1360_Ch06

chapter 6

Experimental program

6.1 Basis

6.1.1 Experimental program purposes

Main Purpose

At this stage of the process development, the main purposeof the experimental program is the collection, the correlation, and the pre-sentation of the

design data

that is specifically needed for the design andoptimization of the new process, as defined and in the

limited range of variablesof practical interest

.It is important to note that the scope of the investigation can depend

also on the

variability

of the particular function under consideration. It canbe found, after the first series of tests, that the already mentioned range ofspecific interest happens to be located in one part of the function wheresharp changes can be seen from a first plotting of all available information.In such case, it is advisable to enlarge the scope of investigation in order toassure a reasonable reliability when interpolating between experimentalpoints. The experimental program, therefore, is formulated in relation to theperceived needs in one particular situation.

Unexpected Problems

Another important purpose includes the

obser-vations

of possible, but unexpected problems, that can occur and that shouldbe dealt with. These can be, for example, difficulties in the separationbetween phases, slow rates of reaction/mass transfer, colloidal precipitates,unwanted color, etc. (see below). The experimental staff should be instructedto observe carefully and to call their supervisor whenever anything seemsunusual. The top R&D managers are often seen circulating between thebenches when such tests are done to obtain a personal appreciation of thebehavior of the reacting or separating mixtures.

Preparation

In addition, the preparation of relatively large represen-tative

samples

of certain products or of certain intermediate phases are oftenneeded for further specialized tests, for market surveys, or just to showaround in the promotion contacts for the project. Therefore, generally allthe materials resulting from these tests should be well packaged, labeled,and stored for future use.

1360_frame_C06 Page 77 Monday, April 29, 2002 3:36 PM

Copyright 2002 by CRC Press LLC

6.1.2 Different sections

The separation of the process into the different sections was already rep-resented in the

process map

(

block diagram

), with the formal

definition

ofthe prevailing chemical mechanism, of the separation between phases, andof the process results expected in each separate section (see Chapter 5,Section 5.1).

Before starting the experimental program, these definitions should bediscussed systematically,

well understood, and agreed

upon among the inven-tors, the process engineers, and the senior experimental staff. The calcula-tion methods that will be requested for

the process design of each operation

will be formulated and agreed upon by the whole process engineeringteam. For this purpose, use the manuals and textbooks listed as referencesin Chapter 5, as well as for those concerned with the separation processeslisted in this chapter.

710

Note that many of the operations in any chemical plant can possibly bedesigned on the basis of

conventional

know-how, with the specific input ofonly a few specific physical properties; for instance, all the sections con-cerned with material handling, liquid flows, blending, packaging, spraying,gas compressing, steamboiler, cooling tower, etc. Thus, the experimentalprogram will not be concerned with such operations at this early stage ofprocess development.

6.1.3 Quantitative data needed for process design

Guided by the definitions, the process engineers also should prepare a listof

all the quantitative data

that will be requested for the process design of eachoperation. Some parts of this design data may be already available in thefiles from the previous analysis of the results of the feasibility tests, from thepromoters own sources, or from earlier publications in related fields. Thesystematic organization of what is available will allow the delimitation ofthe missing data that should be generated in the experimental programpresently considered.

Discontinuities are often found between the sets of data obtained fromdifferent sources, as it may be seen when these are plotted on a commongraph, due to the differences in experimental or analytical techniques. There-fore, in this analysis and determination, it is preferable to allow for significantoverlapping to arrive at a reasonably reliable common function.

(

In the authors considered opinion, there is not much point in designing andstarting any significant experimental program without performing first this processengineering analysis

.)

6.1.4 Format

At the same occasion, it would be useful if the process engineering teamcan specify exactly the preferred format for the results on these data to beused in the experimental reports to allow their direct application in the



process calculations. As there can be many parameters in each stage, the

primary variables

should be indicated in the order used in the calculations(see Section 6.2.1).

This

early

specification

of the format can avoid or reduce the communi-cation problems and the waste of time devoted to clarifications and recal-culations, which often happens between the process engineering team andthe experimental group. In many cases, these two units can also be situatedin geographical locations far apart and may not be able to meet frequentlyface-to-face.

It would also be useful to decide as far in advance as possible thepreferred

order

for the generation and transmittal of

partial data

to theprocess engineering team in

separate, successive, numbered reports

. Thisdemand can appear to be trivial, but this has been, in fact, a sore point inmany projects. If certain parts of the process design work can be startedand advanced before all the data is transmitted in one big bound report,some of the pressure can be relieved from a

serious bottleneck

in processdesign. It generally happens that as soon as the final experimental reportis issued, everybody wants to know all its implications on the new process,the plant under consideration, the economic parameters, etc. On the otherhand, the preparation of serious answers takes time and experienced pro-cess engineers are scarce.

6.1.5 Representative raw materials

As far as possible, these R&D tests should use

representative samples

of theactual raw materials, fuel, water, reagents and additives, filter aid, activecarbon, and IX resins that would be expected to be used in the final plant.As discussed previously, a synthetic mixture of pure laboratory chemicalsfrom the bottles on the shelf cannot duplicate in many cases the exactcomplex physical structure and/or the large number of impurities in theseraw materials. Similarly, whenever it is intended to use combustion gasesin direct contact with the process streams (e.g., in a calciner or a dryer),the exact composition of the fuel can be significant. Such combustion gasescan contain

ash particles

or

gaseous impurities

that can contaminate theproducts or would need to be treated in the waste streams or can accumu-late in the plant.

It has often been found in real cases that the use of

certain raw materials

in such tests did result in

serious, nonexpected problems

. For example, insolvent extraction processes, some impurities can precipitate as fine solidscausing the liquidliquid mixture to emulsify and, thus, preventing thenormal operation of the process. In one particular case, the raw materialcontained an impurity with oxidizing power, which attacked anddestroyed the organic extracting reagent used. In hydrometallurgy andsalt processes, the precipitation of solids that stick or build on the wallsof the equipment and pipes can stop a plant. Some natural streams can,when heated, release some dissolved noncondensable gases, which may



disturb the vaporliquid equilibrium, and/or (at least) need to be collectedand vented properly. Other problems can be the precipitation of colloidsuspensions, coloring phenomena, etc. Such troubles may be seriousenough to kill sooner or later the proposed industrial process at leastin its original form, so it is very important to discover and diagnose themas early as possible. Possible solutions can include pretreatments or evenchanging the source of the problematic raw material.

Unfortunately, it often happens that, despite all reasonable efforts, rep-resentative samples of all the actual raw materials cannot always be readilyprocured from the beginning of the experimental program.

First Situation

This difficult situation has been typically found, forinstance, when a new mineral deposit was being explored and small samplesof the expected raw materials can only be separated and reconstituted fromsmall drilling rig cores in quantities hardly enough for the analytical andpreliminary bench tests.

Second Situation

Another typical case has been the development anddemonstration of a proposed process, which was intended to handle aneffluent stream expected from a future operation that was still at the designor construction stage.

Third Situation

This situation has been also encountered now andagain in the development of large-scale biotechnology processes, which typ-ically consist of two separate parts:

1. The

fermentation

section, which is producing a broth containing avaluable product (e.g., a carboxylic acid)

2. The

recovery

section, intended for the separation of such valuableproduct from the broth into a pure, concentrated, marketable form

These two sections are generally developed and even designed sepa-rately by two specialized groups, and they are often built in different plotsacross the road. Obviously, all the characteristics of the new recoveryprocess are derived from the exact composition of the

expected fermentationbroth

, as defined at the time of the project justification. But it often happenedthat while the recovery group was developing, designing, and buildingtheir processing unit, the fermentation group was continuing their effortsto improve their part. They would aim at a better

productivity

(which is theaverage production

rate per unit volume

of fermentor) and/or a better yieldon the raw materials. This can be quite natural from their point of view,but the resulting changes in the broth composition (mostly with regard tothe associated impurities) can have serious (negative) effects on the recov-ery process, as developed.

A mutual

understanding and coordination

between these two groupsshould be obvious, but often can be delicate in real life and may have to beimposed from above. To be fair, the fermentation group is not alwaysinformed of the downstream development (What do these biologists knowabout our separation processes?). But, at least in one case known to the



author, when the fermentation group was informed that one of the impuritiesgenerated by the microorganism constituted a very serious separation prob-lem, they found a genetic trick to prevent this particular impurity andreplace it with a less problematic one.

In such cases, where

representative samples

of the actual raw materialscannot always be readily procured from the beginning, an experimental pro-gram on synthetic mixtures can only be done as an

exploratory work

aimedat the

preliminary

process design. The results need to be clearly marked assuch, and this situation reflected in the safety factors included in the economicanalysis.

Repeated tests

should be scheduled for later, in the exact selectedconditions, as soon as the actual representative samples can be obtained.

6.1.6 Classification of missing data

The data missing at the beginning of the experimental program can bedivided into three main categories according to the testing techniques thatwill be required in obtaining the results, as discussed below, and the designmethods for:

Operations based on

chemical equilibrium

data, (as reviewed in thecomprehensive book by Henley and Seader

2

Operations based on

dynamic flow conditions

Operations that are

scale-dependant

, i.e., the results depend on the

absolute size

of such equipment

6.2 Chemical equilibrium data

(See basic reference books on separation processes and, in particular, oneson the equilibrium stages, which are the most useful tools in new processdevelopment.

17

)

6.2.1 Vaporliquid equilibrium system

Vaporliquid

(reversible) equilibrium systems are used in unit operations,such as distillation, rectification and stripping, evaporation, and condensa-tion. (Note that

gasliquid

equilibrium systems, which are relevant in unitoperations dealing with scrubbing, cooling, etc. of a gaseous stream, havesome similarity. However, these will be discussed separately, in the nextsection.) Data needed for process design are obtained by correlating the

compositions of both phases

at equilibrium in certain conditions of temperature,absolute total pressure, and the partial pressure of noncondensable gases(inert, nonreactive) that may be present (assuming that such partial pressuredoes not exceed 70 to 80% of the total).

The pair of

compositions for both phases

can be obtained from a

total reflux

test, where the vapors from a boiling liquid phase are totally condensed atthe same absolute pressure and all the condensed liquid is returned to the



boiling liquid. As equilibrium is established, samples are withdrawn fromboth the boiling liquid and the refluxed condensed liquid, and completelyanalyzed for all components. If there are

only two

components, the plottingof the results is straightforward.

But since, in most cases, there are

more than two components

present ineach phase, one has to decide from the start

which two components are thevariables under study

and which other components are to be considered asparameters for the purpose of the present process design, together withthe obvious physical

parameters

, such as the temperature, absolute pres-sure, and partial pressure of noncondensable gases. All the parametershave to be kept constant in each series of tests, to obtain a

cross section fortwo variables

.One may see that the experimental program for a typical system with

four to six components can become very complex unless one limits from thebeginning the

ranges of practical interest

(see Chapter 5, Section 5.1). Once thischosen range is covered with a limited number of experimental points,the numerical results can be interpolated quite safely into a mathematicalfunction by using one of the published theoretical correlation formulas. Thecorrelated function then can be used for the process calculations of the

multiple stages

equilibrium system in one of its forms: countercurrent, cocur-rent, or crosscurrent.

The process design can be done using the theoretical stages concept,and then translated into an equipment design, by relying on the correlationlinking the height of theoretical unit with the operating conditions andthe details of the chosen equipment. Such correlation has been published forseveral basic designs or may be obtained from the suppliers of more spe-cialized equipment.

When there are many condensable components from the beginning (asin petroleum processing, as an extreme case), one may have to cut themixed feed by a coarse separation into two or three ranges (heavies orlights) and to treat

each range as a separate problem

with recycles at thestarting point.

A special case is the concentration of a solution containing nonvolatilesolutes by evaporation of water (or another solvent), leaving the nonvolatilesolutes in the concentrated solution. The vapor phase contains only onecomponent, but the concentrations of the solutes into the liquid phaseincrease gradually, decreasing the partial pressure of the water. In such case,the important data are the quantitative link between the absolute pressureand the boiling temperature of the solution and the concentrations of thesolutes below their saturation limit. These data are essential, for example,for starting the design of energy-efficient,

multiple-effect

evaporators, whichare a critical element in many processes (e.g., salts and sugars).

An equally important result of such tests can be any

observation

aboutthe precipitation of certain solids from the liquid, and the form and behav-ior of such solids, in particular as to their incrustation inside equipmentand pipes, or on heat exchangers surfaces (for their composition, see



Section 6.2.3); as well as the conditions of the release of any noncondens-able gases dissolved in the feed solution.

Another important field of process development is concerned with theseparation between two volatile components that cannot be obtained directlydue to the presence of an azeotrope or another particular feature of theequilibrium curve. (As a reminder, at the azeotropic point, the compositionsof the liquid and of the vapors are identical.)

A well-known case is the system HCl-water (already mentioned inChapter 4, Section 4.3.2) which is dominated by an azeotrope at 20 to22% HCl (the exact number depends on the absolute pressure). Every tonof HCl generated below the azeotrope is accompanied by at least 4 to5 tons of water at its maximum practical concentration and this featuremay prevent or limit its use in other processes. Breaking the azeotropemeans obtaining a more concentrated solution that can be handled atambient temperature (say about 30 to 40% HCl) or even a 100% dry HClgas, if needed.

Such a result is possible by using a cycle of CaCl

2

brine in a close cycle,as the brine absorbs the water and releases the HCl, but this is a compli-cated process with many reflux streams. It is also an expensive process,both in the investment in the HCl-resistant equipment and in the energyconsumption. Another commonly found complication can be the presenceof nonvolatile components in the starting HCl solution, which can accu-mulate in the circulating brine. In such case, the starting solution shouldbe completely evaporated upstream and the heat loads should be redis-tributed. This problem was at the time an open field for creative processdesign, aiming at a better use of the energy and the expensive heat exchang-ers, and of any possible synergetic utilization of sources of low-temperaturewaste heat.

1114

Different aqueous solutions were used, including MgCl

2

and LiCl. It was also proposed to replace the expensive heat exchangersby direct contact heating with organic heat carriers. (See below and alsoChapter 5, Section 5.1.5.)

Direct contact heating technology, with organic heat carriers (stable hydro-carbons, liquid, or vapors)

Certain processes need large heat exchangersmade from expensive materials (resistant to corrosion, such as graphite,glass-lined, tantalum) to introduce heat into the process streams and evap-orate certain components, and similarly for removing heat in condensers. Inother cases, heating of such solutions in a regular heat exchanger wouldprecipitate solids and cause the rapid scaling of such heat exchangers.

One can resort to introducing very hot organic liquid or vapor heat-carrier in direct contact with the process stream to be heated. After heattransfer and equilibration, the organic liquid is separated, removed, washed,and reheated in a separate boiler made of cheaper materials. Although theheat carrier material would have a boiling point much higher than any ofthe components present, it can have a definite vapor pressure in the hotterparts of the equipment, which should be taken into account and includedin the experimental program.



6.2.2 Gasliquid equilibrium system

Gasliquid (reversible) equilibrium systems are relevant in unit operationsdealing with cleaning or cooling of a gaseous stream in contact with a liquidphase. In such operations, the concentrations of certain components thatexist in both the liquid and the gas phase are related by a definite reversibleequilibrium function, for a definite set of parameters.

For example, water and ammonia can be both in a gas stream and in anaqueous solution (or water and HCl, or water and methanol, and the like).The ammonia can be recovered from the gas stream into an aqueous solutionin a packed column where the gas stream (say with a few percents of ammo-nia) will be introduced from the bottom and will flow upwards (exiting with,say, 0.02% ammonia), while water is introduced at the top and flows down-wards, countercurrently. The liquid also may contain nonvolatile solute com-ponents, and the greater part of the gas stream would consist of inertnoncondensable gases.

In principle, such a system can be studied as a vaporliquid equilib-rium system, and a certain number of theoretical equilibrium stages canbe defined to obtain a certain result. But there is a

quantitative difference

compared to a regular vaporliquid equilibrium system. The

kinetics ofreaching such an equilibrium

are much slower, as they depend on the dif-fusion of the relatively few condensable molecules in the bulk of the gasphase until they reach the liquid interface, and possibly also on the resis-tance to mass transfer of the layer adjacent to such interface. So, the

contact conditions

are often

more important

than the theoretical equilibriumand the height of a theoretical equilibrium stage must be

determined exper-imentally

for each of the exact sets of operating conditions and for theexact geometry of the packing in the column. It becomes a completelyempirical design and the position of the equilibrium curve has, in fact,little practical importance.

6.2.3 Liquidliquid equilibrium system

Similarly, a liquidliquid reversible equilibrium system can govern a solventextraction process, which is intended to separate, concentrate, or purify aparticular component from a mixed solution, such as a fermentation broth,a leaching solution from a mineral acid reaction, or a waste stream fromother operations.

One can refer to the basic reference books

1518

and note that the officialdenomination is liquidliquid extraction, but most people in the field keepcalling them solvent extraction processes.

Generally, one is considering two liquid phases, but there also existsinvariant systems with

three liquid phases

at equilibrium, according to Gibbsphases law. At least one of these systems was used in the IMI cleaningprocess for separation of clean phosphoric acid from wet phosphoric acid(see Chapter 4, Section 4.4.3, Reference 26).



Again, in almost all processes of

practical

importance, there are manycomponents in each phase. One has to define all these components and theirranges of concentration and to choose, on one hand, the

variable of majorinterest

and on the other hand, the ranges of parameters (such as the con-centrations or ratios of the other constituents and the physical conditions)that can be covered in a

reasonable

experimental program. If one is not carefulin his choice, the number of tests required can easily shoot up exponentially.

The distribution coefficient of this major variable can be correlatedand used for multiple stage process calculations in the defined ranges ofparameters. The major difference from the vaporliquid

physical

equilibriumsystems is that in most liquidliquid extraction processes, the major variableof interest in any particular process can be either an

ionic

or a

molecular

entity,according to the

chemical

extraction mechanism.Once this procedure is well understood, the bench-scale experimental

program for the development of a separation process based on solvent extrac-tion can be relatively straightforward. The technique of the so-called separa-tion funnels tests is based on equilibration in defined conditions, samplingand analyses, and it can be carried out routinely by laboratory technicians.This fact of life was probably one of the main reasons for the successfuldevelopment of many dozens of new solvent extraction processes in the years1960 through 1980 in various countries. Very promising R&D programs in thisfield are continuing nowadays inside some of the large industrial corporations,although not much is published about that at international conferences.

In this connection, it is important to stress the experimental techniquecalled limiting conditions, which makes it easier to study the effect of onevariable at a time. If, for example, 50 ml of a starting aqueous solution aremixed with 50 ml of a solvent phase, the concentrations of the componentof interest after equilibration will probably change significantly in bothphases. If, for example, a series of tests are done at different temperatures,the quantitative results can be all over the place and a lot of tests will berequired to find a working hypothesis to explain the results. But if 100 mlof the starting aqueous solution are mixed with 1 ml of solvent, the chosencomposition of the aqueous phase will change very slightly, while that ofthe small organic phase can change very significantly. Thus, three to fourtests at different temperatures should give a clear indication of the effect ofthat variable for the particular chosen aqueous composition.

The experimental procedure is also simpler for processes in which thesolvent added is composed of a single component, such as butanol, pentanol,methyl iso butyl ketone (MIBK), and so on. But, for other processes, thecomposition of the solvent phase added can be quite complex by itself andmay present a large number of additional components and parameters, suchas the nature of the extractant (i.e., one particular tertiary amine from thedozens of tertiary amines commercially available), of the modifiers (i.e., oneof many long-chained alcohols available for such duty), and of the diluent(i.e., a light, saturated hydrocarbon), and the relative weight ratios of thesethree classes of components.



In the typical practice of solvent extraction process development, onewould generally start with a screening procedure. (

Even my granddaughterknows by now that, in real life, the Princess would have to kiss many Frogs beforeshe would find, maybe, her Prince

.) This screening procedure is generallystarted with one effective composition formula found in previous publi-cations, or in the suppliers recommendations. This formula may not neces-sarily be the optimal composition for the specific case studied, so that someexploration tests with moderate changes outside this range should be donepreferably before any specific commitment. But the prevailing attitude hasoften been: Lets start with the composition that works, and we will opti-mize later. But as often happens, everybody is too busy later to look backat this exact composition. This is a well-known pitfall.

It has also been observed that the chemical behavior of some extractants(in particular tertiary amines) does change as the new reagent (straightfrom the bottle) is aged after a few dozens cycles of loading/regeneration.This change, which may include a significant shift in the equilibrium curve,can be due most probably to the elimination of some traces of impuritieswhich remained in the new reagent from its synthesis, and possibly alsoto the oxidation of unsaturated bonds in the experimental manipulations.Since the plant will be working eventually with an aged extractant, thetesting conditions and results should reflect that change from the beginning.

An additional form of aging occurs in functioning plants due to accu-mulation of certain impurities in the solvent cycle. Although a continuouspurification procedure is generally used on a side stream, there is an eco-nomic limit to such purification and any plant has to live with a certain levelof impurities in the solvent. This effect is difficult to reproduce from theseearly tests, but has to be accounted for in the design safety factors.

6.2.4 Solidliquid equilibrium system

Solidliquid

reversible

equilibrium data are regulating many processes. Allthe metallurgical transformations relate, in fact, to this field, but the hightemperatures of more than 1000C are considered as a far away situationby most chemists and chemical engineers. Some chemical industries aretouching these high temperatures, but most remain below 200C and, in thisrange, these systems relate to solid dissolution, precipitation, and crystalli-zation, which are widely used in the various inorganic industries, mineraltreatments, and also in the natural sugar and sweetener industries.

In all these processes, the

saturation concentration of one variable component

depends also on the concentration of the other components present, in additionto the physical parameters of temperature, absolute pressure, and non-con-densable gas. The experimental determinations of such saturation concentra-tion can be rather simple, in principle, if only one component is precipitatingor dissolving, while the other components are remaining as parameters eachin its respective phase. (Note that the limiting conditions technique men-tioned above should also be used in such experimental determinations).



However, in many other cases,

two components

may change phase simul-taneously in certain ranges and the experimental program becomes morecomplex. Some solids are also precipitating in the

hydrated form

, taking withthem some of the water into the solid phase and contributing to furtherchanges in the concentrations in the solution. Furthermore, there are impor-tant cases of double-salts precipitation at certain concentrations. For exam-ple, the conditions for the precipitation and for the decomposition of Car-nallite crystals (hydrated double-salt of KCl and MgCl

2

) are the key featuresof the potash industry from the Dead Sea brines as mentioned in Chapter 4.Kainite and Langbeinite crystals are important hydrated double-salts foundin mined potash deposits containing magnesium sulfate and, therefore, theyhave been investigated in great detail to optimize potash recovery processes.Various degrees of

supersaturation

are always of major concern (see below).It is important to note that, in such cases, many of these solid precipitates

can be clearly identified and quantified by established mineralogical tech-niques, in addition to the usual chemical analysis methods. Thus, the collab-oration of a mineralogical laboratory equipped with all the usual microscopesand x-ray diffraction equipment can be a great help in such R&D programs.There was a time when radioisotope tracers were also used in such researchin many research labs, but this technique can be risky if it is not done with allthe special equipment, and it apparently went out of fashion.

6.2.5 Reversible and nonreversible equilibrium

In all the processes mentioned in this chapter, whenever a

reversible equilib-rium

is expected, the determination of the quantitative

relations

of the con-centrations at equilibrium (or at saturation), in various conditions and in thechosen range, is sufficient at least for the preliminary process design anddemonstration stage. However, in many other cases, the reaction involvesfirst a

nonreversible change

(such as one or more strong, one-sided reactions,i.e., neutralization) and then the equilibrium is established between theresulting phases. This order of reactions should be taken into account, andit may complicate further the experimental design.

Therefore, one can appreciate the importance of the process working defini-tion discussed in Chapter 5 in order to keep the experimental program withinaffordable limits.

6.2.6 Chemical equilibrium laboratory tests

As discussed in Chapter 5 in relation to the feasibility tests, most of thechemical equilibrium data can be determined in standard laboratory con-ditions, in rather small batches (in the hundred grams range) unless alarger quantity of one of the resulting phases is required for further testsor evaluation.

Each test consists of bringing into contact, in the specified conditions,proportional quantities of the different inlet streams at the assumed



compositions. After the different reactions and/or mass transfer haveoccurred, the equilibrium is established and the phases are separated (ifneeded); the different phases are sampled and the

compositions and thephysical properties of the different phases

analyzed.But it should be realized that with real systems, the total data collected

from each such test, representing one equilibrium point, amounts to pos-sibly

10 to 20 numbers

. The recording and presentation of all the differentnumerical data sets in the experimental report can be only in the form ofsystematic tables. However, the correlation of this data for all the pointscollected is not straightforward, as it depends mainly on the way in whichthe data will be used in further calculations (sometimes many years later). Thisimportant fact of life should be recognized. Unfortunately, in many casesencountered in the past, certain parts of the data were lost on the way, astheir future use was not clear to those editing the experimental reports.

6.2.7 Experimental difficulties in chemical equilibrium tests

Possible experimental difficulties can derive from the following causes.Establishing Equilibrium Absolute equilibrium is, by definition,

never reached, as its approach is asymptotic. Chemical engineers work withpractical equilibrium, which can differ slightly from the absolute value by afew tenths of one percent, relative. In almost all cases of industrial interest,a practical equilibrium should be reached in a matter of minutes. Therefore,a contact time of 10 to 15 min in a test, before sampling, should assure apractical equilibrium.

In some particular cases, wherever a doubt exists, the tests can berepeated with different contact times, say 3, 8, and 15 min, and the resultscompared and interpolated to determine the contact time required in thisparticular contacting mechanism. A slower mass transfer rate can also resultfrom the adsorption, precipitation, or collection of impurities on the interfa-cial area, or in the adjacent double layer; this situation should be recordedand, if possible, corrected.

At a later stage, when the actual plant design will be considered, theexact contact time required to obtain the desired result will need to bedetermined and optimized in relation to the contacting conditions in thechosen equipment (see Chapter 10, Section 10.5). For some items of equip-ment that are handling very large flows, this contact time can be animportant factor, as every second would be cost significant. But suchoptimization can only be done in direct relation to the type of equipmentchosen and to the designed conditions for the contact and for the subse-quent phases separation.

Supersaturation This is often creating additional complications inequilibrium operations involving solids, by biasing the solubilitys levels.Although the physical causes of natural supersaturation are not reallyknown, there are effective empirical ways to break the supersaturationand reach practical equilibrium (such as seeding, for example). Before



undertaking an extensive testing program, these techniques need to be triedand confirmed in every case to obtain absolute data.

On the other hand, one should remember that a controlled level ofsupersaturation is an essential factor in the design and operation of contin-uous crystallizers and that it can also be put to use for other process sepa-rations. In one specific case, a high level of natural supersaturation wasfound and exploited for an interesting process separation. A double decom-position reaction yielded two solids products, one of them (A) precipitatedimmediately and while the other (B) was maintained in solution by thesupersaturation for a period of time sufficient to separate the (A) solids bycentrifugation.

Sampling Problems Every multiple phase contact/equilibrationshould be followed by a complete phase separation before the resultingphases are sampled. Imperfect phase separation (entrainment of small quan-tities of one phase into another) is a common cause for serious problems,first in the reliability of the data obtained from the experimental work andlater in operation of the continuous industrial equipment.

When small-scale tests at nonambient conditions are done in closed lab-oratory containers, it is not always possible to separate and sample thephases inside. If a mixture is taken and separated outside, i.e., in a centrifugetube, the contact conditions may change (temperature, pressure) and theequilibrium can be shifted before the phases can be separated. This experi-mental problem is not always recognized and may result in erroneous results.In such cases, the size of the test rig may need to be increased.

Analytical Issues By the time the samples have reached the analyticallaboratory, the temperature/pressure conditions have changed and a samplecan separate into a nonhomogeneous mixture of phases. This possibilityshould be suspected and checked in the analytical laboratory before a smallaliquot is taken out for analyses. If this happens, the situation should bereported, as this can have other implications in the plant. Of course, the wholesample should be rehomogenized (by heating or dilution in a solvent) beforethe aliquot is removed.

6.3 Dynamic flow conditionsContinuous reactions or separations, which are dependent on dynamic flowconditions, are generally much more complicated to study or even to fullyunderstand. For example:

A one-phase stream containing a mixture of components is flowingthrough a packed bed of solids with a catalytic action, causing reac-tions between components in the flowing stream.

A countercurrent contact between a gas stream and a liquid stream,which allows reactions to take place, and a small amount of a certaincomponent is changing phase in either direction (see Section 6.2.2).This operation can be done in a packed bed column or in other



types of contact equipment and can be used either to wash anexit gas stream or to strip a liquid stream from an undesiredvolatile contaminant.

Some thermal energy is introduced into a reaction mixture in theform of the combustion gases from a direct flame, from a plasma, orby induction microwaves.

A multiple-phase mixture, resulting from a reaction in a very shortmixing zone, is separated continuously while flowing through a sep-arator, such as a gravity decanter, a cyclone, or a centrifuge.

6.3.1 Design data required

The flow conditions (velocities and paths) determine both the residence timeand the contact conditions affecting the interfacial mass transfer, such as theturbulence or the shearing forces, the differential gravity or G-forces, or atemperature gradient.

The design data required to link quantitatively these flow conditionswith the final results obtained can be either:

1. Rates of reaction and of mass transfer, which determine the chemicalcomposition of the different phases in the resulting stream(s).

2. Physical separation between the different phases in the exiting streams.

6.3.2 Simpler processes

Fortunately, many of these mechanisms of industrial interest were straight-forward enough and have been extensively studied. For example, many ofthe catalytic reactions in a gas mixture flowing in a packed column, or thechanges in a solution flowing through an ion-exchange resin column, can besimulated in relatively small continuous test equipment. The scale up of theperformance of a gas cyclone or a liquid cyclone can be predicted from small-size continuous tests (see Chapter 4, References 37 to 39). The separationresults in a continuous centrifuge (of most types) can be evaluated fromsimple tests with a small, bench-scale machine.

6.3.3 Theoretical models

Theoretical models allow the simulation and data generation from standard-ized batch tests in some other widely used mechanisms that have beenextensively researched. For instance, the mass transfer occurring in the con-tinuous solidliquid and liquilliquid contacting inside mixed vessels canbe reliably designed from the kinetic batch reaction curves obtained in bench-scale tests in well-defined mixing conditions.

Batch Aerobic Fermentation A particular case of increasing industrialimportance is the batch aerobic fermentation involving the mixing of a liquidsolution with dispersed microorganism particles, chemical additives, and air



bubbles. In such a process, from the chemical engineering point of view,oxygen from the air bubbles is continuously dissolved and consumed by themicroorganisms (the bugs in operators jargon), CO2 is generated andevaporated, carbohydrates are reacted and consumed, a soluble valuable(desired) component is produced, and a lot of other side reactions may beoccurring, all simultaneously with the release of heat. A significant coolingcapacity is critical. The flow rate of the air, the pH, and the temperature ofthe mixture are generally maintained and controlled by external means andthe excess gases are vented. Mixing is very important in maintaining theaqueous solution more or less uniform; it can be internal or external and isgenerally combined with the cooling system (jacket or heat-exchangers).

The batch period is a matter of days. Considering a large tonnage plant(say, 50,000 metric tons/year [MTY]) with a batch turnover of, say, 4 days, anda product concentration of the order of 10% in the fermentation broth, the netinternal volume inside all the fermentors is considerable, about 6800 m3 or 34fermentors of 200 m3 each. Therefore, any improvement in the average batchperiod or in the final broth concentration can have a serious economic effect.

The hydrostatic pressure at the bottom of such a large fermentor (say, 5m diameter and up to 10 m in height or even more) is an important operatingparameter. It affects not only the supply pressure and, therefore, the cost ofthe compressed air, but also the solubility of the different gases in the solutionand possibly also the biological functioning of the microorganisms. Differentmodels of large fermentors are used in industry, each with its apparentadvantages and disadvantages. (Only apparent since many of the importantfeatures relevant to their operation have not been released for publicationby the corporations operating them.) In any case, the internal inspection andperiodical cleaning is essential.

Figure 6.1 illustrates the principle of a draft tube circulation with a cool-ing jacket, which is using the inlet air in the draft tube to promote thecirculation of the media. The air/liquid contact inside the draft tube is short,but at a higher turbulence regime. Such type of fermentors have limited sizeand limited cooling capability and, therefore, they were used mostly forsmaller production capacities, since their upscaling is estimated to reach alimit around 60 to 100 m3.

Figure 6.2 shows the main features of a fermentor in which the com-pressed air is sparged from the bottom and an external pumping circuit takesthe media around through the heat exchangers. These features have moredesign options and scale up possibilities, but the passage of the microorgan-isms through a (positive flow) pump and through the heat exchangers hasbeen hotly debated.

The composition of the solution in the batch fermentor is changing allthe time and is monitored by the operator to detect any unexpected trend.As the final trend in composition is asymptotic, the main operating issue ishow and when to stop the reaction (dropping the fermentor), since in manycases, the later period of operation produces little valuable component buta lot of impurities, which can complicate the recovery.



Figure 6.1 Fermentor with a cooling jacket and draft tube circulation.

Figure 6.2 Fermentor with sparging air and external cooling cycle.

gases out

froth

air in

CW out

CW in

coolingjacket

drafttube

gasseparation

air in

CWin

gases out

gasesseparation

cooler



In most new implementations, a batch fermentor is a prudent startingchoice, but it is generally expected that when the industrial process will bewell in hand, a number of such fermentors (four to eight) would be connectedand operated in series in a continuous fashion. This possibility should be abasic condition for the study and that option should be provided in the plantdesign. Note that an industrial setup should also include a smaller specialinoculum fermentor and one or more nonaerated drop-tanks into whichthe content of a finished fermentor is transferred to stop the fermentation,in addition to means for pasteurization (steaming) of all incoming streamsand all equipment and piping, an acceptable waste disposal treatment forthe bugs, and, in many cases, the supply of chilled water.

Once a particular process is defined and a model of fermentor is chosen,the study and design of quite large industrial units can be done from astraightforward quantitative model based on the data generated in a pilotfermentor of 10 to 100 L. Such a pilot is often made in the form of very highvertical glass pipes of 7 to 10 cm diameter, with induced circulation toduplicate the changing hydrostatic pressure effects.

Separation of Solids The rate of separation (or concentration) ofsolids from a slurry in a continuous solidliquid thickener depends on thefiltering velocity of a liquid flow through a dilute solid bed. It has beenmodeled by Kinch18 long ago and can still be calculated from a standardizedslurry settling curve in a 1 L graduated glass cylinder. This method is usedroutinely for the study of the effects of flocculating agents or other pretreat-ments on the settling rate of the slurry and on the capacity of the thickener.(See Chapter 5, Figure 5.2.)

Vacuum Filters Large industrial continuous vacuum filters can bedesigned from standardized, bench-scale, batch-filtering tests.

Rate of Continuous Separation The specific rate for the continuousseparation in industrial liquidliquid settlers can be predicted from stan-dardized batch tests following the Barnea-Mizrahi model.1923

6.3.4 Special test rigs

Despite the above examples of the better-known technologies, there are stillmany cases in which the study of a new system can only be done in a smallspecially designed continuous test rig. In such installations, the contact param-eters and the flow conditions (velocities and paths) should be changeableand controlled exactly and the resulting streams should be separated andsampled, then analyzed.

The design of this special rig should be based on a theoretical model thatwill allow to separate, as far as possible, between the different assumed phys-icalchemical mechanisms, such as:

Mixing and dispersion of phases Flow of the continuous phases near or around the surfaces of dis-

persed solids or liquid particles



Forces causing the physical segregation of dispersed phases, and/orthe coalescence of liquid droplets, etc.

The results of such tests should give a good holding on the scale-upand sizing of conventional industrial equipment, such as mixed tanks, set-tlers, or centrifuges.

Flash Dryer An important example for producing powders fromsolution would be a flash dryer. A more complex case is the drying ofvegetable protein and similar organic concentrates (particularly wheat glu-ten) that has to be done in industry in a set of severe limiting conditions.The high starting moisture gives a messy sticking consistence to the feed inthe dryer, the so-called dry powder must retain a relatively high minimummoisture in order to maintain its activity for later use, and a maximumoperating temperature and a maximum residence time should be maintainedin the heated zone, as any overheating would damage the product.

Such performance can be done, for example, in a so-called ring-dryerin which a very large flow of air is circulated around at a controlled temperature(Figure 6.3). This air stream passes through a mechanical disintegrator intowhich the protein concentrate is injected together with a stream of hotcombustion gases. The small wet particles formed in this very short shock

Figure 6.3 Main elements of a ring dryer.

bag filter

powderproduct

fan

classifier

hot gasessupply

wet gasesoutlet

wet feedmaterial

disintegrator

thermalinsulation



treatment are coated with recycled drier dust and are entrained by the hotair flow into the ring for an average number of turns (residence time). Asmall side stream is removed and directed to a classifier that concentratesthe dried particles collected from the air bleed stream in a bag filter. In viewof all these essential preconditions, such processes can only be studied anddemonstrated in a special pilot rig, which should be flexible enough to repro-duce the different sets of operating conditions, while using real vegetableprotein concentrates. Most suppliers of this type of equipment are equippedwith this pilot rig and it is preferable to work with a selected expert supplier.

Cleaning of Waste Combustion Gases Another example of the need ofspecial test rigs, which have a widely declared importance, but on which relativelylittle process information has been seriously published, is the cleaning of thewaste combustion gases from power plants and large kilns before discharge tothe atmosphere. These combustion gases are discharged from the boiler systemsin very large volumes as hot and corrosive, at a very low positive pressure,and they contain, mainly, a variable concentration of acid sulfur oxides withsome nitrous oxides and various fine ashes and other impurities. Any treatmentshould handle the very large volumes efficiently, and neutralize and eliminatethe a.m. impurities without creating any back pressure that can affect the boilersystems. From a technical point of view, the problem can be solved, but at acost! Various scrubbing systems with slurries of limestone, lime, and dolomitewere proposed and offered commercially, but all had to face the direct corre-lation between the cleaning efficiency and the bottom line cost.

One patented route25 to decrease this overall cost proposed to use ammo-nia as the neutralizing agent and to recover the ammonium salts and usethem as fertilizers. This route required an efficient scrubbing system thatcould assure that:

Objectionable impurities would be completely eliminated. No significant ammonia would remain with the exit gas. A concentrated solution of ammonia salts is obtained. No back pressure is affecting the boiler system.

A new scrubber design26 had to be developed to answer these demands,which is illustrated in principle in Figure 6.4. This is a multichamber, light-weight, horizontal scrubber. In each chamber, there is a large flow of liquidsprayed across the gas flow, maintained at a different concentration. The netflow of the solution is countercurrent to that of the gases. Ammonia gas isintroduced into the hot gas and water is fed from the other side. An auxiliaryfan is used before the chimney. The exit gases can also be reheated, if needed.

6.3.5 Indirect methods

When studying a complicated dynamic mechanism, indirect methods may some-times be used quite successfully, but the convenience of obtaining a lot ofdata by an indirect method may cover a basic difference in the mechanism.



When this author started a M.Sc. graduate research in Chemical Engi-neering a long time ago, he was directed by his tutor, David Hasson, to studythe mechanism of the creation of liquid drops by pressure spraying, by usinga convenient but indirect method.

The conventional experimental method used at the time consisted of acomplicated sampling procedure in order to collect drops on targets, fol-lowed by lengthy manual sorting under the microscope. (All of this was, ofcourse, before the automatic computer sorting available today.) The pro-posed idea was to use hot molten wax as the liquid, so that the drops wouldsolidify as spheres, and the powder could be sampled and separated intosize fractions in a conventional laboratory-sieving machine. This seemed agood idea, and after the usual literature search and study, an experimentalsetup was prepared and tried, but a prosaic problem was immediatelyencountered: the sample of wax powder was warmed and softened by thefriction on the sieve deck, which became rapidly clogged and useless. Thisgraduate researcher almost despaired when, by chance, an old hand visitorpassed through the faculty. Hearing of the problem, he said, Yes, we oncehad something similar and we solved it by adding 'dry ice' to the screens.This dry ice (solid CO2 powder) can be easily produced in the lab from aninverted pressure bottle of liquid CO2, and then sublimized while coolingits surroundings without leaving any traces. This was tried and it workedperfectly, and we started to get nice reproducible results. So, this authorreceived a very useful lesson do not keep your problem to yourself, goand consult experienced professionals.

A lot of results were collected, which correlated nicely with the operatingvariables under study. However, such correlation was completely different

Figure 6.4 Flue gases scrubber.

Am Sulfite Solution out

premixer

last

water makeup

cooling tower

optionheater

fan

fan

flue gases inammonia in

first secondintermediate

compartments

flue gases out



from the partial data published by several previous researchers, indicatinga much smaller drop size and a narrower distribution. So the gist of thethesis was shifted to the explanation of such basic difference, by getting intoa more fundamental account of the different mechanisms that occur success-fully when the liquid is sprayed out under pressure and flows away whileforming filaments and drops. The molten wax method froze literary thedynamic process in its first stage, while with the normal liquids, the largerdrops would be catching up with the smaller ones, collecting and joiningthem, and reaching a larger size distribution (which can be relevant to thespraying of paint or inside gasliquid contactors/scrubbers). So, the con-venient research technique gave very good results,26 but on a completelydifferent situation that can be relevant to certain other applications, such asthe direct spray into a reaction/combustion zone.

Thus, this author got his second lesson in R&D. Before investing a lotof work, time, and energy, one should be reasonably sure that the resultswill remain in the range of interest for the further application considered. Such atypical error is still seen today all around.

6.4 Scale-dependent operationsThese are operations in which the reaction rate, the mass transfer rate, orthe phase separation rate, obtained per unit volume of equipment, willdepend on the actual size of the equipment.

6.4.1 Vertical driving force depending on the hydrostatic height

The effect of the height of a chimney on the resulting draft is well known.The complex effect of the hydrostatic pressure on the operation of industrialaerobic fermentors was already mentioned above.

In a continuous industrial liquidliquid settler (Figure 6.5) operatingunder gravity forces, there are two layers of separated liquids and a layerof mixed phase between them consisting of a dispersion of drops fromone liquid to another. The specific rate for the continuous separation, in m3per m2 of horizontal surface, increases with the thickness of the mixedphase layer (since a higher combined hydrostatic force increases the pres-sure on the drops and accelerates the drop-to-drop coalescence). It is there-fore advantageous to operate with a settler of maximum height which canaccommodate a thicker mixed phase layer. However, there is a diminishingreturn since the quantitative function of the separation rate in m3/(m2 h) isproportional to the mixed phase thickness at the power (0.40 to 0.45). Thismechanism and optimization of such settlers was studied extensively, includ-ing the procedure for scaling up from relatively small batch tests.1923

It was then observed that the separation efficiency of the mixed phaselayer per unit volume increased as its thickness decreased. One would havethought that a flat settler would be the most efficient, but this was, ofcourse, impossible to realize without counting on the minimum volume



taken by the separated layers. This observation led to the invention anddevelopment of improved compact settlers, in which sets of inclinedpartitions, made from thin PVC plates, were installed with 7 to 10 cmdistances between them (Figure 6.6). The volume between two partitionsconstituted a flat settler fed from one direction while the separatedphases were collected on the inclined partitions and drained into verticalchimneys left between the stacks. This addition of stacks of inclinedpartitions increased the overall volumetric separation efficiency of largeindustrial settlers by a factor of about 3. This reduction in the solventinventory in the plant was obviously very important when working onlarge scale with expensive solvents.

6.4.2 Wall effect

This dependence of the results on the absolute size of certain types of equip-ment is explained in most cases by a wall effect and is related to thenonhomogeneity caused by the inside flows near the walls or to significantlarger heat losses through the walls and the like. A larger wall effect can bequantified by a relatively larger ratio of the internal wall surface to thevolume of the equipment. This effect can be more serious whenever theoperation depends on a metastable dynamic balancing (walking a tight

Figure 6.5 Liquidliquid continuous settler.

mixed phase layerheavy liquid drops

passive interface

active interface

feed in

heavy liquid phase

light liquid phase

vent

nitrogen blanket option

apparentinterface

sightglass



rope), in which a relatively short change can cause a collapse, for instancein a fluid bed solid gas contactor (see below).

6.4.3 Crystallizer

For example, it is well known that the average size of the crystals obtainedfrom a continuous industrial crystallizer increases generally with the size ofthe equipment, up to a certain maximum related to the flow mechanism andresidence time. Furthermore, this average crystal size distribution in theproduct can be very important as it determines critically the design and thedaily operation of all the downstream operations involving the crystals, suchas their filtration or centrifuging, washing, drying, screening, marketing, etc.

The driving force for the crystallization is always a certain degree ofsupersaturation which is created purposely by a chemical reaction (additionor decomposition), or by a change of temperature (mostly by cooling), or bya change in concentration caused by the evaporation of water or of anothersolvent. Such supersaturation strives to decrease by precipitation on anyexisting crystal surface.

Figure 6.6 Liquidliquid settler with sets of racks of inclined partitions.

passiveinterface

activeinterface

feed in

heavy liquid phase

light liquid phase

vent

mixed phase

nitrogen blanket option



An additional important factor in the design and the daily operation ofan industrial crystallizer is generally a size classification system between thelarger crystals, which are ready to be removed as product, and the smallercrystals that should be left inside for some additional growing time. Thissize classification can be either on an internal or an external flow cycle, andit has to be done in unfavorable conditions, such as a concentrated, heavyand viscous mother liquor. In some decomposition crystallyzers, such asin the potash industry, there is a further complication as the feed is in theform of larger, lighter crystals, which have to be kept from mixing with theproduct until they are decomposed.

So, there are many types of crystallyzers in use, but their basic princi-ples are similar and relatively simple, and for each practical case the logicalchoice can be reduced a priori to two or three possibilities to be studiedfurther in detail.

Some typical illustrations of the principle of a draft tube crystallizerare shown in Figures 6.7a and 6.7b, and of a dense slurry Oslo-type crys-tallizer for larger crystals in Figures 6.8a and 6.8b.

Coming back to the wall effect on the products size, the prevailingexplanation from experts in this field is that the circulation flow of the slurryinside the crystallizer is slower near the walls. As a result, a relatively largernumber of crystal nuclei does precipitate from the part of mother liquor thatis passing in these regions than in the part remaining in the main cycle flow.There are also more fines generated in a smaller equipment by attritionwith the higher RPMs of the impeller. This larger number of nuclei translatesinto a smaller average size of the crystal product for a fixed productiontonnage. But this is only part of the story.

On the other hand, in most well-designed crystallizers, the amount offines destruction is an effective (although somewhat expensive) operatingtool for increasing the average crystal size by reducing the number of newcrystals that are allowed to develop. Fines destruction is obtained on purposeand on demand by the dissolution of a part of the nuclei by using eitherlocal dilution or heating of the circulating clear mother liquor. In mostprocesses, the amount of fines destruction can be enhanced in the test unitto balance the negative wall effect and to get larger and nicer crystals. Theprediction of the final crystal size distribution in the final industrial unitfrom such small-scale tests does require a lot of experience (and perhapssome guessing) to balance between the contributions of the wall effect andthe fines destruction. Large-scale piloting with real streams, of course,would be much preferable, but in most projects, such piloting is not practicaluntil an advanced stage (if at all).

The final result of this dilemma is that any crystallizer installed in a firstplant with a new process is generally oversized to allow more flexibility inthe level of fines destruction and to be safer as regards the final crystal sizedistribution. As a matter of fact, many industrial crystallizers designed byreputable suppliers for new processes were finally operated at higher capac-ities, up to twice their nominal specification, after the plants experience was



(a)

(b)

Figure 6.7 Draft tube crystallizer.

feed

productslurry

vent

external heateror cooler circuit

feedcrystalsslurryproduct

crystalsslurry

vent

water

wastesolution



optimized and stabilized. (As the manager of one supplier said once, Youvegot a good deal ... why should you complain?)

6.4.4 High-temperature equipment

The study of processes based on high-temperature reactions and transfor-mations is generally done quite effectively batchwise, in small crucibles ina laboratory furnace, indicating the effects of the reactants and of the tem-perature vs. time curve in a controlled environment. This study is mucheasier when it involves a gas reacting with solids or liquid surfaces, sincethe contact is generally good and the diffusion inside and outside of thesolid or liquid is a matter of time. It is also reasonably effective for a solidliq-uid reaction if the solids have been finely ground so that the specific contactsurface is large enough. The main process problem is encountered when thereactants are

all solids

, despite the pretreatment of fine grinding and mechan-ical compaction. In this case, the addition of a

fluxing

compound is requiredthat melts and supplies a film of liquid phase between the solids in whichthe reaction can progress. But such fluxes can generally create other compli-cations downstream.

Figure 6.8a

Crystallizer with external loop.

feed

productslurry

externalheater

or coolercircuit

bleed

to vent orcondenser orvacuum unit

1360_frame_C06 Page 102 Wednesday, May 1, 2002 3:53 PM


As a typical example, the high temperature reaction of zircon (zirconiumsilicate) with calcium oxide to liberate zirconium oxide was mentioned inChapters 1 and 5. Several proposals were published for solidsolid reactionswith small additions of different fluxes, but apparently none of these routescan guarantee the complete elimination of the silica from the zirconia.

The Gorin-Mizrahi patented route mentioned in Chapter 5 involved thereaction of molten CaCl2 in intimate mixture with fine zircon powder andwater vapors (see Figure 6.927). Previous proposals have been to react, athigher temperatures, CaO and zircon in a molecular ratio of more than 3:1to obtain a mixture of CaO,ZrO2 and 2CaO,SiO2, which will have to be treatedby wet acid to separate the ZrO2. Some published trials to operate directlyin a 1:1 ratio to obtain directly ZrO2 were not successful in meeting thedesired purity as regards the very low requirement of residual SiO2. TheGorin-Mizrahi route involved the use of a reacting film of liquid moltenCaCl2 which decomposed at a relative low temperature (less than 1000C),to liberate all the HCl gas and active CaO, and gave an intermediate complex,but well defined, solid double salt of CaO,ZrO2 and CaO,SiO2, on a remaining

Figure 6.8b Crystallizer with external loop.

feed

productslurry

externalheatercircuit

bleed

to vent orcondenser orvacuum unit



core of zircon. In a second kiln at a somewhat higher temperature (about1400C), this intermediate complex was decomposed with the remainingzircon to give solid zirconia and a CaO,SiO2 melt. After quenching andleaching in a dilute HCl solution, only pure ZrO2 remained in the solid form.

The industrial large-scale processes are done in continuous rotatingdrum kilns, calciners, and dryers in which the amount of heat losses throughthe walls is generally quite significant. Large velocity gradients exist along theradius affecting the residence time. Due to these two causes, large temperaturegradients are found both along the axis and the radial dimension.

Thus, the scale up and design of a piece of high-temperature equipmentfrom the results obtained in a small continuous test unit gets more sensitiveas the overall residence time is shorter. On the other hand, over sizing ofthis type of equipment is generally not a practical option, considering thescale of production and the control of the unit cost.

6.4.5 Failure to recognize the wall effect

In certain cases, the wall effect can become an essential component of a reac-tors operation. Failure to recognize that fact in the scale up can be very serious.

When this author was very young, he witnessed such an error in aproject managed by one of the worlds largest chemical companies. A processwas developed in which a decomposition reaction was done at a high tem-perature in a fluid bed reactor, which was maintained in a fluid conditionby a stream of combustion gases introduced from below while the feedsolution was sprayed on the bed from above. The solid product from thedecomposition was collected on the fluid bed particles, which kept growing

Figure 6.9 Triangular diagram ZrO2-SiO2-CaO.

SiO21723 C.

CaO2570 C.

ZrO2 -2715 C.

CaO,ZrO2- 2340 C.

ZrO2,SiO2-1675 C.

various calciumsilicates zone

ZrO2zone

CaOzone

Ca Zirconatezone

CaO,SiO2

1540 C.2 CaO,SiO2



on the upper layers of the bed, and were taken out with a bleed stream ofthe coarser particles accumulated in the bottom part of the bed. The chem-istry was simple, but the local conditions inside the reactor were quiteheterogeneous and required a systematic vertical circulation to take thegrown coarser particles downwards. This circulation was obtained in a pilotreactor of 2-ft diameter, most probably by the wall effect, since the upwardsgas velocity near the wall is always much greater causing a vertical displace-ment in both directions. This pilot fluid bed could be maintained quite stableand the test results obtained were reasonable. However this essential effectwas ignored when the reactor was scaled up and built into a 50-ft diametertower. This catastrophic error led to a complete failure to perform, as thefluid bed was basically unstable with particles growing and growing in theupper layers while smaller particles were removed from the lowest layer. Ifthis issue had been recognized in time, a modified design with inducedcirculation could have been successful, but a new plant was left to rust anda very large investment went down the drain. Needless to say, the reasonsfor this failure have never been publicly explained to the profession, butfollowing such a shock, all the R&D projects in the chemical industry in thearea were shelved for quite a few years.

6.5 Reporting results from the experimental program6.5.1 Frequent partial reports

It has been noted (in Section 6.1.4) that one of the main bottlenecks ofany development project is always the calculation of the practical impli-cations of the results obtained from the experimental work and theirpresentation by the process engineering group. (The professional joke refersto the period in which a process engineer is requested to work 24 hours a dayand then continue the work through the night.) If these results can be madeavailable in a series of successive self-contained reports, each dealingwith one section of the process block diagram, the process engineeringgroup can start to correlate and work out these results as they come,making better use of their limited resources.

It is, therefore, important to agree on a transmittal procedure, whichcan include eventually the transmittal of draft reports (with due reserva-tions) if certain details are still not available. It is also important to identifyclearly these reports as any other project documents with the code number,revision number, date of issue, and name of the responsible person forfurther contacts.

6.5.2 Complete reports on the experiment part

In many projects, it has been seen that such experimental reports werehandled and written as internal memos of current value only. The authorsof these documents seemed to assume that the limited number of readers



should remember all the details of last weeks discussions and, thus,there was no need for further detailing. Such practice often caused seriousmisinterpretations.

But more importantly, these experimental results have been often retrieveda few years later for further studies in order to improve or expand the plantsoperation. In many cases, unfortunately, they could not be used for lack ofcritical factual information. It is therefore very important that all these exper-imental reports are written as self-contained complete scientific reports, whichcan be used also by a new guy who has just arrived on the project.

They should include full details on the purposes, the procedure, thematerials, the sampling and analytical methods, the numerical results, thecalculations procedure, any reference documents, the names of the respon-sible personnel and all the participants, and any observation or reservationor recommendations as regards the value of the results.

The few extra hours required for a complete report would be wellinvested and would be recovered, in any case, when the process packageis prepared (see Chapter 7), although possibly by a different team.

6.5.3 Implications of the results

Finally, it is important as well to prepare and present a comparison of thenumerical values from the experimental results actually obtained in the testswith the assumptions or extrapolations used by the process engineeringgroup in the preliminary process working definition (see Chapter 5, Section 5.1).

Reasonable differences can be expected and the overall effect can beevaluated readily in a recalculation of the balances with the already availablespreadsheets. But if these differences or their implications are larger andmore significant, a review meeting should be called to decide on any changein the program.

6.6 Worth another thought

There is not much point in designing and starting any significantexperimental program without performing first the process engineer-ing analysis and being reasonably sure that the results would remainin the range of interest for the further application considered.

The main purpose of the experimental program is the collection,correlation, and presentation of the design data that is specificallyneeded for the new process design and optimization in the limitedrange of variables of practical interest. Another important purposeis to observe possible, but unexpected, problems that can occur andthat should be dealt with.

If representative samples of the actual raw materials cannot be readilyprocured, an experimental program on synthetic mixtures can onlybe done as an exploratory work for the preliminary process design.



Since, in most cases, there are more than two components presentin each phase, one has to decide from the start which two compo-nents are the variables under study while the other components areto be considered as parameters for the purpose of the present pro-cess design.

Very hot organic liquid or vapor heat carrier can be introduced indirect contact with the corrosive process stream. After heat transferand equilibration, the organic liquid is separated, removed, washed,and reheated in a separate boiler made of cheaper materials.

The experimental technique called limiting conditions make it eas-ier to study, specifically, the effect of one variable at a time.

Many of the solid precipitates can be clearly identified and quantifiedby established mineralogical techniques in addition to the usualchemical analysis methods. The collaboration of a mineralogical lab-oratory can be a great help in a R&D program.

With real systems testing, the total numerical data collected repre-senting one equilibrium point amounts to possibly 10 to 20 num-bers, and its recording and presentation can be only in the form ofsystematic tables.

A 50,000 MTY fermentation plant with a batch turnover of 4 daysand a product concentration of the order of 10% in the fermentationbroth, needs 34 fermentors of 200 m3 each (5 m diameter and up to10 m high). The hydrostatic pressure at the bottom is an importantoperating parameter.

References1. Schweitzer, P.A, Ed., Handbook of Separation Techniques For Chemical Engineers,

McGraw-Hill, New York, 1979.2. Henley, E.J. and Seader, J.D., Equilibrium-Stage Separation Operations in Chem-

ical Engineering, John Wiley & Sons, New York, 1981.3. Davis, G.A., Separation Processes in Hydrometallurgy, Society of Chemical In-

dustry, Ellis Horwood, London, 1987.4. Rousseau, R.W., Handbook of Separation Process Technology, John Wiley & Sons,

New York, 1987.5. Wankat, P.C., Equilibrium Staged Separations, Prentice Hall, New York, 1988.6. McCabe, W.L., Smith, J.C., and Harriot, P., Unit Operations in Chemical Engi-

neering, 5th ed., McGraw-Hill, New York, 1993.7. Humphrey, J.L. and Kelier, G.E., Separation Process Technology, McGraw-Hill,

New York, 1997.8. Seader, J.D and Henley, E.J., Separation Process Principles, John Wiley & Sons,

New York, 1998.9. Khoury, F.M., Predicting the Performance of Multistage Separation Processes, 2nd

ed., CRC Press, Boca Raton, FL, 1999.10. Moriyama, T. and Sakaki, M., Vapor liquid equilibrium of hydrochloric acid-

calcium chloride-water systems (in Japanese), kogyo kagaki zasshi, 64, 1877-1878, 1962, (see also French Patent 979,790, 1965).



11. Tyshotskaya, O.V. and Grinstein, I.M., The system HCl-H2O-CaCl2, sborniktrudy vese. nauchn, issled inst gidro sulfitn prom. (Russian), 13, 184202, 1965.

12. Mizrahi, J., Barnea, E., and Gottesman, E., Production of concentrated HClfrom aqueous solutions thereof, Israel Patent, 36,304, 1972.

13. Lotzch, P. and Scherz, G., System HCl-H2O-MgCl2, Chem Technol. (German),339340, 1973.

14. Kolek, J.F., Hydrochloric acid recovery process, Chem. Eng. Prog., 69, 47-50,1973.

15. Lo, T.C., Baird, M.H.I., and Hanson, C., (Eds.), Handbook of Solvent Extraction,John Wiley & Sons, New York, 1983.

16. Ritcey, G.M. and Ashbrook A.W., Solvent Extraction, Principles and Applica-tion to Process Metallurgy, Vol. 2, Elsevier, Amsterdam, 1984.

17. Rydberg, J., Musikas, C., and Choppin, G.R., Principles and Practice of SolventExtraction, Marcel Dekker, New York, 1992.

18. Godfrey, J.C. and Slater, M.J. (Eds.), Liquid-Liquid Extraction Equipment,John Wiley & Sons, New York, 1994.

19. Barnea, E. and Mizrahi, J., Compact settler gives efficient separation of liquid-liquid dispersions, Proc. Eng., 6063, 1973.

20. Barnea, E. and Mizrahi, J., Separation mechanism of liquid-liquid dispersionsin a deep-layer gravity settler (4-part series), Part 1: The structure of thedispersion band; Part 2: Flow patterns of the dispersed and continuous phaseswithin the dispersion band; Part 3: Hindered settling and drop-to-drop coa-lescence in the dispersion band; Part 4: Continuous settler characteristic,Trans. Inst. Chem. Eng., 53, 61-69, 70-74, 75-80, 83-93, 1975.

21. Barnea, E. and Mizrahi, J., The effects of a packed-bed diffuser precoalesceron the capacity of simple gravity settlers and on compact settlers, paper Proc.Int. Solvent Extraction Conference, Toronto, 374384, 1977.

22. Barnea, E. and Mizrahi, J., A generalized approach to the fluid dynamics ofparticulate systems, Part 1: General correlation for fluidisation and sedimen-tation in solid multi-particle systems, J. Chem. Eng., 5, 171-189, 1973.

23. Barnea, E. and Mizrahi, J., A generalized approach to the fluid dynamics ofparticulate systems, Part 2: Sedimentation and fluidisation of clouds of spher-ical liquid drops, Can. J. Chem. Eng., 53, 461-468, 1975.

24. Clue, A.S., POB 1723, 5816 Bergen, Norway, e-mail [email protected]. Mizrahi, J., A scrubber for the treatment of flue gases, Intern. PCT Patent WO

99/20371, Washington, D.C., Appl. 22.10.97, assigned to Clue, A.S.26. Hasson, D. and Mizrahi, J., The drop size of fan-spray nozzles, Trans. Inst.

Chem. Eng, 39, 415-422, 1961.27. Qureshi, M.H. and Brett, N., Phase equilibria in terniary systems containing

zirconia and silica, Proc. British Ceramic Soc., 67, 1968.



Developing an Industrial Chemical Process, An Integrated ApproachTable of ContentsChapter 06: Experimental program6.1 Basis6.1.1 Experimental program purposes6.1.2 Different sections6.1.3 Quantitative data needed for process design6.1.4 Format6.1.5 Representative raw materials6.1.6 Classification of missing data

6.2 Chemical equilibrium data6.2.1 Vaporliquid equilibrium system6.2.2 Gasliquid equilibrium system6.2.3 Liquidliquid equilibrium system6.2.4 Solidliquid equilibrium system6.2.5 Reversible and nonreversible equilibrium6.2.6 Chemical equilibrium laboratory tests6.2.7 Experimental difficulties in chemical equilibrium tests

6.3 Dynamic flow conditions6.3.1 Design data required6.3.2 Simpler processes6.3.3 Theoretical models6.3.4 Special test rigs6.3.5 Indirect methods

6.4 Scale-dependent operations6.4.1 Vertical driving force depending on the hydrostatic height6.4.2 Wall effect6.4.3 Crystallizer6.4.4 High-temperature equipment6.4.5 Failure to recognize the wall effect

6.5 Reporting results from the experimental program6.5.1 Frequent partial reports6.5.2 Complete reports on the experiment part6.5.3 Implications of the results

6.6 Worth another thoughtReferences

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