Recovery from gas streams

The removal of aromatics and relatively heavy hydrocarbons from gas streams with fixed beds of activated carbon is essentially the same process as. solvent recovery, and similar adsorbents and equipment are used. The principal differences are that in hydrocarbon recovery the feed is typically a natural gas or other combustible gas stream rather than air, and adsorption is usually (but not always) conducted at elevated pressure. The basic design approach for hydrocarbon recovery systems follows the same general logic as that described for solvent recovery systems. A brief outline of the key design steps is given in the Calgon Carbon Corporation bulletin, Heavy Hydrocarbon Removal or Recovery from Gas Streams (1987). 

Calgon Carbon Corporation, 1987, Heavy Hydrocarbon Removal or Recovery from Gas Streams, Bulletin 23-66b. 

The US petroleum refining industry generates sales of over 730 billion with only about 143 plants. It employs 62,000 people. About 90% of the produets used in US are fuels of whieh 43% is gasoline. Figure 13.20.1 illustrates how the produets breakdown. The proeess is described in detail in Chapter 3. Emissions of hydrocarbons to the atmosphere occur at almost every stage of the production process. Solvents are produced in various processes and they are also used to extract aromatics Irom lube oil feedstock, deasphalting of lubricating base stocks, sulfur recovery from gas stream, production of solvent additives for motor fuels such as methyl tert-butyl ether and tert-amyl methyl ether, and various... 

Separation of CO2 from gas streams is required in four areas (1) purification of natural gas (gas sweetening), (2) separation of CO2 from enhanced oil recovery (FOR) gas streams, (3) removal of CO2 from flue gas, and (4) removal of CO2 from biogas. A fifth area vital for the space age should be mentioned removal of CO2 from life support systems onboard space ships, and also in submarines. All these applications have different specifications for the purified gas or for the recovered CO2, and future membrane applications will most likely be based on tailor-made materials. 

Porous ceramic or micrometallic filters are very effective for recovering entrained fines from gas streams (228,252). Multiple installations are required because it is necessary to blow back each filter element periodically to dislodge the catalyst cake that builds up on the surface and leads to increased pressure drop. Filters have been used for catalyst recovery in other fluid-catalyst processes where high cost or other considerations justify extraordinary measures to minimize catalyst losses. However, this expedient has not been employed in commercial fluid cracking units because losses are readily controlled to a reasonable level by simpler means. In fact, intentional discard of catalyst is often practiced, in addition to normal losses, in order to maintain catalyst quality at a high level by permitting increased additions of fresh catalyst. 

The problem of the removal of aerosol particles from gas streams has become of increasing importance from the standpoint of public health and the recovery of valuable products. Technology of controlling the aerosol particles or improving the liquid phase of aerosol is very important in many industrial processes such as oil and petroleum, electronic, mining, and food, as well as waste products like noxious emission of aerosol in chemical plants. There are several ways for this purpose among which fibrous filters are more popular so that it is obvious to try to improve their efficiency. The efficiency of collection and the pressure drop are the most important practical considerations in the design of these fibrous filters [2], Various... 

The removal of dust particles, typically 1 to 1,000 microns in diameter, from gas streams (also called gas cleaning) is accomplished on an industrial scale by four main types of equipment (1) bag filters, (2) cyclones using centrifugal force, (3) electrostatic precipitators, and (4) venturi scrubbers using washing with a liquid. Reasons for dust collection include air-pollution control, elimination of safety and health hazards, recovery of a valuable product, improvement in the quality of other products, and reduction of equipment maintenance. Typical ranges of particle size that can be efficiently removed by each of the four methods are as follows ... 

When hot raw gas is to be cooled directly or if it has already been cooled in a waste-heat recovery system, it will normally be treated in a wet scrubber. Wet scrubbers are devices that utilize gas/ liquid contacting to cool the gas stream, condense high-boiling hydrocarbons, dissolve some constituents, and separate particles from gas streams. There are many different wet scrubber designs, but all utilize similar mechanisms. 

---