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China's nitrogen and methanol industries

Summary

China has been the site of much of the recent plant construction activity in the nitrogen and methanol industries as it struggles to keep pace with burgeoning demand. However, problems of feedstock availability and the impact of international competition as the country liberalises are also making themselves felt.

Abstract

China’s nitrogen industry has had to develop from small beginnings – in 1949, fertilizer production in China was less than 6,000 t/a nutrient. However, imported Soviet technology led to the development of domestic urea and SSP plants, with production rising to 80,000 t/a nutrient by 1955. Domestic technology led to the development of ammonium bicarbonate capacity in 1958, and over subsequent years this became the dominant method of nitrogen production, achieving more than half of N capacity. Likewise the fused calcium magnesium phosphate (FCMP) process made it possible to use phosphate rock with a high silica and calcium content, which is difficult to beneficiate, to produce phosphate fertilizer. Consequently a lot of small FCMP plants were also built. However, difficulties with transportation of fertilizer led to the development of small, dispersed plants in over 1,800 locations. The result was that, while fertilizer production reached 1.72m t/a by 1965, apart from 30 medium-size (60,000 t/a) ammonia plants, all other sites were producing less than 10,000 t/a each.

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Ammonia as a refrigerant

Summary

Although a minority use for ammonia, refrigeration has made something of a comeback since the banning of various chlorofluorocarbons under the Montreal Protocol.

Abstract

Something in the region of 130m t/a of ammonia is produced globally. Of this, about 80% is used for agriculture, either directly applied as in the US, or via conversion into urea, ammonium nitrate, diammonium phosphate or other fertilizers. Of the remainder, most is used industrially for chemical production. Only about 2% is used as a refrigerant.

This was not always the case. In the 1950s, for example, ammonia was used as the primary refrigerant in 20% of marine cargo ships, and carbon dioxide formed most of the rest. It is still common in industrial refrigeration systems, and may represent 10–15% of commercial refrigeration systems. However, during the 1960s both ammonia and CO2 were increasingly replaced as refrigerants by the new generation of chlorofluorocarbon (CFC), hydrochlorofluorocarbon (HCFC) and hydrofluorocarbon (HFC) based compounds, beginning with freon. Use of ammonia as a refrigerant dropped to a low point in the early 1990s when less than 0.5% of ammonia went towards this use.

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Compact heat exchangers in ammonia/urea production

Summary

In ammonia and urea plants the predominant heat transfer equipment has traditionally been the massive shell-and-tube heat exchanger. Jakob Liedberg, of Alfa Laval, shows how compact plate heat exchangers have made inroads in even these exacting applications

Abstract

The shell-and-tube heat exchanger is the most tried and tested of all heat exchanger designs. It dates back to the beginnings of the industrial revolution, when it was the basis of some of the earliest industrial steam boilers. It was the centrepiece of almost, if not absolutely, every steam locomotive that was ever built. It is still the most widely used single design type for heat exchangers. Its biggest advantage was its robustness and durability. But in relation to its heat transfer capacity it is both bulky and exceedingly massive.

Plate heat exchangers are now a fully accepted alternative in liquid/liquid heat transfer applications in the ammonia and urea processing industries. Conventional gasketed plate-and-frame heat exchangers are widely used in applications such as secondary cooling systems, where sea water is used as the cooling medium. These heat exchangers are also used as interchangers in absorption/stripping systems for gas cleaning, where they recover energy and thus improve the overall operating economics of the plant. Gasketed heat exchangers are also easy to clean and repair, since they can be completely disassembled.

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Performance is the key

Summary

The perfect ammonia synthesis catalyst is nowhere nearer, but incremental improvements in catalyst technology and operating practice in the various process stages are enhancing plant efficiency and economics.

Abstract

A conventional steam-reforming ammonia plant contains seven catalytic stages – feedstock desulphurisation, primary reforming, secondary reforming, HT CO shift, LT CO shift, methanation and synthesis. Some plants may have an eighth stage – prereforming – between desulphurisation and primary reforming.

Recent developments to the ammonia flow sheet may omit one or other of the seven stages. For example, plants using pressure-swing adsorption or a cryogenic gas purification process do not need a methanation stage, which is only there to keep CO and CO2 out of the synthesis reactor, since the residual levels of carbon oxides from these process stages are already as low as can be attained by methanation.

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Urea beside the sea

Summary

Representatives from Stamicarbon's client companies, licensed contractors and equipment suppliers were treated to a comprehensive programme of papers and round-table discussions at Stamicarbon's upbeat Urea Symposium in Scheveningen in May.

Abstract

The Dutch chemical manufacturer DSM has been a urea manufacturer since the infancy of that industry and has run the entire gamut of urea synthesis processes as both developer and operator, from the original “once-through” plants, through partial- and total-recycle processes, to the advanced stripping process of today. It was, indeed, the first to introduce the stripping principle, which notably improved the economics of the urea synthesis process.

Because the conversion of ammonium carbamate into urea is incomplete, to obtain the urea product it is necessary to decompose the unchanged ammonium carbamate back into carbon dioxide and ammonia. In a total recycle process that is done by letting the pressure down in stages and heating the reaction medium, then condensing the resulting gases back to reform liquid ammonium carbamate and pumping it back into the previous stage. In a stripping process the bulk of the carbamate decomposition is done in the synthesis loop at the synthesis pressure by exposing the heated solution to a rapid flow of incoming carbon dioxide feed. To compensate for the resultant lowering in the partial pressure of ammonia over the carbamate solution, more carbamate decomposes. The resulting decomposition products pass with the carbon dioxide feed to the high-pressure carbamate condenser, where the carbon dioxide reacts with the incoming ammonia to form fresh ammonium carbamate solution, which continues to the synthesis reactor.

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