Feed-gas mixtures

An adsorbent attracts molecules from the gas, the molecules become concentrated on the surface of the adsorbent, and are removed from the gas phase. Many process concepts have been developed to allow the efficient contact of feed gas mixtures with adsorbents to carry out desired separations and to allow efficient regeneration of the adsorbent for subsequent reuse. In nonregenerative appHcations, the adsorbent is used only once and is not regenerated. 

Because this reaction is highly exothermic, the equiUbrium flame temperature for the adiabatic reaction with stoichiometric proportions of hydrogen and chlorine can reach temperatures up to 2490°C where the equiUbrium mixture contains 4.2% free chlorine by volume. This free hydrogen and chlorine is completely converted by rapidly cooling the reaction mixture to 200°C. Thus, by properly controlling the feed gas mixture, a burner gas containing over 99% HCl can be produced. The gas formed in the combustion chamber then flows through an absorber/cooler to produce 30—32% acid. The HCl produced by this process is known as burner acid. 

Example 6 Solvent Rate for Absorption Let us consider the absorption of acetone from air at atmospheric pressure into a stream of pure water fed to the top of a packed absorber at 25 C. The inlet gas at 35 C contains 2 percent by volume of acetone and is 70 percent saturated with water vapor (4 percent H2O by volume). The mole-fraction acetone in the exit gas is to be reduced to 1/400 of the inlet value, or 50 ppmv. For 100 kmol of feed-gas mixture, how many Idlomoles of fresh water should be fed to provide a positive-driving force throughout the pacldug How many transfer units will be needed according to the classical adiabatic method What is the estimated height of pacldug required if Hqq = 0.70 m ... 

The left-hand side of Eq. (14-55) represents the efficiency of absorption of arw one component of the feed-gas mixture. If the solvent oil is denuded of solute so that Xo = 0, the left-hand side is equal to the fractional absorption of the component from the rich feed gas. When the number of theoretical plates N and the hquid and gas rates L i and G, f have been fixed, the uractional absorption of each component may be computed directly and the operating lines need not be placed by trial and error as in the graphic approach described earlier. 

A slurry bed reactor is in a pilot stage investigation. This type is characterized by having the catalyst in the form of a slurry. The feed gas mixture is bubbled through the catalyst suspension. Temperature control is easier than the other two reactor types. An added advantage to slurry-bed reactor is that it can accept a synthesis gas with a lower H2/CO ratio than either the fixed-bed or the fluid-bed reactors. 

For this reason, operation around atmospheric pressures is typical. Space velocity should he high to avoid the reaction of ammonia with oxygen on the reactor walls, which produces nitrogen and water, and results in lower conversions. The concentration of ammonia must he kept helow the inflammahility limit of the feed gas mixture to avoid explosion. Optimum nitric acid production was found to he obtained at approximately 900°C and atmospheric pressure. 

Figure 7.7 Influence of the increasing molar ratio of water and carbon monoxide, and of the addition of 50% H2 to the feed gas mixture on the CO conversion in WGS reaction over Cu0 2Ce08O2 y, catalyst at different feed compositions with SV = 5000 hr1. The solid lines are model fits assuming first-order reversible kinetics. The dotted lines represent the equilibrium conversions for the specific feed compositions. (Reprinted from [51 ]. With permission from Elsevier.)...

Step 1 Feed gas mixture flows into column 2 while product flows out. A portion of this product is passed through column 1 at a lower pressure acting as a purge. 

A catalyst design strategy based on ternary metal oxides with catalytic and chemical constraints limited the desired set of catalyst compositions to about 100000, a number that could be tested in the scanning mass spectrometer [46]. A feed gas mixture of C2H6, 02, and Ar (4 1 5) was created using a set of digital... 

In the discussion of concentration polarization to this point, the assumption is made that the volume flux through the membrane is large, so the concentration on the permeate side of the membrane is determined by the ratio of the component fluxes. This assumption is almost always true for liquid separation processes, such as ultrafiltration or reverse osmosis, but must be modified in a few gas separation and pervaporation processes. In these processes, a lateral flow of gas is sometimes used to change the composition of the gas on the permeate side of the membrane. Figure 4.14 illustrates a laboratory gas permeation experiment using this effect. As the pressurized feed gas mixture is passed over the membrane surface, certain components permeate the membrane. On the permeate side of the membrane, a lateral flow of helium or other inert gas sweeps the permeate from the membrane surface. In the absence of the sweep gas, the composition of the gas mixture on the permeate side of the membrane is determined by the flow of components from the feed. If a large flow of sweep gas is used, the partial... 

Figure 8.15 The effect of stage-cut on the separation of a 50/50 feed gas mixture (pressure ratio, 20 membrane selectivity, 20). At low stage-cuts a concentrated permeate product, but only modest removal from the residue, can be obtained. At high stage-cuts almost complete removal is obtained, but the permeate product is only slightly more enriched than the original feed...

Because the membrane selectivity and pressure ratio achievable in a commercial membrane system are limited, a one-stage membrane system may not provide the separation desired. The problem is illustrated in Figure 8.16. The target of the process is 90% removal of a volatile organic compound (VOC), which is the permeable component, from the feed gas, which contains 1 vol% of this component. This calculation and those that immediately follow assume a feed gas mixture VOC and nitrogen. Rubbery membranes such as silicone rubber permeate the VOC preferentially because of its greater condensability and hence solubility in the membrane. In this calculation, the pressure ratio is fixed at 20... 

Once the external parameters (pco, p0,- and T) have been established to initiate autonomous kinetic oscillations, these can usually be sustained for periods of time as long as desired, provided that the surfaces are prevented from becoming contaminated (in particular by carbon, originating from spurious traces of hydrocarbons in the feed gas mixture) and that the partial pressures are not drifting off (31). With Pt(l 10) a complication may arise insofar that during the course of the reaction the surface struc-... 

The MSC membranes are produced by carbonization of PAN, polymide, and phenolic resins. They contain nanopores, which allow some of the molecules of a feed gas mixture to enter the pore structure at the high pressure side, adsorb, and then diffiise to the low pressure side of the membrane, while excluding the other molecules of the feed gas. Thus, separation is based on the difference in the molecular sizes of the feed gas components. The smaller molecules preferentially diffuse through the MSC membrane as shown by Table 4 [16,17]. 

The SSF membranes, which are produced by carbonization of PVDC, contain nanopores that allow all of the molecules of a feed gas mixture to enter the pore structure. However, the larger and more polar molecules are selectively adsorbed on the carbon pore walls at the high pressure side, and then th dif se selectively to the low pressure side. The smaller molecules are enriched at the high pressure side. These membranes can be used to enrich H2 from mixtures with C1-C4 hydrocarbons or from mixtures with CO2 and CH4. They can also be used to separate CH4-H2S and H2S-H2 mixtures. Table 5 compares performances of SSF carbon and polymeric PTMSP membranes for H2 enrichment from FCC off gas [15]. Clearly, the SSF membrane is much superior for this application. 

Recompression may be an issue, for example, in the case of hydrogen separation. In the majority of cases, most of the hydrogen from a hydrogen-containing feed gas mixture appears in the permeate which is on the lower pressure side of the membrane. Most applications, however, require the hydrogen so obtained to be at high pressures for subsequent processing. Thus, in a case like this, recompression-related costs must be included for an equitable comparison and these costs sometimes are critical to the economic feasibility of the gas separation process. 

The basic concept of a H2 PSA process is relatively simple. The impurities from the H2-containing feed gas mixture are selectively adsorbed on a micro- and meso-porous solid adsorbent (zeolites, activated carbons, silica and alumina gels) at a relatively high pressure by contacting the feed gas with the solid in a packed column of... 

The nanoporous carbon membrane consists of a thin layer (<10pm) of a nanoporous (3-7 A) carbon film supported on a meso-macroporous solid such as alumina or a carbonized polymeric structure. They are produced by judicious pyrolysis of polymeric films. Two types of membranes can be produced. A molecular sieve carbon (MSC) membrane contains pores (3-5 A diameters), which permits the smaller molecules of a gas mixture to enter the pores at the high-pressure side. These molecules adsorb on the pore walls and then they diffuse to the low-pressure side of the membrane where they desorb to the gas phase. Thus, separation is primarily based on differences in the size of the feed gas molecules. Table 7 gives a few examples of separation performance of MSC membranes. ° Component 1 is the smaller component of the feed gas mixture. 

Removal of trace organic and inorganic impurities from a gas stream by an activated carbon is one of the oldest appH cations of adsorption technology. The other applications of Table 22.1 where the adsorbed components are present in dilute or bulk quantities in the feed gas mixture were developed during the last 30 years. 

Figure 12.6 shows a process flow diagram of the AIMS (some details concerning gas delivery to the system has been excluded for simplification). The key components include (a) a feed gas manifold, which consists of a pressure regulators and a bank of mass flow controllers (M FCs) for on-line generation of the reaction feed gas mixture (b) a reactor manifold, which consists of dedicated MFCs that meter the feed gas mixture to individual parallel-operating microreactors and (c) an on-line reactor feed and product gas analysis system. A brief description of the various components that perform these various functions is given below. 

Valves (SOVs) (this assembly is referred to as the Redwood flow manifold). The composition of the reactor feed gas mixture is defined by setting the flow rates for each of the process gases using these dedicated mass flow controllers (MFCs). 

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