Pilot testing of specific process operations

Pilot testing of some process operations may be required to confirm detailed quantitative specifications for particular pieces of equipment items, when these operate with the exact streams of the new process. Examples are given below.

9.4.1 Multiple-effects evaporator

The basic design of a multiple-effects evaporator for budgeting purposes

could be based on bench-scale equilibrium data, but the reliable detailed design of an industrial installation will require the experimental determina- tion of certain quantitative factors that could be still unknown, such as, for instance:

• What heat transfer coefficient can be obtained with different concen- trations of the solution (density, viscosity) and with different veloc- ities in the tubes, and what will remain from the heat exchanger’s performance after a few hundred hours of operation and the resulting deposits (coating) on the heat exchange surface?

• What would be the possible effect of any noncondensable gases or soluble impurities dissolved in the feed solution on the behavior of the solution boiling inside, such as frothing, splashing, precipitation, and possible encrustation of solids?

• What would be the frequency, method, and ease of internal cleaning and how would that affect the average number of working hours for design?

• What would be the “external” behavior of the concentrated solution, once it is removed from the evaporation conditions (depressuriza- tion, cooling)?

Furthermore, larger quantities of concentrated solution may be required for testing of the downstream operations relative to this evaporator, such as a crystallizer, a flaker, a spray-dryer, and so on.

These requirements necessitate the continuous operation of a pilot evapora- tor, equipped with all the instrumentation for collecting the necessary data, while storing the resulting concentrated solution in suitable containers. The test period would be relatively long (a few weeks, for example) and sufficient quantities of the starting solution needed, of the actual composition or as close as possible (with due reservations). Such piloting can be done in a specialized R&D institute, or in cooperation with a potential equipment supplier, who could rent a portable pilot installation and operate it.

9.4.2 Liquid–liquid contacting battery

Another example of a major package is a “multiple-stage, counter-current, liquid-liquid contacting battery” for a solvent-extraction process. For design- ing a “horizontal” battery of mixers–settlers, in which each stage is assumed to be practically at equilibrium, all the process aspects can be calculated reliably from the results of bench-scale equilibrium tests, such as the number of theoretical stages, the concentrations, the mass transfer rate in a mixed vessel, and the liquid–liquid separation rate (see Chapter 6, Sections 6.2.3 and 6.4.1). This choice of equipment was therefore popular for implementa- tion of new processes (Chapter 4, Reference 13) and it would probably still be the best choice for a small number of stages (say three to five).

However, when a larger number of stages is required, with larger flow- rates and more costly solvents, the option of a mixers–settlers battery could present significant disadvantages as compared to a continuous vertical col- umn-contactor, or to a set of centrifugal extractors. These disadvantages could be, for example, a bigger internal inventory of solvent, a larger hori- zontal area in the plant’s layout, the need for more intermediate pumps, and so on. These issues have been discussed extensively in international confer- ences and some recent papers relevant for industrial equipment are listed in the references.11–14 _{Cusack et al.}11 _{presented the development of a new high} capacity column design from an analysis of previous models (Koch). Axial mixing in large-scale packed extractors is detailed in Reference 12. Lo13 reported on an experimental comparison among three different existing models of columns, his conclusions for one particular case and the scale-up procedures to be used for an industrial project. Movsowitz et al.14 _reported on a rather exceptional case in which a uranium plant in Australia worked

for two years in two parallel lines, one with four mixers-settlers and the second with two Bateman columns and then it was decided to use (of course) more columns for their new expanded plant.

Several suppliers offer vertical columns/contactors, each with their own proprietary design and know-how. None of these columns can be reliably sized without extensive pilot testing for each case with the actual materials, in order to determine: acceptable velocities, height of a theoretical contact stage, behavior of the phases mixture (observations), starting procedure until a steady-state operation is reached, and so on. Therefore, each supplier is organized with its own portable pilot installation and expert staff, which can be hired by a prospective client to conduct such tests with his own materials, for process demonstration and equipment sizing. In many cases, the hiring fee for the pilot is deducted from the purchase price of the industrial equip- ment, if a deal is reached.

But for the process engineering group, the main issue before ordering such equipment for a novel process is to understand the internal mecha- nisms, which are generally not entirely published. For example, how the performance is scaled-up and what can be modified if the results obtained during start-up are unsatisfactory?

9.4.3 Main problems for piloting

The above typical examples emphasize the two main problems related to the piloting of specific process operations, from the point of view of the implementing corporation:

• The investment in a new, owned pilot would be expensive, require in-house expertise and a relatively long time to start and, therefore, would be justified only for a long-term continuing R&D program in this particular field. Otherwise, after the conclusion of these series of tests, this pilot installation could remain unused for a long time. On the other hand, these pilot tests could be possibly done in cooperation of a pre-selected supplier, as most suppliers of specialized equipment have their own pilot installations. However, such preselection could impose many formal limitations, which should be clear and accept- able from the beginning.

• The procurement of a sufficient quantity of representative feed solu- tion may be difficult and may need to be produced in another pilot, according to the upstream operations (before the one under consid- eration). This condition may require a more comprehensive and lengthy program.

9.5 Modeling

The methodology and technique for the development of a dynamic mathe- matical model that can simulate a specific process have been occupying the

attention of the chemical engineering scientific community for the last two or three decades, and have evolved rapidly with the advancement of com- puterized resources. This is one of the most popular (fashionable?) academic fields in chemical engineering faculties and many commercial programs are also offered to the professionals. There are good textbooks and publications and there is therefore no need to recapitulate them here.1–4

However, any such model can only be as good and accurate as the numer- ical data in its data base and this weak point has discouraged many developers of novel processes. It is therefore important to recognize the importance and the need for such a tool, by considering it a long-term investment in the process development program. Thus, the model can be started and run first on the basis of “reasonable assumptions” and the significance of the results can be studied and understood. Then, after the first runs, a list should be prepared of certain data with significant leverage, which should preferably be confirmed and completed by additional bench-scale tests (see Section 9.6.3 below). The model will therefore be progressively improved.

A dynamic mathematical model, as the quantitative basis of this specific process, will be used first for the important, but not critical, task of the engineering design of the instrumentation and control systems and for deci- sions concerning the volumes of buffer tanks. (This design is not critical because it is dealing with relatively wide ranges.) However, at a later stage of the plant’s design (see Chapter 10, Section 10.2), this dynamic mathemat- ical model should be used for a critical task, in order to evaluate the conse- quences of any change in: the composition of the raw materials, the concen- tration of possible impurities, the kinetics of mass transfer, or the quality requirements from the new products.

Finally, after the plant’s start-up, this model should be expanded to correlate and interpolate the operating plant’s results. This expansion will hopefully culminate in the achievement of new process know-how for the developing corporation.