Once-through cyclone

Pneumatic dryers are much more compact and easy to control. The low material volumetric concentration in the dryer enables equilibrium conditions to be reached very quickly. As the material is once-through with the drying medium, the exhaust air is humid and thus the exhaust temperature cannot be too low, otherwise, condensation may occur in the separation cyclone. This will result in significant heat loss as well... 

According to the results reported by Jacob (1978), in a once-through steam generator about 45% of the I activity carried by the main steam is separated in the cyclone downstream of the high-pressure part of the turbine together with the... 

In some circumstances it may not be desirable to use once-through air as the transport gas (e.g. for the risk of contamination of the factory with toxic or radioactive substances for risk of explosion an inert gas may be used in order to control humidity when the solids are moisture sensitive). In these cases a closed loop system is used. If a rotary positive displacement blower is used then the solids must be separated from the gas by cyclone separator and by inline fabric filter. If lower system pressures are acceptable (0.2 bar gauge) then a centrifugal blower may be used in conjunction with only a cyclone separator. The centrifugal fan is able to pass small quantities of solids without damage, whereas the positive displacement blower will not pass dust. 

As mentioned in Chap. 1, cyclones work as a result of the centrifugal forces acting on the particles suspended in the swirling gas stream. This causes the particles, which are denser than the gas, to move outward to the cyclone wall, along which they are transported downward to the dust exit. The cleaned gas leaves near the centerline, in a reverse-flow cyclone through the roof. In a once-through or flow-through cyclone, the cleaned gas exits out the bottom. ... 

Fluidized combustion of coal entails the burning of coal particles in a hot fluidized bed of noncombustible particles, usually a mixture of ash and limestone. Once the coal is fed into the bed it is rapidly dispersed throughout the bed as it bums. The bed temperature is controUed by means of heat exchanger tubes. Elutriation is responsible for the removal of the smallest soHd particles and the larger soHd particles are removed through bed drain pipes. To increase combustion efficiency the particles elutriated from the bed are coUected in a cyclone and are either re-injected into the main bed or burned in a separate bed operated at lower fluidizing velocity and higher temperature. 

The fine particle airstream from the cyclone was sampled by two total filters in parallel. A Millipore Fluoropore 47 mm diameter Teflon filter with a 1 pm pore size was used for the first seven samples. Subsequent samples were obtained with a 0.4 pm pore size 47 mm Nuclepore polycarbonate filter because particle absorption measurements and elemental analysis by particle induced X-ray emission (PIXE) were easier and more accurate using the Nuclepore filters. In parallel with the Nuclepore filter, a TWOMASS tape sampler collected aerosol using a Pallflex Tissuequartz tape. The aerosol deposit area was 9.62 cm on the Nuclepore and Millipore filters and 0.317 cm on the Tissuequartz tape. The flow rate was 16-20 1pm through the Nuclepore and Millipore filters and 10 1pm through the Tissuequartz tape. Each Millipore or Nuclepore filter was placed in a labeled plastic container immediately after collected, sealed with Parafilm, enclosed in a ziplock bag, and placed in a refrigerator in the trailer. The tape in the TWOMASS sampler was advanced between samples. The tape sample was removed about once every 8-10 weeks and stored similarly to the Nuclepore filters. The TWOMASS was cleaned at that time. All samples were stored in an ice chest during the return trip to Caltech. Field blanks were handled identically to the samples. Of approximately 100 filter samples collected in 1979, 61 were selected for analysis. The 61 were chosen to span the variation in bjp and to obtain representative seasonal and diurnal samples. Sample times varied from 6 to 72 hours, with an average of 20.1 hours. 

The collection of the pyrolysis oils is difficult due to their tendency to form aerosols and also due to the volatile nature of many of the oil constituents. As the aerosols agglomerate into larger droplets, they can be removed by cyclonic separators. However, the submicron aerosols cannot be efficiently collected by cyclonic or inertial techniques, and collection by impact of the aerosols due to their Brownian or random motion must be utilized. A coalescing filter is relatively porous, but it contains a large surface area for the aerosol particles to impact by Brownian motion as they are swept through by the pyrolysis gases. Once the aerosol droplets impact the filter fibers, they are captured and coalesce into large drops that can flow down the fibers and be collected. 

Once the liquid phase is separated from the gas, it is diverted to impinge on the wall of a glass vessel like a simple test tube or collection bottle. The liquid runs down the wall while the gas exits the top. The carbon dioxide expands 500 times. Under typical operating conditions, 25-30 L/min of gas exits out the top of the test tube or bottle. The waste gas is diverted through a waste collection container and then vented outside. A small amount of the modifier, like methanol, is present in this waste stream (due to the vapor pressure of the compound). As with the cyclone separator, the user is never exposed to the effluent of the chromatograph. 

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