Column sequence control

Figure 6. Column sequence control (for complex solvent sequences.)...

He) was introduced into the column, and the column pressure reached the adsorption pressure Pa. This process took about 5 seconds. Valve V-2 was then opened, and adsorption in the column from the inlet gas took place (adsorption step). During this period, the flow rate and concentration of CO, Gu and C/ were measured. (2) Valves V-1 and V-2 were closed, V-4 was opened, and the column was evacuated (desorption step). At the end of the desorption step, the pressure was below 13 Pa after 600 seconds. (3) As V-3 was also opened under evacuation, helium was supplied as a countercurrent puige to remove CO thoroughly (countercurrent purge step). (4) With V-3 closed and V-4 still open, the column was re-pressurized to the adsorption pressure with helium. The measurement conditions are summarized in Table 1, and the samples that were screened are listed in Table 2. The adsorption temperature was one parameter examined in this study, and a sequence controller was programmed for each set of conditions, so that steps (1) ( 4) were repeated over more than 4 hours. The total amount of desorbed gas was determined from the gas collected at the exit of the rotary flowm er. 

If one is going to control column sequence, this allows one to utilize complex solvent sequences as well (Figure 7). For example, for certain samples one will have to put on a sample after that separation is complete, one will have to flush that bed structure and then one will have to re-equilibrate that bed structure before the next sample can be added. If one uses a segmented column system, one can operate each column independently in terms of the operation sequence, and optimally get the downstream side of the system functioning at its maximum because, at each step, there will be a product coming out of the bed. 

The most effective control structure is one in which the column feed is used to control temperature while recycling products back to the upstream feed unit. This strategy can be applied to multiple column sequences in a plantwide environment. 

We will start this chapter by explaining how the heterogeneous azeotropic distillation works in separating a mixture with an azeotrope. Three systems with different RCMs will be used as examples to illustrate the column sequence for the separation. After that, we will focus on the detailed design and control of the isopropanol-water system, which is a system that exhibits the most complex RCM. 

In this chapter, design and control of the IPA dehydration process via extractive distillation will be studied. Since, in this distillation system, entrainer selection is an important step before working on the optimal design of the column sequence, we will start by comparing two alternative entrainers for this separation system in the following section. 

Two kinds of startup and shutdown are commonly encountered, depending on whether foe column base is full ( wet ) or empty ( dry ) when foe colunrn is shut down. As will be seen, wet column startups and shutdowns are much faster. The same basic override system permits automatic startups regardless of whether they are wet or dry. It should be noted that no formal program or sequence control is necessary. Instead, as soon as steam and feed are available, foe column comes on line at foe maximum speed permitted by foe constraints. Depending on circumstances, a column may or may not follow foe same sequence of constraints from startup to startup. 

The scan area is recognized as a sequence of points set out in rows and columns and detected in a raster-like marmer under adjustable computer control [3]. 

The Aromax process was developed in the early 1970s by Toray Industries, Inc. in Japan (95—98). The adsorption column consists of a horizontal series of independent chambers containing fixed beds of adsorbent. Instead of a rotary valve, a sequence of specially designed on—off valves under computer control is used to move inlet and withdrawal ports around the bed. Adsorption is carried out in the Hquid phase at 140°C, 785—980 kPA, and 5—13 L/h. PX yields per pass is reported to exceed 90% with a typical purity of 99.5%. The first Aromax unit was installed at Toray s Kawasaki plant in March 1973. In 1994, IFP introduced the Eluxyl adsorption process (59,99). The proprietary adsorbent used is designated SPX 3000. Individual on-off valves controlled by a microprocessor are used. Raman spectroscopy to used to measure concentration profiles in the column. A 10,000 t/yr demonstration plant was started and successfully operated at Chevron s Pascagoula plant from 1995—96. IFP has Hcensed two hybrid units. 

Sequential design. When a particular procedure is always executed in sequential order, for example, the start-up of a distillation column, a similar sequential arrangement of the controls will help to ensiure that parts of the sequence are not omitted. 

The embedded model approach represented by problem (17) has been very successful in solving large process problems. Sargent and Sullivan (1979) optimized feed changeover policies for a sequence of distillation columns that included seven control profiles and 50 differential equations. More recently, Mujtaba and Macchietto (1988) used the SPEEDUP implementation of this method for optimal control of plate-to-plate batch distillation columns. 

---