Scrubbing with amines

Field experience has shown that the drum in which this separation is made has a tendency to carry over distillate into the hydrogen-rich gas. The gas is scrubbed with amine to remove hydrogen sulfide. The oil in the gas is picked up by the amine and flows to the sulfur recovery plant. On one unit such an incident resulted in a fire at the tail end of the sulfur plant. 

Usually, natural gas treatment on the basis of thermal process engineering takes place in three steps (see Fig. 7.9). The first step that may consist of partial steps just like all other subsequent steps, serves the preparation of the crude gas for its processing. Here, for example, acid-forming gas components, such CO2, H2S and other sulphuric compounds are removed. Usually, chemical scrubbing with amines (MEA, DEA, MDEA) is applied in which the adsorbent is being regenerated. Then the natural gas is dried. In case of moderate water dew point requirements, glycol is used as wash liquor. The lowest water contents (< 1 ppm) are achieved with the application of zeolitic molecular sieves. Finally, mercury is removed in case aluminium will be used as material of construction for equipment. Mercury in contact with aluminium may lead to catastrophic corrosion. 

The overall rate of a homogeneous reaction hke that of ammonia is determined by a nonlinear combination of effects of diffusion and chemical reaction. The effects of such a reaction on the rates of mass transfer are analyzed in the first two sections of this chapter. In Section 17.1, we describe the simplest case, that of a first-order irreversible chemical reaction. We also summarize extensions of this case, extensions that produce significant gains only at the cost of major effort. In Section 17.2, we describe some results for second-order reactions. In Section 17.3, we apply these ideas to a specific case, that of H2S scrubbing with amines. 

Amine Guard A process for extracting acid gases from refinery streams by scrubbing with an alkanolamine. Many such processes have been developed, this one was developed by the Union Carbide Corp. and uses monoethanolamine. It has been used to purify hydrogen produced by steam reforming. In 1990, over 375 units were operating. 

Optisol A process for removing acid gases and sulfur compounds from gas streams by scrubbing with an aqueous solution of an amine and a proprietary physical solvent. 

UCAR [Union Carbide Carbon dioxide] A process for removing carbon dioxide from gas streams by scrubbing with methyl diethanolamine. Use of a proprietary corrosion inhibitor permits higher concentrations of the amine to be used than in similar processes. 

The hydrogen-rich gas stream from the high-pressure separator is recycled back to the reactor feed by using a recycle compressor. Sometimes with sour feeds, the first-stage recycle gas is scrubbed with an amine system to remove hydrogen sulfide. If the feed sulfur level is high, this option can improve the performance of the catalyst and result in less costly materials of construction. 

Hydrogenation of C02 can also be shifted to the right by further reactions of HCOOH, which may be simply its neutralization with abase, or reactions with amines or alcohols, yielding formamides or formate esters, respectively. Such an effect of amines or aminoalcohols is especially important since often these compounds are key ingredients of scrubbing mixtures used to recover carbon dioxide from flue gases. Furthermore, in laboratory research, hydrogenation of C02 is studied in the presence of bases, and while the product is usually mentioned as formic acid, in fact it is a formate salt or an azeotropic mixture, such as that with triethylamine. 

The carbon dioxide may then be. removed by scrubbing with caustic solutions or, preferably, with amines such as ethanolamines. 

In a conventional combustion process, such as a power station boiler, the carbon dioxide is contained in the exit flue gas. The concentration is generally fairly low and ranges from about 4 vol.% for a combined-cycle system operating on natural gas to 12-14 vol.% for a traditional boiler fired by pulverized coal. The exhaust can be scrubbed with an amine solution, typically monoethanolamine, which is then heated to release the absorbed carbon dioxide. The low concentration of carbon dioxide in the exit gas means that a huge volume would have to be handled and this would entail the installation... 

For difficult cleanup jobs, a vertical tower is used instead of a mix valve and settler. It will provide several theoretical equilibrium stages. The mix valve provides one stage. Figure 14-3 shows a typical arrangement propane scrubbing to remove H2S with amine. Carry-over of amine in the treated propane and high residual H2S are the two usual complaints. 

The Allied Chemical—C.P.I. process, which was developed by T. O. Wentworth,is based on recycle of NH3 that is separated from the reactor effluent gases by scrubbing with monoethanolamine. The carbon dioxide is absorbed in the amine, and the gaseous ammonia is recycled to the reactor. (See Fig. 28.20.) The reactor effluent first is decomposed then the urea solution is removed and concentrated. A specially designed reactor... 

Example 17.3-1 Carbon dioxide absorption with amines In many industrial processes, carbon dioxide must be removed from a gas mixture. This removal is frequently accomplished by scrubbing with aqueous solutions of pH 8 to 10 containing compounds like... 

Scrubbing andDesliming. Sylvinite ores in North America contain 1—6 wt % water-insoluble clays. A significant portion of these clays is less than 0.002 mm in diameter. If not removed or controUed in some manner, clay bodies that are dispersed in the flotation solution, ie, brine saturated with KCl and NaCl, absorb the amine coUector, which is added to effect flotation separation, and the coUector is rendered ineffective. Clay is the most troublesome impurity encountered in the processing of sylvinite ore. 

Corti and Manfrida [2] have also done detailed calculations of the performance of plant A2. They drew attention to the need to optimise the amines blend (including species such as di-ethanolamine and mono-ethanolamine) in the absorption process, if a removal efficiency of 80% is to be achieved and in order to reduce the heat required for regenerating the scrubbing solution. Their initial estimates of the penalty on efficiency are comparable to those of Chiesa and Consonni (about 6% compared with the basic CCGT plant) but they emphasise that recirculation of water from... 

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