Extraction Capacity of Carbon Dioxide

Monomer extraction from latex products involves mass transfer in a heterogeneous system. Three phases have to be considered the polymer particles, the aqueous phase, and the CO2 phase (see Fig. 14.3). In this work, the film model has been used to describe the diffusion of MMA in this three-phase system, for which the following assumptions have been made  

The mass transport flux of MMA in each phase is defined as  

The equilibrium equation of MMA between two phases is defined by a partition coefBcient  

To quantify the extraction capacity of carbon dioxide, the partition coefficient of MMA between the water and the C02-phase (m2) is a prerequisite. Since this partition coefficient is not reported in literature, m2 has been measured in a laboratory scale extraction unit at different temperatures and CO2 pressures and has also been predicted using the Peng-Robinson equation of state. 

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As a general rule, the respiration of individual cells decreases as the concentration of carbon dioxide in the medium increases. Fish show a lessened capacity to extract oxygen from their environment with increasing amounts of carbon dioxide present. On the other hand, many invertebrates show marked increases in respiratory rate (or ventilation) with increased amounts of the gas In their surroundings. 

The operating conditions for the extraction column were selected from the experimental data previously measured by choosing the conditions that yield the more favorable capacity and selectivity of carbon dioxide for the FFA fraction. We selected a pressure of 21 MPa and a temperature of 323K. A flowsheet of the proposed extraction process is shown in Figure 1. The separation of the oil components from the solvent stream exiting the extraction column is made in two steps, named SCi and SCm, in order to regenerate the most part of the solvent. The raffinate stream, S2, flows to a third separation column, SCD, to regenerate the carbon dioxide solubilized in that stream. 

Supercriticial Solvents. Although it was known in 1879 that supercritical fluids had solvent properties (180), supercritical extraction was not extensively developed until the early 1980s. This method uses organic or inorganic compounds as solvent, at or usually above their critical temperature and pressure where they are known as supercritical fluids. In a supercritical fluid state, common gases such as carbon dioxide have the properties and extractive capacity of a liquid. The compound most used in supercritical extractions is carbon dioxide. Carbon dioxide can exist as a gas, liquid, or solid, depending on pressure and temperature conditions. However, at or above its critical point. CO2 can only exist as a supercritical fluid. 

The instrumentation for SFE can be relatively simple as shown in Figure 29-10. Instrument components include a fluid source, most commonly a lank of carbon dioxide a syringe pump having a pressure rating of at least 400 atm and a flow rate for the pressurized fluid of at least 2 mL/min a valve to control the flow of the critical fluid into a heated extraction cell having a capacity of a few milliliters and an exit valve leading to a flow restrictor that depressurizes the fluid and transfers it into a collection device. In the simplest instruments, the flow restrictor is 10 to 50 cm of capillary tubing. In modern sophisticated commercial instruments, the restrictors are variable and controlled manually or automatically. Several iastrument manufacturers offer various types of SFE apparatus. ... 

In view of the attractive features of carbon dioxide, it seems certain that any further hop extraction plants will employ CO2 as have all the recently built plants. Because of the flexibility available in the choice of operating conditions for a supercritical plant and the possibility of using any spare capacity for other purposes such as decaffeination of tea or coffee, it is also probable that any future hop extractors will be basically supercritical, but capable of operating under liquid or near-liquid conditions. 

Calculations of treatment costs for supercritical soil remediation were made with a computer model that evaluates the capital and operating costs depending on plant capacity, carbon dioxide conditions for extraction and separation, operating conditions, soil transport and pretreatment and other boundary conditions like maintenance, depreciation or insurance. Some additional important parameters for the plant design are given in table 1. 

Except for ethanol, all modifiers enhance the solvent capacity of the extraction fluid at 2 mol% and 4 mol%. Still, large enhancements only occur with carbon dioxide modified with cyclohexanon and especially with acetonitrile. 

The extraction of lipids and alkaloids with compressed carbon dioxide was performed at 298 K and 313 K using the pressures 8, 10, 15 and 20 MPa in an apparatus similar to that schematised in Figure 1. The extraction cell is a stainless steel vessel with 220 cm3 of capacity specially built for this purpose. 

The concept, presented in the paper, forecasts a big profit when using new coal-fired power plants without any atmospheric emissions and injection of high pressure carbon dioxide, produced in the plant, into old oil fields for almost total oil extraction. With a capacity of 14 GW in Europe it is possible to extract 300 Mt of oil. The concept is undoubtedly worth being funded for further studies for the design of a demonstration plant. 

The first report on DFA formation in higher plants dates back to 1933, when Schlubach and Knoop [23] isolated a compound tentatively identified as a-D-fructofuranose p-o-fmctofuranose l,2 2,l -dianhydride (10, also known as DFA I) from Jerusalem artichoke. Alliuminoside, a difructofuranose 2,6 6,2 -dianhydride for which configuration at the glycosidic linkages was not determined, was isolated from tubers of Allium sewertzowi [24], However, the fact that these results have not been further confirmed throws some doubt onto whether the DFAs were actually from plant origin or were formed by the presence of microorganisms. The enzymic formation of a-o-fructofuranose p-o-fructofuranose l,2 2,3 -dianhydride (1, DFA HI) in sterilized homogenates of the roots of Lycoris radiata, a plant use in China as a traditional folk medicine, unequivocally demonstrated the capacity of this plant to produce this particular DFA [25]. The compound was further extracted from the intact bulbs by supercritical carbon dioxide and its structure unequivocally established by NMR [26]. 

Much of the commercial interest has been in the food and pharmaceutical industries. Here, the major driving force is the desire to have conpletely natural processes, which cannot contain any residual hydrocarbon or chlorinated solvents tHumphrey and Keller. 19971. Supercritical carbon dioxide has been the SCF of choice because it is natural, nontoxic, and cheap, is conpletely acceptable as a food or pharmaceutical ingredient, and often has good selectivity and capacity. Currendy, supercritical CO2 is used to extract caffeine from green coffee beans to make decaffeinated coffee. Supercritical CO2 is also used to extract flavor conpounds from hops to make a hop extract that is used in beer production. The leaching processes that were replaced were adequate in all ways except that they used solvents that were undesirable in the final product. 

Plants need water and carbon dioxide along with sunlight for photosynthesis. Shortage of water in the soil and low insolation slow down photosynthesis. Crop yield depends also on the availability of minerals (fertilizer). The utiUzable water available to the plant is the difference between the retention capacity of the soil and the limit of extraction, and this capacity is dependent on the type of soil. In arid zones, the water which is available to the plant is only a fraction of the water received by the soil because the latter ends up in different places, for instance, as runoff water, seepage water lost or diverted, or water which is a constituent part of the soil and is not extractable by the roots. If the water extracted by the roots is insufficient, the plant will wilt and may eventually reach the permanent wilting point. Each plant requires a certain depth of soil for occupation by its roots and the water... 

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