Propane methane recovery

Several companies have proposed vapor permeation modules to operate heavy hydrocarbon recovery from small natural gas flowstream since the 1990s MTR Inc., Borsig and GKSS. All the membranes in operation are based on sihcone selective layer which offer mixed-gas propane/methane separation selectivity comprised between 3 and 5 and butane/methane separation selectivity ranging from 5 to 10. 

As indicated in Table 4, large-scale recovery of natural gas Hquid (NGL) occurs in relatively few countries. This recovery is almost always associated with the production of ethylene (qv) by thermal cracking. Some propane also is used for cracking, but most of it is used as LPG, which usually contains butanes as well. Propane and ethane also are produced in significant amounts as by-products, along with methane, in various refinery processes, eg, catalytic cracking, cmde distillation, etc (see Petroleum). They either are burned as refinery fuel or are processed to produce LPG and/or cracking feedstock for ethylene production. 

Essentially all of the methane [74-82-8] is removed ia the demethanizer overhead gas product. High recovery of ethane and heavier components as demethanizer bottoms products is commonplace. The work that is generated by expanding the gas ia the turboexpander is utilized to compress the residue gas from the demethanizer after it is warmed by heat exchange with the inlet gas. Recompression and deUvery to a natural gas pipeline is performed downstream of the plant. A propane recovery of 99% can be expected when ethane recoveries are ia excess of 65%. 

The Cj plus bottoms from the demethanizer then go to the deethanizer. A propylene-propane bottoms product containing 90-92% propylene is obtained which may either be sold, used directly as propylene- 90, or further purified. The ethylene-ethane overhead from the deethanizer is separated in the splitter tower yielding a 99.8% overhead ethylene product at -25°F. The ethane bottoms at -l-18°F may either be sent to fuel gas or used as feed to an ethane cracking furnace. Overall ethylene recovery in these facilities is about 98%. The product is of very high purity with less than 50 parts per million of non-hydrocarbon contaminants and a methane plus ethane level below 250 ppm. 

The earliest application of a moving bed in which solids moved with respect to the containing vessel was reported in the late 1940s. A typical application was the recovery of ethylene from gas composed mainly of hydrogen and methane, and with some propane and butane. The unit shown diagrammatically in Figure 17.26, taken from the work of Berg(45), is known as the hypersorber. 

Van Sint Annaland [28] proposed a four-step process for propane dehydrogenation coupled with combustion of methane or propane the phases of the process are illustrated in Fig. 1.15. Different process schemes are possible, depending on the sequence of the phases within one cycle. In any case, the complete cycle is symmetric, which is favorable with respect to heat recovery. 

The overhead product of the crude unit contains hydrogen, methane, carbon dioxide, hydrogen sulfide, and hydrocarbons up to butanes and some pentanes. It is usually sent to a set of distillation columns known as a saturate gas plant for recovery of propane and butane for sale. The lighter gases are then used as refinery fuel. 

Some recovery of reservoir pressure and decrease in viscosity of the residual petroleum in the reservoir may be obtained by returning natural gas to the formation. High formation pressures contribute to the solubility of methane, ethane, propane, etc., in the residual petroleum, which brings about the decreased viscosity. Carbon dioxide injection is also used, which accomplishes similar objectives as natural gas return [19], at the same time as offsetting releases of carbon dioxide by fossil fuel combustion (sequestration). Nitrogen has also been used for this purpose [20]. 

Within different lean oil rate regions, different specifications provide "more unique" definitions of the column performance. For instance, at a lean oil rate of around 700 kmol/h, the key component is ethane. Therefore in this region the column performance is better defined by specifying the ethane recovery or concentration than, say, the methane concentration in the overhead. This is because at about this lean oil rate, the dependence of the ethane recovery or concentration on the lean oil rate is stronger than that of the methane concentration in the overhead. At lower lean oil rates, propane becomes the key component hence, its concentration or recovery would be a better variable for defining the column performance. 

H-Oil unit are processed for sulfur recovery and then sent for separation through the gas recovery facilities associated with the steam cracker. Remaining unconverted residue from the H-Oil operation is used as a fuel oil component for plant fuel. Ethylene is manufactured by steam cracking of ethane, propane, naphtha, and distillate, and products from these operations are separated in conventional gas recovery facilities. Hydrogen for H-Oil is partially supplied by by-product recovery from steam cracker and H-Oil off-gases supplemented by steam reforming of methane. The heavy oils produced in steam cracking of naphtha and distillate are blended with the H-Oil residue to yield plant fuel. 

If one variable is specified, the lean oil rate and the column performance are determined. For instance, if the mole fraction of methane in the overhead is specified at 0.9, the lean oil rate required to meet this specification is 440 mol/hr. At this lean oil rate, the mole fraction of ethane in the overhead is 0.06, the recovery of methane in the liquid is 8%, that of ethane 35%, propane 84%, n-butane 100%, and so on (Figure 8.4). 

Refinery and Chemical Plant Gas Recovery. It is becoming increas-in y useful to recover substantially aU of the propane and even ethane (or ethylene). Higher absorber pressures and larger circulations of oil may be used to capture the propane or ethane, but it is also necessary to Use refrigeration if ethane is to be recovered. The Tennessee Gas Trans-mistion Co. cools the entire gas stream (750 MMscf) to minus 96 F with temperatures of minus 121 F at some points. The plant can be operated so that substantially only methane is returned to tiie company pipeline. A somewhat similar operation is shown in the gas recovery part of Figs. 204 and 20-5 under the discussion of Ethylene Plants (Chap. 20). 

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