Entrained solids, removal

By-product HCl from the heavy organic-chemicals industry (p, 798) now accounts for over 90% of the HCl produced in the USA, Where such petrochemical industries are less extensive this source of HCl becomes correspondingly smaller. The crude HCl so produced may be contaminated with unreacted CI2, organics, chloro-organics or entrained solids (catalyst supports, etc,), all of which must be removed. 

The use of steam washing equipment. Apart from the removal of entrained solids and water, steam washers are quite effective at removing vaporous silica carryover (typically 60-70% removal). 

Instead of filtration or centrifuging as a means of separating the crystallized wax from oil, an electrical precipitation method has been proposed, similar to that employed in desalting of crude or removal of entrained solids from gases. 

The dry product is primarily collected in cyclone collectors (a few bag houses still remain), sieved, and finally packaged in moisture barrier containers. The exit air from the dryer often has to be treated to meet local pollution control laws. While many of the older dryers use gas incineration, as energy costs have increased these incineration systems have become quite costly to operate. New dryer installations use scrubbing systems (e.g., aqueous/chemical sprays) to remove entrained solids and gaseous volatile flavors. 

The fluidized bed reactor is a vertical steel vessel to which TDF is fed through a side port. A fluidized bed of TDF is maintained with hot air. The abrasive action of the fluidized particles erode the char from the TDF, reducing the tire material to small pieces. As the TDF decomposes, ash and char are swept out of the reactor with the fluidizing air. The biggest disadvantages of a fluidized bed system are the need to remove entrained solids from the vapors, and the need to maintain the hot, fluidizing gas. 

Example. Bitumen is recovered in the form of a froth when a separation-flotation process is applied to surface mined oil sand. Once de aerated, this bituminous froth is a W/O emulsion from which the water must be removed prior to upgrading and refining. At process temperature (80 °C) the emulsion viscosity is similar to that of the bitumen, but the density, due to entrained solids, is higher. Taking t) = 500 mPa-s and f> = 1.04 g/mL, the rate of creaming of 20 pm diameter water droplets under gravitational force will be very slow ... 

Crude desalting is a water-washing operation done at the refinery to further clean up the crude oil before processing. The crude oil pretreated by field separators will still contain water and entrained dirt. Water-washing removes much of the water-soluble minerals and entrained solids. 

Gas exiting the gasifier flows through a cyclone for removal of entrained solids (char, ash, and sorbent) and then to the product gas... 

Desalting is a water-washing operation performed at the production field and at the refinery site for additional crude oil cleanup (Fig. 13.2). If the petroleum from the separators contains water and dirt, water washing can remove much of the water-soluble minerals and entrained solids. If these crude oil contaminants are not removed, they can cause operating problems during refinery processing, such as equipment plugging and corrosion as well as catalyst deactivation. 

This process can be operated on all types of coal without pretreatment. Dried, pulverized coal and oxygen are converted in a horizontal, entrained-flow gasifier at about 1820°C and near-atmospheric pressure. The raw gas is quenched with water to solidify entrained molten ash, scrubbed to remove entrained solids, and purified to remove hydrogen sulfide and a controlled quantity of carbon dioxide. The resulting product is used as synthesis gas. 

A brick lining has the secondary effect of protecting the lead from abrasion by contained slurries or suspended matter. The protective surface film of insoluble salts can be thinned or removed by such abrasion, and so the resistance to acids seriously affected. Figure 12-7 shows graphically how the velocity of a 20% sulfuric acid solution at 77°F, without any entrained solids, passing over the face of a lead lining can cause increasing corrosion as velocity increases. 

The process of nucleation apparently is dominant in removal of the entrained solid salts, whereby most of the residual ash also is removed by scrubbing liquid with low contact time. Whatever the reason, the process is eflFective at low pressure loss. 

Fluidization allows better removal of the heat generated by the reaction, by means of coils sunk in the catalyst mass, conveying a coolant fluid and producing high-pressure steam. However, it requires auxiliary units, including cydone separators to recover entrained solid particles, reactant injection and distillation systems, etc. 

In U.S. plants hydrofluorination is carried out in two stirred fluidized-bed reactors in series, with counterflow of solids and gases. The bed to which UO2 is fed and from which exhaust gases are discharged runs at 300°C, partially converts UO2 to UF4, and reduces the HF content of the effluent gases to around 15 percent. The bed to which anhydrous HF and the partially converted UO2 are fed runs at 500°C and converts more than 95 percent of the UO2 to UF4. To prevent caking of the fluidized beds, it has been found necessary to provide each reactor with a vertical-shaft, slow-speed stirrer to scrape the reactor walls. Production rates around 700 to 900 kg/h are obtained in 0.75-m-diameter reactors. Effluent gases are filtered to remove entrained solids, cooled to condense aqueous HF, and scrubbed to remove the last traces of HF. 

Petrov and Petrov (1998) developed a molecular hydrodynamic theory of film deposition during removal. Their theory correctly assumes a flow pattern - which we identified as a split streamline - between the solid substrate and the monolayer in Figure 10.5 (c). This pattern is indeed the necessary pattern for successful deposition during removal, but it is not the only flow pattern for solid removal at all dynamic contact angles. Petrov and Petrov (1998) address the kinetics of water removal between the solid and the monolayer and the formation of wet or dry monolayers depending on the amount of water entrained. 

This example ignores the entrainment of liquor with the solids removed from the system and any wash water or solution added to improve the separation. These are important questions in process design, as is the proper water balance in the crystallizer if the best results are desired. If the purpose of the Glauber s salt crystallizer is only to concentrate the sulfate purge stream, very high quality will not be as important. 

The term entrainment will be used here to describe the ejection of particles from the surface of a bubbling bed and their removal from the vessel in the fluidizing gas. In the literature on the subject other terms such as carryover and elutria-tion are often used to describe the same process. In this section we will study the factors affecting the rate of entrainment of solids from a fluidized bed and develop a simple approach to the estimation of the entrainment rate and the size distribution of entrained solids. 

The processes are basically the same with little variation in flow scheme. The sour gas containing HaS and/or COa enters the plant through a scrubber, which removes any free liquids and/or entrained solids. The sour gas then enters the bottom of the absorber and flows upward in countercurrent contact with the descending aqueous amine solution. Sweetened gas leaves the top of the absorber and flows to a dehydration unit, where saturation water from the aqueous amine solution is removed. 

Referring to Figure 4-20, which represents a typical Phosam plant, the coke-oven gas (after cooling and cleaning to remove entrained solids, water, and tar) enters the bottom of the absorber where it is contacted countercurrently with an aqueous solution of ammonium phosphate. About 99% of the ammonia is removed from the gas stream in the absorber. The ammonia-free product gas is suitable as feed for a variety of processes to remove hydrogen sulfide and other impurities. The ammonia-rich solution is drawn off the bottom of the absorber for regeneration. 

In FBMRs, the membranes are inserted inside the fluidized catalyst bed, serving as a product extractor or a reactant distributor. Figure 7.1 shows a typical FBMR structure for selective removal of a product (hydrogen) [4,5]. Pd-membrane tubes are placed vertically in the FBMR.The reactant gas is fed through the gas distribution plate at the bottom of the reactor to fluidize the fine particulate catalysts. Entrained solids are separated from the reaction product gas stream by internal cyclone separator and then returned to the reactor catalyst bed. 

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