Enthalpy cost calculations

Example 17.2 Enthalpy-Based Steam Cost Calculations... 

Positive enthalpy and entropy terms were calculated meaning that, contrary to self-organisation processes in apolar solvents, the enthalpic gain and entropic cost are completely overriden by solvation effects. Attempted inclusion of even small diatomic guests leads to a widening and thus a destabilisation of the cap-... 

Thns, for a valne of G/L of 4.0, the valnes of Z/HTU for XJX = 0.1 are 1.49 for 85°F and 1.95 for 75°F. The height of the tower at 85°F is 1.48 x 23.5 = 34.8 ft, whereas at 75°F it becomes 1.95 x 23.5 = 45.8 ft. This shows that setting the GIL ratio to 4.0 instead of 2.0 wonld resnlt in a shorter tower. The amount of air requirecf, however, is not doubled 34.8 times 2 divided by 49.5 = 1.40. Thus, at this higher air loading rate, only 40% more air is reqnired. To find the optimum GJL ratio, the entire design mnst be priced and the minimnm cost tower selected. This requires repetitive calculations using a computer and incorporating reasonably accurate cost data as well as mass transfer, enthalpy transfer, and pressure drop characteristics on the detailed analysis. 

A desirable goal is to compute a thermodynamic energy such as the molecular atomization energy or the enthalpy of formation, with chemical accuracy, which means an accuracy of 1 kcal/mol. Currently available functionals in DFT cannot do this. High-level methods such as CCSD(T), QCISD(T), CISDTQ, and MP6 with large basis sets can do this but are much too costly to be feasible except for quite small molecules. The aim of the compound methods G3 and CBS discussed in this section is to achieve 1 kcal/mol accuracy with a computational time that allows calculations on molecules containing several nonhydrogen atoms. 

Bond-dissociation enthalpies are for breaking bonds, which costs energy. In calculating values of use positive BDE values for bonds that are broken and negative values for bonds that are formed. 

G2. [Note This problem is quite extensive.] Biorefineries producing ethanol by fermentation have several distillation columns to separate the ethanol from the water. The first column, the beer still, is a stripping column that takes the dilute liquid fermenter product containing up to 15% solids and produces a clean vapor product that is sent to the main distillation column. The main column produces a distillate product between about 65 mole % and the ethanol azeotrope, and a bottoms product with very litde ethanol. The calculated diameter of the main distillation column is much greater at the top than elsewhere. To reduce the size and hence the cost of the main column, one can use a two-enthalpy feed system split the vapor feed into two parts and condense one part, then feed both parts to the main column at their optimum feed locations. This method reduces the vapor velocity in the top of the column, which reduces the calculated diameter however, a few additional stages may be required to obtain the desired purity. 

The surface area calculation for heat transfer equipment is the most importent part of cost targeting. For simplicity, the area model (Townsend and Linnhoff, 1983a, 1983b) is used to explain how surface area is calculated directly from composite curves. To do this, utilities are added to composite curves to make heat balance between hot and cold composites. Then the balanced composite curves are divided into several enthalpy intervals, and each enthalpy interval must feature straight temperature profiles (Figure 9.11). 

There could be several hot and cold streams within an enthalpy interval. For each heat exchanger involving hot stream i and cold stream j in Ath interval, the surface area and cost can be calculated via equations (9.1) and (9.2) ... 

The most common method for steam pricing in the process industry is based on the steam enthalpy. The method is straightforward for a given cost of steam at generation pressure, the price of steam at lower pressure is based on the amount of heat available with this steam in comparison with the heat available for the steam at the generation pressure. In other words, HP steam is generated from boilers and the cost of HP steam can be calculated via equation (17.3). Then, MP and LP steam costs are prorated from the HP cost based on the ratio of enthalpy values. It seems the idea makes sense however, there are fundamental flaws. The method and its limitations can be best illustrated via the example presented next. 

Solution. If BFW conditions are used as the basis to measure the heat available in steam, the heat available with a steam is equal to the enthalpy difference between the steam and BFW. Thus, the costs for MP and LP can be calculated based on the enthalpy ratio as... 

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