Feed gases

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. 

Absorber oil units offer the advantage that Hquids can be removed at the expense of only a small (34—69 kPa (4.9—10.0 psi)) pressure loss in the absorption column. If the feed gas is available at pipeline pressure, then Httle if any recompression is required to introduce the processed natural gas into the transmission system. However, the absorption and subsequent absorber-oil regeneration process tends to be complex, favoring the simpler, more efficient expander plants. Separations using soHd desiccants are energy-intensive because of the bed regeneration requirements. This process option is generally considered only in special situations such as hydrocarbon dew point control in remote locations. 

An analytical model of the process has been developed to expedite process improvements and to aid in scaling the reactor to larger capacities. The theoretical results compare favorably with the experimental data, thereby lending vahdity to the appHcation of the model to predicting directions for process improvement. The model can predict temperature and compositional changes within the reactor as functions of time, power, coal feed, gas flows, and reaction kinetics. It therefore can be used to project optimum residence time, reactor si2e, power level, gas and soHd flow rates, and the nature, composition, and position of the reactor quench stream. 

Butanes are recovered from raw natural gas and from petroleum refinery streams that result from catalytic cracking, catalytic reforming, and other refinery operations. The most common separation techniques are based on a vapor—Hquid, two-phase system by which Hquid butane is recovered from the feed gas. 

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. 

Spira.1- Wound Modules. Spiral-wound modules were used originally for artificial kidneys, but were fuUy developed for reverse osmosis systems. This work, carried out by UOP under sponsorship of the Office of Saline Water (later the Office of Water Research and Technology) resulted in a number of spiral-wound designs (63—65). The design shown in Figure 21 is the simplest and most common, and consists of a membrane envelope wound around a perforated central coUection tube. The wound module is placed inside a tubular pressure vessel, and feed gas is circulated axiaUy down the module across the membrane envelope. A portion of the feed permeates into the membrane envelope, where it spirals toward the center and exits through the coUection tube. 

Spiral-wound modules are much more commonly used in low pressure or vacuum gas separation appHcations, such as the production of oxygen-enriched air, or the separation of organic vapors from air. In these appHcations, the feed gas is at close to ambient pressure, and a vacuum is drawn on the permeate side of the membrane. Parasitic pressure drops on the permeate side of the membrane and the difficulty in making high performance hollow-fine fiber membranes from the mbbery polymers used to make these membranes both work against hollow-fine fiber modules for this appHcation. 

Gas Separation. During the 1980s, gas separation using membranes became a commercially important process the size of this appHcation is stiH increasing rapidly. In gas separation, one of the components of the feed permeates a permselective membrane at a much higher rate than the others. The driving force is the pressure difference between the pressurized feed gas and the lower pressure permeate. 

Quench Converter. The quench converter (Fig. 7a) was the basis for the initial ICl low pressure methanol flow sheet. A portion of the mixed synthesis and recycle gas bypasses the loop interchanger, which provides the quench fractions for the iatermediate catalyst beds. The remaining feed gas is heated to the inlet temperature of the first bed. Because the beds are adiabatic, the feed gas temperature increases as the exothermic synthesis reactions proceed. The injection of quench gas between the beds serves to cool the reacting mixture and add more reactants prior to entering the next catalyst bed. Quench converters typically contain three to six catalyst beds with a gas distributor in between each bed for injecting the quench gas. A variety of gas mixing and distribution devices are employed which characterize the proprietary converter designs. 

Figure 8 shows the characteristic sawtooth temperature profile which represents the thermodynamic inefficiency of this reactor type as deviations from the maximum reaction rate. Catalyst productivity is further reduced because not all of the feed gas passes through all of the catalyst. However, the quench converter has remained the predominant reactor type with a proven record of reflabiUty. 

The vertical tube (water-cooled) generator consists of two concentric tubes the outer of which is cooled with water and acts as the ground electrode. Feed gas is introduced into the top of the inner stainless steel tube (which serves as the high voltage electrode), exits at the bottom of the outer tube, flows upward through the aimular space (which contains the electric discharge), and emerges at the top of the outer tube into a product gas manifold. 

Feed Ga.s Purifica.tion. Because nickel-based reforming catalysts are quite sensitive to sulfur, halogen, and heavy metal poisons which may be found ia natural gas, a feedstock purification system is normally required. Sulfur compounds, ia both organic and inorganic forms, are the most common... 

Chlorides may be found in natural gas, particularly associated with offshore reservoirs. Modified alumina catalysts have been developed to irreversibly absorb these poisons from the feed gas. 

Incorporation of a feed gas saturator cod in the convection section of the primary reformer allows for 100% vaporization of the process condensate. The steam is used as process steam in the reformer. 

The feed gas is iatioduced neai the lotoi axis. Enriched and depleted gases are extracted by stationary pitot-like scoops. The location and shape of these tubes, and the baffles within the rotor, gready effect the gas dow which recirculates within the rotor, reaching enrichment equiUbtium at a given feed rate. A vacuum is maintained around most of the rotor. The UF leakage around the stationary axial post is confined to the top of the case by the use of a molecular pump. 

A vapor-phase process primarily for ECC off-gas feeds was developed by Sinopec Technology Company based on a 2eoHte catalyst of the Pentasd type (24,25). It reHes on frequent regeneration of the catalyst to minimi2e pretreatment of the ECC off-gas and allows the impurities in the feed gas to react with ben2ene to form by-products. Consequently, the product yield and purity are low. Joint licensing by ABB Lummus Crest and Sinopec was announced in 1994. 

Other components in the feed gas may react with and degrade the amine solution. Many of these latter reactions can be reversed by appHcation of heat, as in a reclaimer. Some reaction products cannot be reclaimed, however. Thus to keep the concentration of these materials at an acceptable level, the solution must be purged and fresh amine added periodically. The principal sources of degradation products are the reactions with carbon dioxide, carbonyl sulfide, and carbon disulfide. In refineries, sour gas streams from vacuum distillation or from fluidized catalytic cracking (FCC) units can contain oxygen or sulfur dioxide which form heat-stable salts with the amine solution (see Fluidization Petroleum). 

The Claus process converts hydrogen sulfide to elemental sulfur via a two-step reaction. The first step involves controUed combustion of the feed gas to convert approximately one-third of the hydrogen sulfide to sulfur dioxide (eq. 9) and noncatalytic reaction of unbumed hydrogen sulfide with sulfur dioxide (eq. 10). In the second step, the Claus reaction, the hydrogen sulfide and sulfur dioxide react over a catalyst to produce sulfur and water (eq. 10). The principal reactions are as foUow ... 

Cupric ion concentration is kept at an acceptable but low level by direct air oxidation of the solution. SoHds formation from sulfides in the feed gas is also possible therefore, pretreatment for sulfur removal is required. 

As in the case of the salt complexation processes, the cryogenic systems require prepuriftcation of the feed gas. Bulk water, hydrogen sulfide, and carbon dioxide are removed by standard techniques. Final removal of these materials is accompHshed by adsorption. After prepuriftcation, the gases are ready for cryogenic processing. 

The carbon monoxide product is removed from the top of the column and warmed against recycled high pressure product. The warm low pressure stream is compressed, and the bulk of it is recycled to the system for process use as a reboder medium and as the reflux to the carbon monoxide column the balance is removed as product. The main impurity in the stream is nitrogen from the feed gas. Carbon monoxide purities of 99.8% are commonly obtained from nitrogen-free feedstocks. 

A flow diagram for the system is shown in Figure 5. Feed gas is dried, and ammonia and sulfur compounds are removed to prevent the irreversible buildup of insoluble salts in the system. Water and soHds formed by trace ammonia and sulfur compounds are removed in the solvent maintenance section (96). The pretreated carbon monoxide feed gas enters the absorber where it is selectively absorbed by a countercurrent flow of solvent to form a carbon monoxide complex with the active copper salt. The carbon monoxide-rich solution flows from the bottom of the absorber to a flash vessel where physically absorbed gas species such as hydrogen, nitrogen, and methane are removed. The solution is then sent to the stripper where the carbon monoxide is released from the complex by heating and pressure reduction to about 0.15 MPa (1.5 atm). The solvent is stripped of residual carbon monoxide, heat-exchanged with the stripper feed, and pumped to the top of the absorber to complete the cycle. 

The carbon monoxide purity from the Cosorb process is very high because physically absorbed gases are removed from the solution prior to the low pressure stripping column. Furthermore, there is no potential for oxidation of absorbed carbon monoxide as ia the copper—Hquor process. These two factors lead to the production of very high purity carbon monoxide, 99+ %. Feed impurities exit with the hydrogen-rich tail gas therefore, the purity of this coproduct hydrogen stream depends on the impurity level ia the feed gas. 

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