Counter current flow

Here the reacting mixture and coolant flow in opposite directions for counter current flow of coolant and reactants. At the reactor entrance, V = (), the reactants enter at temperature and the coolant exits at temperature T a,. At the end of the reactor, the reactants and products exit at temperature T. while the ctx)lanl enters at 

Again we write an energy balance over a differential reactor volume to arrive at. 

Solution to a counter current flow piobletn to find the exit conversion and temperature requires a trial-and-ermr procedure. 

Consider an exothennic reaction where the coolant stream enters at the end of the reactor (V - V() at a temperature say MX) K. We have (o carry out a irial-and-crmr procedure to find the temperature of the coolant exiling the reactor. 

Assume an exit coolant temperature at the feed entrance (X = 0. V = 0) to the reactor to be 

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Counter-current flow. Structured packings. Gauze-type with triangular flow channels, Bravo, Rocha, and Fair correlation... 

The best known use of the hairpin is its operation in true counter-current flow which yields the most efficient design for processes that have a close temperature approach or temperature cross. However, maintaining countercurrent flow in a tubular heat exchanger usually implies one tube pass for each shell pass. As recently as 30 years ago, the lack of inexpensive, multiple-tube pass capability often diluted me advantages gained from countercurrent flow. 

Spray Towers A spray tower consists of an empty shell into the top of which the liquid is sprayed by means of nozzles of various kinds the droplets thus formed are then allowed to fall to the bottom of the tower through a stream of gas flowing upwards. The use of sprays appears to offer an easy way of greatly increasing the surface area exposed to the gas, but the effectiveness of the m.ethod depends on the production of fine droplets. These are difficult to produce and suffer from the disadvantage that they are liable to entrainment by the gas even at low gas velocities. The surface area may also be reduced as a result of the coalescence of the droplets first formed. As a consequence of these effects, the large increase in surface area expected may not be achieved, or if achieved m.ay be accompanied by serious entrainment and internal circulation of the liquid so that true counter-current flow is not obtained. A single spray tower is suitable for easy absorption duties. For difficult duties, a number of towers in series can be used. 

A typical equilibrium diagram is shown on Figure 8. As noted earlier, a counter-current flow scrubber provides the highest efficiency of operation. The respective concentrations and rates, as shown in Figure 9 provide the basis for a material balance across this scrubber design. 

Fluid Iron Ore Reduction (FIOR) is a process for reducing ore to iron with a reducing gas in a fluid bed. For thermodynamic efficiency, iron ore reduction requires counter current flow of ore and reducing gas. This is achieved in FIOR in a multiple bed reactor. Precautions are necessary to prevent significant back mixing of solids between beds, since this would destroy counter current staging. 

BATCH COOLING EXTERNAL HEAT EXCHANGER (COUNTER-CURRENT FLOW), NON-ISOTHERMAL COOLING MEDIUM... 

Batch heating/cooling of fluids external heat exchanger (counter-current flow) non-isothermal cooling medium... 

Figure 2-2. Change in AT over distance, counter-current flow of fluids.

L the shell-side fluid makes one pass from inlet to outlet. With a longitudinal baffle, and with the nozzles placed 180° around the shell, the shell-side fluid would be forced to enter at the left, flow to the right to get around the baffle, and flow to the left to reach the exit nozzle. This would be required to approximate true counter-current flow, which was assumed in the heat transfer equations of Chapter 2. 

A physical solvent process is shown in Figure 7-6. The sour gas contacts the solvent using counter-current flow in the absorber. Rich solvent from the absorber bottom is flashed in stages to a pressure near atmos... 

Oxidized solution is delivered from the pumping tank to the top of the absorber tower, where it contacts the gas stream in a counter-current flow. The reduced solution flows from the contactor to the solution flash drum. Hydrocarbon gases that have been dissolved in the solution are flashed and the solution flows to the base of the oxidizer vessel. Air is blown into the oxidizer, and the solution, now re-oxidized, flows to the pumping tank. 

Figure 8-4 shows a typical trayed contactor in which the gas and liquid are in counter-current flow. The wet gas enters the bottom of the contactor and contacts the "richest glycol (glycol containing water in solution)... 

Figure 8-4. Typical glycol contactor in which gas and liquid are in counter-current flow.

For counter-current flow of the fluids through the unit with sensible heat transfer only, this is the most efficient temperature driving force with the largest temperature cross in the unit. The temperature of the outlet of the hot stream can be cooler than the outlet temperature of the cold stream, see Figure 10-29 ... 

Eor one shell and multipass on the tube side, it is obvious that the fluids are not in true counter-current flow (nor co-current). Most exchangers have the shell side flowing through the unit as in Eigure 10-29C (although some designs have no more than two shell-side passes as in Eig-ures 10-IJ and 10-22, and the tube side fluid may make two or more passes as in Eigure 10-IJ) however, more than two passes complicates the mechanical construction. 

F = Correction factor to LMTD for counter-current flow for various mechanical pass configurations, see Figures lO-SfA-J. 

Note F = 1.0 for pure counter-current flow. As co-current flow increases in design arrangement (not flow rate), the F is reduced, and the exchanger efficiency Mis, to a usual practical lower limit of 0.75-0.80 ... 

ATi , = Log mean temperature difference for counter-current flow, °F. 

F = MTD correction factor, dimensionless, corrects log mean temperature difference for any deviation from true counter-current flow. 

Kolar and Broz (K4) have described a theoretical analysis of counter-current flow of liquid and gas through a packed bed. A relationship has been derived between holdup of liquid, flow rates of fluids, and physical properties of fluids. The relationship contains three parameters, the values of which must be determined by experiment. Experimental data are not presented. 

Describe the advantages and disadvantages of the following reactor types with reference to heat and mass transfer. For each reactor discuss one reaction for which it may be appropriate to use that reactor, (a) fluidized bed reactor, (b) A continuous counter-current flow reactor, (c) A monolith reactor. 

Counter-current flow is the most efficient method and the most commonly used. It will give the greatest concentration of the solute in the extract, and the least use of solvent. 

For a feed rate 2000 kg/h of solution, composition 30 per cent w/w MEK, determine the number of stages required to recover 95 per cent of the dissolved MEK using 700 kg/h TCE, with counter-current flow. 

Before equation 12.1 can be used to determine the heat transfer area required for a given duty, an estimate of the mean temperature difference A Tm must be made. This will normally be calculated from the terminal temperature differences the difference in the fluid temperatures at the inlet and outlet of the exchanger. The well-known logarithmic mean temperature difference (see Volume 1, Chapter 9) is only applicable to sensible heat transfer in true co-current or counter-current flow (linear temperature-enthalpy curves). For counter-current flow, Figure 12.18a, the logarithmic mean temperature is given by ... 

The usual practice in the design of shell and tube exchangers is to estimate the true temperature difference from the logarithmic mean temperature by applying a correction factor to allow for the departure from true counter-current flow ... 

Figure 12.18. Temperature profiles (a) Counter-current flow (/>) 1 2 exchanger (c) Temperature cross...

An economic exchanger design cannot normally be achieved if the correction factor Ft falls below about 0.75. In these circumstances an alternative type of exchanger should be considered which gives a closer approach to true counter-current flow. The use of two or more shells in series, or multiple shell-side passes, will give a closer approach to true counter-current flow, and should be considered where a temperature cross is likely to occur. 

When the fluid being vaporised is a single component and the heating medium is steam (or another condensing vapour), both shell and tubes side processes will be isothermal and the mean temperature difference will be simply the difference between the saturation temperatures. If one side is not isothermal the logarithmic mean temperature difference should be used. If the temperature varies on both sides, the logarithmic temperature difference must be corrected for departures from true cross- or counter-current flow (see Section 12.6). 

The temperature correction factor, Ft, will normally be higher with plate heat exchangers, as the flow is closer to true counter-current flow. 

Channels per pass 19 Thermal plates 21 Plates total Counter-current flow... 

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