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CO2 removal systems for ammonia plants – a survey

Summary

The economics of hydrogen and ammonia plants heavily depends on the efficiency of systems used for removal of carbon dioxide from process gas. Over the past 30 years there have been many innovations in this section of the plant, from ordinary water wash to hot pot solutions to primary and secondary amines. As well as the absorption solution for absorption, tower design, packing, etc are also important. Here, M.P. Sukumaran Nair of Fertilizers And Chemicals Travancore Ltd traces these developments and, based on operating experiences, assesses how common problems can be addressed to achieve a reliable, efficient and sustainable operation.

Abstract

Separation of carbon dioxide from ammonia synthesis gas to make it fit for the synthesis reaction and recovering it to meet the later requirements of urea production is a major process step in the design of an ammonia plant. There has been a continuous development in technology for carbon dioxide (CO2) removal, with the prime focus on achieving higher purity of outlet gas, reduction in energy consumption and ensuring trouble-free operation, in the most economical manner.

Proven technologies are available for physical, chemical and mixed modes of absorption of CO2 from the process gas, which is usually a mixture of hydrogen, nitrogen, and small quantities of carbon monoxide, methane and argon. Most of the physical absorption units are associated with ammonia plants based on partial oxidation of fuel oil or coal, and the physical absorbent solution is methanol. It is capable of accepting a wide range of impurities, consumes relatively low energy compared to water wash and amine wash and the outlet gas is discharged almost dry and with the desired purity. Physical absorption processes are based on the fact that the CO2 molecule is more polar in nature and regeneration is achieved by simple flashing of the loaded solution. There is no additional energy input requirement for regeneration. However, later development of ammonia process technology based on natural gas feedstock and the evolution of a new generation chemical absorption technologies for CO2 removal rendered physical absorption processes unattractive in large capacity plants.

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Coal as a feedstock

Summary

With natural gas prices rises in much of the world, attention has begun to focus on the possibility of using alternative feedstocks for ammonia production. Gasified coal has long been used as a source of syngas, and new environmental concerns have given fresh impetus to this technology.

Abstract

Most ammonia and urea is produced from natural gas. However, in theory any hydrocarbon feedstock can be used provided that it is capable of being oxidised to synthesis gas (syngas). India, for example, with a shortage of natural gas, makes great use of naphtha. However, with natural gas (and indeed naphtha) prices high in many parts of the world, there is growing interest in alternative feedstocks for ammonia production. This article takes a look at some of the alternatives.

Table 1 shows the current split in global ammonia capacity. Where plants have the capacity to use more than one type of fuel, the primary fuel has been taken. As can be seen, of a total of 150m t/a of capacity, over 70% is derived from natural gas.

There are three main routes for producing an ammonia synthesis gas with the required 3:1 H2:N2 ratio;

  • Steam reforming of natural gas or other light hydrocarbon
  • Partial oxidation of heavy hydrocarbons
  • Partial oxidation (or other gasification process) of coal

 

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Economics of LTS catalysts in ammonia plants

Summary

David Borzik of MissChem Nitrogen, Inc., and David Rice of Süd Chemie Inc. discuss how low temperature shift (LTS) catalyst performance can affect plant economics, and show the savings that can be made from higher activity catalysts.

Abstract

In today's challenging business environment, it is important to understand how catalyst performance affects the cost of ammonia production. In order to determine the effects of aging front-end ammonia plant catalysts, a paper study was done by calculating front-end heat and material balances based on varying levels of activity in each catalyst service. These heat and material balances were then used as inputs to a synthesis loop model in order to calculate relative energy consumptions. The results of this study are summarized below.

The referenced study also used equivalent annual cost (EAC) as a tool for evaluating the cost of catalyst options with different lives. The equivalent annual cost of a project is equal to the annual payment of an annuity that has the same present value as the project's cash flows discounted at the same rate over the same time period.

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Re-using melamine off-gas

Summary

Recycling ammonia and carbon dioxide generated in a melamine plant to a urea plant is not too much of problem in a closely-integrated urea-melamine operation, but it is less straightforward if the process units are not close together. The Pul/awy Nitrogen Works in Poland has come up with its own way round this problem.

Abstract

There are various ways of re-using the off-gases (ammonia and carbon dioxide) from pressurized melamine units by introducing them directly to the reaction system of a urea plant or, where the condensation system is operated under a pressure equal to the pressure in the urea reactor, into the high-pressure carbamate condenser. But those ideas are only economically attractive when the melamine plant operates at or above the pressure in the urea plant. In any case, even where the process conditions permit it, it is undesirable to recycle the off-gases from a melamine plant to a urea plant unless the two plants are sited very close together because of the dangerous nature of the off-gases.

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Piping and vessel safety in nitrogen plants

Summary

Considering how highly stressed they are, nitrogen chemicals plants are remarkably safe and reliable, thanks partly to the various codes and standards relating to pressure equipment design and fabrication and partly to systematic inspection and maintenance. Nevertheless, failures still occur occasionally in piping and vessels, not always those operating under the highest pressures.

Abstract

The only chemical routes for fixing atmospheric nitrogen that have been developed to the commercial scale to date consume large amounts of high-level energy from external sources – electrical energy in the case of the original Birkeland-Eyde process, which made nitrogen oxides in an electric arc, chemical and thermal energy in the case of ammonia synthesis. Nowadays, of course, the ammonia synthesis route is the only one used, and in its almost 90-year history it has become so sophisticated that the net specific energy consumption is now not all that far from the theoretical minimum. But this energy economy in the process has only been achieved at the cost of elaborate and expensive hardware to maximise the recovery of reaction heat which would otherwise be wasted.

An ammonia plant comprises several individual process stages operating at pressures ranging between about 30 bar and 80–300 bar, depending on the design philosophy of the process, and temperatures from 1,000ºC down to around 50ºC. These take place in a variety of reactors and mass transfer devices whose design and operating conditions are optimised for the specific unit process. To adjust the process gas temperature between the stages and to recover or supply heat, as appropriate, there is an array of inter-stage waste heat boilers, steam superheaters, economisers, gas-gas heat exchangers, water-cooled coolers and ammonia-cooled chillers, and pressure increases between stages are provided by turbocompressors (for gases) or (normally) centrifugal pumps for liquids.

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