Miniplate

FIG. 36 Synergistic mixture of alkane- (paraffin) sulfonates (PS) and fatty alcohol ether sulfates (FAES). Cleaning effect in miniplate test at 50°C, tap water (12° German hardness), 0.075 g of active surfactant mixture per liter. 

When implemented at the miniplant reactor scale, there was good agreement between the model predictions and measurements of residual monomer as shown in Figure 4. 

Rinard, I. H., Miniplant design methodology, in Ehreeld, W., Rinard, I. H., Wegeng, R. S. (Eds.), Process Miniaturization 2nd International Conference on Microreaction Technology, IMRET 2, Topical Conf. Preprints, pp. 299-312, AIChE, New Orleans (1998). 

Figure 5.2-1 illustrates the iterative and interactive nature of process development for fine chemicals. As shown, a process is evaluated at any stage to decide whether to continue or to stop process development and abandon it. In the development of fine chemistry processes, the sequence of steps is often laboratory-miniplant-commercial production. It must be emphasized that in process development for fine chemicals efforts in various sectors, including process design, are often made in parallel and not necessarily in sequence. 

This application was performed on a lab-scale reactor and later in a miniplant-scale reactor. The reaction studied was the vapor phase catalytic amidation/cyclization step in a pesticide process. As shown in Figure 2, two reactions are taking place on the catalyst bed. 

Whereas in a micro plant only the most important process steps and some of the recycle loops are tested, in the miniplant the complete process with all recycle loops is executed and processed continuously for several weeks. For environmental and economic reasons, solvents and unconverted reactants are recycled. Here, it is especially important to find out if small impurities below the detection limit will possibly accumulate or form deposits. To minimize the up-scaling risk for the final production plant, often a pilot plant is the next scale-up step. Now the final product is produced and often first samples are sent to customers. Every step in this scale-up procedure is followed by an evaluation step defined in a number of evaluation studies, which assist in the decision as to whether the project has to be terminated, repeated or further processed. 

With the introduction of advanced process simulation tools [25], the so-called integrated process development established. Here the most time-consuming pilot plant phase is bypassed by a combined approach of experimental miniplant testing and numerical process modeling. 

Miniplant Technology - A Model for the Micro Structured Reactor Plant Concept... 

As miniplant technology must be considered as a model for micro structured reactor plant technology, a closer look into its working principles is very illustrative. All characteristics of a miniplant were described by the inventors of this technology, Dow, in 1979 [28] and BASF [29], which also introduced the combined simulation/ experimentation concept (Figure 4.2). 

The above-mentioned integrated process development combines simulation tools with miniplant equipment to bypass the set-up of a pilot-scale plant Similar to this combined approach, a micro structured reactor plant can bridge laboratory-scale and micro-scale development. One could think of a future micro-integrated process... 

It must be pointed out again that even today confusion of terms can be observed when chemical engineers discuss miniplants or microplants . In most of these cases they identify with the terms mentioned above chemical plants made of glassware with volumes in the range of up to a few liters. To summarize, at that early stage no specialized micro structured reactors for production purposes were available. Most of the fabricated micro structured devices were made in terms of micro fabrication capabilities and not adapted to the chosen chemical process. It is no wonder that at first visionary theoretical work either had to be based on conventionally fabricated chemical reactors or did not outline reactor design in detail [30],... 

Paradigm Change Drives Miniplant Design Methodology... 

The miniplant concept was proposed first by Ponton at the University of Edinburgh (UK) in 1993 [1,58-60], The potential benefits of distributed processing, as outlined above and reported elsewhere [2, 4], were identified, in particular focusing on an increase in process safety. However, the critical evaluation of the miniplant concept also revealed a number of potential drawbacks. For instance, the specific production costs of a small production units not only depend on reactor miniaturization, but are also determined by control instruments and other peripheral equipment. 

These disadvantages should be overcome by miniplants, generally defined similar as in [1, 60], strictly based on a modular design. In order to adapt to varying needs, several modules may be operated in parallel. Typical module capacities range from 100,000 to 1,000,000 lb/yr. Operation of the miniplants should be so reliable and simple that the majority of these plants can be operated by personnel not specially skilled in process technology. Start-up and shutdown have to be performed fast to allow just-in-time production. The entire plant should be transportable including footprint and containment volume. 

Another issue with the miniplant concept relies on standardization, rendering a different strategy for increasing throughput. Whilst current scale-up processes are based on an incremental progression of know-how, this should be achieved in miniplants in one stage. The small standardized modules should, at best, be fabricated using mass production techniques. 

Encapsulation in containment vessels allows near zero emission rates to be reached in miniplants, e.g. by purging the reactor with an inert gas sent to a scrubber. This embedding of the miniplant should also dramatically reduce the risk of explosion or environmental contamination in case of an accident Even if modules of the miniplant are damaged or break, the robust encasement will be mechanically and chemically stable enough to prevent pollution. 

Ponton [60] discusses exemplarily a miniplant concept for performing the Andrussov process, yielding hydrogen cyanide from methane, oxygen and ammonia with a platinum catalyst. HCN is a widely used but highly toxic chemical which requires extreme safety issues, in particular when it is transported or shipped. A miniplant should allow one to produce this toxic material from comparably low toxic ammonia and methane directly on-site at the customer and on-demand in small or even bigger quantities. 

Following a similar motivation, a schematic flow scheme of a miniplant for hydrogen cyanide formation via the Degussa route was proposed in [30],... 

Figure 4.8 Flowsheet of an HCN miniplant [60] (by courtesy of Springer Verlag).

The volume flow in a typical miniplant is of the order of 101 h 1. The limiting factor is the gravity-driven flow in the separation units, for example, a rectification column. As separation units usually accompany a chemical process, this flow limit dominates the overall capacity of a miniplant. It is surprising that the flow rate is not limited here by the pressure loss. 

In a microstructured reactor plant, in contrast, the flow rate will be dominated by the pressure loss. Typical pressure losses in micro devices are of the order of 1 bar at a flow of 11 h 1 (water) [50,93], If sufficient pump capacity is available, the pressure loss in a micro structured device is limited by the mechanical stability of the reactor housing, which is often made of steel and hence a loss of several bar is certainly acceptable. Even the combination of up to 10 different micro devices only amounts to about 10 bar in this example. The main advantage of a micro structured reactor plant is that the flow rate can be adjusted more freely because the flow is pressure driven and not influenced by a single gravity-driven device as in a miniplant. 

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