Бурение грунтовых зондов, установка энергетических колодцев

The steady increase in the metal content of liquid partial-oxidation feedstocks over the years has led to a developing awareness of the necessity to consider nickel and iron carbonyl formation in the raw synthesis gas. Nickel and iron carbonyl are toxic gaseous compounds that form during the cooling of the raw gas and pass on in the raw gas to the treating units. Depending on the treatment scheme, there may be a need for special handling to avoid problems.

Table 6-4 shows some of the principal chemical and physical data of these gases (IPCS 1995, 2001; Kerfoot 1991; Lascelles, Morgan, and Nicholls 1991; Wildermuth etal. 1990).

The formation of nickel and iron carbonyls can take place in the presence of gase­ous carbon monoxide in contact with metallic nickel or iron or their sulfides. Industri­ally hydrogen sulfide or carbonyl sulfide are used as catalysts for the production of nickel carbonyl from active nickel. Ammonia has also been used as a catalyst. Given that all three of these gases are present in the raw synthesis gas, one needs to anticipate some carbonyl formation in a partial oxidation gas containing as much as 50 mol % CO if the feedstock contains significant quantities of nickel or iron.

The reactions leading to the formation of carbonyls in a partial oxidation unit are shown in Table 6-5 together with their equilibrium data.

Figure 6-12 shows a plot of the equilibrium concentrations of nickel and iron carbonyls against temperature for various CO partial pressures. From these plots one can see that carbonyl formation increases with increasing pressure and decreasing temperature, whereby nickel carbonyl formation takes place already at significantly higher temperatures than iron carbonyl formation. Based on this data and a plant pressure of 60 bar and 45 mol% CO in the raw gas, one could expect the formation of 1 ppmv Ni (CO)4 from nickel sulfide below about 380°C and 1 ppm (v) Fe (CO)5 from iron sulfide below 40°C. The corresponding temperatures for carbonyl forma­tion from the metals are somewhat higher. Although the kinetics of the reactions, particularly at lower temperatures, may prevent equilibrium conditions arising in practice, these tendencies correspond with industrial practice (Soyez 1988; Beeg,

Schneider, and Sparing 1993). Carbonyl formation takes place in the cold section of the plant. And because of the lack of solubility of the carbonyls in water, they leave the partial oxidation unit with the raw gas.

Formation of carbonyls can be inhibited to some degree by the presence of free oxygen. There is, however, no recorded instance of such an approach being taken in any gasification unit.

The consequences of any metal carbonyl slip into the gas treatment units depend very much on the treatment scheme. Quench cooling leads to a lower carbonyl formation than the use of a syngas cooler, since much of the metals removal takes place at higher temperatures. This applies particularly to iron carbonyl formation. Nonetheless, in one plant with quench cooling and subsequent raw gas shift, significant depositing of nickel sulfide on the shift catalyst led to reduced catalyst life (BASF undated). This is caused by the reverse of reaction 6-6, decomposition of the carbonyls on heating in the shift unit.

As described in Table 6-4, the carbonyls are not soluble in water. They are not removed from the gas by amine washes. Most physical-chemical washing systems will also allow the carbonyls to pass through the absorber and appear in the clean gas, so that depending on the application, other problems may occur downstream.

Carbonyls are soluble in physical washes such as Rectisol and can be completely removed from the synthesis gas this way. It is, however, necessary to consider the subsequent fate of the metals. The relative partial pressures of carbon monoxide and hydrogen sulfide in liquor containing the dissolved carbonyls is substantially different to that of the raw gas, so that the reactions 6-6 and 6-7 are driven towards the left, particularly on heating the liquor for regeneration. The subsequent precipitation of the sulfides can cause problems, such as fouling of heat exchangers. If decomposition of the carbonyls is suppressed in the acid-gas removal unit, then they will appear in the sour-gas stream and may deposit on the Claus catalyst in the sulfur recovery unit. The various licensors of such physical wash processes have developed methods to control this phenomenon.

Effects of Carbonyls

Iron carbonyl can present problems in the methanol synthesis and was a regular diffi­culty in the older high-pressure processes because of its formation if CO came into contact with iron in the loop equipment. Irrespective of its origin, iron carbonyl will decompose at the conditions of the methanol synthesis (50-100 bar, 250°C) leaving iron deposits on the methanol catalyst. The iron will then catalyze Fischer-Tropsch reactions, contaminating the methanol with unwanted hydrocarbons (Supp 1990; Skrzypek, Sloczynski, and Ledakowicz 1994). Skrzypek and colleagues report that nickel carbonyl has the same effect. Carbonyls can act as a poison on other synthesis catalysts. This must be reviewed on a case-by-case basis.

In an IGCC situation, if carbonyls are permitted to enter the gas turbine, they will decompose at the high temperatures prevailing in the burners. There is a potential, then, for the metals to deposit on the turbine blades, causing imbalance. Care is generally exercised, therefore, to avoid this.

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