Composition Analyzers

A number of composition analyzers used for process monitoring and control require chemical conversion of one or more sample components preceding quantitative measurement. These reactions include... 

In the dry process the limestone and clay materials are crushed and stored in separate bins and their composition analyzed. After the composition is known, the contents of the bins are blended to achieve the desired ultimate cement characteristics. The blend is ground to a mesh size of 100-200. This small mesh size maximizes the contact between individual particles. 

Mantle reservoirs. The only quasi-systematic studies of igneous materials have centered on the mantle in particular mid-ocean ridge basalts (MORE), ocean island basalts, and mantle peridotites. After reporting one MORE analysis in Chan and Edmond (1988), the first full study of MORE (Chan et al. 1992) reported three apparently imaltered Atlantic basalts and one from the East Pacific Rise, with a range in 8 Li of +3.4 to +4.7 (Fig. 5). Subsequent studies have increased the global range of samples, the diversity of bulk compositions analyzed. 

To illustrate the concept, consider a single distillation column with distillate and bottoms products. To produce these products while using the minimum amount of energy, the compositions of both products should be controlled at their specifications. Figure 8.13u shows a dual composition control system. The disadvantages of this structure arc (1) two composition analyzers are required, (2) the instrumentation is more complex, and (3) there may be dynamic interaction problems since the two loops are interacting. This system may be difficult to design and to tune. 

The disturbance must be detected. If we cannot measure it, we cannot use feedforward control. This is one reason why feedforward control for throughput changes is commonly used, whereas feedforward control for feed composition disturbances is only occasionally used. The former requires a flow-measurement device, which is usually available. The tatter requires composition analyzer, which may or may not be available. 

The development of digital control computers and of chromatographic composition analyzers has resulted in a large number of control systems that have discontinuous, intermittent components. The nature of operation of both of these devices is such that their input and output signals are discrete. 

Figure 3.2. Flowsketch of an olefins plant and specifications of the ethylene product. AR designates a composition analyzer and controller (after Skrokov (Ed.), Mini- and Microcomputer Control in Industrial Processes, Van Nostrand I Reinhold, New York, 1980).

Derivative control is sensitive to noise that is made up of random higher frequency perturbations, such as splashing and turbulence generated by inflow in the case of liquid level control in a vessel, so that it is not satisfactory in such situations. The variety of composition controllers arises because of the variety of composition analyzers or detectors. 

Kikkoman Corp., a soy sauce making company, developed an automatic chemical composition analyzer of soy sauce (Figure 9) [16]. The analyzer consists of an InfraAlyzer 400 or InfraAlyzer 500, a temperature controller, an automatic sampler and pumps. A certain amount of soy sauce collected by the automatic sampler is sent to the NIR analyzer at a constant flow rate by a pump through a temperature controller at 20°C. NIR measurement is made automatically. After the NIR measurement, the sample cell and tube are washed with cleansing liquid. It takes about 3 minutes to analyze one sample including washing process. 

Figure 9. Automatic chemical composition analyzer of soy sauce.

Physical property measurements are sometimes equivalent to composition analyzers, because the composition can frequently be inferred from the measurement of a selected physical property. 

A number of composition analyzers used for process monitoring and control require chemical conversion of one or more sample components preceding quantitative measurement. These reactions include formation of suspended solids for turbidimetric measurement, formation of colored materials for colorimetric detection, selective oxidation or reduction for electrochemical measurement, and formation of electrolytes for measurement by electrical conductance. Some nonvolatile materials may be separated and measured by gas chromatography after conversion to volatile derivatives. 

Wheat SFF material was kindly donated by Amylum UK (Nesle, France). The material had a dry matter (DM) content of 18.5% and was stored in buckets at -18°C. The composition, analyzed by combined acid hydrolysis with 1% H2S04 followed by enzymatic hydrolysis with cellulolytic and hemicellulolytic enzymes, is given in Table 1. 

Figure 2-5 Chromatogram of Milk Fat Fatty Acid Composition Analyzed as Butyl Esters on a 30-m Capillary Column. Source Reprinted from R.G. Ackman, Animal and Marine Lipids, in Improved and Technological Advances in Alternative Sources of Lipids, B. Kamel and Y. Kakuda, eds., p. 298, 1994, Aspen Publishers, Inc.

Both of the control structures discussed in Sec. 2.7.2 (CSl and CS4j work because they detect the inventories of the reactant components A and B in the system and bring in fresh feed streams to balance the consumption of the two components. Structure CSl does this by using the liquid level in the reflux drum of the second column as an indicator of the amount of A in the system and the liquid level in the base of the first column as an indicator of the amount of B in the system. Structure CS4 uses a composition analyzer to measure directly the concentration of one of the reactants in the reactor. But both of these structures lack a direct handle on production rate. 

Many industrial columns use temperatures for composition control because direct composition analyzers can be expensive and unreliable. Although temperature is uniquely related to composition only in a binary system (at known pressure), it is still often possible to use the temperatures on various trays up and down the column to maintain approximate composition control, even in multicomponent systems. Probably 75 percent of all distillation columns use temperature control of some tray to hold the composition profile in the column. This prevents the light-key (LK) impurities from dropping out the bottom and the heavy-key (HK) impurities from going overhead. 

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. 

The four fresh reactant feed streams must be managed in an appropriate way to satisfy overall component balances. Fortunately, composition analyzers are available. Figure 8.2 gives a sketch of the process with nomenclature and the values of flowrates, compositions, temperatures, and pressures at the initial steady state (Mode 1). 

Step 5. The final isobutane product is the distillate from the DIB column, and we want to keep the composition of the nC4 impurity at 2 mol %. Nothing can be done about the propane impurity. Whatever propane is in the fresh feed must leave in the product stream. Because the separation involves two isomers, the temperature profile is flat in the DIB column. Use of an overhead composition analyzer is necessary. 

If we select temperature, we would like the reactor flow and composition to be nearly constant and we are constrained by the upper reactor temperature limit of 1300°F. If we select toluene composition, we can control it either directly or indirectly. If directly, a reactor feed composition analyzer is needed and is used to adjust either the fresh toluene feed rate or the total reactor toluene feed rate. If indirectly, the separation section is used as an analyzer for toluene. This allows us to control the total flow of toluene to the reactor (recycle plus fresh). Fresh toluene feed flow is used to control toluene inventory reflected in the recycle column overhead receiver level as an indication of the need for reactant makeup. Controlling the total toluene flow sets the reactor composition indirectly and is advantageous because it is less complicated and does not require an on-line analyzer. 

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,... 

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