Steady States in Distillation

Guttinger T.E. and Morari M. (1997). Predicting multiple steady states in distillation singularity... 

Extended life for the zeolite Beta catalyst has been demonstrated in this work using the same, or similar, crude tBA feedstocks to those employed in Table 1. Isobutylene generation has been monitored over ca. 500 to 1000 hours of service, under steady state reactive distillation conditions, without significant losses in activity or changes in product compositions. 

In a steady state continuous distillation with the assumption of a well mixed liquid and vapour on the plates, the holdup has no effect on the analysis (modelling of such columns does not usually include column holdup) since any quantity of liquid holdup in the system has no effect on the mass flows in the system (Rose, 1985). Batch distillation however is inherently an unsteady state process and the liquid holdup in the system become sinks (accumulators) of material which affect the rate of change of flows and hence the whole dynamic response of the system. 

Simulation, Modeling, and Design Feasibility Because reaction and separation phenomena are closely coupled in a reactive distillation process, simulation and design are significantly more complex than those of sequential reaction and separation processes. In spite of the complexity, however, most commercial computer process modeling packages offer reliable and flexible routines for simulating steady-state reactive distillation columns, with either equilibrium or kinetically controlled reaction models... 

Gorak, A., Simulation Methods for Steady State Multicomponent Distillation in Packed Columns, The Institution of Chemical Engineers Symposium Series No. 104, Distillation and Absorption 1987, A413-A424 (1987). 

This operating condition may be approached in a number of ways. A continuous distillation column may be placed on total reflux operation by setting the external rates equal to zero. This type of operation is commonly used in the startup of either a continuous or a batch distillation column. After a given quantity of feed has been introduced into the column, it may be allowed to come to a steady state in which no streams enter or leave the column. As will be demonstrated below, the precise steady state which is reached depends upon the holdup on each plate as well as the holdups in the accumulator and the reboiler. 

Chen, R, et al.. Multiple steady states in reactive distillation Kinetic effects. Computers and Chemical Engineering, 2002, 26(1) 81 93. 

The design equations for a single-feed distillation column using these variables have been formulated by Barbosa and Doherty (1988a,b,c) (see also Rev, 1994). The procedure has been extended to multiple reactions by several authors (see, e.g., Ung and Doherty, 1994a,b,c, and Kolah et al., 1996). The appearance of multiple steady states in the solution of the DCR equations has also been considered (Huan et al., 1995) in methyl-/ert-butyl ether production. 

Guttinger T.E. and Morari M. (1999a). Predicting multiple steady states in equilibrium reactive distillation. 1. Analysis of non-hybrid systems. Industrial and Engineering Chemistry Research 38 (4), 1633-1648. 2.1, 2.9, 2.9.1.1, 2.9.2, 4.2... 

Higler A., Taylor R. and Krishna R. (19996). Nonequilibrium modelling of reactive distillation mutiple steady states in MTBE synthesis. Chemical Engineering Science 54, 1389-1395. 2.9, 2.9.1.2, 2.9.3,... 

Flow rates of isoamylene fraction, methanol and TAME fraction are measured using tensiometer balances, and reflux and distillate flow rates using a Coriolis-type flowmeter. The samples are analysed off line. Process data, such as temperatures, flows, levels and pressures are stored and visualised by a monitoring system based on microprocessor controller MSC68. This gives a way to identify steady states in the column. 

Figures 12.13 and 12.14 give responses for feed flowrate and feed composition disturbances. Stable regulatory control is shown with product purities held close to their specifications for all of these large disturbances. Figure 12.13 gives the responses of the system for step changes in the setpoint of the feed flow controller at 0.2 h. The solid lines are for a 10% increase, the dashed lines are for a 20% increase, and the dotted lines are for a 20% decrease. Increasing the feed results in increases in both distillate product streams, as expected. The two temperatures in the extractive column are held at their setpoint values after a short transient period, as is the average temperature in the solvent recovery column. The purities of the two products are held quite close to their desired values. The largest offset in the acetone product purity occurs for the 20% decrease in feed flowrate where it drops to 99.4 mol%. The system comes to a new steady state in less than 2 h.

The previous section showed no surprises regarding the effect of tray holdup. In this section we look at changing the number of reactive trays. Intuition would lead us to think that the more trays the better. This is certainly the case in conventional distillation. However, as we will see, this is not the case with a steady-state reactive distillation column for this type of reaction (two reactants, two products). 

RGA Example In order to illustrate use of the RGA method, consider the following steady-state version of a transfer function model for a pilot-scale, methanol-water distillation column (Wood and Berry, Terminal Composition Control of a Binaiy Distillation Column, Chem. Eng. Sci, 28, 1707, 1973) Ku = 12.8, K = -18.9, K. i = 6.6, and Koo = —19.4. It follows that A = 2 and... 

In operation, a batch of liquid is charged to the pot and the system is first brought to steady state under total reflux. A portion of the overhead condensate is then continuously withdrawn in accordance with the established reflux pohcy. Cuts are made by switching to alternate receivers, at which time operating conditions may be altered. The entire column operates as an enriching section. As time proceeds, composition of the material being distilled becomes less rich in the more volatile components, and distillation of a cut is stopped when accumulated distillate attains the desired average composition. 

Consider azeotropic distillation to dehydrate ethanol with benzene. Initial steady-state conditions are as shown in Fig. 13-108. The overhead vapor is condensed and cooled to 298 K to form two hquid phases that are separated in the decanter. The organic-rich phase is returned to the top tray as reflux together with a portion of the water-rich phase and makeup benzene. The other portion of the water-rich phase is sent to a stripper to recover organic compounds. Ordinarily, vapor from that stripper is condensed and recycled to the decanter, but that coupling is ignored here. 

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