Cryogenic unit

In the newer Sasol plants the tailgas from the FT reactors is first water-washed (to extract the remnants of water-soluble oxygenated compounds), then treated in a Benfield unit (to remove all CO ) and then fed to a cryogenic unit where the gas is separated into four streams, a CH -rich, a H -rich, a C and a C /C stream. 

The stream from the cryogenic unit which is rich in C /C-olefins can be fractionated and selectively hydrogenated (to remov traces of dienes) to yield the pure olefins. Common uses of propene are the production of polypropylene, acrylonitrile, cumene etc. Butene can be catalytically dehydrogenated to butadiene which is used in the production of synthetic rubbers. 

Due to increased feedstock costs, some new ammonia plant designs use fuel more efficiently but their capital cost may be higher. The recovery of the hydrogen and ammonia from the synthesis purge gas by a cryogenic unit or a membrane system results in an ammonia capacity increase of about 5%57. 

Atarget of 25% recycling for 2003, aiming for 50% by 2005, has been set. This is based on a mechanical cryogenic unit which will require additional capacity or be augmented via a solvent based process (www.vinyl2010.org). 

Over time turbo-expander systems have improved in efficiency and can be used to extract ethane by inclusion of gas-to-gas heat recovery systems. These are variously described as cryogenic systems or cold boxes and are similar in operation to the cryogenic units used for the production of LNG. The use of cold-boxes permits pre-cooling of the gas before the turbo-expander and hence an overall colder operation, this is illustrated in Figure 3.3. 

The purified gas is fed into the Synthol and fixed-bed reactors. The products from the reactors are cooied and separated in a water phase, oil phase and tail gas. The + Ca olefinic products from the tail gas are separated in an oil absorption tower and oligomerized over an acidic catalyst to gasoline. Tite remaining tali gas can be treated in a cryogenic unit to provide methane and hydrogen, which is partly used as fuel gas or feedstock for ammonia synthesis. The remainder is steam-reformed over nickel catalysts to give CO/H3. 

Chiyoda Process [1137], In this process the traditional fired primary reformer is also replaced by an exchanger reformer and the heat balance requires excess air in the secondary reformer with the consequence of a cryogenic unit as final step in the makeup gas preparation to remove the surplus of nitrogen. Additionally, gas turbines are proposed as drivers for the process air compressor and synthesis gas compressor with the hot exhaust being used for steam generation and feed gas preheating. 

Typically, natural gas will also be dried to remove water vapor using a glycol system, a cryogenic unit, or molecular sieve adsorbents, and it may be treated for the removal of mercury, arsenic (often as arsine, ASH3), or phosphorous (often as phosphine, PH3). Various adsorption units are available for the removal of traces of these metals. 

Accounts for tbe fact lhai feed gas contains some heavy hydrocarbons which membranes retain as a small extra benefit. rProcess details not Specified standard cryogenic unit presumably considered. 

Gas chromatography was performed on a Carlo Erba 4160 series GC equipped with a flame ionization detector, a Carlo Erba Model 200 cryogenic unit, and a Houston Instruments B-5000 strip chart recorder. A J and W 30M x 0.32 mm, 1.0 Lim film thickness, DB-5 capillary column was employed using a splitless injection at 20 C and programming 20 to 180°C at 10°/min. Standard curves were obtained... 

The cryogenic units cannot be expanded if it is not foreseen during the design phase. 

The proposed benefits of such technologies are related to lower oxygen production costs, lower usage of water compared to a typical cryogenic unit, smaller footprint, and fuel flexibility [94]. Typical inputs and outputs are shown in Figure 18.12. Depending on the exact chanical composition, ITMs operate in the range from 600°C to 900°C. 

The cryogenic units cannot be expanded if it is not foreseen during the design phase. Generally they can be over-dimensioned and a capacity increase is often obtained without modification to the cold box itself through addition of a tail gas compressor. 

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