Andre B. Haan - Process Technology: An Introduction (de Gruyter Textbook)

(A.1)

: Flow sheet of an ammonia plant. Adapted from [213].

The purified natural gas, which contains about 80 % methane, is converted to hydrogen and CO, as given in eqs. (A.2a) and (A.2b). The latter is called the water-gas shift reaction:

(A.2a)

(A.2b)

Both of these reactions are endothermic, which means that a lot of heat is required to allow these reactions take place. High temperature, low pressure, and high steam-to-carbon ratios favor the reactions.

In the primary reformer the feed of methane and steam is led through nickel alloy tubes and placed in a furnace to heat the gas to 750850C at a pressure of 30-40 atmosphere. The nickel content of the alloy, which also acts as a catalyst, is in the range of 2030 %. Most of the methane is converted, but there is still a large amount of CO in the outlet gas. Side reactions taking place are carbon formation by decomposition of higher hydrocarbons and by CO reduction and disproportionation. Most of this takes place in the first section of the reformer, when the hydrogen content is still low. Promoters, such as potash, are used to suppress the carbon formation, which decreases the cracking of higher hydrocarbons. The reformer type used is a so-called Kellogg box type. In the box reformer the catalyst tubes are arranged in parallel, single-width rows heated from both sides, either by gas or liquid fired burners, located in the furnace arch.

In the secondary reformer the reaction is continued at higher temperatures (up to 1000 C) to achieve a low methane slip (0.20.3 vol%). Again the catalyst used is nickel, this time on an alumina support to withstand the high temperatures.

A.1.4 Shift conversion

Because the ammonia synthesis catalyst is very sensitive for poisoning by CO and CO2, these gases have to be removed. The easiest way to do this is to convert the carbon monoxide to carbon dioxide, absorbing the dioxide with a suitable absorbent. Unfortunately, the conversion of CO leads to equilibrium. At 300 C, only 85 % conversion is obtained, which even decreases at higher temperatures. Usually the conversion is separated into two parts: a high temperature conversion (HTS) to remove the bulk of the CO, and a low temperature conversion (LTS) for the remainder. The HTS takes place at 350-400 C over a magnetite catalyst stabilized with chromium oxide. The main reason to apply high temperatures is the high reaction speed, so the conversion of the bulk of the CO2 takes place very rapidly. The LTS converter operates at temperatures of 200-250 C in the presence of a CuO catalyst supported on alumina and zinc oxide. The converter design is based on an already used catalyst system, so that when the catalyst load is fresh (and thereby more active), the operating temperature can be lower than design temperatures. Gradually raising the temperature compensates for the lower activity of the used catalyst, and thus a constant outlet composition is achieved.

A.1.5 Carbon dioxide removal

Carbon dioxide is an acidic gas which reacts reversibly with aqueous solutions of alkanol amines. In general, solutions of weak bases (15-40 wt%) in water are applied for the removal of CO2 from gases. Strong bases are inapplicable, because they would give irreversible .

: CO2 removal system.

A.1.6 Final purification

Because oxygen containing compounds poison the ammonia syntheses catalyst, they must be efficiently removed. This is done in three steps: dehydration, and excess nitrogen removal. The reactions taking place in the methanation are exactly the reverse of the reforming reactions at a temperature of 300400 C. They almost go to completion, reducing the CO and CO2 content to a few ppm. Dehydration is done by the use of molecular sieves, usually at the interstage of the syntheses gas compressorto reduce volume requirement. Excess nitrogen removal is necessary because there are some processes which use excess air (and thus excess nitrogen), and the nitrogen has a negative influence on the ammonia production. This is usually carried out by cryogenic purification, which involves cooling and stripping of the partly liquefied stream. Even here the CO content is lowered, about 50 %

A.1.7 Ammonia syntheses and recovery

, the reaction will lead to an equilibrium, and therefore not all of the raw material can be converted. A higher conversion can be reached by introducing a recycling loop. The exact recycling stream in proportion to the product stream has to be determined after economic considerations. After mixing with the recycling loop, compression takes place to 220 bar and a temperature of 500 C. By doing this the speed of the reaction is increased and the equilibrium is shifted to a more favorable ammonia production. As previously discussed, the syngas is dried at this stage using molecular sieves, after which the ammonia from the recycle is recovered. This reduces the required refrigeration, because the converter effluent is not saturated by the syngas addition, and cooling water can be used instead of more complicated refrigeration methods. This method leads to a very pure syngas, which increases catalyst lifetime (even up to 20 years) due to the lack of contaminants. To maintain a high partial pressure of the reactants, the inerts entering with the syngas have to be removed using a purge stream, as will be discussed later on: