Down-stream processing/unit operation

Down-stream processing/unit operations (minimizing the number of stages necessary to obtain the product in the state desired by the customer). 

Modern separation methods in down-stream processing include high-speed counter-current chromatography, supercritical fluid extraction, nanotube membrane ultraflltration. But the general principles of separation methods (unit operations) have been the same for the past century (see the article Chemical Engineering Science in Part 6). 

The general surroundings influencing the operating conditions An up-stream unit that feeds the process A down-stream unit when the outflow of the process is a manipulated inflow for the down-stream unit... 

Typical values of the stream factor are in the range of 0.96 to 0.90. Even the most reliable and well-managed plants will typically shut down for two weeks a year for scheduled maintenance, giving an SF = 0.96. Less reliable processes may require more downtime and hence lower SF values. The stream factor represents the fraction of time that the process unit is on-line and operating at design capacity. When estimating the size of equipment, care must be taken to use the design flowrate for a typical stream day and not a calendar day. Example 8.8 illustrates the use of the stream factor. 

This is illustrated by operation of the desalter, which is usually the first processing unit in the refinery proper. Its function is to reduce the content of bottom sediment and water (BS W) from the crude charge to the crude still. Water (generally brine) causes corrosion in units down-stream of the desalter as a result of decomposition of chlorides to hydrochloric acid at the elevated processing temperatures. Addition of alkali to the desalter reduces hydrolysis of calcium and magnesium chlorides and consequently results in less hydrochloric acid being formed in the crude still overheads, etc. 

Adsorption is very different from absorption, distillation, and extraction. These three processes, detailed in the five previous chapters, typically involve two fluids flowing steadily in opposite directions. In absorption, a gas mixture flows upward through a packed column while an absorbing liquid trickles down. In distillation, a liquid mixture is split into a more volatile liquid distillate and a less volatile bottoms stream. In extraction, two liquid streams move countercurrently to yield an extract and a raffinate. To be sure, in some cases, the contacting may involve near-equilibrium states, and in other cases it may be described with nonequilibrium ideas like mass transfer coefficients. Still, all three units operations involve two fluids at steady state. 

In the major catalytic processes of the petroleum and chemical industries, continuous and steady state conditions are the rule where the temperature, pressure, composition, and flow rate of the feed streams do not vary significantly. Transient operations occur during the start-up of a unit, usually occupying a small fraction of the time of a cycle from start-up to shut-down for maintenance or catalyst regeneration. 

The process centers on a fixed-bed downflow reactor that allows catalyst replacement without causing any interruption in the operation of the unit (Figure 9-28). Feedstock is introduced to the process via a filter (backwash, automatic) after which hydrogen and recycle gas are added to the feedstock stream which is then heated to reactor temperature by means of feed-effluent heat exchangers whereupon the feed stream passes down through the reactor in trickle flow. Sulfur removal is excellent (Table 9-18), and substantial reductions in the vanadium content and asphaltenes content are also noted. In addition, a marked increase occurs in the API gravity, and the viscosity is reduced considerably. 

The management of impurities is an issue of greatest significance in Process Design. Impurities affect the product quality, but also can generate troubles in operation and maintenance, and cause environmental damages. The inventory of impurities is a plantwide control problem, because it involves reactors and separators connected through recycles. Ideally, each component should be followed in each unit and each. stream. Downs (1992) pointed out that the material balance must be preserved not only from an overall viewpoint, but also for each component. He presented a qualitative... 

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