On temperature control

Current Development. Following success of the prototype IPU a second more comprehensive facility was commissioned. This is capable of up to four pumps of mixed peristalitic or diaphragm types, each linked to specific feed vessels on individual balances. The whole is interfaced to an IBM AT computer (see Figure 7) which in addition to Intelligent liquid additions, has the capacity to absorb modules from the work on temperature control and stirring in a full multi-tasking computer-assisted system, as mentioned above. 

Figure 3.10. Condensers, (a) Condenser on temperature control of the PF condensate. Throttling of the flow of the HTM may make it too hot. (b) Condenser on pressure control of the HTM flow. Throttling of the flow of the HTM may make it too hot. (c) Flow rate of condensate controlled by pressure of PF vapor. If the pressure rises, the condensate flow rate increases and the amount of unflooded surface increases, thereby increasing the rate of condensation and lowering the pressure to the correct value, (d) Condenser with vapor bypass to the accumulator drum. The condenser and drum become partially flooded with subcooled condensate. When the pressure falls, the vapor valve opens, and the vapor flows directly to the drum and heats up the liquid there. The resulting increase in vapor pressure forces some of the liquid back into the condenser so that the rate of condensation is decreased and the pressure consequently is restored to the preset value. With sufficient subcooling, a difference of 10-15 ft in levels of drum and condenser is sufficient for good control by this method.

On temperature control of the vapor leaving the reboiler or at some point in the tower,... 

In all these cases the reflux rate is simply set at a safe value, enough to nullify the effects of any possible perturbations in operation. There rarely is any harm in obtaining greater purity than actually is necessary. The cases that are not on direct control of reflux flow rate are (g) is on cascade temperature (or composition) and flow control, (h) is on differential temperature control, and (i) is on temperature control of the HTM flow rate. 

The heat exchange medium may be liquid or gaseous with the tolerance on temperature control given as 0.5°C, which is not particularly easy to achieve with simple apparatus. 

The final subject discussed in this chapter is the issue of reactor scaleup. Moving from a laboratory test tube in a constant temperature bath to a 20-L pilot plant reactor to a 200,000-L commercial plant reactor involves critical design and control decisions. One major problem is the reduction of the heat transfer area relative to the reactor volume (and heat transfer duty) as we move to larger reactors. This has an important effect on temperature control and reactor stability. 

Figure 3.77 Running relay-feedback test on temperature controller.

An increase in temperature is accompanied by a decrease in viscosity. For Newtonian materials, this relationship can be approximated to the Arrhenius equation. To obtain an accuracy of 1% in the measurements of the viscosity of water requires a temperature control of 0.3°C. The temperature dependence increases with an increase in viscosity. Thus, the demands on temperature control are even higher for more viscous materials. The shearing of the samples itself also generates heat. To ensure that the heat is not effective in altering the temperature of the sample and equipment, the heat has to be removed quickly. 

Anticholinergic drugs clearly may cause problems in patients with closed-angle glaucoma (or a narrow angle between the iris and cornea), paralytic ileus, pyloric stenosis, or urinary retention. Because of their effects on temperature control they may be undesirable in patients with pyrexia (especially children) and during very hot weather. 

For example, assume that you want to perform tests on the plant, represented by Figure 15.74. The plant is a simple distillation column with overhead accumulator pressure controlled by moving the hot vapor bypass, bottoms level maintained by bottoms product draw rate, and the overhead accumulator level controlled by adjusting the overhead product draw rate. Reflux is on flow control, and the reboiler is on temperature control. Typical move sizes for this plant are shown in Table 15.12. 

F. Roozeboom and N. Parekh, Rapid Thermal Processing Systems A Review with Emphasis on Temperature Control, / Vac. Sci. Technol. B., 8, pp. 1249-1259,1990. 

A research program based on temperature control was devised. I designed the equipment and supervised the operation. It took three years of steady work on nickel catalyst to prove to my satisfaction that it could be brought back to activity by regeneration in situ, and consequently a large plant was built. The plant performed as expected, but the discovery of large quantities of crude oil rendered uneconomic this plant and all others making synthetic liquid fuels from coal. 

For an example, size a reflux accumulator for a depropanizer to be installed in an existing large refinery gas plant. Assume that 900 gpm of reflux is pumped back on temperature control. Also, 500 gpm of product propane is fed to a new ethylene unit. Product flow is on level control with unit alarm. 

Most column control schemes (see Sec. 16.5) use the composition (or temperature) controller to manipulate either the reflux or reboil, directly or indirectly. The stream which is not controlled is commonly "free, i.e., on flow control. This "free stream is usually manipulated during flood testing, while the stream on temperature control will be automatically adjusted to maintain product composition. For instance, if reflux rate is on temperature control and reboil rate is on flow control, flood testing is performed by raising the reboil rate. This warms up the control tray and increases condensation. The temperature controller will call for more reflux, and the column will reach new stable conditions with both reboil and reflux increased. 

---