External heat exchangers

Another method for increasing heat transfer area is to circulate, usually by pumping, the process liquid from the reactor through an external heat exchanger. The heat exchanger 

X iterative loop to find diameter using direct substitution dguess= (2 vr/pi) 0.3333 eirror=10 while ercooO. 011 1 vesse 1 2Mguess  

However, there is an important dynamic effect as the size of the heat exchanger is increased. The larger holdup in the heat exchanger introduces more dynamic lag in the heat transfer process, which could degrade dynamic performance. We observed this in the jacket-cooled system discussed in Section 3.1.5. The smaller the thickness of the jacket, the better the temperature control. 

The external heat exchanger is assumed to be tube-in-shell with 0.0254-m-diameter tubes, 5 m in length. Given a desired total heat transfer area, the number of tubes and 

Volume per tube = Heat-transfer area per tube = Number of tubes = 

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Process. A typical indirect hydration process is presented in Eigure 1. In the process, propylene reacts with sulfuric acid (>60 wt%) in agitated reactors or absorbers at moderate (0.7—2.8 MPa (100—400 psig)) pressure. The isopropyl sulfate esters form and are maintained in the Hquid state at 20—80°C. Low propylene concentrations, ie, 50 wt %, can be tolerated, but concentrations of 65 wt % or higher are preferred to achieve high alcohol yields. Because the reaction is exothermic, internal cooling coils or external heat exchangers are used to control the temperature. 

If severe heat-transfer requirements are imposed, heating or cooling zones can be incorporated within or external to the CSTR. For example, impellers or centrally mounted draft tubes circulate Hquid upward, then downward through vertical heat-exchanger tubes. In a similar fashion, reactor contents can be recycled through external heat exchangers. 

Fig. 9. Methods of heating tanks (a) external heat exchanger (b) serpentine steam cods and (c) plate cods.

Internal Coil or Jacket Plus External Heat Exchange. 11-19... 

External Heat Exchanger Isothermal Heating Medium... 

Another procedure, which is more accurate for the external-heat-exchanger cases, is to nse an equivalent value for MC (for a vessel being heated) derived from the following energy balance ... 

Interna] Coil or Jacket Plus External Heat Exchanger This case can be most simply handled by treating it as two separate problems. M is divided into two separate masses Mi and (M — Mi), and the appropriate equations given earlier are apphed to each part of the system. Time 0, of course, must be the same for both parts. 

When the solute has a large heat of solution or when the feed gas contains high percentages of the solute, one should consider the use of internal coohng coils or intermediate external heat exchangers in a plate-type tower to remove the heat of absorption. In a packed tower, one could consider the use of multiple packed sections with intermediate hquid-withdrawal points so that me hquid coiild be cooled by external heat exchange. 

FIG. 23-1 Heat transfer to stirred tank reactors, a) Jacket, (h) Internal coils, (c) Internal tubes, (d) External heat exchanger, (e) External reflux condenser. if) Fired heater. (Walas, Reaction Kinetics for Chemical Engineers, McGraw-Hill, 1959). 

The fuel is usually natural gas. The energy liberated by reaction may be directly recovered in the process or indirectly recovered by suitable external heat exchange (see Fig. 25-14). 

FIG. 27-48 Simplified flow diagram for circulating AFBC (with external heat exchanger). 

Reactors may be operated batchwise or continuously, e.g. in tubular, tubes in shell (with or without internal catalyst beds), continuous stirred tank or fluidized bed reactors. Continuous reactors generally offer the advantage of low materials inventory and reduced variation of operating parameters. Recycle of reactants, products or of diluent is often used with continuous reactors, possibly in conjunction with an external heat exchanger. 

Fig ure 6-9. An autothermal reactor with an external heat exchanger. 

BATCH HEATING EXTERNAL HEAT EXCHANGER, ISOTHERMAL HEATING MEDIUM... 

When cooling a batch with an external heat exchanger and an isothermal cooling medium, the equation is ... 

Calculate the time required to cool the batch described in Example 7-12 if an external heat exchanger with a heat transfer surface of 200 ft (18.58 m ) is available. The batch material is circulated through the exchanger at the rate of 25,000 Ib/hr (11,339.8 kg/hr). The overall... 

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