... 

Many industrial processes take place in open systems in which material enters and leaves the system through process streams and in which energy can cross system boundaries as heat and work. At any instant, a complete identification of the state requires specification of values for such variables as temperatures, pressures, compositions, and flow rates. However, because of the stuff equations in 3.6.1, not all of these quantities are independent. So we have here the same kinds of questions addressed in 3.1 How many interactions are available to change the state How many independent variables must be specified to identify the state of an open steady-flow system The discussion here extends that in 3.1 from closed systems to open ones however, the discussion remains limited to systems composed of a single homogeneous phase with no chemical reactions. The extensions to multiphase systems are given in 9.1 and to those having chemical reactions in 10.3.1... 

Analysis 2. In this second analysis, we consider situations in which the heat duty is unimportant and can be eliminated. In these cases we take the entire exchanger as the system. Now the system involves two substances, so C = 2, and it has two inlets plus two outlets, so Np = 4. We also have the following external constraints no shaft work (1), no heat transfer between system and surroundings (1), and only one component in each of the four streams (4). So the number of interactions available for manipulating this system, given by (3.6.12), is... 

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