Gas recycling calculations

First catalyst bed exit gas recycle calculations also give steady-state exit gas compositions and quantities. These allow calculation of ... 

Appendix Y Cooled first catalyst bed exit gas recycle calculations... 

Third catalyst bed exit gas recycle calculations (Fig. Y.2) are done as described above, but all three beds must be included in the calculations. Computer calculations are required. 

One of the authors would specifically like to thank his son George Davenport and his nephew Andrew Davenport for their help with (i) wet sulfuric acid and (ii) cooled catalyst bed exit gas recycle calculations. 

Clearly, these figures suggest that the plant is very sensitive to the amount of flue gas recycled. There appears to be no full parametric or economic calculation published in the literature for this FG/TCR cycle, which suggests that it has not been considered as an attractive option. 

The cold gas recycle (CGR) ratio values (Figure 4) are metered values and are more consistent than the HGR and total gas recycle ratio values which were calculated from gas analyses. Although the calculated total recycle gas flow rate was erratic, catalyst bed temperatures were uniform and easily controlled by varying recycle rate and bed inlet tem-... 

Table 2.12 shows the table stream calculated with Aspen Plus for toluene fresh feed of 100 kmol/h, purge fraction of 0.06 and ratio of hydrogen/toluene in the inlet reactor mixture of 5. In these conditions, the gas recycle rate is about ten times the molar flow rate of the inputs. 

We begin with a fairly simple process consisting of a reactor, condenser, separator, compressor, and stripper with a gas recycle stream (Fig. 8.1). This process was developed and published by Downs and Vogel (1993) as an industrial plantwide control test problem. A FORTRAN program is available from them that does the derivative evaluations for the process. The user must write a main program that initializes the simulation, does the controller calculations, performs the numerical integration, and plots the results. 

Manual calculations are also very useful when solving flowsheets that use recycle and purge. Purge streams are often withdrawn from recycles to prevent the accumulation of species that are difficult to separate, as described in Section 2.15. A typical recycle and purge flow scheme is illustrated in Figure 4.42. A liquid feed and a gas are mixed, heated, reacted, cooled and separated to give a liquid product. Unreacted gas from the separator is recycled to the feed. A make-up stream is added to the gas recycle to make up for consumption of gas in the process. If the make-up gas contains any inert gases, then over time these would accumulate in the recycle and eventually the reaction would be slowed down when the partial pressure of reactant... 

The over-all heat-transfer coefficients for the fixed-bed and hot-gas-recycle systems were calculated from a correlation of heat transfi through packed beds. A relatively high transfer coefficient of 50 Btu/(hr) (sq ft) ( F) is obtained for the hot-gas-recycle system because of the high linear velocity of the gas. A uniform amount of reaction has been assumed through the catalyst bed. When the reaction occurs nonuniformly and a large amount of conversion takes place in a limited area, as is often the case near the point of entry of the fresh gas, the gradients are higher. 

Type of reactor Hourly space velocity Gas recycle ratio, recycle gas fresh gas Type of heat transfer Sstimated over-all heat nsfer ooefificient, Btu/(hr)(sq ft)CV) Calculated avg catalyst temperature gradient, F... 

Figure 27.3 Schematic cooled first catalyst bed recycle process with 660 K feed gas and 700 K recycle gas. The calculation method is described in Appendix Y.

Steady-state first catalyst bed exit gas temperature with 20% recycle of cooled first catalyst bed exit gas is calculated as follows. 

The schematic diagram of the experimental setup is shown in Fig. 2 and the experimental conditions are shown in Table 2. Each gas was controlled its flow rate by a mass flow controller and supplied to the module at a pressure sli tly higher than the atmospheric pressure. Absorbent solution was suppUed to the module by a circulation pump. A small amount of absorbent solution, which did not permeate the membrane, overflowed and then it was introduced to the upper part of the permeate side. Permeation and returning liquid fell down to the reservoir and it was recycled to the feed side. The dry gas through condenser was discharged from the vacuum pump, and its flow rate was measured by a digital soap-film flow meter. The gas composition was determined by a gas chromatograph (Yanaco, GC-2800, column Porapak Q for CO2 and (N2+O2) analysis, and molecular sieve 5A for N2 and O2 analysis). The performance of the module was calculated by the same procedure reported in our previous paper [1]. 

Ethylene is to be converted by catalytic air oxidation to ethylene oxide. The air and ethylene are mixed in the ratio 10 1 by volume. This mixture is combined with a recycle stream and the two streams are fed to the reactor. Of the ethylene entering the reactor, 40% is converted to ethylene oxide, 20% is converted to carbon dioxide and water, and the rest does not react. The exit gases from the reactor are treated to remove substantially all of the ethylene oxide and water, and the residue recycled. Purging of the recycle is required to avoid accumulation of carbon dioxide and hence maintain a constant feed to the reactor. Calculate the ratio of purge to recycle if not more than 8% of the ethylene fed is lost in the purge. What will be the composition of the corresponding reactor feed gas ... 

As seen in Figure 8.6, the cathode recycle case demonstrates the potential for reasonably high efficiency without the expense or complexity of an exhaust gas recuperator, but at the expense of GT power, as shown in Figure 8.7 comparison (the SOFC power is the same for both cases, 240 MW). A high temperature blower however, could also be expensive. The blower power required is included in the efficiency calculations (70% blower efficiency). The back pressure that must be overcome is considered to be the fuel cell pressure drop only, which is likely optimistic. 

The Vload factor is used to determine what is known as jet flood. Jet flood is simply the liquid jetting, causing liquid to recycle from one tray back to the tray above, from which the liquid passed. In some cases jetting can be so severe that it blocks the gas passage with pressure buildup. The following equations calculate Vload ... 

Equation (8.5) does not account for the volumetric efficiency (VE) loss of reciprocating-type compressors. Volumetric efficiency is simply the clearance allowed in the compressor cylinder head in which this compressed gas volume is allowed to mix with the inlet gas to be compressed in the next compression stroke. Thus this compressed gas in the cylinder clearance is recycling, which makes the output of the compressor less efficient, the larger this clearance volume. Volumetric efficiencies usually range fron 6 to 20%. An average reciprocating compressor VE should be 10%. For conservative cost calculating, use 15% of overall GHP for the VE efficiency. 

Guess the temperature of the cold-shot stream Tcs- This is the temperature of the stream after the recycle gas (at an initially unknown temperature and flowrate coming from the compressor) is combined with the two fresh feedstreams at 313 K. An iteration loop is used throughout this step until the guessed value and the calculated value are sufficiently close. 

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