2. DUSTS AND TAR REMOVAL
Nearly all manufactured gas contains some dust as it leaves the production unit. In addition, gas made from coal in coke-ovens or coal gasifier units (as differentiated from that made from hard coke) also often contains small amounts of tar or resins. In most cases where the gas contains both contaminants, the tar is removed along with the dust. Some of the German installations, the tar is removed along with the dust. Some of the German installations, however, include a separate electrostatic tar precipitator installation which operates in the conventional manner well known in the United States.
Three methods of dust and tar removal are in general use in Germany as follows: mechanical separators such as cyclone separators, baffled separators, traps (both wet and dry) and filters; (2) Electrostatic precipitators; and (3) Water washing methods.
The mechanical separators used in Germany are of various designs and were made in Germany by several companies. A good description of the methods usually employed is given in a report dated December 12, 1936 by Dr. Geister of I.G. Farbenindustrie entitled "Entstaubung", or translated, "Dust Removal." A copy of this report is contained in the Appendix of this report and is not described further here.
A special filter found to be in use which is claimed to have particular advatages as a mechanical separator for gas cleaning is the "Oppauer Schachtfilter" in which the filter medium is a slowly moving bed of granulated coke installed in a vertical cylindrical chamber and resting on a movable grato. The gas enters the bottom and flows to the top. The coke is shakon from the grate into water, agitated and washed, then returned to the top of the bed by an hydraulic convoyor, which also serves to keep the filter bed wet. The process is described in a report by Dr. sachsse entitled "Removal of Carbon Black and Dust from Gases with the Oppauer Schacht-filter" attached in the appendix.
Other filters that may be mentioned in passing are the Viscin air filter frequently used on the air inlet of the oxygen plants, the synthetic silk bag filters used in the separation of carbon black from gases cracked in the electric arc, and fibre glass either dry or saturated with an oil or adhesive material.
The principle of the electrostatic process is well known so details of constructions were not obtained.
When electrostatic precipitation is not used, the last traces of dust in the gases are usually removed by a water wash. This water wash may be carried out in a spray tower, an ordinary packed column, may be combined with an absorption purification system for removal of H2S, CO2, NH3 etc. or may be performed in a mechanically agitated washer-cooler such as the patented Theissen unit. Where a gas absorption-purification system such as the Alkazid unit is also used for dust removal, a filter is included in the process through which either continuously or intermittently a part of the circulating absorption solution is passed to remove the accumulated sludge.