The Vacuum Pump

The vacuum pump keeps both chamber and ice condenser sufficiently free from residual gases so that the water vapor is able to flow unimpeded from the drying material process. 

In practice, the freeze-drying process of solutions proceeds as following  

The bulk solution is dispensed into the vial from a 0.5 to 2.5 cm height. The amount of solution in the vial is very important for efficient drying. The vials are loosely capped with the special rubber stoppers, and then are put into the vacuum chamber. 

The vials with the product are gradually frozen below the eutectic point of the solution (usually -30 to -40 °C). 

The condenser is turned on, and the condenser indicator is observed until it reaches its maximum value (usually -50 °C). 

---

The Z-spray inlet/ionization source sends the ions on a different trajectory that resembles a flattened Z-shape (Figure 10.1b), hence the name Z-spray. The shape of the trajectory is controlled by the presence of a final skimmer set off to one side of the spray instead of being in-line. This configuration facilitates the transport of neutral species to the vacuum pumps, thus greatly reducing the buildup of deposits and blockages. 

Of course, some substances are sufficiently volatile that a heated inlet line can be used to get them into a mass spectrometer. Even here, there are practical problems. Suppose a liquid or solid is sufficiently volatile, that heating it to 50°C is enough to get the vapor into the mass spectrometer through a heated inlet line. If the mass spectrometer analyzer is at 30°C, there is a significant possibility that some of the sample will condense onto the inner walls of the spectrometer and slowly vaporize from there. If the vacuum pumps cannot remove this vapor quickly, then the mass... 

The Z-spray source utilizes exactly these same principles, except that the trajectory taken by the ions before entering the analyzer region is not a straight line but is approximately Z-shaped. This trajectory deflects many neutral molecules so that they diffuse away toward the vacuum pumps. 

In a world increasingly conscious of the dangers of contact with chemicals, a process that is conducted within the walls of a vacuum chamber, such as the VDP process for parylene coatings, offers great advantages. Provided the vacuum pump exhaust is appropriately vented and suitable caution is observed in cleaning out the cold trap (trace products of the pyrolysis, which may possibly be dangerous, would collect here), the VDP parylene process has an inherently low potential for operator contact with hazardous chemicals. 

This reduction in permeabiUty due to cake consoHdation or coUapse may be so large that it may nullify or even overtake the advantage of using high pressures in the first place and there is then no reason for using the generally more expensive pressure filtration hardware. While a simple Hquid pump may be cheaper than the vacuum pump needed with vacuum filters, if air displacement dewatering is to foUow filtration in pressure filters, an air compressor has to be used and is expensive. 

Du Pont called this new lubricant material Krytox (64,65) and initially it had such extraordinary properties that it sold for 200/kg ( 187kg ca 1993). Krytox was and is used ia most of the vacuum pumps and diffusion oil pumps for the microelectronics iadustry ia this country and ia Japan because it produces no hydrocarbon (or fluorocarbon) vapor contamination. It has also found important appHcations ia the lubrication of computer tapes and ia other data processiag appHcations as weU as military and space appHcations. 

The primary sources of contamination in ion implantation come from metal atoms that may be etched off reactor fixtures, such as reactor wads, wafer holder, cHps, and so on. The pump oils used by the vacuum pumps may be a source of hydrocarbon contamination. The dopant sources themselves are not a significant source of contamination because unwanted ions are separated out from the beam during beam analysis. 

The process operates at 1 kPa (10 mbars) and 450 kW of power. When the condenser temperature reaches 580°C, the power is reduced to 350 kW. Cooling water is appHed to the condenser, throughout distillation, by means of sprays. Normally distillation takes 10—12 hours and the end point is signified by an increase in furnace temperature and a decrease in vapor temperature to 500—520°C. At this point the power is turned off and the vacuum pump is shut down. Nitrogen is then bled into the system to prevent oxidation of 2inc. 

The dezincing chamber is set first in the drossed lead bath, then water connections are immediately made in order to prevent the formation of steam within the water jacket. While the temperature is being raised, the vacuum pump is placed in operation and the agitator started. The temperature is then raised to 600°C and held throughout the operation. 

Because of the low efficiency of steam-ejector vacuum systems, there is a range of vacuum above 13 kPa (100 mm Hg) where mechanical vacuum pumps are usually more economical. The capital cost of the vacuum pump goes up roughly as (suction volume) or (l/P). This means that as pressure falls, the capital cost of the vacuum pump rises more swiftly than the energy cost of the steam ejector, which iacreases as (1 /P). Usually below 1.3 kPa (10 mm Hg), the steam ejector is more cost-effective. 

Other factors that favor the choice of the steam ejector are the presence of process materials that can form soflds or require high alloy materials of constmction. Factors that favor the vacuum pump are credits for pollution abatement and high cost steam. The mechanical systems require more maintenance and some form of backup vacuum system, but these can be designed with adequate reflabiUty. 

For low pressure pipelines that have ports open to the atmosphere, eg, sewers or closed effluent culverts, samplers are designed to sample through manholes. In a typical system, the Hquid is lifted through a suction line into the sampling chamber under vacuum. When filled, the vacuum shuts off, and the sample drains into a sample jar. A secondary float prevents any Hquid from reaching the vacuum pump. The suction line then drains by gravity back to the source. 

The prevacuum technique, as its name implies, eliminates air by creating a vacuum. This procedure faciUtates steam penetration and permits more rapid steam penetration. Consequendy this results in shorter cycle times. Prevacuum cycles employ either a vacuum pump/steam (or air) ejector combination to reduce air residuals in the chamber or rely on the pulse-vacuum technique of alternating steam injection and evacuation until the air residuals have been removed. Pulse-vacuum techniques are generally more economical vacuum pumps or vacuum-pump—condenser combinations may be employed. The vacuum pumps used in these systems are water-seal or water-ring types, because of the problems created by mixing oil and steam. Prevacuum cycles are used for fabric loads and wrapped or unwrapped instmments (see Vacuum technology). 

The vacuum pump is usually of the steam-jet type if high-pressure steam is available. If nigh-pressure steam is not available, more expensive mechanical pumps may be used. These may be either a water-ring (Hytor) type or a reciprocating pump. 

By far the largest load on the vacuum pump is water vapor carried with the noncondensable gases. Standara power-plant practice assumes that the mixture leaving a surface condenser will have been cooled 4.2°C (7.5°F) below the saturation temperature of the vapor. This usually corresponds to about 2.5 kg of water vapor/kg of air. One advantage of the countercurrent barometric condenser is that it can cool the gases almost to the temperature of the incoming water and thus reduce the amount of water vapor carried with the air. 

Figure 18-95 also contains a schematic layout of the equipment which is required for all bottom-feed leaf tests. Note that there are no valves in the drainage line between the test leaf and the filtrate receiver, nor between the filtrate receiver and the vacuum pump. 

Hand-crimp the hose in back of the test leaf, and then turn on the vacuum pump and regulate the bypass valve on the pump to give the desired vacuum level in the receiver. 

Adjustment may also be required for differences in altitude between the test site and the commercial installation. In general terms, if the plant elevation is higher, the vacuum pump size must be increased, and conversely. 

Required vacuum pump capacity = 2.65/4.29 = 0.62 mVmiu X m of total filter area. AUow for pressure drop within system when specifying the vacuum pump. See next example. 

In the flow schematic (Fig. 22-80), the condenser controls the vapor pressure of the permeating component. The vacuum pump, as shown, pumps both hqiiid and vapor phases from the condenser. Its major duty is the removal of noncondensibles. Early work in pervaporation focused on organic-organic separations. Many have been demonstrated few if any have oeen commerciaHzed. Still, there are prospects for some difficult organic separations. 

Replacement of the steam-jet ejector with a vacuum pump. The distillation operation will not be affected. The operating cost of the ejector and the vacuum pump are comparable. However, a capital investment of 75,000 is needed to purchase the pump. For a five-year linear depreciation with negligible salvage value, the annualized fixed cost of the pump is I5,000/year. 

Operating the column under atmospheric pressure thereby eliminating the need for the vacuum pump. Here a simulation study is needed to examine the effect of pressure change. 

A constant nitrogen addition into the discharge line from the vacuum pump to the vacuum pump discharge drum/seal drum system. 

A vacuum pump seal drum design which provides a liquid seal (hydraulic flame arrester) to mitigate flame propagation backward into the vacuum system. The seal liquid is an organic stream (mostly Cg aromatics) that comes from the vacuum pump discharge drum overflow. 

Calculate section by section from the process vessel to the vacuum pump (point of low est absolute pressure). 

Assume a velocity, v, ft/sec consistent with Figure 2-46. Use Table 2-21 for short, direct connected connections to the vacuum pump. Base the final specifications for the line on pump specifications. Also the diameter of the line should match the inlet connection for the pump. General good practice indicates that velocities of 100 to 200 ft/sec are used, w ith 300 to 400 ft/sec being the upper limit for the rough vacuum classification. 

The suction pressure required ai the vacuum pump (in absolute pressure) is the actual process equipment operating pressure minus the pressure loss between the process equipment and the source of the vacuum. Note that absolute pressures must be used for these determinations and not gauge pressures. Also keep in mind that tlie absolute pressure at the vacuum pump must always be a lower absolute pressure than tlie absolute pressure at the process. 

Figure 2-47. Acceptable pressure losses between the vacuum vessel and the vacuum pump. Note reference sections on figure to system diagram to illustrate the sectional type hook-ups for connecting lines. Use 60% of the pressure loss read as acceptable loss for the system from process to vacuum pump, for initial estimate. P = pressure drop (torr) of line in question Po = operating pressure of vacuum process equipment, absolute, torr. By permission, Ryans, J. L. and Roper, D. L., Process Vacuum System Design Operation, McGraw-Hill Book Co., Inc., 1986 [18].

Capacity of the vacuum pump at the working vacuum Example Vessel size = 200 ft ... 

Occasionally the odor of hydrogen cyanide can be detected during the distillation, even when a trap filled with sodium hydroxide pellets precedes the usual trap cooled in dry ice and acetone to protect the pump. For safety, the vacuum pump should be placed in a hood, or provision should be made for the pump exhaust to be vented into a hood or out-of-doors during the distillation. 

A vacuum system is required to handle 10 g/s of vapour (molecular weight 56 kg/kmol) so as to maintain a pressure of 1.5 kN/m2 in a vessel situated 30 m from the vacuum pump. If the pump is able to maintain a pressure of 0.15 kN/m2 at its suction point, what diameter pipe is required The temperature is 290 K, and isothermal conditions may be assumed in the pipe, whose surface can be taken as smooth. The ideal gas law is followed... 

An additional difficulty, especially noticeable when the product ion, NO+ in this case, has a stable neutral counterpart is the presence of background ions. The vacuum pump used in this experiment, a titanium... 

As shown schematically in Fig. 2.4a, the reactant gases are introduced in the upstream side, then flow down the reactor tube, and exhaust downstream through the vacuum pump. 

---