Recirculation ratio

JCj is the local recirculation ratio equal to q airrec/ fairexh Jairrec rate through the unit (cleaner), m s ... 

The recirculation ratio for a unit is the lb rate of liquid leaving the outlet compared to the lb rate of vapor leaving. The liquid recirculation flow rate entering the unit is set by the differential pressure driving the system. 

The tubes are usually 1 V4 -in. O.D. but never smaller than 1-in. O.D. because the flow contains vapor as well as liquid. The recirculation ratio i.e., liquid-to-vapor ratio in the outlet, is seldom less than 5 and more often is 10-15, sometimes reaching 50. 

Note that for preliminary calculations, the frictional resistances in piping can be neglected but should be included in final calculations, particularly at high recirculation ratios. 

If the driving force. Step 12, does not equal or slightly exceed the total of resistances in Step 11, the unit should be rebalanced that is, shorter tubes used to give less pressure drop, lower recirculation ratio used to give less pressure drop, or a larger number of total tubes used to give less pressure drop. The liquid can be raised... 

Because the resistances are now considerably less than the available driving force, the new estimate for the recirculation ratio must be considerably greater than 20 1. 

After several trials to obtain a balance, try a recirculation ratio of 73 1 and an exchanger with 178, 12 BWG, 8-ft long tubes. 

With the driving force being slightly greater than the total resistance to flow, the recirculation ratio will be greater than the trial value of 73. For purposes of design, this is a satisfactory basis to proceed. 

This compares to assumed value of 0.001 + 0.001 = 0.002 The Kern method is usually easier to handle for pressure systems than for vacuum systems. The recirculation ratio is higher and, therefore, requires more trials to narrow-in on a reasonable value for the low pressure systems. The omission of two-phase flow in pressure drop analysis may be a serious problem in the low pressure system, because a ratio on the high side may result, causing a high hj value. In general, however, for systems from atmospheric pressure and above, the method usually gives conservative results when used within Kern s limitations. 

Calculate the liquid flow rate leaving the reboiler for the vapour rate and recirculation ratio. 

The amount of liquid vaporized in the reboiler should not be more than 80%, otherwise this will tend to lead to excessive fouling of the reboiler. For kettle reboilers, there is no recirculation. But for thermosyphon reboilers, a recirculation ratio can be defined as ... 

This usually lies between 0.25 and 6. The greater the value of recirculation ratio, the less fouling there is in the reboiler. Lower values tend to be used in horizontal thermosyphons and higher values (greater than 4) used in vertical thermosyphons. The recirculation ratio is a degree of freedom at the discretion of the designer. This should be fixed later when the detailed design is carried out. 

The mean temperature difference should be less than 35 to 55°C. This will avoid excessive fouling and excessive vaporization per pass (i.e. low recirculation ratio), leading to poor heat transfer in the upper parts of the tubes as heat transfer to a liquid annulus is replaced by heat transfer to a mist. 

A) Determine the recirculation ratio required to give 90% efficiency ... 

A trickling filter is designed to treat 435 m /day of acid waste stream generated from a synthetic organic chemical plant. The BOD5 of the acid waste before and after the primary sedimentation is 3250 mg/L and 2850 mg/L, respectively. The efficiency of the filter at a recirculation ratio of 4.5 is 92%. If the depth of filter is restricted to 1.6 m and the value of the constant in Eckenfelder s equation is n = 0.5, determine the value of constant assuming the hydraulic loading rate = 17.5 m /m day. 

We have proposed a fluidized bed type process, which can be applied to phosphate removal from wastewater containing phosphate 2-23 mg/jg as P.By the results of experiments using equipment of capacity l-4m3 /day, factors such as supersaturation, recirculation ratio and space velocity were recognized to affect crystallization rate or phosphate removal efficiency. By mathematical analysis, we could obtain the characteristic equation for fluidized bed process, to agree well with experimental results. 

Figure 8 shows effect of recirculation ratio and space velocity. Horizontal line shows total PO4 as P in the influent and vertical line does total phosphate in the effluent. Black circles were data obtained from the experiments in case that recirculation ratio was 0 and space velocity 38 1/h. White circles were obtained, in case that recirculation was 2 and space velocity 13 1/h. Solid line and dotdash line shows theoretical values calculated from eq.(8), to agree well with experimental results. Increasing in recirculation ratio and decreasing in space velocity was recognized to improve phosphate removal efficiency. 

We have proposed fluidized bed process and by the results of experiments, factors such as supersaturation, recirculation ratio and space velocity affected crystallization rate or phosphate removal efficiency, and experimental results agreed well with calculated values from characteristic equation. 

Figo8 Effect of recirculation ratio and space velocity... 

In recirculation reactors, a recirculation ratio Rc (ratio of the total flow returned to the inlet and the total outlet flow) larger than 20 is frequently recommended [11, 16]. This criterion is not complete as is apparent from the literature [17-19]. One can demand that the rate over the catalyst bed may not change more than 5%, thus for an nth order isothermal reaction,... 

This implies that the necessary recirculation ratio is not a fixed value, but depends on the reaction under consideration and the conversion level. At low conversions, the recirculation rate need not be high according to this criterion (good mixing, eq 5, is still required to avoid dead zones in the reactor), and a differential PFR model can be used. At high conversions, the recirculation rate must increase. It can easily be seen that a recirculation ratio of 20 limits the conversion for a first-order and a second-order reaction to 50% and 25%, respectively. 

The problem in the verification of these criteria is the calculation of the recirculation ratio. No general relationship is valid, and it must be estimated for each reactor configuration by experimentation (see Refs 16 and 18). 

In CSTRs one has one degree of freedom more available to minimize the temperature gradients, the recirculation ratio Rc. Increasing the total flow rate through the bed enhances the rate of heat supply or removal. If the temperature gradient over the bed is only affected by the heat production (consumption) through reaction and heat removal (supply) by the total flow through the bed, the criterion for sufficient isothermal behavior (less than 5% deviation in reaction rate, eq 6) is... 

External recirculation, also represented schematically in Fig. 7.18, is an alternative to achieve perfect mixing, which is convenient when only gas streams are involved. The minimum external recirculation ratio depends on the reaction... 

A municipal wastewater having a BOD of 250 g/m3 is to be treated by a two-stage trickling filter. The desired effluent quality is 25 g/m3 of BOD. If both of the filter depths, De, are to be 1.83 m and the recirculation ratio, R, is 2 1, find the required filter diameters. Assume the following design assumptions apply. 

The experiments with the inert tracer may only show that the time, necessary for the fluid in the reactor to be well mixed, is much smaller than the average residence time. When a chemical reaction takes place, an additional time-scale, the time constant of the chemical reaction, appears. This time characterizes the reaction rate and can be defined as the time in which the reaction proceeds to a certain conversion, say 50%. For many practical heterogeneous catalytic reactions, the reaction time is so short that reactants entering the reactor may be converted without being mixed, for example, during the first cycle. For such fast reactions, of course, the reactor cannot be considered as gradient-free, whatever the recirculation ratio is. 

The found inequalities show that the minimum recirculation ratio depends strongly on the conversion achieved and the kinetics of the reaction. As an example we will apply... 

The criteria found for power law reactions show that the recirculation ratios of an order of 10, which are usually sufficient to observe ideal mixing behavior in tracer experiments, may not be satisfactory under reaction conditions. For example, assume that isothermal conditions in the bed have been achieved and the error due to a concentration nonuniformity should not exceed 5% (6 = 0.05). In this case the required recirculation ratio is determined from... 

Thus, if the total reactor conversion is 90% the minimum recycle ratio for a first-order reaction should be at least 90 and for a second order reaction 180. This simple example demonstrates that large conversions in recycle reactors are as unfavorable as very small conversions in a packed bed differential reactor. If one is interested in the reaction rate at small concentrations of the reactants, these concentrations in the recycle reactor should be achieved by small or moderate inlet concentrations but not through a large conversion. Large recirculation ratios can be achieved by decreasing of the bed depth. 

The total amount of air introduced into the flotation tank comes from the air saturation tank and from the influent feed. Because the pressure in the influent feed is the same atmospheric pressure in the flotation tank and, because the temperature in the flotation tank may be assumed equal to the temperature of the influent feed, any air that happens to be with the influent does not aid in floating the solids. This source of air may therefore be neglected. Letting R be the recirculation ratio and g be the influent flow, the amount of air introduced into the flotation tank A is... 

---