Residence decanters

Complex aromatic raw materials such as petroleum resids, decant oils, coal, and coal tars have been employed for many years by the earbon industry and eontinue to be used extensively in the fabneation of eoke, earbon, and artifieial graphite [1], These same feedstoeks also have the potential for use in produeing "advaneed" earbon produets such as carbonaceous mesophase, fibers, and beads 12-4],... 

Decanters are used to separate liquids where there is a sufficient difference in density between the liquids for the droplets to settle readily. Decanters are essentially tanks which give sufficient residence time for the droplets of the dispersed phase to rise (or settle) to the interface between the phases and coalesce. In an operating decanter there will be three distinct zones or bands clear heavy liquid separating dispersed liquid (the dispersion zone) and clear light liquid. 

In procedure A, a sample is mixed with pentane and centrifuged, after which the resid or asphalt solution is decanted and the precipitate is washed twice with pentane, dried, and weighed. For toluene-insoluble constituents, a separate sample of the resid or asphalt is mixed with pentane and centrifuged. The precipitate is washed twice with pentane, once with toluene-alcohol solution, and once with toluene. The insoluble material is then dried and weighed. In procedure B, procedure A is followed except that instead of pentane, a pentane-coagulant solution is used. 

To obtain the gold in a perfectly pure stats, however, the precipitated powder is boiled with hydrochloric acid of specific gravity 1 1, which is then decanted, and the resides is boiled twice with fresh acid. The gold is not washed between these successive treatments with add, which remove the last traces of iron, and nearly all tlie chloride of silver,... 

Aldolization, Hydrogenation and Refining. In the second processing step, aldolization (1 0), n-butyraldehyde is added to aqueous sodium hydroxide which is recycled from the decanter. The caustic concentration in the cycle is maintained at 3% by adding makeup caustic. Residence time in the aldol reactor is typically about 60 seconds. Reaction temperature is controlled between 110°C and 120°C at pressures of 50 to 70 psi. The reactor effluent is fed to a decanter to separate the 2-ethylhexenal (EPA) from the caustic solution recycle. Water produced by dehydration of the aldol dilutes the cycle and is purged after decantation. 

Liquid-hquid separators are also called decanters or settlers. The flow to the settler consists of a dispersed phase and a continuous phase, and the function of a settler is to coalesce and separate the dispersed phase from the continuous phase. The separator volume must be sufficiently large to allow sufficient time for the dispersed-phase drops to reach the liquid-liquid interface and coalescence. Thus, the residence time has two components. These are the time required for the droplets to reach the interface and the time required for the droplets to coalesce. 

An oil-water mixture is separated in a decanter. The properties of oil and water from an example by Hooper and Jacobs [22] are summarized in Table 6.4.1. If the residence time required for coalescence is 5.0 min, obtained from experiments, find the dimensions of the decanter. 

The emulsion leaving the reactor enters a settler. Residence times there often average up to 60 min to permit separation of the two liquid phases. Most of the acid phase is recycled to the reactor, being injected near the eye of the impeller. The hydrocarbon phase collects at the top of the decanter it contains unreacted isobutane, alkylate mixture, often some light n-paraffins, plus small amounts of di-isoalkyl sulfates. The sulfates must be removed to prevent corrosion problems in the distillation columns. Caustic washes are often employed to separate the sulfates they result in destruction of the sulfates. Acid washes have the advantage that most of the sulfates eventually react to reform sulfuric acid, which is reused, and to produce additional alkylate product. 

Feed >10% v/v, or contamination present, or ill-behaved, size as coalescence controlled. For vertical decanters, allow a total residence time that depends on density difference and interfacial surface tension. For a typical 0.5-m height of coalescent band (or a decanter of 0.7-m height), use an overflow total flow rate velocity of 1.5 (Ap/0.1)° L/s m, where the density difference is in units of Mg/m to determine the horizontal cross-sectional area. For horizontal configurations, use half of the vertical overflow velocity. Can add parallel plates or high- and low-energy combination coalescer promoters (see size increase, liquid-liquid. Section 16.11.9.2). 

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