Bulk removal

Adsorption systems employing molecular sieves are available for feed gases having low acid gas concentrations. Another option is based on the use of polymeric, semipermeable membranes which rely on the higher solubiHties and diffusion rates of carbon dioxide and hydrogen sulfide in the polymeric material relative to methane for membrane selectivity and separation of the various constituents. Membrane units have been designed that are effective at small and medium flow rates for the bulk removal of carbon dioxide. 

Natural gas contains both organic and inorganic sulfur compounds that must be removed to protect both the reforming and downstream methanol synthesis catalysts. Hydrodesulfurization across a cobalt or nickel molybdenum—zinc oxide fixed-bed sequence is the basis for an effective purification system. For high levels of sulfur, bulk removal in a Hquid absorption—stripping system followed by fixed-bed residual clean-up is more practical (see Sulfur REMOVAL AND RECOVERY). Chlorides and mercury may also be found in natural gas, particularly from offshore reservoirs. These poisons can be removed by activated alumina or carbon beds. 

This process uses propylene carbonate as a physical solvent to remove CO2 and H2S. Propylene carbonate also removes C2+ hydrocarbons, COS, SO2, CS2, and H2O from the natural gas stream. Thus, in one step the natural gas can be sweetened and dehydrated to pipeline quality. In general, this process is used for bulk removal of CO2 and is not used to treat to less than 3% CO2, as may be required for pipeline quality gas. The system requires special design features, larger absorbers, and higher circulation rates to obtain pipeline quality and usually is not economically applicable for these outlet requirements. 

The physical solvent sulfolane provides the system with bulk removal capacity. Sulfolane is an excellent solvent of sulfur compounds such as H2S, COS, and CS2. Aromatic and heavy hydrocarbons and CO2 are soluble in sulfolane to a lesser degree. The relative amounts of DIPA and sulfolane are adjusted for each gas stream to custom fit each application. Sulfinol is usually used for streams with an H2S to CO2 ratio greater than 1 1 or where it is not necessary to remove the CO2 to the same levels as is required for H2S removal. The physical solvent allows much greater solution loadings of acid gas than for pure amine-based systems. Typically, a Sulfinol solution of 40% sulfolane, 40% DIPA and 20% water can remove 1.5 moles of acid gas per mole of Sulfinol solution. 

Mixed solvents are combinations of physical and chemical solvents which increase the flexibility of treating (1). The chemical solvent allows for treatment of lower-pressure streams while the physical solvent allows for bulk removal of the acid gas. 

Selectivity increases complexity and/or residual impurities Staged operation and cooling for high purity Commercial high acid-gas pressure bulk removal favored 1,2,3, 4, 5... 

More selective than MeOH Selectivity or high purity require high complexity Commercial - high acid-gas pressure, bulk removal favored 5... 

Can be selective, with stages and recycle High solvent cost, moderate complexity for bulk removal Commercial - designed for high acid-gas pressures, bulk removal 1,2,3,4,5... 

Activated carbon, used commercially to remove small quantities of H2S from synthesis gas (3), is useful as a follow-on unit to a bulk-removal process. Operation is cyclic, with periodic regeneration of the carbon by steam or inert gas. 

In principle, the combination of membranes for bulk removal of the carbon dioxide with amine units as polishing systems offers a low-cost alternative to all-amine plants for many streams. However, this approach has not been generally used because the savings in capital cost are largely offset by the increased complexity of the plant, which now contains two separation processes. The one exception has been in carbon dioxide flood enhanced oil-recovery projects [49,54], in which carbon dioxide is injected into an oil formation to lower the viscosity of the oil. Water, oil and gas are removed from the formation the carbon dioxide is separated from the gas produced and reinjected. In these projects,... 

Depending on source, geographic location, and the extent of extraction, the acid gas content of fuel gases often exceeds pipeline specifications. Certain natural gases and landfill gases can contain up to 50% carbon dioxide. Bulk removal of both carbon dioxide and hydrogen sulfide from such sources, i.e., the process of sweetening, not only improves the fuel value of the gas, but also helps reduce corrosion of pipelines and transmission equipment. Membranes are suitable for this application especially where the scale is relatively small and the economics favor scalable membrane systems. 

Microfiltration as pretreatment to RO is used for bulk removal of suspended solids and bacteria. Microfiltration is a barrier technology that can in many cases replace conventional clarification and filtration. Advantages of MF over conventional clarification and filtration include the following ... 

After this bulk removal of the carbon oxides, the typical synthesis gas still contains 0.2-0.5vol% CO and 0.005 0.2 vol% CO2. All oxygen-containing compounds have to be reduced to a very low ppm level, as they are poisonous toward the ammonia synthesis catalysts. Methanation is the simplest method to reduce the concentrations of the carbon oxides well below 10 ppm, and is widely used in steam-reforming plants. 

Total has also long been interested in the bulk removal of acid gases notably, in gas fields operated in the Far East, containing over 40% CO2, as well as in the Caspian sea... 

The cost of acid gas removal depends strongly on its concentration and the need for downstream compression. In the case of very sour gas the combination of a bulk removal step ahead of the final sweetening unit can reduce the overall add gas removal cost. If the acid gas is re-injeded the bulk removal process should offer... 

Physical (organic) solvents are also employed for the bulk removal of H S and/ or CO2 from gas streams [1, 2]. In a physieal solvent, aeid gases dissolve without... 

This approach is, of course, only possible with solid-state ISE and only with those rapidly electrochemically regenerated. Unless removed by purging, oxygen is always present in the solution and alleviates the need for addition of a redox mediator that could alter the rest potential of the antimony electrode and interfere with the pH. Moreover, the normalization of the approach curve with the limiting current in the bulk removes the need for the knowledge of the bulk concentration and diffusion coefficient of oxygen. 

Goddin63 presented a detailed cost analysis for recovering C02 from casinghead gas where bulk removal is effected by membrane separation followed by a conventional gas treating process. His analysis showed that for feed gas containing more than 30% C02, the process heat requirements for a membrane separation process are approximately 30 to 40% lower than for a cryogenic process. When compared to an acid gas removal process using DEA as a solvent, the membrane separation process requires only 20 to 25% of the process heat requirement. 

---