Heat of compression

Compression of CNG to 20 MPa requires four stage compression. Provision of such facilities is costly, and it is an energy consuming process. There is also a substantial heat of compression which results in a temperature rise of the compressed gas. This means that in practice less than 230 VfV are stored when a CNG vessel is filled to 20 MPa unless the filling process is carried out isothermally. 

Kompressibilitat, /. compressibility. Kompressions-warme, /. heat of compression. 

Isothermal compression takes place when the heat of compression is removed during compression and when the temperature of the gas stays constant. The characteristic equation is... 

The distance of D between the two parts of the curve indicates the proportion of flash gas at that point. The condenser receives the high-pressure superheated gas, cools it down to saturation temperature, condenses it to liquid, and finally suhcools it slightly. The energy removed in the condenser is seen to he the refrigerating effect plus the heat of compression. 

Compression in space D by the piston B the heat of compression is removed by water cooling (or air cooling in the mini-versions). 

Intuitively, one would expect a volume contraction on forming a strongly bonded compound from the elements. Indeed, Richards 190, 191) regarded heats of formation as heats of compression. The fractional volume contraction, AV = (molecular volume - 2 atomic vol-ume)/2(atomic volume), has been related to formation heats for NaCl or CsCl type structures 151). Even nonpolar compounds in the condensed state cohere in close-packed arrays. The packing density of difluorine, derived from the ratio of the van der Waals envelope to the molecular volume, is especially low, and a larger contraction would be expected for fluorides than for other halides. This approach has yet to be systematically examined. 

Although isothermal compression is desirable, in practice the heat of compression is never removed fast enough to make this possible. In actual compressors only a small fraction of the heat of compression is removed and the process is almost adiabatic. 

In the hydrogen filling station layout by Ferrel et al. (1996), a maximum pressure of 56.9 MPa is selected, although BOC did not take into account the effects of heat of compression. Usually the maximum pressure has to be about 25% above the pressure level of the vehicle tank. For a pressure of 43.8 MPa at 85 °C, the pressure in the storage banks must be about 54.8 MPa, which is a good fit for the 56.9 MPa assumed. 

Overloading Excessive heat of compression due to an excessively high pressure ratio. Check the pressure value adjustments and the tightness of the vacuum chamber ... 

Heat of Compression. The turbomachinery is operated at a sufficiently high pressure ratio in order to provide the required temperature to the cathode inlet. Compressor airflow is used to maintain the temperature difference between the cathode air inlet and cathode air outlet (fuel cell air AT). 

Recuperation. A recuperator is used to exchange heat between the turbine exhaust and compressor exit air. As in the case for the heat of compression, fuel cell AT is managed via the compressor air mass flow. In the simulations, the recuperator was not able to provide all of the heat necessary to preheat the cathode flow to the desired temperature. Methods for adding additional heat to the system are supplementary fuel added to a combustor before the fuel cell or after, or cathode recycle can also be used to make up the difference. Here, supplementary fuel was added after the fuel cell, to the post-combustion system. 

As shown in Figure 8.8, the amount of recycle necessary to maintain a cathode inlet temperature of 973 K is high and decreases as PR increases until the point where the heat of compression is reached where no recycle is necessary. 

In order to clarify these ideas, we need to compare the irreversible entropy productions (or the exergy destruction) in cycles that utilize regenerative heating of compressed air, thermal recuperation in the form of evaporation and superheating of the methanol fuel, and chemical recuperation through either reforming or cracking reaction with methanol. The next section presents such a comparison in a simplified form to illustrate the utility of thermodynamic analyses. 

Case 1 Regenerative heating of compressed air from 650F to 800F with liquid methanol at 77F as the fuel. 

The purpose of the final cooler is to remove the heat of compression added by the exhauster and to cool the gas to its final temperature so that downstream absorbers will operate more efficiently. Final cooling is typically achieved by direct contact with the cooling medium, either water or oil. An important function of final cooling is removal of naphthalene. In final coolers using wash oil, the naphthalene dissolves in the oil, and a side stream is steam stripped to remove the naphthalene. If water cooling is used, the condensed naphthalene must be absorbed by contacting the water with tar. The tar is continuously exchanged with fresh tar to prevent naphthalene buildup. 

Gas (known as wet gas because it contains hydrocarbon liquids to be extracted) enters the turbo-compressor and is compressed to typically 100 bar. The heat of compression is removed in an inter-stage cooler. The gas is then passed to an expander, which is coupled to the... 

---