Ethane Separation

Ethane recovered from natural gas is mainly used in ethylene plants in which ethylene and other light alkenes are produced from ethane and even higher hydrocarbons. Since today ethylene plants are only efficient with annual capacities starting at 500 000 t of ethylene, even the demand of natural gas in the preceding ethane recovery is high. With a typical ethane content of a mole fraction of 5% in the natural gas, about 1 200 000 m h ofnatural gas are required for the ethylene production mentioned before. Large petrochemical complexes process about two or three times this amount. 

Similar to the propane separation, with this simple process the economic ethane yield can only be brought to slightly over 90%. For higher yields, the further development of the GSP will be useful, which is known under the term RSV (Recycle Split Vapoiu) (US patent 5,568,737 of the Elcor Corporation, see Fig. 7.15). 

with the otherwise same basic process, a split flow of the compressed sales gas is recycled, completely condensed in the heat exchanger (7), subcooled and finally fed to column (4) as reflux. This reflux is significantly leaner in ethane compared to the reflux used in the GSP-process. Thus, better retention of the ethane at the top of column (4) is possible owing to which efhane yields between 95 and 99% are economically achievable. 

With the liquefaction of natural gas, an energy density is obtained that corresponds to about three times the pressure storage at 200 bar. This fact is favourably used for storage and transport purposes. 

Apart from serving as permanent energy store, LNG is also an alternative to the natural gas transport via pipeline. In case the laying of pipelines is not possible for geographical or political reasons, or if distances between source and consumer significantly exceed 3000 km, today natural gas is usually liquefied near the source, transported by ship as LNG and converted into its gaseous state again near the consumer. Liquefaction plants of this kind serve the permanent basic supply of gas customers and are therefore called Base-Load-Plants. 

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C2 fraction without acetylene to ethylene/ethane separation... 

For separation of liquefied gases, the critical temperature of the distillate may be lower than the cooling water temperature, and refrigeration is nseded. The economic balance is still primarily between the first favorable and first unfavorable effects, but the refrigeration complicates the analysis. Optimization is required for selecting the best pressure, and can be lengthy and tedious if correctly performed. Shortcuts often lead to nonoptimum conclusions. Each case must be considered on its own merits. An example of such an optimization for an ethylene-ethane separation column, as well as of some optimization pitfalls, is described elsewhere (7). 

Saudi Arabia s policy has been to develop a national petrochemical industry that would sell its products worldwide. More than Qatar and Kuwait, it had abundant supplies of ethane and methane extracted from gases that were being flared. The ethane separation capacities of its refineries alone accounted for a potential of 3.5 million tons a year of ethylene. 

Reactor effluent is chilled and light-ends are separated from the C2-hydrocarbons. The demethanizer overhead is processed for ethylene recovery while the bottoms is sent to ethylene/ethane separation. An open heat-pump splitter is applied, thus sending ethylene product to the gas pipeline from the discharge of the ethylene-refrigerant compressor. 

Economics The advantages of this process are low equipment costs (viz. the deethanizer system and ethylene/ethane separation) and reliability of the acetylene hydrogenation due to low excess hydrogen at the reactor inlet. The refrigeration compressor benefits from low specific power and suction volume, while the cracked-gas compressor processes above-ambient-temperature gas. 

Sungpet A, Way JD, Thoen PM, and Dorgan JR. Reactive polymer membranes for ethylene/ethane separation. J Membr Sci 1997 136 111-120. 

Teramoto M, Takeuchi N, Maki T, and Matsuyama H. Ethylene/ethane separation by facilitated transport membrane accompanied by permeation of aqueous silver nitrate solution. Sep Purif Technol, 2002 28(2) 117-124. 

M. Teramoto, S. Shimizu, H. Matsuyama, N. Matsumiya, Ethylene/ethane separation and concentration by hoUow-fiber facilitated transport membrane module with permeation of silver nitrate solution, Sep. Purif. Technol. 44 (2005) 19-29. 

Total reflux startup is most attractive in large superfractionators which use high reflux ratios (e.g., isobutane-normal butane ethylene-ethane separation) and/or in heat-pumped columns. Such columns take from a few hours to a couple of days to start up and stabilize. Due to the high reflux ratio, they are relatively insensitive to feed variations. These features make them ideal candidates for total reflux startups. Total reflux startup is least attractive when the ratio of reflux to feed is low (in such cases, most of the stabilizing can only be performed after feed is introduced), and when the column is easy and trouble-free to start up. 

Ploegmakers J., Japip S., Nijmeijer K. 2013. Mixed matrix membranes containing MOFs for ethylene/ethane separation. Part A Membrane preparation and characterization. Journal of Membrane Science 428 445 53. 

Bux H, Chmehk C, Krishna R, Caro J. Ethene/ethane separation by the MOE membrane ZlE-8 Molecular correlation of permeation, adsorption, diffusion. J Membr Sci 2011 369(1-2) 284-289. 

Ploegmakers, J., Jelsma, A.R.T., Ham, A.G.J.v.d. and Nijmeijer, K. (2013) Economic evaluation of membrane potential for ethylene/ethane separation in a retrofitted hybrid membrane-distillation plant using unisim design. Industrial Engineering Chemistry Research, 52 (19), 6524—6539. 

Rungta, M., Zhang, C., Koros, W.J. and Xu, L. (2013) Membrane-based ethylene/ethane separation The upper bound and beyond. AIChE Journal, 59, 3475-3489. 

For facilitated transport of ethylene through silver ion-containing perfluorosulfonic acid ion-exchange membranes, high ethylene/ethane separation factors are obtained by Way and coworkers in Chapter 19. Ethylene of greater than 99 percent purity is obtained from a 50 50 mixture of ethylene and ethane at ambient temperature with feed and permeate pressures of one atmosphere. 

Figure 2. Ethylene/Ethane Separation Factors for Ag -Form Nafion 117 and Dow Membrane at Various Hydration Temperatures...

Figures 6 and 7 present the influence of ethylene partial pressure and temperature on the ethylene/ethane separation factor. Both figures show the usual observation that permeability of the reactive gas increases with decreasing partial pressure. Consequently, the separation factor increases with decreasing ethylene partial pressure for both Ag -form Nafion 117 and Dow membranes.

Figure 6. Ethylene/Ethane Separation Factor of Ag -Form Nafion 117 as a Function of Temperature...

Figure 12 shows how the ethylene/ethane separation factor decreases with increasing ethylene partial pressure. These data were obtained by performing experiments where the composition of the ethylene/ethane binary fe was varied at four feed and sweep gas total pressures. The decrease of the separation factor is a consequence of the carrier saturation phenomena commonly seen in flicilitated transport which causes the ethylene permeability to decrease with increasing ethylene partial pressure. 

The concept of using Ag+ in liquid membranes to promote facilitated transport of simple gaseous alkenes, specifically ethylene/ethane separations, began with papers by LaBlanc et al. (6) and Teramoto et al. (7,8). Interest in this process waned somewhat when it was discovered that Ag+ formed explosive side products with acetylene which was present in the feed stocks. Despite this potential problem, researchers at BP America developed a Ag- --based separation process for propene/propane separation (9). 

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