Feed gas concentrations

The Stretford Process sweetens and also produces sulfur. It is good for low feed gas concentrations of H2S. Economically, the Stretford Process is comparable to an amine plant plus a Claus sulfur recovery plant. Usually, the amine/Claus combination is favored over Stretford for large plants. Stretford can selectively remove H2S in the presence of high CO2 concentrations. This is the process used in the coal gasification example in the Introduction. 

The Q3D model rests on the following idea. Along-the-channel models clearly show [10,159] that the variation ofthelocal current density and feed gas concentration along the channel are negligible on a length of the order of the MEA thickness. This permits one to neglect the z component of fluxes in mass and current conservation equations, written for... 

The outline of calculations is as follows. Let the cell voltage be fixed. For given along-the-channel profiles of feed gas concentrations, the internal problem returns the profile of the local current density. This local current density is then used to update the concentrations of feed gases in the channels. This procedure is repeated until convergence is reached. Thereby a point on the voltage current curve and a map of parameters in a cell cross section is obtained. 

Equations (138) and (139) allow to calculate va(z) and vc(z), if j(z) is known (the latter is determined from the solution of the internal problem). Then one can find the feed gas concentration along the channel. For example, in the cathode channel we have... 

Jets, venturis very soluble gas only with target species Henry s law constant <10 kPa/mol fraction feed gas concentration >1 vol%. Efficiencies 50% to 85%. 

Phases gas-liquid, liquid-liquid. Gas-liquid residence time, very short. Reaction rates, very fast need rapid absorption. Very high gas capacity. Used for neutralization reactions with one of the reactants in the gas phase. For surface area see size reduction, Section 16.11.8.2. Gravity spray surface area 30 to 70 vrFIvrF target species Henry s law constant 10 to lO kPa/mol fraction feed gas concentration 0.3 to 4 vol%. Venturi jet nozzle surface area 200 to 2500 mVm very soluble gas only with target species Henry s law constant <10 kPa/mol fraction feed gas concentration >1 vol%. Liquid-liquid surface area 7 to 75 vrVIvrV. Related topics are solvent extraction (Section... 

Phases gas-liquid, liquid-liquid. Gas liquid residence tune, short. Use for very fast reactions all reaction is in the liquid film and is mass transfer-controlled. Gas-liquid surface area, max. observed 800 m /m with the usual range 200 to 500 m /m or slightly higher than a bubble column. Target species Henry s law constant <10 kPa/mol fraction feed gas concentration <1 vol%. Not for foaming, corrosive, or particulates. Liquid-liquid can operate as gravity flow or with fluid pulsed operation. For pulsed operation surface area 75 to 3000 m /m. Related topics are solvent extraction (Section 16.11.4.10). 

The main product, Cr02Cl2 can be formed during the course of PCE decomposition and thus Cr loss can occur on the catalyst surface. However, after the reaction the serious accumulation of carbon and chlorine and the reduction of BET surface area of the catalyst were hardly observed. This indicates that carbon and chlorine depositions on the catalyst surface may not be the main cause of the catalyst deactivation for the present reaction system. It should be noted that about 2.8 wt.% of Cl was observed on the surface of Cr0x/Al203 catalyst after 30 h of on-stream time at 10,000 ppm of the feed gas concentration to the reactor system. 

Countercurrent packed column target species Henry s law constant < 10 kPa/ mol fraction feed gas concentration < 1 vol%. Efficiencies 95+%. vulnerable to plugging. OK for foaming and corrosive. 

Insufficient absorption or off-specification for exit scrubbed gas feed gas concentration off spec/feed gas temperature or pressure outside operating window for amine absorbers > 50 °C for H2S and < 24 °C for C02/feed gas pressure has de-creased/[solvent flowrate too low] for glycol dehydration 12.5 to 25 L TEG per kg water removed/[solvent incorrect] /incorrect feed tray location/[column operation faulty] /absorber operating conditions differ from design/[absorber malfunction]. ... 

ZnS is generated at the adsorbent surface as a new chemical species. Therefore, Eq. (34.2) must be attributed to chemisorption, which is a sub-class of adsorption. The maximum possible sulfur loading is 33%, which corresponds to the complete conversion of ZnO to ZnS. If the amount of sulfur in the feed gas concentration is small enough, on-site regeneration can be dispensed with. After breakthrough, the absorbent bed must be replaced by a fresh one. In such a concept, ZnO is used as a poHshing bed. 

When treating Claus unit tail gas, the process is capable of overall sulfur recoveries of up to 99.6% The number of reactors included is dependent on the feed gas concentration and the required sulfur recovery. Catalyst selectivity is maximized at a MODOP reactor outlet temperatures of 250°-270°C (482°-518 F). As the oxidation reaction is highly exothermic, additional stages must be employed to limit the reactor outlet temperature to below 320°C (608°F) at high concentrations of H2S in the feed gas. 

Because the rate of the hydrosulfide oxidation reaction increases as the pH of the wash liquor drops, a high CO2 feed gas concentration is favorable to the kinetics of the process however, a low pH adversely affects H2S absorption. 

---