Oxygen based process

There are 12 producers of ethylene oxide ia the United States. Table 9 shows the plant locations, estimated capacities, and types of processes employed. The total U.S. production capacity for 1992 was ca 3.4 x 10 metric tons. The percentages of total domestic production made by the air- and oxygen-based processes are ca 20 and 80%, respectively. The largest producer is Union Carbide Corp. with approximately one-third of the United States ethylene oxide capacity. About 94% of domestic ethylene oxide capacity is located on the Gulf Coast near secure and plentiful ethylene suppHes. Plans for additional U.S. production ia the 1990s have been announced by Union Carbide (incremental expansions), Eormosa Plastics (at Pt. Comfort, Texas), and Shell (at Geismar, Louisiana) (101). 

Figure 3 shows a simple schematic diagram of an oxygen-based process. Ethylene, oxygen, and the recycle gas stream are combined before entering the tubular reactors. The basic equipment for the reaction system is identical to that described for the air-based process, with one exception the purge reactor system is absent and a carbon dioxide removal unit is incorporated. The CO2 removal scheme illustrated is based on a patent by Shell Oil Co. (127), and minimises the loss of valuable ethylene in the process. 

For the same production capacity, the oxygen-based process requires fewer reactors, all of which operate in parallel and are exposed to reaction gas of the same composition. However, the use of purge reactors in series for an air-based process in conjunction with the associated energy recovery system increases the overall complexity of the unit. Given the same degree of automation, the operation of an oxygen-based unit is simpler and easier if the air-separation plant is outside the battery limits of the ethylene oxide process (97). 

From the preceding discussion, it is clear that no meaningful generalizations can be made regarding the overall superiority of either the air- or oxygen-based process. 

COPE [Claus Oxygen-based Process Expansion] A modification of the Claus process, which improves the recovery of the sulfur. The combustion stage uses oxygen instead of air. Introduced in 1985 and now licensed by Air Products Chemicals and Goar, Allison Associates. In 1990, six units were operating in the United States. 

The air-based process was the first to be commercialized in 1937, some years after the discovery of the reaction by Lefort in 1931. The oxygen-based process introduced in 1958, was however more competitive and has become increasingly preferred for new installations and for the revamping of earlier ones. It is now estimated that the use of air is only competitive for production capacities of 20 kt/a or less, for which an oxygen separation unit would be too expensive. 

Conversion of air-based processes into oxygen-based processes in vapor-phase oxidation to reduce polluting emissions. Examples are (i) synthesis of formaldehyde from methanol, (ii) etheneepoxidation to ethene oxide and (iii) oxychlorination of ethene to 1,2-dichloroethane. 

Component Air-based process Oxygen-based process... 

Table 5.3 shows typical vent compositions of air- and oxygen-based processes for fluid-bed processes (operation with almost stoichiometric feedstock). 

Plant design for the direct oxidation of propene would most likely be based on pure oxygen feed, rather than air, to gain yield advantage and lower capital costs. The minimal purge gas flow in an oxygen-based process makes it economically feasible to use a ballast gas (diluent) other than nitrogen. 

In this chapter, each oxygen-based process is described in detail and compared to the older air-based technology to illustrate the advantages of oxygen. This may suggest air-based oxidations which will switch to high-purity oxygen in the future. 

A simplified process flow diagram of the air-based ethylene oxidation process is shown in Figure 2 [3J. Only the reaction section is shown as the recovery section is identical for both the air- and oxygen-based processes. 

A simplified process flow diagram showing the reaction section of the oxygen based process is illustrated in Figure 3 [3]. 

Figure 3 Flow diagram for ethylene oxide oxygen-based process.

The catalyst for the oxygen-based process has a selectivity of ethylene to ethylene oxide which varies with different licensors between 70 and 80%. A selectivity of 75% is used in carrying out the following material balance calculation. The balance of the ethylene forms a small amount of byproducts, acetaldehyde and formaldehyde, but the primary by-product is carbon dioxide. For simplification it is assumed that the balance of the ethylene forms carbon dioxide. The following equations apply ... 

The total installed cost for the air-based plant is nearly 50% greater than for an oxygen-based plant. The capital cost for an air separation unit is not included in the estimate for the oxygen-based process. It is assumed that oxygen is purchased over the fence as a raw material and the cost is accounted for in the production cost estimate as the cost of feedstock. 

Comparative cost estimates are presented in Table 4 for ethylene oxide processes. The higher cost of ethylene feedstock for the air-based process is a reflection of lower overall yield. More ethylene is required to compensate for the quantity that is oxidized to carbon oxides. This cost advantage for the oxygen-based process is partially offset by the cost of the oxygen and the higher cost for methane ballast gas and other chemicals for the carbon dioxide removal system. 

The oxygen-based process benefits from a by-product credit for carbon dioxide. High purity carbon dioxide is recovered from the recycle gas and sold for use in carbonated beverage and dry ice manufacture. This revenue partially offsets the higher cost for oxygen although the air-based process has a significant by-product credit for steam. 

A utility credit is realized in both the air-based and the oxygen-based processes. Because of the highly exothermic nature of the oxidation reaction, a large quantity of steam is generated in both cases. The utility credit... 

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