Liquid circulation

Liquid is circulated through the column in order to heat up the column or to flush pockets of undesirable materials (e.g., water from a hydrocarbon column chemicals from a previous campaign) prior to feed introduction. The circulation liquid must he compatible with the chemicals which are normally in the column. Some key considerations are described below and illustrated in Fig. 11.8. Depending on the nature of the circulation, several of the guidelines in Sec. 11.6 also apply to liquid circulation. 

The circulating liquid must remain fluid without solidifying or becoming excessively viscous even at the lowest temperature expected at the coliunn internals before liquid circulation is conunenced. This is especially important in winter startups in cold climates. It may pay to warm up the colunm internals and piping by passing hot air or steam through the column just before liquid circulation is started. 

Alternatively (or additionally), a less viscous circulating liquid can be used. It is ofl n possible to use a low-viscosity, freeze-resistant liquid in the initial circulation steps, and to replace it by a compatible high-boiling, decomposition-resistant liquid in the hotter steps. 

Flush condenser if condenser circuit has low points. Ensure a sufficiently high circulation rate. 

When liquid circulation is used for column dehydration (in addition to heating), it is common to perform the heating and dryout in steps of cold, warm, and hot circulation. The cold circulation ( 100°F) sweeps water to low points, from where it must be drained. All the drains must be operated until no more water comes out any plugged drains must be unblocked. During the warm circulation, the temperature is gradually raised to about 180°F. This step melts any residual ice and vaporizes remaining pockets of water. 

---

Forced circ. One theoretical tray Handles high viscous solids-containing liquids Circulation controlled Higher transfer coefficient Highest cost with additional piping and pumps Higher operating cost Requires additional plant area... 

Liquid circulated in reboiler 4 vapor generated in reboiler 1... 

A common type of heat exchanger used in industrial ventilation is the plate fin-and-tube heat exchanger (Fig. 9.7). Liquid or gas flows in the tubes, with a gas or a liquid circulating outside the tubes between the plates. 

Jones, A.G., 1985a. Liquid circulation in a draft-tube bubble column. Chemical Engineering Science, 40, 449-462. 

Installation of a valve in the liquid circulation line as shown on the illustration can aid in overcoming instability and variations in reboiler duty. 

In an airlift fermenter, mixing is accomplished without any mechanical agitation. An airlift fermenter is used for tissue culture, because the tissues are shear sensitive and normal mixing is not possible. With the airlift, because the shear levels are significantly lower than in stirred vessels, it is suitable for tissue culture. The gas is sparged only up to the part of the vessel cross section called the riser. Gas is held up, fluid density decreases causing liquid in the riser to move upwards and the bubble-free liquid to circulate through the down-comer. The liquid circulates in airlift reactors as a result of the density difference between riser and down-comer. 

In a humidifier in which the make-up liquid is only a small proportion of the total liquid circulating, its temperature approaches the adiabatic saturation temperature 0S, and remains constant, so that there is no temperature gradient in the liquid. The gas in contact with the liquid surface is approximately saturated and has a humidity Jf... 

Cavitations generate several effects. On one hand, both stable and transient cavitations generate turbulence and liquid circulation - acoustic streaming - in the proximity of the microbubble. This phenomenon enhances mass and heat transfer and improves (micro)mixing as well. In membrane systems, increase of fiux through the membrane and reduction of fouling has been observed [56]. 

Due to the liquid circulation in stirred tanks which transports all particles with a certain frequency through the impeller zone, they undergo the maximum shear stress. 

Gas holdup and liquid circulation velocity are the most important parameters to determinate the conversion and selectivity of airlift reactors. Most of the reported works are focused on the global hydrodynamic behavior, while studies on the measurements of local parameters are much more limited [20]. In recent years, studies on the hydrodynamic behavior in ALRs have focused on local behaviors [20-23], such as the gas holdup, bubble size and bubble rise velocity. These studies give us a much better understanding on ALRs. 

The energy of liquid circulation pump was calculated by Eq. (1), and the energies of blower and vacuum pump were calculated by Eq. (2). 

Fig.6 and Fig.7 illustrate the effect of draft-tube diameter on liquid superficial velocity, liquid circulating flowrate and gas hold-up. Results show that the liquid superficial velocity in the riser increases with increasing the draft-tube diameter while the liquid velocity in the... 

In the case of the full bubble column, the bottom gas phase mixed region is coincident with the impeller, and no significant gas mixing occurs below the impeller, in the lower region of the tank, as shown in Fig. 5.125. The dashed arrows in Fig. 5.125 and Fig. 5.126 represent mass transfer interchange between the gas space and liquid volumes of the tank. The liquid circulation, which is not shown, is identical to Fig. 5.124. 

Kettle reboilers have lower heat-transfer coefficients than the other types, as there is no liquid circulation. They are not suitable for fouling materials, and have a high residence time. They will generally be more expensive than an equivalent thermosyphon type as a larger shell is needed, but if the duty is such that the bundle can be installed in the column base, the cost will be competitive with the other types. They are often used as vaporisers, as a separate vapour-liquid disengagement vessel is not needed. They are suitable for vacuum operation, and for high rates of vaporisation, up to 80 per cent of the feed. 

Freedman, W., and Davidson, J. F., Hold-Up and Liquid Circulation in Bubble Columns, Trans. Instn. Chem. Engrs., 47 T251 (1964)... 

Miyahara, T., Hamaguchi, M., Sukeda, Y., and Takahashi, T., Size of Bubbles and Liquid Circulation in a Bubble Column with a Draft Tube and Sieve Plate, Can. J. Chem. Eng., 64 718 (1986)... 

The thermostatting systems that are applied maximize the Joule heat dissipation through cooling either air (forced air convection or with a Peltier device) or liquid (Peltier device) circulation. In most of the comparisons of temperature control by air and liquid circulation, it was shown that liquid-thermostatting systems perform best [31,77]. 

Equations (89)-(92) have been verified by the authors, both for the final overall values and for the influence of individual variables. Their typical results for inviscid fluids are presented in Fig. 15, which shows a good agreement between the theoretical and the experimental values. The authors have obtained higher bubble volumes than those of Davidson and Schuler (D9) under otherwise similar conditions. This is probably due to absence of liquid circulation in the present condition. 

Liquid circulation from the reactor to the heat exchanger is flow-controlled. Condensate level in the condensate drum is controlled by manipulating BFW (boiler feed water). 

The air-lift consists of two pipes, intercoimected at top and bottom. In one of the pipes (the riser) air is sparged at the bottom. The air rises and escapes at the top. Therefore, nnder most circnmstances there is no air present in the other pipe (the downcomer). The density difference between riser and downcomer canses an intensive liquid circulation. Two designs can be used, i.e., the internal (Figure 11.8A) and the external loop reactor (Figure 11.8B). 

Since the liquid circulates, there must be certain locations in the cell at which the forward and backward flows of the liquid are equal. An alternative to the averaging procedure suggested in the example is to do the particle tracking at a location at which the medium experiences no net flow. 

---