Once-Through Process

Fig. 5. Product yields for the EDS process in a 34-kg/d pilot plant A, with and B, without the Elexicoking option where I represents gas D, naphtha and , oil. The pressure for the once-through process is 10.3 MPa, for the recycle, 13.8 MPa. NRT, nominal residence time S/C/B,...

Clear solution processes used are either once through or regeneration (double alkali). Once-through processes, as shown in Figure 4, generally use caustic or soda ash in the following reactions. Ammonia could also be used under special circumstances where there is a ready market for ammonium sulfate. 

Sometimes the solution used is spent byproducts, like caustic, from nearby process units. Generally, the absorbent is purchased and the solution is made up on site. Once-through processes are used when there is a large process wastewater stream available to dilute the dissolved solids concentration to an acceptable level. 

Figure 4. Clear solution once-through process...

Preparation. The current basic comml procedure, the once-through process (Fig 1), uses liq ammonia and liq carbon dioxide at 1750 to 3000psiand 160—200°. These constituents react to form amm carbonate which is decompd at about 80psi to urea and w. Several variations of the process include a partial recycle process and, the most commonly used technique, the total recycle process. Fig 2 is a diagram of the latter process as adapted for use by the Chemical... 

The shift from air-based, once-through processes to oxygen-based recycle processes, and the corresponding change from reactant-lean to oxidant-lean processes. This not only considerably reduces the emissions and makes purge streams more concentrated and hence more easily combusted but also may lead to improved selectivity and productivity. Examples are the oxychlorination of ethylene to 1,2-dichloroethane and the epoxidation of ethylene. 

First, as a once-through process, which lasted 8 hours and gave far better yields for the An(III) recovery than the hot test performed in 1999 with nPr-BTP. More than 99.9% of Am(III) and more than 99.8% of Cm(III) were recovered from the genuine feed. An(III) losses in the raffinate solution and in the spent solvent, reached respectively 180 and 13 pg/L for Am(III) and 23 and 1 pg/L for Cm(III). A mean DF of 150 was estimated for An(III) versus Ln(III), but higher values were calculated for lighter Ln(III), which were also the major components initially present in the feed. Hence, the... 

In a catalytic membrane reactor the pressure difference between feed and permeate could be adjusted such that high selectivity and high conversion in a once through process is obtained (Fig. 29d). The amount of catalyst necessary and the required residence time of the gas would be less than in conventional fixed bed reactors since the diffusional resistance is overcome by the external pressure gradient. The above advantages are already partly exploited by the use of macro-porous catalyst pellets, mentioned in Section 10.1.2.3 [19, 20]. 

Economics By operating the reactor at a lower conversion to increase selectivity and subsequent recycle, the total yield can be increased by 20% compared to once through processes. This in combination with oxygen enrichment can raise throughput very substantially. The detailed economics suggest that the capital investment can be decreased by 10% to 20%, while the operating costs by about 10% compared to once-through processes. 

Mitsubishi Gas Chemical proposes two alternatives for upgrading the aromatic 0 cut, according to whether or not the m-xylene can be commercialized directly. The simplest, or once-through process, does not include an isomerization stage. The more general process includes this possibility, so that the following sections can be distinguished ... 

A once-through process will be used in combination with tri-generation as in this way the product output of the plant can be optimised for market prices or needs. The co-production of electricity, heat, and liquid fuels leads to higher efficiencies and lower overall costs. 

In the case of a once-through process and tri-generation between 30 to 50 % of the energy of the gas is converted into liquids and the rest is converted into electricity and heat in a combined cycle. About 30% of the energy content of the gas converted into liquids is released in the form of heat, which can be used to generate additional electricity with an integrated steam cycle. 

These facts have led to the review and finally the development of an alternative approach and in particular the possibilities of recovering vanadium for metallurgical use. Figure 100 illustrates the principles of the soot ash removal unit. The carbon slurry from the SGP unit is flashed to atmospheric pressure in the slurry tank (a). The slurry is then filtered on an automatic filter (b) to recover a filter cake with about 80% residual moisture and a clear water filtrate. The filter cake is subjected to a controlled oxidation process in a multiple-hearth furnace (c). This type of furnace, which is well established in many industries and specifically in the vanadium industry, allows combustion of the carbon to occur under conditions where the vanadium oxides neither melt nor corrode. This is not an easy task if one considers the problems of burning a high-vanadium fuel oil in a conventional boiler. The product is a vanadium concentrate, which contains about 75% V2O5. Compared to the old naphtha extraction-based recycle system, the new once-through process consists of only two proces-... 

In addition to a small quantity of dissolved gas, mainly carbon dioxide, the raw methanol depending upon the CO2 content of the synthesis gas contains between 3.5 and 12 wt. % water as well as a number of impurities varying with the selectivity of the catalyst used and the CO content of the reactw feed gas. Crude methanol produced from synthesis gas with low CH4 content recovered from coal generally contains up to 3 500ppm byproduct in the case of a process operating with a gas loop. Byproduct formation rises to ten times this value in once through processes. Table 3.7 shows a typical crude methanol composition as can be expected on a catalyst with good selectivity and a CO partial pressure of about 6 bar in the reactor feed gas. 

Since the amount of ammonium carbamate that can be absorbed in the absorber solution described above is limited by its solubility in the system H20-urea-NH3, part of the ammonia and carbon dioxide cannot be recycled and must be used in the production of a coproduct nitrogen material, As in the once-through process, the operation of the urea plant still must coincide with that of the coproduct plant. 

Table 5. Overview of Performance of Representative Catalysts for Propane Ammoxidation to Acrylonitrile under Once-Through Process Feed Conditions...

Finally, an interesting variant of the once-through process has been proposed to produce multiple products, i.e. by varying propane/ammonia in the feed, different acrylonitrile/acrylic acid ratios can be achieved, which provides an interesting opportunity to optimize the operation of the plant, based on changes in feedstock, product price, and demand. In this sense, it is illustrative of the parallel between... 

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