Computer, control

On-line, dosed-loop computer control frequently does more harm than good. This is not a popular opinion, but it is my opinion. Perhaps my observations only apply to refinery process units, rather than chemical plants in general. But 1 know what I ve seen. 

I have often been persuaded that computer control is really working very well on a particular process unit. But more often than not, a closer investigation reveals the truth of what I learned in 1968. An experienced, intelligent, committed operator, fully involved in his or her duties, will outperform computer control on most occasions. 

Shinsky of Foxboro, a famous expert in process control, once explained to me that most control problems are really process limits. Mr. Shinsky said that for computer control to succeed, we must update the physical plant limitation data. Nothing has changed in 30 years to make his wise words less true. 

Many computer models fail to represent reality because of faulty input of feed compositions. Common causes of wrong input of feed 

One of my friends, Steve, made such an error in designing a debutanizer. He failed to account for the methane and ethane in the existing debutanizer feed stream. The sample from the 25-psig feed vessel was taken in a bottle. The lighter components weathered off prior to lab analysis. The results were, for Steve, rather catastrophic. The debutanizer overhead product could not be fully condensed. The noncondensable vapor pressured up the overhead reflux drum. The noncondensables were vented, along with 30 percent of the butane, to 

when I come into the control room and note the reflux rate is falling when it should be rising. I ll hear, The computer handles the reflux rate. It s not my responsibility. This is abusive use of computer technology. The operator has been turned into an observer and is no longer actively controlhng the process. 

Because of the low cost of computer hardware, computer control is now widely used. Computer control schemes can be powerful and flexible. They have the potential to make a process easier to control and more profitable. The different types of computer control are described below. 

As process variables change it may be advantageous to alter setpoints in certain loops. This is accomplished by the supervisory computer which monitors process variables and uses a mathematical model of the process to estimate optimal settings. Sophisticated control strategies are possible because settings of several loops can be altered simultaneously. The task of the plant 

DDC has most of the advantages of supervisory control but with extra flexibility in choice of control strategies. Any control mode can be simulated— the control engineer is not restricted to PID. However, because none of the control loops can function independently of the central computer, elaborate measures may have to be taken to allow for computer failure. In critical applications a standby computer would be provided. 

3 Distributed digital control. DDC became economically practical with the advent of mini-computers, and the concepts have been extended by the use 

An evaporative crystallization process is used to produce Na2S04 crystals from aqueous solution. Fresh feed, 22% Na2S04, is mixed with recycle solution and fed to an evaporator where it is concentrated to 45% This stream is cooled and crystals of Na2S04 are removed in a filter unit as a wet 

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Labor costs tend to be higher (although this can be overcome to some extent by use of computer control). 

Drill ships are used in deep water and remote areas, and these vessels are equipped with a drilling unit positioned in the middle of the ship. Positioning is achieved dynamically by computer controlled thrusters. The ample storage space allows operation for long periods of time without re-supply. 

The measurements are done at a table with two in X- and Y- direction moveable axes. The measured structures, by an Aluminium-alloy, are situated at the X-axis. The sensor at the Y-axis scans the structure step by step. The position and the electoral signal are measured for every step. A computer controls the movement of the sensor and the data acquisition. 

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]. 

This is a fairly accurate and convenient method for measuring the surface tension of a liquid-vapor or liquid-liquid interface. The procedure, in its simpli-est form, is to form drops of the liquid at the end of a tube, allowing them to fall into a container until enough have been collected to accurately determine the weight per drop. Recently developed computer-controlled devices track individual drop volumes to = 0.1 p [32]. 

Since the drop volume method involves creation of surface, it is frequently used as a dynamic technique to study adsorption processes occurring over intervals of seconds to minutes. A commercial instrument delivers computer-controlled drops over intervals from 0.5 sec to several hours [38, 39]. Accurate determination of the surface tension is limited to drop times of a second or greater due to hydrodynamic instabilities on the liquid bridge between the detaching and residing drops [40],... 

A typical noisy light based CRS experiment involves the splitting of a noisy beam (short autocorrelation time, broadband) into identical twin beams, B and B, tlnough the use of a Michelson interferometer. One ami of the interferometer is computer controlled to introduce a relative delay, x, between B and B. The twin beams exit the interferometer and are joined by a narrowband field, M, to produce the CRS-type third order polarization in the sample ([Pg.1209]

Memfield successfully automated all the steps m solid phase peptide synthesis and computer controlled equipment is now commercially available to perform this synthesis Using an early version of his peptide synthesizer m collaboration with coworker Bemd Gutte Memfield reported the synthesis of the enzyme ribonuclease m 1969 It took them only SIX weeks to perform the 369 reactions and 11 391 steps necessary to assemble the sequence of 124 ammo acids of ribonuclease... 

Derivative methods work well only when sufficient data are recorded during the sharp rise in plT occurring near the equivalence point. This is usually not a problem when the titration is conducted with an automatic titrator, particularly when operated under computer control. Manual titrations, however, often contain only a few data points in the equivalence point region, due to the limited range of volumes over which the transition in plT occurs. Manual titrations are, however, information-rich during the more gently rising portions of the titration curve before and after the equivalence point. 

There are a variety of possible linked scanning methods, but only those in more frequent use are discussed here. They differ from the linked scanning methods used in triple quadrupole instruments and ion traps in that two of the three fields (V, E, and B) are scanned simultaneously and automatically under computer control. The most common methods are listed in Table 34.1, which also defines the type of scanning with regard to precursor and product ions. 

Linking requires that two of the fields be changed simultaneously and automatically under computer control. Fields V and E or E and B are generally linked. 

By automating the linked scanning under computer control, a complete mass spectrum can be scanned for metastable ions in just a few seconds. 

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. 

The use of automatic machine tools, often computer controlled and programmable, requires abrasive elements which have performance and lifetime reproductibihty. Natural abrasives suffer on both counts performance is limited and inconsistent quaUty leads to unpredictable lifetimes. Manufactured abrasives have both superior performance and consistency consequendy, they have largely replaced the natural ones. 

Manufacturing processes have been improved by use of on-line computer control and statistical process control leading to more uniform final products. Production methods now include inverse (water-in-oil) suspension polymerization, inverse emulsion polymerization, and continuous aqueous solution polymerization on moving belts. Conventional azo, peroxy, redox, and gamma-ray initiators are used in batch and continuous processes. Recent patents describe processes for preparing transparent and stable microlatexes by inverse microemulsion polymerization. New methods have also been described for reducing residual acrylamide monomer in finished products. 

The entire continuous automatic process is computer controlled so that continuous performance information is available. Pressure reHef is permitted wherever possible to minimize the likelihood of a detonation. Continuous-screw extmsion processes may be employed for making nitrocellulose single-. 

The next step is to apply a number of loss control credit factors such as process control (emergency power, cooling, explosion control, emergency shutdown, computer control, inert gas, operating procedures, reactive chemical reviews), material isolation (remote control valves, blowdown, drainage, interlocks) and fire protection (leak detection, buried tanks, fire water supply, sprinkler systems, water curtains, foam, cable protection). The credit factors are combined and appHed to the fire and explosion index value to result in a net index. 

Computer controls are likewise used for stove operation, to control deUvery of the hot blast. High hot blast temperatures are generally desirable, as these reduce the coke rate. Control of the flame temperature in the raceway is effected by controlled additions to the hot blast, primarily of moisture. Injectants into the tuyeres such as coal, oil, and natural gas are often used to replace some of the coke. The effect of these injectants on flame temperature must be accounted for, and compensation is performed by lowering moisture or adding oxygen. 

A computer-controlled bandpass filter system controls the size of the acquired spectral window. Typically, this is set to about 120% of the desired sweep width. Only frequencies within these limits are allowed to reach the ADC. Those frequencies outside the limits would only contribute to the noise in the final spectmm. The need for this system is dictated by the nonselective nature of the excitation rf pulse. 

Full computer control of sector instmments has made this type of instmment much easier to operate than previously. These instmments require more maintenance than lower resolution instmments, however. 

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