Light ends recovery fractionation

Light Ends Recovery, Fractionation, and Conversion Propylenes and butylenes may be recovered for feed to a polymerization plant for production of high octane... 

Light ends recovery and fractionating equipment is necessary after the Powerformer and on the pipestill overhead stream to separate the effluent mixtures into the desired boiling range cuts. 

The cracked products pass out through two stages of cyclones which collect entrained catalyst and return it to the dense bed. Velocities at the outlet of the dense bed are normally 2.0-3.0 ft./sec. Upon leaving the cyclones, the vapors go to the primary fractionator which separates the heavy products from the gasoline and lighter components. The light products go on to the light ends recovery unit. The heavy material is separated and either recycled to the reactor or withdrawn from the system. 

This section covers the various steps used to recover and separate into useful products the desirable light ends fractions present in the large volumes of effluent gases produced by the various refinery processes. The recovery is usually accomplished by a combination of compression and absorption. The subsequent separation into useful fractions is invariably carried out by distillation, usually in combination with distillates in the gasoline boiling range which are recovered with the light ends fractions. 

The reactor effluent is cooled and fed to the ethylene separator for recovery of unreacted gaseous ethylene. The liquid phase is filtered to remove small amounts of polymer and then treated with aqueous caustic to remove the catalyst. The dissolved light ends (C2 and C4 olefins) are separated by suitable fractionating towers in series. A portion of the ethylene is purged to remove methane and ethane, and the remaining ethylene is recycled to the compressor. The butene-1 is removed to storage. 

Understand Process Characteristics H2, H2S, and NH3 and light ends are removed from reaction effluents through a series of separation and flashes, resulting in the reaction products in a liquid form, which goes to the stripper, the feed heater, and then to the main product fractionator. The task of the product fractionation is to separate different products based on their product specihcations such as distillation endpoint, ASTM D-86 T90% or T95% point, and so on. Side draws from the column go to the product strippers where kerosene and diesel products are made. The net draw from the column bottom is called unconverted oil (UCO), which is recycled back to the reaction section for nearly complete conversion. There are two pump-arounds, namely, kerosene and diesel pumparounds, as a main feature of heat recovery from the main fractionation column. 

The prime product is the steel, which is magnetically separated and then cleaned. The mixed non-ferrous metals are cleaned and resold. The residue is fluff, in light or heavy fractions, made up of glass, fabrics, rubber, lubricants and miscellaneous dirt as well as plastics. Conventionally, both categories of fluff have ended up as landfill. The systems as designed therefore make no provision whatever for the recovery of polymeric material. Meanwhile the mountains of fluff accumulate (see Fig. 8.2). 

Conversion units may employ a full-fledged fractionation train, with a preflash tower to remove light ends an atmospheric fractionator to separate light naphtha, heavy naphtha, middle distillates, and unconverted oil and a vacuum tower to maximize the recovery of diesel. Some hydrocrackers use the atmospheric tower to produce full-range naphtha, which is then separated into light and heavy fractions in a naphtha splitter. 

This type of column is found in the light ends section of the refinery gas plant and processes only identifiable components. It is also the common type of fractionator found in the distillation section of natural gas liquids recovery facilities or, for that matter, in most chemical plants where the process objective is to make a separation between two components. The design procedure for calculating this type of tower is outlined in the following discussion. 

To obtain subtractions LDL was first isolated in the conventional manner between density 1.019 and 1.063 and its density readjusted to 1.050. It was then sandwiched into the center of a five-step gradient as described by Teng et al. [1] and centrifuged in a swinging bucket SW 50.1 rotor for 40h at 10°C. At the end of the 40 h two clearly separated bands were visible towards the top of the tube, which were called light LDL and heavy LDL under some circumstances a third band of heavier LDL is visible, but in most people one sees just two bands (Fig, 1). Using a Beckman fraction recovery system, it is possible to take off the gradient in 0.5-ml fractions, and thus recover these two fractions separately. 

Purification. For polymerization, butadiene that is at least 99 mol% pure is required. Although alkynes are the most troublesome impurities, separation of the butadiene from other C4 products is also necessary. Simple fractional distillation is effective for removing the light (C3) and heavy (C5) ends from butadiene, but not for removing the various C4 species because of the closeness of the boiling points to each other and to butadiene. Further complicating purification, butadiene forms azeotropes with re-butane and 2-butene. The most widely used recovery systems are extraction with aqueous cuprous ammonium acetate (CAA) and solvent extractions with furfural, acetonitrile, dimethylformamide, dimethylacetamide, or AT-methylpyrrolidinone (65,66). 

---