Decompression rate

The reaction rate constant and the diffusivity may depend weakly on pressure (see previous section). Because the temperature dependence is much more pronounced and temperature and pressure often co-vary, the temperature effect usually overwhelms the pressure effect. Therefore, there are various cooling rate indicators, but few direct decompression rate indicators have been developed based on geochemical kinetics. Rutherford and Hill (1993) developed a method to estimate the decompression (ascent) rate based on the width of the break-dovm rim of amphibole phenocryst due to dehydration. Indirectly, decompres-... 

Index Entries Lipase activity high pressure decompression rate C02 SCCOz exposure time. 

Many enzymes are stable and catalyze reactions in supercritical fluids, just as they do in other non- or microaqueous environments (7). Enzyme stability and activity may depend on the enzyme species, supercritical fluid, water content of the enzyme/support/reaction mixture, decompression rates, exposure times, and pressure and temperature of the reaction system. 

To understand the potential of pressure application to enzyme processes and to help elucidate the reaction mechanism as well as a rational design of alcoholysis reactors for future scale-up, we investigated the influence of temperature, pressure, exposure times, and decompression rates on the activity of a commercial immobilized lipase (Novozym 435) activity in high-pressure C02 medium. 

Lipase (approx 1.0 g) was charged into the cell, and the temperature established in the experimental design was reached. Afterward, the system was pressurized and maintained at a constant temperature and pressure for a preestablished exposure time. Typically, the pressure elevation time was <0.5 min and was not included in the pressure holding time because of its comparatively short duration. Then, at the decompression rates (10-200 kg/ [m3min]) defined, the system was depressurized and the lipase activity was measured. The loss of lipase activity was defined as the difference between the activity at the beginning and at the end of the process. 

Run Temperature (T) (°C) Initial pressure (bar) Exposure time (t) (min) Decompression rate (R) (kg/[m3min]) Reduced density (RD)... 

A Taguchi experimental plan with two levels and four variables (temperature, exposure time, decompression rate, and reduced density) was adopted. The experimental plan, covering the variable ranges commonly usedfor transesterification reactions (1), is presented inTable 1. The experiments were accomplished randomly, and duplicate runs were carried out for all experimental conditions leading to an average reproducibility better than 5%. The activity loss was then modeled empirically in order to determine the influence of the process variables on main and cross-interaction parameters. 

The experimental results obtained are presented in Fig. 2. One can observe from Fig. 2 that temperature, reduced density, and exposure time influenced positively the activity loss whereas decompression rate had a weak negative effect. 

Table 2 presents the results obtained in the statistical modeling. Temperature and reduced density had a pronounced effect on enzyme activity loss, both showing a positive effect. At this point, it is important to mention that the cross-interaction temperature-exposure time had a significant negative effect. In the range investigated (10-200 kg/[m3-min]), the decompression rate had a weak negative effect on loss of enzyme activity. The same effect was observed with the exposure time (60-360 min). 

We conclude that a commercial immobilized lipase from C. antarctica (Novozym 435) was stable in SCC02 for all experimental conditions investigated. Based on the results obtained here and comparison of them with the results obtained by other investigators, it can be concluded that the magnitude of pressure, temperature, decompression rate, and exposure time needed to inactivate the enzyme strongly depends on the nature and the source of enzyme and, primarily, whether the enzyme is in its native or immobilized form. For the purpose of using this enzyme to catalyze the transesterification reaction of vegetable oils in order to produce esters, the results obtained herein are relevant, because the immobilized lipase can be used with low activity loss at typical conditions of temperature and pressure employed in many biotransformations of raw materials. 

Hermetically sealed assemblies were not as compatible and, as expected, did not tolerate the pressure of the cleaning process. A motor stater, composed of wire windings potted in place with an unfilled epoxy was also found to be incompatible with the cleaning process. In this case, the carbon dioxide would fill voids in the epoxy and then during depressurization cause fi cturing at these voids. Later this problem was alleviated by cleaning at a lower pressure and decreasing the decompression rate. 

The rate of decompression can also have an effect on the ability of the compacts to consolidate (form bonds). Based on the liquid-surface film theory, the rate of crystallization or solidification should have an effect on the strength of the bonded surfaces. The rate of crystallization is affected by the pressure (and the rate at which the pressure is removed). High decompression rates should result in high rates of crystallization. Typically, slower crystallization rates result in stronger crystals. Therefore, if bonding occurs by these mechanisms, lower machine speeds (lower rates of... 

The sawtooth waveform can be augmented to create a trapezoidal profile. Like the sawtooth profile, the trapezoidal profile uses constant compression and decompression rates. However, between the compression and decompression segments an intentional dwell time is incorporated. The dwell time is often defined as the time that the moving punches remain stationary at their furthest point of travel. This type of profile is useful when studying either the effects of compression rate or dwell time on powder compression behavior. 

Figure 1. Hysteresis isotherm of polymer 3S on a water/air interface. The compression and decompression rate is 50 Mm/min.

Decompression sickness refers to other physiological effects that can occur during ascent to the water s surface—the decompression phase. If the decompression rate is too fast, the absorbed nitrogen will preferentially form bubbles. This process has variable results, depending on the size and location of the bubbles in the body. If they develop in the muscles or spine, extreme pain and muscle contraction can result—a consequence that has earned the term the bends. Bubbles can also block the bloodstream or cause pressure in the brain. 

Independently of the definition of amorphous polymorphism chosen, experiments suggest that the relationship between LDA and HDA ( first-order like ) is different from the relationship between HDA and VHDA ( continuous ), at least at 140K [65,66]. Therefore, the possibility that there exists a second critical point in metastable water within the No man s land is still open and remains the topic for future works. However, the continuous nature of the HDA-VHDA transformation strongly questions the possibility of a third critical point in metastable water. Experiments at slower compression/decompression rates and at higher temperatures (>140K) may help to elucidate this issue. 

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