Demethanization

Dry inlet gas that has been dehydrated by molecular sieves (qv) or alumina beds to less than 0.1 ppm water is spHt into two streams by a three-way control valve. Approximately 60% of the inlet gas is cooled by heat exchange with the low pressure residue gas from the demethanizer and by external refrigeration. The remainder of the inlet gas is cooled by heat exchange with the demethanized bottoms product, the reboiler, and the side heater. A significant amount of low level refrigera tion from the demethanizer Hquids and the cold residue gas stream is recovered in the inlet gas stream. 

The two portions of the feed stream recombine and flow into the high pressure separator where the Hquid is separated from the vapor and is fed into an intermediate section of the demethanizer with Hquid level control. The decrease in pressure across the level-control valve causes some of the Hquid to flash which results in a decrease in the stream temperature. The pressure of the vapor stream is decreased by the way of a turboexpander to recover... 

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%. 

Recoveries of 90—95% ethane have been achieved usiag the expander processes. The Hquid product from the demethanizer may contain 50 Hquid vol % ethane and usually is deHvered by a pipeline to a central fractionation faciHty for separation iato LPG products, chemical feedstocks, and gasoline-blending stocks. 

Although current United States synthetic capacity for isoprene is based entirely on dehydrogenation of refinery isoamylenes and demethanation of... 

The system just described is for the so-called low pressure demethanizer. Some Hcensors offer a high pressure demethanizer design (3.5 MPa), combined with either front-end deethanization or depropanization. 

Front-end hydrogenation is also possible. This approach uses a different type of catalyst, and the reactor is located upstream of the demethanizer. For this design, a deethanizer or depropanizer tower is located upstream of the demethanizer to remove heavy fractions. This approach has been utilized by some Hcensors with some success. 

Predemethanization. The conventional design employs a single-step multiple feed tower. Utilization of a second tower upstream of the existing primary tower reduces the load on the primary tower. This reduces the propylene refrigeration power and reduces the propylene chilling loads. This approach is typically uneconomical with low pressure demethanizers, but has been combined with high pressure demethanizer systems by some Hcensors. 

Demethanizer Overhead Expander and Multifeed Fractionation. Incorporation of an expander into the conventional high pressure demethanizer system eliminates bottlenecks in the refrigeration system, the demethanizer condenser, and charge gas compressor. It reduces the cost by lowering the refrigeration power. Multiple feed deethanization and ethylene fractionation debottlenecks the deethanizer, ethylene fractionator, and the refrigeration systems, thereby reducing power consumption. 

Tower Internals and Equipment Modification. Tower capacity expansion can be achieved through the use of random or stmctured packing, or through the use of higher capacity trays such as the UOP multiple downcomer tray. Packing has been used in the gasoline fractionator, water quench tower, caustic and amine towers, demethanizer, the upper zone of the deethanizer, debutanizer, and condensate strippers. Packing reduces the pressure drop and increases the capacity. 

The turboexpander lowers the temperature of the product to -100°F, causing it to liquify. Now at 350 psig pressure, the liquid from this process enters the demethanizer tower where it mingles with the previously introduced stream of liquid. The turboexpanders provide a 92% recovery rate while the former system, a backup Joule-Thomson valve, was able to provide only a 60% recovery rate. The volume of gas entering the turboexpanders can vary up to 10% yet, the different flowrates do not significantly affect the efficiency of these units, which are rated at 2,400 hp at 16,000 rpm. 

The liquid collected at the bottom of the demethanizer tower is a mixture of ethane, propane, butane, and condensate (EPBC), which is taken off in a stream and pumped—as a liquid, at 1,000 psig—to a customer facility. Another part of the EPBC is introduced into a deethanizer tower. The stream of EPBC liquid entering the deethanizer tower is further separated into PBC liquid and pumped to the El Paso Natural Gas facility in Gallup, New Mexico. EP (ethane and propane)... 

A methane gas stream taken off the demethanizer process, and still at 350 psig, is compressed via byproduct energy from the turboexpanders and raised to 410 psig. The gas is then introduced into a 15,000 hp compressor and raised to 850 psig for delivery back to the El Paso Natural Gas Company. The 60 psig boost by each turboexpander represents a 15% reduction in required horsepower. This amounts to considerable energy saved and is yet another reason why the turboexpander is useful in a cryogenic process of this type. 

Pattinson. Scott, Changes in Demethanizer Reboiler Solve Efficiency Problems, Oil and Gas Journal, May I, 1989, p. 102. 

The gases are again dried and then further compressed to about 550 psi. Separation of hydrogen and methane take place in the demethanizer and in its preflash system. Three successive Golder preflash steps are used in this separation, with propylene as refrigerant, then ethylene, and finally a self-generated methane refrigerant at -200 F. 

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. 

A typical ethane cracker has several identical pyrolysis furnaces in which fresh ethane feed and recycled ethane are cracked with steam as a diluent. Figure 3-12 is a block diagram for ethylene from ethane. The outlet temperature is usually in the 800°C range. The furnace effluent is quenched in a heat exchanger and further cooled by direct contact in a water quench tower where steam is condensed and recycled to the pyrolysis furnace. After the cracked gas is treated to remove acid gases, hydrogen and methane are separated from the pyrolysis products in the demethanizer. The effluent is then treated to remove acetylene, and ethylene is separated from ethane and heavier in the ethylene fractionator. The bottom fraction is separated in the deethanizer into ethane and fraction. Ethane is then recycled to the pyrolysis furnace. 

The demethanizer distillation column of an ethylene process works at extremely low temperatures. The feed is cooled with extremely small temperature differences of the order of 1°C to minimize the refrigeration costs associated with the cooling. What type of heat exchanger would you expect to be used for this duty ... 

Demecarium bromide, 4 360t Dementholized mint oil (DMO), 24 514 Demerara Sugar, 23 482 Demethanizer overhead expander, 10 616 Demethanizers, 10 613-614 Demethoxylation, during alkaline pulping, 21 23-24... 

The liquid streams from separators 7-11 enter the demethanizer column (unit 13). The top product of this column is methane, which is sent to the fuel gas stream via the cold section and drying/precooling section. 

The bottom product of the demethanizer column enters the C2 splitter column (unit 15) as a feed. The top product of this column is cooled and compressed and subsequently stored as ethylene product. The bottom product of the C2 splitter column is... 

The demethanizer, deethanizer, and debutanizer are fractionating columns that separate the lighter and heavier compounds from each other. Traces of triple bonds are removed by catalytic hydrogenation with a palladium catalyst in both the C2 and C3 stream. Cumulated double bonds are also hydrogenated in the C3 fraction. These are more reactive in hydrogenation than ethylene or propylene. The C2 and C3 splitters (Fig. 8.4) are distillation columns that can be as high as 200 ft. The mechanism of cracking was previously discussed in Chapter 7, Section 6. 

Figure 8.26. A cascade refrigeration system employing ethylene and propylene for condensing the overhead of a demethanizer at — 145°F. The diagram is somewhat simplified.

Berry and co-workers have reported a new method for the synthesis of moderate to high molecular weight polygermanes in very high yields by the catalytic coupling of tertiary germanes.83 This ruthenium-catalyzed process appears to be the first example of a catalytic demethanative coupling, in which element-element bonds are produced with the concurrent elimination of CH4 [Eq. (58)]. 

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