Steam feed requirement

A comparable statement can be made with regard to the power apphed to a mechanical recompression evaporator.) In summary, the steam flow required to increase the sohd content of the feed from Xq to X. is... 

A higher steam/hydrocarhon ratio favors olefin formation. Steam reduces the partial pressure of the hydrocarbon mixture and increases the yield of olefins. Heavier hydrocarbon feeds require more steam than gaseous feeds to additionally reduce coke deposition in the furnace tubes. Liquid feeds such as gas oils and petroleum residues have complex polynuclear aromatic compounds, which are coke precursors. Steam to hydrocarbon weight ratios range between 0.2-1 for ethane and approximately 1-1.2 for liquid feeds. 

It is not the intention here to consider in detail the subject of boiler feed-water conditioning and treatment for nuclear plant, but the general principles may be noted. Essentially, the same objectives apply as in fossil-fuelled plant, embodied in the three aims to minimise corrosion, deposition and steam-carryover. Requirements are more stringent in nuclear plant because there is no possibility of repairing tubes which have failed, let alone those which have suffered either deposition or corrosion. Again, certain tubes in nuclear plant have very modest design corrosion allowances so that only minimal loss of thickness from any cause can be tolerated. 

Assuming axisymmetric flow (i.e., r and z as independent variables, neglecting any 6 variations), state (not derive) the full mass-continuity and momentum equations that describe the flow in the annulus. Identify the dependent variables. Considering the characteristics and order of the system, state and discuss a set of boundary conditions that could be used to solve the system. Clearly, some approximation is required around the steam-feed entrance to retain axisymmetry. 

Varying the enthalpy, or heat content, of the feed is an additional independent variable that an operator, or process design engineer, can use to optimize fractionation efficiency. An additional benefit of feed preheat is that a lower-level temperature heat source can be used. If valuable 100-psig steam is required for the reboiler, then low-value 20-psig steam might be adequate for the feed preheat exchanger. 

Assuming the feedstock is methane, which is the major component of natural gas, the theoretical feed requirement would be equivalent to one-fourth of the potential hydrogen production or 16,713 SCF CH /ST NH3(15.2 MM BTU/ST). However, the actual process consumes on the order of 22,420 SCF CHi+/ST NH3 or about 20.4 MM BTU/ST NH3 (LHV). The required quantity of feed depends on the process design criteria chosen for the methane conversion in the reforming section, the efficiency of CO conversion, degree of CO2 removal and the inerts (CHi+ + Ar) level maintained in the ammonia synthesis loop. Thus, the potential hydrogen conversion efficiency of the feedstock in the steam reforming process is about 75%. Table 3 shows where the balance of the feed is consumed or lost from the process. 

Table XV provides a summary of the coal moisture, steam feed rate and exit gas pressure transient response runs showing the time required to reach the given condition. It provides a rough estimate for the values of the bifurcation points for these runs. Thus, the bifurcation point for the coal moisture step change runs lies between 30.08 and 31.05 wt % moisture. For the steam feed rate changes, it lies between -2.25% and -2.50%, and for the exit gas pressure, it is bracketed by the 3.14 MPa (31 atm) and 3.24 MPa (32 atm) values.

In the shift conversion step, carbon monoxide reacts with steam to form equivalent amounts of hydrogen and carbon dioxide. Upon cooling of the effluent gas, most of the unreacted steam is condensed and separated as process condensate. Modem ammonia plants utilize a two-step, in-series shifting, carried out at high and then low temperatures to increase conversion efficiency. Use of the dual-shift conversion system lowers overall plant steam requirements, and the lower CO leakage results in reduction in plant feed requirements due to more complete conversion of CO to hydrogen. Under normal operating conditions there is no emission from the shift converters. 

Temporal product (Hy, CO, CO2 and CH4) distribution was, however, dependent on the feed composition and temperature. Stoichiometrically, propane steam reforming requires a steam-... 

The steam to carbon ratio (S/C ratio) is the ratio of the moles of steam to atoms of carbon in the reformer feed. The S/C ratio, in conjunction with temperature and pressure, affects hydrogen yield, H2/CO ratio of the syngas product and methane conversion. The minimum S/C ratio for methane is about 1.7. However, excess steam is required to prevent carbon formation, avoid catalyst deactivation and adjust product Hj/CO [31. As a result, actual S/C ratios for steam reforming of methane are typically between 3.5 and 5.0. 

High pressure steam generators require very good quality feed water. Thus, its overall hardness should be less than 20-30 milligrams/kg, and the silicic acid content less than 30 milligrams/kg. Therefore, water must be chemically purified before it can be fed into the boilers. Water purification system uses the widely accepted sodium cation exchange method with subsequent degasiEcation. Other methods may also be used as required by the quality of source water. 

For steam generation preheat boiler feed water with available low temperature process streams, maximize the use of heat transfer surfaces by optimizing sootblowing frequency and decoking of tubes, flash blowdown to produce low pressure steam if required. 

The methane combustion configuration is sketched in Figure 3.14 and consists of two sections [44]. Hydrogen permselective membranes are integrated in a fiuidized reform-ing/shift top section where ultra-pure H2 is extracted and the energy required for the SR is supplied via in situ methane oxidation in a separate fluidized bottom section, where oxygen is selectively fed to the methane/steam feed via oxygen permselective membranes. 

In the oxidation section, CH4 is partially oxidized in order to achieve the high temperatures required for O2 permeation through the perovskite membranes and to simultaneously preheat part of the CH4/steam feed. 

Figure 3.2 shows the equilibrium conversion of methanol steam reforming as a function of the S/C ratio of the feed [25]. It is obvious that the maximum hydrogen concentration in the reformate is gained at S/C 1. However, to minimize the carbon monoxide concentration in a practical system, a surplus of steam is required. Therefore, in practice, systems operate at S/C ratios ofbetween 1.3 and 2.0. Elevated pressure also decreases the selectivity towards carbon monoxide [49]. 

The addition point of gaseous fuels requires careful consideration to avoid homogeneous reactions upstream of the reformer vith autothermal reforming and partial oxidation. Commercial flame arresters are normally not capable of operating under the elevated temperatures of the fuel processor. Microchannels are known to act as flame arresters (see Section 6.3.2) and may be inserted into the tubing system to avoid uncontrolled reaction of the fuel/air mixture. For liquid fuels, which are usually injected into the pre-heated steam feed or even into the air/steam feed mixture, either cooled injection nozzles [567] or the application of steam jackets may be used to ensure stable operation of the nozzle. 

Stability and sealing of these ceramic membranes in the reactor) and that the entire process is carried out in a single unit. The disadvantage is the higher required steam feed ratio. Nevertheless, the overall energy efficiencies of both concepts are very comparable and much higher than that can be obtained with the conventional process, especially when scaled down and particularly when including CO2 capture. 

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