Fractionation operating problems

Jet Flood. Flooding generally occurs by jet flood or downcomer backup. Reference 15 gives Equations 1, 2, and 3 for Jet flood, using Ballast trays. 

The larger value from Equations 1, 2, and 3 applies. Equation 2 applies only for liquid rates less than 0.50GPM/L , where L ,i is the weir length in inches. Equation 3 applies where the dowmcomers are unusually small relative to the required downcomer area. 

Downcomer Backup Flood. For downcomer backup. Equation 4 can be used. Reference 15 states that if the downcomer backup for valve trays exceeds 40% of tray spacing for high vapor density systems I3.01bs/ft-), 50% for medium vapor densities, and 60% for vapor densities 

Tower Operations. The tower operator can quickly determine which type of flooding will tend to be the limiting one for a particular system. If a rigorous computer run is available for the anticipated or actual operation, the operator can quickly calculate the expected limiting column section. The operator can then provide DP cell recording for the entire column and limiting section(s). As mentioned previously, a DP cell is the best measure of internal traffic and flooding tendency. 

Flooding across a column section reflects itself in an increase in pressure drop and a decrease in temperature difference across the affected section. Product quality is also impaired, but it is hoped that the other indicators will allow correction of the situation before major change in product quality. When a column floods, the levels in the accumulator and bottom often change. It can occur that the accumulator fills with liquid carried over while the reboiler runs dry. 

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Kister says that d/b plots are primarily used when there is a tight spec, on a nonkey component or a concern about the distribution of an intermediate key component. His book shows d/b curves for various feed stage locations on a plot of the mole ratio of a reference component in the distillate to the bottom product, versus the relative volatility of each component to this reference component. This plot is made on log-log paper. The optimum feed produced a curve closest to linear. The d/b plot is suggested as a troubleshooting tool in the subsection of the Troubleshooting section, Fractionation Operating Problems. ... 

Operational problems in size fractionation by membrane filters... 

Fats are chemically triglycerides and can be regarded as the esters produced by the reaction of fatty acids with the trihydric alcohol glycerol. In practice, oils and fats are the product of biosynthesis. Some sugar confectionery contains oils or fats whereas other products, e.g. boiled sweets, are essentially fat-free. The traditional fat used in sugar confectionery is milk fat, either in the form of butter, cream, whole milk powder or condensed milk. Milk fat can only be altered by fractionating it. and while this is perfectly possible technically, there must be sufficient commercial and technical benefits to make it worthwhile. One problem with fractionation operations is that both the desirable and the undesirable fractions have to be used. 

Figure 14.2. Operational problems in size fractionation by membrane filters, (a, b, c) Size distributions of iron oxyhydroxyphosphate particles obtained by transmission electron microscopy (true distribution) and syringe filtration on Nucleopore polycarbonate filters and Schleicher-Schuell cellulose ester depth filters. Iron particles formed at the oxic-anoxic interface of eutrophic Lake Bret (Switzerland), (d) Fraction of iron particles retained on 3.0-pim membranes, as a function of flow rate (j) Nucleopore polycarbonate, and (2) Schleicher-Schuell cellulose nitrate. In the absence of coagulation or adsorption, no particle should be retained. (From Buffle et al., 1992.)...

Operational problems due to high fractions of water in the mobile phase... 

To continue with the ethylbenzene/styrene separation of the immediately previous problems, now consider the use of a random packing for the fractionator. In Problem 12.8, for 50-mm metal Pall rings, a column diameter of 3.21 m (10.5 ft) would be required to operate at 80% of flood. Now estimate the efficiency of this same packing for the same service. The basic data are given in Problems 12.6 through 12.9. A packed height of 6.0 m will be assumed. 

Water can cause severe operational problems in services which are not meant to handle it. One refiner (296) stated that 99 percent of his fractionator upsets are due to water other refiners (296) agreed with this evaluation. Most problems occur when the column separates water-insoluble materials such as hydrocarbons. The main adverse effects of water in such services are pressure surges, flooding, cycling, corrosion, hydrates, and off-spec products. Typical sources of water in this t5q>e of services are... 

It was pointed out earlier that a compression screw pump was used to provide catalyst circulation in the initial 100 /D plant and that it caused serious operating problems. Fortunately the concept of the standpipe to build up pressure was conceived at this opportune moment and was soon put to use. It may seem surprising, but the standpipe concept was only accepted after considerable persuasion and discussion. Confirmation was needed, so a standpipe 100 feet high was set up on one of the refinery fractionation columns at Baton Rouge, filled with catalyst, and aerated. Pressure gauges confirmed the calculated buildup in pressure, and when a valve at the bottom was opened the catalyst ran out as though it were a liquid. 

So when two phases, a and p, are in equilibrium, the state is identified by values for C properties those C properties could be the pressxue plus (C - 1) independent mole fractions r for the a phase. But although situations having T = 2 require only C properties to identify the state, we often need to know more to satisfy the needs of a design or operational problem. For example, in addition to pressure and r , we may also need to know temperature and the composition of the other phase xP. Values for these additional properties can be computed by solving the equilibrium conditions... 

D17. An extractive distillation system is separating conpound A from C with a solvent. Solvent flow rate is S, feed flow rate is F, entering solvent is pure, and entering feed is a saturated liquid with mole fractions and Zq. Because of plant operating problems, the distillate flow rate has to be shut down (D = 0). The values of F, S, Q, and Qr are not changed. After a new steady state is reached, predict the values of B and the mole fractions in the bottoms stream. 

As indicated earlier, there is a delicate balance between the concentration of resins and asphaltenes in petroleum fractions. Any interruption in the ratio of asphaltenes resins can cause operational problems because of asphaltene precipitation (coke formation) and plant shutdown. It is therefore crucial, if coke formation is to be minimized, to monitor the ratio of asphaltenes resins during the thermal processing of bitumens and heavy oils. Clarke and Pruden developed a heat transfer analysis technique that can detect the onset of asphaltene precipitation. They showed that precipitated asphaltenes could be re-peptized using polynuclear aromatic compounds such as phenanthrene. It has been demonstrated in the 5000 bbl/d CANMET hydrocracking unit... 

The asphaltenes are arguably the most complex fraction of oil. In variations of pressure, temperature or composition of oil, asphaltenes tend to associate and precipitate causing several costly operational problems from transport to refining (Trejo et al, 2007 Lira-Galeana and Duda, 2006). 

The actual mechanism of formation of the solid fraction in the pyrolysis of PE and the influence of reaction temperature and reactor geometry are still open problems. Their elucidation is important in order to minimize solid formation in gasification and pyrolysis reactions that may cause sooting and fouling operating problems of these processes. 

Equilibrium catalyst attrition index and average particle size distribution (APS) indicate changes in the rate of catalyst attrition. Further analysis of APS for any catalyst that is carried forward into the fractionator, present in the slurry, or which leaves the unit via the regenerator stack can identify problems associated with catalyst quality or cyclone operation. Problems include operation at greater than design feed, catalyst rates or cyclone maloperation. APS is also important in predicting the fluidization properties of the catalyst inventory. 

Maintenance costs account for a large fraction of the total operating expenditure (opex) of a project. Because of the bath tub curve mentioned above, maintenance costs typically increase as the facilities age just when the production and hence revenues enter into decline. The measurement and control of opex often becomes a key issue during the producing lifetime of the field as discussed in Section 14.0. However, the problem should be anticipated when writing the FDP. 

Fine grinding and air classification make possible the production of some cake flour from hard wheat and some bread flour or high-protein fractions from soft wheat. AppHcation of the process theoretically frees the miller from dependence on different wheats, either hard or soft, that change each crop year. The problem is how to market the larger volume of low protein or starch fractions at prices adequate to justify the installation and operation of the special equipment (46). 

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