Resid entrainment

The operating engineer should check the color of the HVGO periodically. A darkish yellow color is normal black gas oil indicates resid (and metals) are being entrained into the gas oil. Some of the more common causes of resid entrainment are ... 

The demister pad is partially coked. Quite often, the demister spray header (see Fig. 13-1) is designed for too large a wash oil flow. At low flow, the wash oil does not distribute to the ends of the spray header, and the peripheral area of the demister dries up and cokes. The sections of the demister still open are exposed to velocities high enough to promote resid entrainment. A high AP across the demister shows that coke plugging is the problem. An examination c the demister before it is removed from the tower will indicate which areas are not being wetted with wash oil from the spray header. This information should then be used to revamp the spray header. 

As well as preventing liquid carry over in the gas phase, gas carry undef must also be prevented in the liquid phase. Gas bubbles entrained in the liquid phase must be given the opportunity (or residence time) to escape to the gas phase under buoyancy forces. 

Flotation. Flotation (qv) is used alone or in combination with washing and cleaning to deink office paper and mixtures of old newsprint and old magazines (26). An effective flotation process must fulfill four functions. (/) The process must efficiently entrain air. Air bubble diameter is about 1000 p.m. Typically air bubbles occupy 25—60% of the flotation cell volume. Increa sing the airRquid ratio in the flotation cell is said to improve ink removal efficiency (27). (2) Ink must attach to air bubbles. This is primarily a function of surfactant chemistry. Air bubbles must have sufficient residence time in the cell for ink attachment to occur. (3) There must be minimal trapping of cellulose fibers in the froth layer. This depends on both cell design and surfactant chemistry. (4) The froth layer must be separated from the pulp slurry before too many air bubbles coUapse and return ink particles to the pulp slurry. 

Still another process, called BI-GAS, was developed by Bituminous Coal Research in a 73 t/d pilot plant in Homer City, Peimsylvania. In this entrained-bed process, pulverized coal slurry was dried and blown into the second stage of the gasifier to contact 1205°C gases at ca 6.9 MPa (1000 psi) for a few seconds residence time. Unreacted char is separated and recycled to the first stage to react with oxygen and steam at ca 1650°C to produce hot gas and molten slag that is tapped. 

In the liquid-hquid extraction area, in the mining industry, coming out of the leach tanks is normally a slurry, in which the desired mineral is dissolved in the liquid phase. To save the expense of separation, usually by filtration or centrifugation, attempts have been made to use a resident pump extraction system in which the organic material is contacted directly with the slurry. The main economic disadvantage to this proposed system is the fact that considerable amounts of organic liquid are entrained in the aqueous slurry system, which, after the extraction is complete, is discarded. In many systems this has caused an economic loss of solvent into this waste stream. 

The three main types of reactors shown in Fig. 27-6 are in aclual commercial use the moving bed, the fluidized bed, and the entrained bed. The moving bed is often referred to as a. fixed bed because the coal bed is kept at a constant height. These differ in size, coal feed, reactant and product flows, residence time, and reaction temperature. 

For vaporAiquid separators there is often a liquid residence (holdup) time required for process surge. Tables 1, 2, and 3 give various rules of thumb for approximate work. The vessel design method in this chapter under the Vapor/Liquid Calculation Method heading blends the required liquid surge with the required vapor space to obtain the total separator volume. Finally, a check is made to see if the provided liquid surge allow s time for any entrained water to settle. 

Most theoretical studies of heat or mass transfer in dispersions have been limited to studies of a single spherical bubble moving steadily under the influence of gravity in a clean system. It is clear, however, that swarms of suspended bubbles, usually entrained by turbulent eddies, have local relative velocities with respect to the continuous phase different from that derived for the case of a steady rise of a single bubble. This is mainly due to the fact that in an ensemble of bubbles the distributions of velocities, temperatures, and concentrations in the vicinity of one bubble are influenced by its neighbors. It is therefore logical to assume that in the case of dispersions the relative velocities and transfer rates depend on quantities characterizing an ensemble of bubbles. For the case of uniformly distributed bubbles, the dispersed-phase volume fraction O, particle-size distribution, and residence-time distribution are such quantities. 

The FCC process is used worldwide in more than 300 installations, of which about 175 are in North America and 70 in Europe. Figure 9.10 shows the principle of an FCC unit. The preheated heavy feed (flash distillate and residue) is injected at the bottom of the riser reactor and mixed with the catalyst, which comes from the regeneration section. Table 9.5 gives a typical product distribution for the FCC process. Cracking occurs in the entrained-flow riser reactor, where hydrocarbons and catalyst have a typical residence time of a few seconds only. This, however, is long enough for the catalyst to become entirely covered by coke. While the products leave the reactor at the top, the catalyst flows into the regeneration section, where the coke is burned off in air at 1000 K. 

Sufficient residence time must be allowed in the downcomer for the entrained vapour to disengage from the liquid stream to prevent heavily aerated liquid being carried under the downcomer. 

The efficiency of settling chambers increases with residence time of the waste gas in the chamber. Because of this, settling chambers are often operated at the lowest possible gas velocities. In reality, the gas velocity must be low enough to prevent dust from becoming re-entrained, but not so low that the chamber becomes unreasonably large. The size of the unit is generally driven by the desired gas velocity within the unit, which should be less than 3 m/s (10 ft/sec), and preferably less than 0.3 m/s (1 ft/sec). 

Vapor-Liquid Gravity Separator Design Fundamentals The critical factors in the performance of a horizontal separator are the vapor residence time and the settling rate of the liquid droplets. However, two other factors enter into the design—the vapor velocity must be limited to avoid liquid entrainment, and there must be sufficient freeboard within the vessel to allow for a feed distributor. For vertical separators, the design is based on a vapor velocity that must be less than the settling velocity of the smallest droplet that is to be collected, with due allowance for turbulence and maldistribution of the feed. The vapor residence time is a function of the vapor flow rate (mass), vapor density, and volume of vapor space in the separator, based on the following ... 

The recommended method is from Guidelines for Pressure Relief and Effluent Handling Systems (AIChE-CCPS, 1998). It is an improvement over the method presented in the 7th edition of this Handbook. The procedure involves calculating a terminal velocity for a selected droplet size, then providing enough residence time in the vapor space to allow the droplets to fall from the top of the vessel to the level of liquid collected. Also, the vapor velocity in the separator must not exceed the value above which liquid may Be entrained from the liquid surface in the separator. The tank is treated as a simple horizontal cylinder, neglecting the volume of liquid in the heads. 

The residence time rCd = rH/aCd and the limiting concentration CinCd/aCd are divided by a factor of 30 relative to a non-reactive case, e.g., chlorine. Entrainment by sediments flushes the excess Cd 30 times faster and decreases Cd steady-state concentration 30 times relative to a sediment-free lake. [Pg.351]

---