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Ind. Eng. Chem. Res. 2007, 46, 2268-2271
New Metrics for Evaluating the Performance of Membrane Operations in the Logic of Process Intensification Alessandra Criscuoli*,† and Enrico Drioli†,‡ Institute on Membrane Technology, ITM-CNR, Via Pietro Bucci Cubo 17/C, 87030 Rende (CS), Italy, and Department of Chemical Engineering and Materials, UniVersity of Calabria, Via Pietro Bucci Cubo 42/A, 87030 Rende (CS), Italy
The aim of this work is to discuss the role of membrane operations for re-designing industrial productions in the logic of the process intensification. In particular, new metrics for comparing membrane performance with that of traditional operations are tentatively proposed. The comparison is performed in terms of the following: productivity/size ratio; productivity/weight ratio; flexibility; modularity. Referring to the flexibility of the plant, two different aspects are considered: the ability to handle variations that might occur during the life of the plant (such as variation of the pressure, the temperature, or the feed composition) and the ability to be applied in different cycles of production. The modularity takes into account the changes of the plant size related to variations of the plant productivity. As a case study, the new metrics are applied to the sparkling water production. Although the new metrics have been defined for membrane operations, the proposed approach can be more generally applied for evaluating the “degree of intensification” achievable with any other process of interest. Introduction The development of improved systems of production is an important issue for reaching the objective of a sustainable growth. The achievement of an industrial growth able to preserve and/or improve the quality of life without compromising the needs of future generations is, in fact, one of the main challenges of modern society. In past years, efforts have been made for defining/identifying indicators to measure the “sustainability” of industrial processes and, in particular, their impact on three specific areas: environment, economy, and society.1-5 Environmental indicators take into account the resource usage (energy, material, and land) and the emissions, effluents, and waste related to a certain production (values are often calculated per kilogram of product). A similar approach has been followed in developing the so-called “green chemistry”.6 Economic indicators refer to investments, profits, values, and taxes, whereas social indicators mainly consider the employment situation, the health and safety at work, and the benefits/troubles for the community. A strategy strictly related to the sustainability concept is that of process intensification. According to the process intensification strategy, in fact, future industrial cycles of production must provide higher production capacities, reduced energy and raw materials consumption, increased safety, and reduced equipment size and waste production.7-10 Membrane operations are well-known for their compactness (they have a high area/volume ratio), their flexibility, and their modularity, all characteristics that can help in reaching the objectives of the process intensification. In past years, both the improvements reached in membrane properties and the development of new membrane systems (e.g., membrane contactors) result in a higher presence of membrane operations in industrial
processes. For example, the water deoxygenation step in the electronic industry for semiconductor manufacturing is today carried out by membrane contactors that find application also in the beverage market.11 To estimate the convenience of a membrane process with respect to a conventional one, the exergy analysis and the substitution coefficient calculation are used.12-13 The exergetic efficiency of a process is obtained by calculating the entropic losses (or production of entropy): the exergy destroyed in entropy production corresponds to a lower work obtained or a higher work required. Therefore, the exergy analysis helps in identifying the parts of the plant less efficient. Concerning the substitution coefficient (CS), it compares the primary energy saved to the electrical energy consumed in cycles that utilize electricity-consuming operations, like membranes, in substitution of conventional thermal operations. Usually, the substitution results to be convenient when the CS is greater than 10.5 MJ/kWh (2.5 Mcal/kWh). In this work, new metrics for comparing membrane performance with that of traditional units are proposed. With respect to the existing indicators the new metrics take into account the size, the weight, the flexibility, and modularity of the plantssall voices that play an important role in the evaluation of the overall performance in the logic of the process intensification. These metrics do not replace the existing ones, because they refer to other aspects of the production plants. Therefore, in the final evaluation of the process performance, the new metrics must be considered together with the environmental, economic, and society indicators. As a case study, the application of the new metrics to the sparkling water production is presented and discussed. Definition of New Metrics
* To whom correspondence should be addressed. Tel.: +39-0984492118/2034. Fax: +39-0984-402103. E-mail:
[email protected] (A.C.);
[email protected] (E.D.). † ITM-CNR. ‡ University of Calabria.
The first metric proposed compares the productivity (P)/size ratio (PS) of membrane units with that of conventional operations (eq 1). Future plants should be characterized by high productivities and low sizes; therefore, when the PS metric is
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higher than 1, membrane operations should be preferred, whereas, for PS values lower than 1, traditional units should be chosen.
PS (productivity/size ratio) )
P/size(membranes) (1) P/size(traditional)
Another important parameter in installing new plants is the weight of the operating units involved, especially for installations off-shore or for plants that are built in remote areas. To take into account this aspect in making the plant design, the productivity/weight ratio (PW) is defined (see eq 2) that compares the productivity/weight ratio of membrane units with that of conventional systems. Plants with high productivities and low weight are preferred; then, a PW higher than 1 is in favor of membranes, whereas a PW lower than 1 means that traditional systems are performing better.
PW (productivity/weight ratio) )
P/weight(membranes) P/weight(traditional) (2)
Plants of future generations should be flexible as much as possible. First of all, they should be able to handle variations that might occur during the life of the plant (such as variation of the pressure, the temperature, or the feed composition), to reduce the costs associated with changes/modifications of the existing operating units. The flexibility metric (see eq 3) compares membranes and traditional devices in terms of variations for which the existing plants are able to work, without requiring any type of adjustment. As for the previous metrics, membranes have to be preferred when a value higher than 1 is obtained.
flexibility1 )
variationshandled(membranes) variationshandled(traditional)
M (modularity) )
|area2/area1(membranes) - MI| |vol2/vol1(traditional) -MI|
(6)
The new proposed metrics have been calculated for the sparkling water production, as reported in the next section. Case Study: Sparkling Water Production The sparkling water production is usually carried out in packed columns where the water to be carbonated is deoxygenated by sending as stripping stream, carbon dioxide. The deoxygenated water, slightly carbonated, is then carbonated up to the desired value by injecting under pressure carbon dioxide in the pipeline. Figure 1 shows a scheme of the traditional process. The same operation can be carried out by using membrane contactors. In this case hydrophobic membranes are employed to keep in contact the water stream and the carbon dioxide. Due to a difference in partial pressures, carbon dioxide is transferred from the gas side to the liquid and, simultaneously, the oxygen is removed from the water to the gas side. With this system, the deoxygenated water at the exit of the membrane module already contains the desired value of dissolved carbon dioxide, and there is no need of further CO2 injections (see Figure 2). The proposed new metrics have been calculated by considering the same feed stream characteristics (Table 1) and the same targets (Table 2) for the two systems. Concerning the membrane system, hollow fiber modules of 130 m2 in series have been considered, each one with a size of 0.08 m3 and a weight of 100 kg. The internal diameter of the fibers was about 200 µm and the pore size around 0.03 µm. Data on the traditional system have been obtained from industrial companies. In Tables 3 and 4 the carbon dioxide flow rate and pressure, the size, and the weight required, respectively, for the membrane
(3)
Referring to the flexibility of the plant, the ability to be applied in different cycles of production is another important aspect to be considered. If the plant is converted for a different production, the presence of versatile operating units makes easier and cheaper the change. Equation 4 expresses the flexibility as the ratio between the number of processes that membrane units are able to perform and that related to traditional systems. Again, a flexibility value higher than 1 is in favor of membranes. Figure 1. Scheme of the sparkling water production by traditional systems.
flexibility2 )
Nprocesses performed(membranes) Nprocesses performed(traditional)
(4)
A typical property of membrane operations is the modularity. The modularity takes into account the changes of the plant size due to variations of the plant productivity. To compare the modularity of membranes with that of traditional units, a modularity metric is defined (see eq 6). Given a variation of the plant productivity (e.g., from productivity1 to productivity2s see eq 5), this metric compares the variations of the area (for membranes) with those of the volume (for conventional systems). The membrane system has a higher modularity if the modularity metric is lower than 1; modularity values higher than 1 are in favor of the traditional system.
productivity2 MI (modularity index) ) productivity1
(5)
Figure 2. Scheme of the sparkling water production by membrane systems. Table 1. Feed Stream Characteristics water flow ratesproductivity (m3/h) temperature (°C) pressure (bar) O2 content (ppm)ssaturation value
30 20 1 9.24
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Table 2. Target Values for the Exit Stream water flow rate - Productivity (m3/h) temperature (°C) O2 content (ppm) CO2 content (g/L)
30 20
