Column processes

Cryogenic distillation has been used extensively ia the processiag of natural gas for nitrogen removal and for helium recovery (22—23). Two basic processes are now used for nitrogen rejection from natural gas— the single-column heat-pumped process and the double-column process. Eadier processes utilized multistage flash columns for helium recovery from natural gas (24). 

In a simple single-column process, although the oxygen purity is high, the nitrogen effluent stream is impure. The equilibrium vapor... 

Gaillard, J.-F. and Treguer, P. (1997). Antares I France JGOFS in the Indian Sector of the Southern Ocean Benthic and water column processes. Deep-Sea Res. 1144, 951-1176. 

Yield/selectivity/conversion - benchmarking to industrial bubble-column processing... 

GL 14] [R 1 ] [P 13] A yield of 85% was obtained by micro flow processing similarly to large-scale bubble column processing [57]. Selectivity was much better since less than 0.05% dichloroacetic acid was formed, whereas conventional processing typically gives 3.5%. 

Nooraii, A., Barton, G., and Romagnoli, J. A. (1994). On-line data reconciliation, simulation and optimising control of a pilot-scale distillation column. Process Syst. Eng. PSE 94 2, 1265-1268. 

The baseline process, including the pressure sintering step, was demonstrated with both simulated high level waste and under hot cell conditions using a waste solution prepared from typical spent light water reactor fuel. A batch contacting method using sodium titanate was also evaluated, but the overall decontamination factor was much lower than obtained in the column process. 

However, a larger portion of materials which may be of concern as trace components will not be extractable or generally soluble in aqueous systems. This presents the necessity to concentrate components contained in organic solvents. When the organic solution is too complex, or the components sought are too volatile to allow concentration simply by evaporation, then more complex means must be pursued. Under these circumstances, liquid column processes are again commonly used. Alumina, silica gel,... 

The use of UTEVA-Resin, an extraction chromatographic resin to be described in more detail below, has also been described for robotic separations of actinides in a glove box64-66 These authors designed their process to allow columns to run dry between steps to simulate what could happen in an unattended open-column process. In addition, corrosive solvents were avoided. They reported >90% recoveries and fractions that were suitable for thermal ion mass spectrometry (TIMS) source preparation without further purification or treatment. 

The single-column process (Figure 1) is similar to that of Jones et al. (1). This process is useful for bulk separations. It produces a high pressure product enriched in light components. Local equilibrium models of this process have been described by Turnock and Kadlec (2), Flores Fernandez and Kenney (3), and Hill (4). Various approaches were used including direct numerical solution of partial differential equations, use of a cell model, and use of the method of characteristics. Flores Fernandez and Kenney s work was reported to employ a cell model but no details were given. Equilibrium models predict... 

The two-column process (Figure 2) is the heatless adsorption process of Skarstrom (5 ) This process can perform both recovery and purification of a light component Local equilibrium models of this process have been presented by Shendalman and Mitchell (6), Chan et al (7), and Knaebel and Hill (8) In each case the method of characteristics was used. Models with finite mass transfer rates have been published by Kawazoe and Kawai (9 ), Mitchell and Shendalman (10), Chihara and Suzuki (11), and Richter et al (12). In these models the method of characteristics and direct integration of partial differential equations were employed ... 

Two-Column Process The heatless adsorption process (Figure 2) has two columns Each column undergoes a four-step cycle. Three steps involve the same sequence as the single column process pressurization with feed, feed at high pres-... 

Modeling of this process proceeds in the same way as for the single-column process described above except for introduction of a purge step. The process is started with one column equilibrated with feed at high pressure and the other at low pressure Eqs (16), (20) and (22) are used as before with renumbering of cells and mole fractions at appropriate points in the cycle, but the purge step is inserted into the sequence at the appropriate point ... 

A total material balance around one column of the two-column process at cyclic steady state is... 

Eqns (16), (20) and (22) were integrated numerically to obtain the separation performance of the one- and two-column processes The GEAR package (13) was used for the integration after determining that it was faster than, say, Runge-Kutta methods For all calculations N 50 and e - 0.40 Dimensionless parameters varied were 8, ph pL Yf and, for the two-column process, H. Combinations of the parameters of 6 and Pr/Pl were chosen to correspond to the me thane-helium system on BPL carbon Adsorption isotherm data for methane at 25°C (14) were represented by... 

An example of the effect of pressure on the performance of the single column process for the methane-helium separation is shown in Table 1. Using Figure 4(b) and 4(d), the cuts corresponding to 80 percent recovery of helium were determined as a function of yF and Pr L Then the enrichments and in... 

Effect of Pressure on Single-Column Process for Helium-Methane Separation... 

The curves for H = 0 represent the performance of the single-column process. It is apparent that the single-column process is superior at large product cuts in terms of both enrichment and recovery. For small cuts, the two-column process is superior. 

The equations for a local equilibrium cell model of pressure swing adsorption processes with linear isotherms have been derived. These equations may be used to describe any PSA cycle composed of pressurization and blowdown steps and steps with flow at constant pressure. The use of the equations was illustrated by obtaining solutions for a single-column recovery process and a two-column recovery and purification process. The single-column process was superior in enrichment and recovery of the light component at large product cuts. The two-column process was superior at small cuts ... 

For the large throughputs that are common today, the double-column process is preferred over the single-column process, since it does require smaller columns with smaller column diameters, which makes the transportation... 

Schematic of double-column process. (Adapted from Ullmann s Encyclopedia of Industrial Chemistry, Vol. A22, VCH Publishers, Inc., New York, 1993.)...

---