Recycle lines

Fresh reducing gas is generated by reforming natural gas with steam. The natural gas is heated in a recuperator, desulfurized to less than 1 ppm sulfur, mixed with superheated steam, further preheated to 620°C in another recuperator, then reformed in alloy tubes filled with nickel-based catalyst at a temperature of 830°C. The reformed gas is quenched to remove water vapor, mixed with clean recycled top gas from the shaft furnace, reheated to 925°C in an indirect fired heater, and injected into the shaft furnace. For high (above 92%) metallization a CO2 removal unit is added in the top gas recycle line in order to upgrade the quaUty of the recycled top gas and reducing gas. 

Example 10.11 The piping in the recycle reactor of Example 10.9 has been revised to lower the recycle line and pump volume to 100 cm. What eflbct will this have on the exit concentration of component A if all other conditions are held constant ... 

Recycle Line Problems Seal Flush Problems... 

Alternatively, for nonequilibrium process streams, where a pumped reactor sample recycle line is available, in-line fiber-optic transmission cells or probes (Figures 5.26 and 5.27) can be used to minimize sample transport. It is highly desirable that some form of pumped sample bypass loop is available for installation of the cell or probe, so that isolation and cleaning can take place periodically for background reference measurement. 

Hydrogen recycle line for a reformer unit, to cool the catalyst... 

The characteristic time-scales mentioned above take into account some but not all practical considerations. For example, really intense stirring (rpm > 500) in the CSTR is not recommended for in situ studies since a deep vortex ivill be formed in the liquid, gas ivill be entrained, and tivo-phase flow w ill occur in the recycle line. Also, two-phase flow will generally cause cavitation in a mechanical pump (possibly stopping flow) and induce irreproducible spectroscopic measurements. 

Note that the parameter VL/VR in a slurry reactor is the fractional liquid holdup. In the packed bubble bed reactor and the trickle-bed reactor, under complete recycling of the liquid stream, VL/VR is the ratio of total volume of the liquid that is processed (recycled) to the volume of reactor, and is always greater than 1. By recycling, it is possible to process a larger volume of liquid than the reactor volume by having a surge tank in the recycle line. 

Polymer-Assisted Ultrafiltration of Boric Acid. The Quickstand (AGT, Needham, MA) filtration apparatus is pictured schematically in Figure 3. The hollow fiber membrane module contained approximately 30 fibers with 0.5 mm internal diameter and had a nominal molecular weight cut-off of 10,000 and a surface area of 0.015 m2. A pinch clamp in the retentate recycle line was used to supply back pressure to the system. In a typical run, the transmembrane pressure was maintained at 25 psig and the retentate and permeate flow rates were 25 ml/min and 3 ml/min, respectively. Permeate flux remained constant throughout the experiments. 

Figure 4.43. Plant layout for reactive processing. 1 - tanks for reagents 2 - heat exchangers 3 - low-pressure pumps 4 - high pressure metering pumps 5 - low-pressure recycling line 6 - high-pressure recycling line 7 - chamber for shock mixing 8 - mold.

It is important to identify not only what may happen (consequences) and why (causes), but also what should be done (actions) to prevent serious accidents. Examples of operability studies in particular situations are included in Coulson and Richardson, Volume 6 (1983 pp. 292-294, 296298). Modifications initiated in one section of the plant can have effects in other sections, it is therefore necessary to consider the effects of any changes that are made on the overall operation of the plant. The existence of recycle lines requires careful consideration in the study. A second operability study may need to be performed after all the actions have been incorporated into the design and included on the flowsheet. 

The recycle line is described by plug flow so that it represents some delay in recycling the oversize fraction of particles. 

An 18-inch (450 mm) section of 6-inch (150 mm) diameter carbon steel fluid coker piping unexpectedly ruptured. The gross failure of this slurry recycle line allowed a high initial release rate of hydrocarbons. A vapor cloud formed engulfing an area that included the light oil product pumps. 

Because the solids path does not contain any sort of mechanical closure, the characteristic pressure distribution plotted in Fig. 11 is obtained. The distribution of solids between the fluidized bed and the recycle line is directly related to this pressure distribution. Operating properties differ from one recycle design to another [56]. 

One of the cationic surfactants was evaluated In a commercial alkylation unit at the Baytown refinery. A parallel test was conducted In which two reactors received the same feed and fresh acid. Figure 19. The modifier was injected into an acid recycle line on reactor 8, to rapidly bring its concentration to working strength and then the rate was lowered to maintain the concentration. After 11 days the additive concentration was doubled and after 19 days its addition was switched from reactor 8 to reactor... 

For a first approximation to the solution, we will assume that essentially all the methanol condenses, with only trace amounts appearing in the recycle line. We will also assume that most of the water condenses and that very small amounts of carbon monoxide, carbon dioxide, hydrogen, methane, and nitrogen dissolve in the condensate. To account for methanol and water vapor in the recycle gases and the solubility of the gases in the crade methanol, we would have to include phase equilibrium relationships in the analysis. As stated earlier, several condensable byproducts, high and low-boiling compounds in the cmde methanol, are present in small amounts, as shown in Table 3.5.1. We will not consider these compounds in the synthesis-loop analysis. 

The heart of this facility is a refractory-lined reactor cylinder with 20 cm in-diameier and a total height of about 6.5 m. The internal refractory forms two concentric cylinders (each one of thickness 6.5 cm) with different thermal conductivity and wearing properties. Additionally, the reactor tube, the cyclone and the recycling line are protected with outside insulation in order to minimise heat losses. The gasifier components (raiser, cyclone and standpipe) are equipped with ten thermocouples (K-type) and pressure transducers. The thermocouples probes are connected to a computer based data acquisition and control system. In addition, the feed rates of gasification... 

Figure 4 shows the gasifier installation with the reactor in the middle, the feedstock hopper in the background right-hand side and the recycling line in the foreground left-hand side. 

The continuous enzyme membrane reactor (CMR). (1) Temperature-controlled water-bath (2) Feed tanig (3) Stirrer motor for feed tank (4) Feed pump (5) Feed inlet line to the reaction vessel (6) Reaction vessel (7) Magnetic stirring table (8) Prefilter (9) Recycle pum (10) Flowmeter (11) Membrane inlet pressure gauge (12) Hollow fiber membrane cartridge (13) Membrane outlet pressure gaug (1 Pressure adjusbneut valve (15) Retentate recycle line (16) Air bath environment (17) Pemieate (product) line (18) Permeate collection vessel (19) Electronic balance... 

In the first case (Fig. 5.19a) the recycle pump is at a fixed position between two columns. Since all columns are moving upstream according to the SMB principle the recycle pump performs the same migration. This means that the flow rate of this pump has to be adjusted depending on the section it is located at present. This design results in a locally increased dead volume because of the recycle line and the pump itself. Such an unequal distribution of the dead volume can be compensated by asynchronous shifting of the external ports, as introduced by Flotier and Nicoud (1995). 

Concentration profiles in the recycle line are measured and collected during a cycle. Since this measurement point is fixed in the closed-loop arrangement, the sampled signal includes information from all three sections. During the start-up phase, an online estimation of the actual model parameters is started in every cycle. The quadratic cost functional... 

To predict the evolution of the process, the actual state (i.e. the concentration profiles over the columns) is needed. It is computed by simulation of the process model using the measurements in the recycle line as input functions. As the model is adapted online, this is sufficiently accurate. 

An analysis based upon the Fisher information matrix showed that the process model is highly sensitive to the Henry coefficients, mass transfer resistances and reaction rate. These parameters are, therefore, re-estimated online at every cycle. Figure 9.8 compares the concentration profiles collected in the recycling line with the simulated ones. The parameters were initialized with the values given in Table 9.1. 

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