Distillation columns quality control

In the first approach, we may consider a classical standalone control structures, as displayed in Fig. 13.5. Reactor feed is on flow control (PI), and outlet stream on level control (PI). For the distillation column a classical inventory control is column pressure with condenser cooling (PI), base level with bottom product (P), and reflux drum level with distillate (P). Quality control loops are top composition with reflux and bottom composition with reboiler duty, both as PI controllers. 

The product flow of a distillation column that is controlled by an energy balance control scheme may fluctuate, since it is controlled by a level controller consequently it will affect the downstream process unit. If a distillation column is controlled by a material balance control scheme, the distillate flow is affected by the slow quality control loop and disturbances will be smoothed and only partially propagated to downstream process units. Therefore, when the product flow is not allowed to fluctuate or if the one product flow is much larger than the other, a material balance control scheme is selected. 

Devise a control system for the distillation column described in Chapter 11, Example 11.2. The flow to the column comes from a storage tank. The product, acetone, is sent to storage and the waste to an effluent pond. It is essential that the specifications on product and waste quality are met. 

Since we have two control degrees of freedom, our objectives in distillation are to control the amount of LK impurity in the bottoms product ( b.lk) and the amount of HK impurity in the distillate ( 5>Hk) Controlling these compositions directly requires that we have composition analyzers to measure them. Instead of doing this, it is often possible to achieve fairly good product quality control by controlling the temperature on some tray in the column and keeping one manipulated variable constant. Quite often the best variable to fix is the reflux flowrate, but other possibilities include holding heat input or reflux ratio constant. 

We briefly discussed in Chap. 2 (Sec. 2.5) one important aspect of distillation column control when one of the products leaving the column is a recycle stream. Should the composition of this recycle stream be controlled Probably not from the perspective of the isolated column because the control loop holding the composition of the other stream leaving the column, which is a product from the plant, could then be more tightly tuned. However, the plantwide control perspective may show that the performance of the reactor can be improved by holding the purity of the recycle stream more constant, and this could result in smaller disturbances to the column. The overall effect may be better product quality control even though the product-quality loop is less tightly tuned. 

In this chapter we have presented some fundamental concepts of distillation control. Distillation columns are without question the most widely used unit operation for separation in the chemical industry. Most final products are produced from one end or the other of a distillation column, so tight control of product quality requires an effective control system for the column. However, the column is usually an integral part of an entire plant, so its control scheme must also be consistent with the plantwide control structure. 

Step 5. The distillate stream from the product column is salable benzene. Benzene quality can be affected primarily by two components, methane and toluene. Any methane that leaves in the bottoms of the stabilizer column contaminates the benzene product. The easy separation in the stabilizer column allows us to prevent this by using a temperature to set column steam rate (boilup). Toluene in the overhead of the product column also affects benzene quality. In this column the separation between benzene and toluene is also fairly easy. As a result, we can control product column boilup by using a tray temperature. To achieve on-aim product quality control, we most likely would use an on-line overhead composition analyzer to adjust the setpoint of this temperature controller,... 

The flowsheet is completed with P-type controllers for level in the reflux drum and bottoms, and PI controller for pressure. These controllers ensure the basic Inventory control, but are not sufficient for quality control. Therefore, we are interested by distillate flow rate and purity faced with disturbances in the feed. Fig. 4.7 presents the open loop response to feed variation of +/- 10%. Increasing the feed to 110 kmol/hr gives an increase in purity over 99%, but a decrease of the distillate rate to less than 47.5 kmol/hr. After reset to initial conditions, the feed is reduced to 90 kmol/h. This time the distillate rate increases at 52.5 kmol/hr, but the purity drops dramatically to 86%. This behaviour seems somewhat strange, so the reader is encouraged to find a physical explanation. The need for quality control in a distillation column is obvious. This issue will be treated in the Examplel2.2. 

Controlled variables (outputs). The selection of the controlled variables should be based on process engineering judgement, as well as the type and location of the measurements. In this case the controlled variables refer to quality control, as the purity of products from distillation columns, or having a plantwide character, as the concentration of key impurities on selected process streams, or temperatures regarding the reaction system, etc. 

Manipulated variables (inputs). These are available degrees of freedom left after considering the basic control. The selection of manipulated variables could also be the object of a local controllability analysis. A typical example is the use of SVD for the selection of the sensitive stage for inferential quality control in a distillation column. 

Let s consider a simple one-feed two-product distillation column. When examined as stand-alone item with a fixed feed, its control is typically a 5x5 multivariable problem. Among the five controlled variables three are for the basic inventory pressure, level in reflux drum and reboiler. The two remaining are for quality eontrol purity of distillate and bottom products. The five manipulated variables are distillate flow D, bottom product B, boilup V or reboiler duty Qr, condenser duty and reflux flow L. The combination of controlled and manipulated variables may lead to several control structures. Here we present some typical situations useful for dynamic simulation. 

Let s take a look at the control loops. Now the make-up stream of reactant B is fed on level controller LCf of the buffer tank. The flow rate of the exit stream (Recycle) is set at a constant value by a simple specification on the stream s script. There are also two level controllers LCt and LCb for the top and bottom inventories of the distillation column, which manipulate the distillate and bottom products, respectively. Besides, the top column pressure is kept constant by means of the condenser duty. Quality control is implemented only for the bottom product, the reflux being fixed. We considered a composition measure with a first-order lag transmitted to a controller that manipulates the reboiler duty. 

The quality of the intermediate DCE must fulfil strict purity specifications. Low impurity levels imply high energetic consumption, but higher impurity amounts are not desired for operation. The intermediate DCE is conditioned mainly in the distillation column S2. In the bottom product the concentration of the two bad impurities E and E must not exceed an upper limit, of 100 and 600 ppm, respectively, while the concentration of the good impurity E must be kept around optimal value of 2000 ppm. Because these impurities are implied in all three reaction systems through recycles that cross in the separation system, their inventory is a plantwide control problem. The problem is constraint by technological and environmental constraints, as mentioned. 

An analysis of the degrees of freedom indicates as a first choice manipulated variables belonging to the column S2, used for quality control D2- distillate flowrate, SS2 - side stream flowrate, and Q2 - reboiler duty. We may also consider manipulated variables belonging to the column S4, adjacent and connected with S2 by a recycle, but dynamically much faster. Thus, supplementary outputs are D4-distillate flow rate, and Q4-reboiler duty. Hence, the inputs are the variables D2, SS2, Q2, D4 and Q4. 

An operator requests that the logic of a control loop on a distillation column be changed in order to minimize product quality variations. 

There is only one reactant that is fed to the column. The two products are removed out of the two ends of the column. Olefin metathesis is an example of this type of reactive distillation column. Figure 9.2 illustrates this system and gives an effective control scheme. A C5 olefin reacts to form a light C4 olefin, which is removed in the distillate, and a heavy C6 olefin, which is removed in the bottoms. The two temperature controllers are used to maintain conversion and product quality. The production rate is set by a feed flow controller. 

One of the best ways of improving product quality at both ends of the distillation column is to reduce variability, as shown in Figure 7.5 that will reduce the failure rate. Robinson described the benefits from improved dryer control, such as reduced variability in product moisture content and reduced energy consumption because the average concentration of moisture in the product could be run closer to the upper specification limit. 

Van Kampen, J. A. Automatic Control by Chromatograph of the Product, Quality of a Distillation Column, Convention on Advances in Automatic Control, Nottingham, England, April, 1965. 

Betlem, B.H.L. (2000) Batch distillation column low-order models for quality control. Chemical Engineering Science, 55, 3187-94. 

One conld also try to estimate the quality ( inferential control ) by using a suitable algorithm that uses conventional measured values of for example temperatme, pressure, etc. A simple example is the pressure compensation of temperatme measurements on the tray of a distillation column. For binary mixtures this yields a measure of the composition, for a multi-component system it is an approximate measure of the composition. 

The purity can be expressed in the concentration of the key component the heavy key component in the top product and the light key component in the bottom product. Let us assume that there is a strict quality requirement for the top product purity, which can be met by the right operation of the distillation column. There are no demands on the bottom quality however, there is the potential that some valuable top product disappears with the bottom product. Consequently, we would like to control the bottom temperature, which is to some extent an indicator of the bottom purity. 

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