Distillation pressure control

Chin, T G., Guide to Distillation Pressure Control Methods, Hydrocarbon Processing, October 1979, p. 145. 

The feed flow is often not controlled but is rather on level control from another column or vessel. The liquid product flow s (distillate and bottoms) are often on level rather than flow control. Top vapor product is, however, usually on pressure control. The reflu.x is frequently on FRC, but also may be on column TRC or accumulator level. 

Factors of importance in preventing such thermal runaway reactions are mainly related to the control of reaction velocity and temperature within suitable limits. These may involve such considerations as adequate heating and particularly cooling capacity in both liquid and vapour phases of a reaction system proportions of reactants and rates of addition (allowing for an induction period) use of solvents as diluents and to reduce viscosity of the reaction medium adequate agitation and mixing in the reactor control of reaction or distillation pressure use of an inert atmosphere. 

But, suppose we must maintain a constant pressure in the wet-gas drum. The pressure in this drum may be controlling the pressure in an upstream distillation column. To hold a constant pressure in the drum, we will have to resort to spillback suction pressure control, illustrated in Fig. 28.5. 

Most distillation columns are operated under constant pressure, because at constant pressure temperature measurement is an indirect indication of composition. When the column pressure is allowed to float, the composition must be measured by analyzers or by pressure-compensated thermometers. The primary advantage of floating pressure control is that one can operate at minimum pressure, and this reduces the required heat input needed at the reboiler. Other advantages of operating at lower temperatures include increased reboiler capacity and reduced reboiler fouling. 

Optimization and vacuum control strategies (a) minimizing (floating) pressure by maximizing coolant valve opening, (b) floating pressure control of partial condenser with vapor distillate, (c) floating pressure control when the distillate is both vapor and liquid. 

The control of the separation section is presented in Figure 10.11. Although the flowsheet seems complex, the control is rather simple. The separation must deliver recycle and product streams with the required purity acetic acid (from C-3), vinyl acetate (from C-5) and water (from C-6). Because the distillate streams are recycled within the separation section, their composition is less important. Therefore, columns C-3, C-5 and C-6 are operated at constant reflux, while boilup rates are used to control some temperatures in the lower sections of the column. For the absorption columns C-l and C-4, the flow rates of the absorbent (acetic acid) are kept constant The concentration of C02 in the recycle stream is controlled by changing the amount of gas sent to the C02 removal unit The additional level, temperature and pressure control loops are standard. 

The gas and electrolyte mixture flows through two openings in the front plate and passes to separator where the gas is separated from the electrolyte. The gases are then washed with distilled water and led to the gas holders. The hot electrolyte flows to the coolers and after cooling passes to the lower channels of the electrolyzer and then to the individual chambers. The feeding water is brought by a funnel to the upper part of the separator where it is first used to wash the gas. The separating columns connected by an overflow pipe also act as pressure controllers. 

Dimethyl Tellurium Difluoride1 3 The apparatus shown in the Figure is set up. 4.7 g (30 mmol) of dimethyl tellurium and 175 ml of fluorotrichloromethane are placed in the 500 ml reaction flask which is cooled to — 78° in an ethanol/dry ice bath. The rotameter and pressure controls are set to deliver a fluorine/nitrogen (or argon) mixture (1/10, v/v). The gas mixture is passed at a flow rate not exceeding 100 m//min through a metal spiral immersed in a — 78° cooling bath and then into the stirred solution of dimethyl tellurium. The reaction is complete after 75 mmol of fluorine have been passed into the flask. Dimethyl tellurium difluoride precipitates as a white solid. The solvent is removed by vacuum distillation under an inert atmosphere and the moisture-sensitive residue is pure dimethyl tellurium difluoride yield 1.3 g (22%) m.p. 82° sublimes at 40D/0.001 torr. 

One of the challenging aspects of distillation column control is the many limitations imposed on the operation of the column. There are hydraulic constraints, separation constraints, heat-transfer constraints, pressure constraints, and temperature constraints. We recommend the excellent books by Kister (1992 and 1990) on distillation design and operation. 

Pressure control with vapor distillate product... 

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