Vacuum carbonization

Vacuum carbonate An improved version of the Seabord process for removing hydrogen sulfide from refinery gases, in which the hydrogen sulfide is stripped from the sodium carbonate solution by steam instead of by air. Developed by the Koppers Company, Pittsburgh, in 1939 two plants were using this process in the United States in 1950. 

In this process liquid air is used for condensing the vapours in order to preserve the high vacuum carbonic acid has been used by other investigators, and Levene and van Slyke have recently employed sulphuric acid, cooled by a freezing mixture, as an absorbent for this purpose. 

As catalysts are generally insulating, it is necessary to metallise the surface to be examined and ensure that it is earthed in order to enable the flow of charge from the electron beam. A metallization layer (generally made of carbon) is deposited via thermal evaporation under vacuum (carbon has the advantage of not being particularly absorbent vis-a-vis the incident electrons and vis-ii-vis the characteristic X-rays). The electrical contact with the sample holder is created using a silver or carbon lacquer. 

Coking produces a blast furnace coke feed substantially free of sulfur. However, the gaseous product, coke oven gas, has a sulfur gas content of 900-1, lOOg/m (at 15°C, 1 atm) [31]. This is mainly hydrogen sulfide, which may be removed either by the vacuum carbonate or Stretford processes. The sulfur gas removal efficiency of the Koppers Company s vacuum carbonate process is about 90%, which produces sulfuric acid, whereas the Stretford process can achieve 99% containment to a sulfur product (Chaps. 3 and 9). The choice of desulfurization process depends on the efficiency required and the sulfur product desired. Condensible hydrocarbons such as benzene (and other aromatics) and phenols have always been recovered by condensation, etc. [34]. 

Hot carbonates are well suited for the removal of C02 at moderate or high levels in the presence of little or no H2S. The process acquired its name from the use of elevated temperatures in both the absorber and the regenerator (110—115°C). Hot carbonates such as the Benfield and the Koppers Vacuum Carbonate utilize K2C03 to remove H2S, COS, and C02 from gas streams [35]. Their heat requirements and high solvent circulation make hot carbonates more expensive than other acid gas removal processes. Other hot carbonate processes, including the Catacarb and the Giammarco-Vetrocoke processes, use catalysts, corrosion inhibitors, and/or activators to enhance the removal of the acid gases. Hot carbonate-promoted systems are able to decrease the C02 level from 1% to 0.1%. Promoters include DEA, amine borates, and hindered amines [36]. 

The major absorption bands occurring in CO2 in the infrared are at 2.7, 4.3, 15, and 100 /X [10, 11], These bands are so narrow and weak that solid CO2 can be considered to be highly transparent in the infrared. The data on solid CO2 films shown in Fig, 4 are consistent with a model of a film with an absorption coefficient with little wavelength dependence and a vacuum-carbon dioxide film reflection coefficient of 0.10. 

EDX analysis The tissue sections were coated with a thin layer of carbon in a high vacuum carbon evaporator and mounted on the stage of a scanning electron microscope (SEM) for elemental analysis. 

Processes that use aqueous ammonia to remove H2S and CO2 are still offered for coke-oven gas purification, and many such plants are in operation in the U.S. and Europe however, it appears that the trend for new operations is toward the use of other absorbents. Other absorption processes that may be applicable for COG purification include the Takahax, Stretford, Vacuum Carbonate, Potassium Carbonate, and Sulfiban (MEA) processes. These processes can be designed to avoid serious adverse effects of trace impurities in the gas, and generally provide somewhat higher H2S removal efficiency than ammonia scrubbing. The processes are described in detail in other chapters (see index). 

Although the Vacuum Carbonate process is still commercially available, in recent years it has been replaced in many coke-oven gas treating installations by processes based on the oxidation of hydrogen sulfide to elemental sulfur in the liquid phase (see Chapter 9). 

Figure 5-37. Vacuum Carbonate (hot-activation) gas-purification plant. Koppers Company, Inc.

Figure 5-39. Koppers two-stage Vacuum Carbonate process. Diagram courtesy of ICF Kaiser Engineers... 

Principal reactions occurring in the Vacuum Carbonate process are NajCOj + CO2 + H2O = 2NaHC03... 

Table 5-13 Vacuum Carbonate Process Solution Analysis ...

Table 5-14 Vacuum Carbonate Process Absorber Data ...

At plant A, the HCN that is removed in the Vacuum Carbonate process is recovered as a by-product. In most other plants, this substance is either removed and burned (to permit the H2S to be utilized in conventional acid plants), or it is destroyed in a specially designed sulfuric acid plant, which utilizes the H2S. 

Coke-oven gas normally meets tiiese specifications after it has passed through benzene washers. If the Vacuum Carbonate process is located ahead of the benzoie washos, which is... 

Analysis of Acid Gas From Plants Using Vacuum Carbonate Process ... 

Corrosion is generally not a major difficulty with Vacuum Carbonate plants because of die relatively low temperatures employed. Mild steel is the primary material of construction, although, as previously mentioned, wooden and ceramic packings and stainless-steel actifier trays are also used. Stainless steel is also used for minor components of the vacuum and other pumps, thermometer wells, and instrument dl hragtns (Smith, 1953). 

Operating ReqtdremerUs. Comparative operating data for a Koppers two-stage Vacuum Carbonate plant and a single-stage Vacuum Carbonate plant, both with Koppers heat exchange systems to conserve steam, are presented in Table 5-16 (Koppers, 1978). The... 

Table 5-16 Comparative Operating Data for a Koppers Two-Stage Vacuum Carbonate Plant and a Single-Stage Vacuum Carbonate Plant with Koppers Heat Exchange Systems ...

Coke-Oven Gas Pressure to Vacuum Carbonate Plant, ... 

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