Downward flow

The space immediately above the resin bed may or may not be filled with Hquid in downward flow systems, depending on the design. If not filled, water entering the column from the top and impinging on the upper surface of the resin bed forms hills and valleys unless the flow is dispersed over the cross-sectional area. A distributor similar to the one used to collect resin below the bed, or splash plate, is placed a short distance above the resin bed to improve the distribution of the process stream flow. 

After the SO converter has stabilized, the 6—7% SO gas stream can be further diluted with dry air, I, to provide the SO reaction gas at a prescribed concentration, ca 4 vol % for LAB sulfonation and ca 2.5% for alcohol ethoxylate sulfation. The molten sulfur is accurately measured and controlled by mass flow meters. The organic feedstock is also accurately controlled by mass flow meters and a variable speed-driven gear pump. The high velocity SO reaction gas and organic feedstock are introduced into the top of the sulfonation reactor,, in cocurrent downward flow where the reaction product and gas are separated in a cyclone separator, K, then pumped to a cooler, L, and circulated back into a quench cooling reservoir at the base of the reactor, unique to Chemithon concentric reactor systems. The gas stream from the cyclone separator, M, is sent to an electrostatic precipitator (ESP), N, which removes entrained acidic organics, and then sent to the packed tower, H, where SO2 and any SO traces are adsorbed in a dilute NaOH solution and finally vented, O. Even a 99% conversion of SO2 to SO contributes ca 500 ppm SO2 to the effluent gas. 

Separation of a chemical species from a mixture of similar compounds can also be achieved by melt crystallization, which is, for example, an important means of separatingpara- s.yXen.e (p-xylene) from the ortho and meta isomers. -Xylene is crystallized at the top of a vertical column and crystals are moved downward countercurrentiy to Hquid. The Hquid flowing upward is generated by adding heat to melt the crystals at the bottom of the column a portion of the melt is removed as product and the remainder flows up the column to contact the downward-flowing crystals. Effluent mother Hquor, consisting almost entirely of the ortho and meta isomers of xylene, is removed from the top of the column. 

F Absorption, co-current downward flow, random packings... 

For pressure drop and holdup in inclined pipe with upward or downward flow, see Beggs and Brill ]. Pet. Technol, 25, 607-617 [1973]) the mechanistic model methods referenced above may also be apphed to inchned pipes. Up to 10° from horizontal, upward pipe inclination has httle effecl on holdup (Gregory, Can. J. Chem. Eng., 53, 384-388 [1975]). 

Bucket elevators, skip hoists, and cranes are used for top feeding of the furnace. Retention and downward flow are controlled by timing of the bottom discharge. Gases are propelled by a blower or by induced draft from a stack or discharge fan. In normal operation, the downward flow of sohds and upward flow of gas are constant with time, maintaining ideal steady-state conditions. 

Plate-Column Capacity The maximum allowable capacity of a plate for handling gas and liquid flow is of primaiy importance because it fixes the minimum possible diameter of the column. For a constant hquid rate, increasing the gas rate results eventually in excessive entrainment and flooding. At the flood point it is difficult to obtain net downward flow of hquid, and any liquid fed to the column is carried out with the overheaa gas. Furthermore, the column inven-toiy of hquid increases, pressure drop across the column becomes quite large, and control becomes difficult. Rational design caUs for operation at a safe margin below this maximum aUowable condition. 

Downward flow of both fluids imposes no restriction on the gas rate, except that the pressure drop will be high. On the whole, the trickle bed is preferred to the flooded bed. 

In vertical downward flow as well as in upward and downward inclined flows, the flow patterns that can be observed are essentially similar to those described above, and the definitions used can be applied. Experimental data on flow patterns and the transition boundaries are usually mapped on a two dimensional plot. Two basic types of coordinates are generally used for this mapping - one that uses dimensional coordinates such as superficial velocities, mass superficial velocities, or momentum flux and another that uses dimensionless coordinates in which some kind of dimensionless groups are used as coordinates. The dimensional coordinates maps are inherently limited to the range of data and flow conditions under which the experiments were conducted. In spite of this limitation, it is widely used because of its simplicity and ease of use. Figure 24 provides an example of such a map. 

The term three-phase fluidization requires some explanation, as it can be used to describe a variety of rather different operations. The three phases are gas, liquid and particulate solids, although other variations such as two immiscible liquids and particulate solids may exist in special applications. As in the case of a fixed-bed operation, both co-current and counter- current gas-liquid flow are permissible and, for each of these, both bubble flow, in which the liquid is the continuous phase and the gas dispersed, and trickle flow, in which the gas forms a continuous phase and the liquid is more or less dispersed, takes place. A well established device for countercurrent trickle flow, in which low-density solid spheres are fluidized by an upward current of gas and irrigated by a downward flow of liquid, is variously known as the turbulent bed, mobile bed and fluidized packing contactor, or the turbulent contact absorber when it is specifically used for gas absorption and/or dust removal. Still another variation is a three-phase spouted bed contactor. 

The only force opposing the downward flow of the heated air or upward flow of the cooled air is a buoyancy force. In their analysis, Helander and Jakowatz also suggested accounting for inertial forces due to the entrainment of room air. However, this suggestion is not in an agreement with a principle of momentum conservation used in most of the existing models for isothermal jets. 

A downward flowing exhaust through the grizzly and into the hopper directly counteracts the plume velocity. 

Class U The Class U (Types A, Bl, B2, and biological safety cabinets provide personnel, environmental, and product protection. Airflow is drawn around the operator, through the hood opening and into the front grill of the cabinet, which provides personnel protection, in addition, the downward flow of HEPA-filtered air provides product protection by minimizing the chance of cross-contamination along the work surface of the cabinet. Because cabinet air has passed through the exhaust HEPA filter, it... 

Circulates by axial flow parallel to the shaft and its flow pattern is modified by baffles, normally a downward flow. 

Upward flow should be avoided as the film coefficient falls considerably below the value for the downward flow however, see later section for details. 

Figures 12-106, 12-107, and 12-107A show an exploded view of a typical positive displacement blower. The impeller lobes rotate in opposite directions on parallel mounted shafts. Figure 12-107. One shaft serves as the drive shaft and drives the other through the gears. A timing hub allows for adjusting the timing angle of the lobes. The rotation may be either for upward, downward, or side gas flow. When liquid is entrained in the gas, the downward flow is preferred to assist in case drainage.

Bioreactors a) batch stirred tank b) continuous stirred tank c) continuous packed-bed i) downward flow, ii) upward flow and iii) recycle d) continuous fluidised-bed e) continuous ultrafiltration. Redrawn from Katchalski - Katzir E. (1993) Trends in Biotechnology II, 471-477. 

Most boiler plants with electrical power generating facilities employ surface condensers. These are shell-and-tube heat exchangers in either one-, two-, or four-pass configurations. Surface condensers typically receive cooling water on the tube-side and steam on the shell-side of the heat exchanger. The LP turbine steam generally is received at the top of the condenser and proceeds through the condenser in a downward flow, while the FW turbine exhaust steam enters at the side. 

By far the major portion of the available gas-absorption data have been obtained for countercurrent flow, which is the normal mode of operation for packed-bed absorbers. Special mention may be made of the results of Dodds et al. (D6), who examined mass transfer by the absorption of gas in liquid under cocurrent downward flow at flow rates higher than those corresponding to the flooding point for countercurrent operation. 

Lapidus (LI) described liquid residence-time distribution studies for air-water and air-hydrocarbon in cocurrent, downward flow through a column of 2-in. diameter and 3-ft height. Spherical glass beads of 3.5. mm diameter and cobalt molybdate catalyst cylinders of -in. diameter were used as packing materials. 

Glaser and Lichtenstein (G3) measured the liquid residence-time distribution for cocurrent downward flow of gas and liquid in columns of -in., 2-in., and 1-ft diameter packed with porous or nonporous -pg-in. or -in. cylindrical packings. The fluid media were an aqueous calcium chloride solution and air in one series of experiments and kerosene and hydrogen in another. Pulses of radioactive tracer (carbon-12, phosphorous-32, or rubi-dium-86) were injected outside the column, and the effluent concentration measured by Geiger counter. Axial dispersion was characterized by variability (defined as the standard deviation of residence time divided by the average residence time), and corrections for end effects were included in the analysis. The experiments indicate no effect of bed diameter upon variability. For a packed bed of porous particles, variability was found to consist of three components (1) Variability due to bulk flow through the bed... 

Weekman and Myers (W2) examined the fluid-flow characteristics of cocurrent downward flow of gas and liquid. The pulsing effect first noted by Larkins et al. was also observed in this work. Pressure-drop data could be correlated satisfactorily by a relation similar to those used for two-phase flow in pipes. Surface-active agents were observed to have a pronounced influence upon flow regime transition and pressure drop. 

A high vapour velocity upwards tends to increase the thickness of the film and thus reduce h though the film may sometimes be disrupted mechanically as a result of the formation of small waves. For the downward flow of vapour, TEN Bosch<5 9 has shown that h increases considerably at high vapour velocities and may increase to two or three times the value given by the Nusselt equation. It must be remembered that when a large... 

In a packed distillation column, the vapour stream rises against the downward flow of a liquid reflux, and a state of dynamic equilibrium is set up in a steady state process. 

FIGURE 11.5 Rotating-disk column with countercurrent downward flow of a liquid and upward flow of a gas or lower-density liquid. 

The bubbles play the role of the gas phase. The role of the liquid is played by an emulsion phase that consists of solid particles and suspending gas in a configuration similar to that at incipient fluidization. The quasi-phases are in cocurrent flow, with mass transfer between the phases and with a solid-catalyzed reaction occurring only in the emulsion phase. The downward flow of solids that occurs near the walls is not explicitly considered in this simplified model. 

Gel Filtration. The lyophilized protein was redissolved in 50 mM phosphate buffer, pH 7.4 0.15 m NaCl 0.013 % sodium azide and loaded on a Superdex 75HR1030 column equilibrated with the same buffer. Elution was downward flow (0.15 ml/min) and 0.25 ml fi actions were collected. Fractions with pectin lyase activity were combined, dialyzed against distilled water and used in the next step. To estimate the molecular mass of PNL, the column was calibrated with standard proteins (Sigma MW-GF-70 Albumin, 66,000 Da Carbonic Anhidrase, 29,00 Cytochrome, 12,400 and Aprotinin, 6,500). The proteins were eluted in the conditions described above and their volumes (F ) were calculated fi om the peak maximum of the absorbance at 280 nm. The partition coefficient was obtained fi om the relationship where F, represents the bed volmne of column and F the void volume (which was calculated using blue dextran. Sigma). The molecular mass was determined using a standard curve of vs the logarithm of the molecular masses of the standards [28, 29]... 

Figure 2. Gel filtration. The dry residue obtained after ammonium sulfate precipitation was redissolved in 50 mM phosphate buffer, pH 7.4 0.15 M NaCl 0.013 % sodium azide, which was loaded on a Superdex 75HR1030 column equilibrated with the same buffer. Elution was downward flow (0.15 ml/min) and 0.25 ml fractions were collected. The fractions were assayed for protein content (— ) and PNL activity (- - ).

---