GB2160516A

GB2160516A - Ammonia plant re-vamp process

Info

Publication number: GB2160516A
Application number: GB08514520A
Authority: GB
Inventors: Frank Clifford Brown; Christopher Leslie Winter; Trevor Williams Nurse
Original assignee: Humphreys and Glasgow Ltd
Current assignee: Humphreys and Glasgow Ltd
Priority date: 1984-06-07
Filing date: 1985-06-07
Publication date: 1985-12-24
Also published as: GB8414491D0; GB2160516B; CA1301429C; GB8514520D0

Abstract

Light hydrocarbon feedstock is fed directly into the secondary reformer with excess of air over that to produce a stoichiometric mixture, the heat load of the existing primary reformer is maintained close to the original figure for increases in plant throughput of over 50%. After operating the heat recovery and shift system in a conventional manner the extra gas is treated in a PSA unit to remove any unwanted impurities and sufficient nitrogen so that the gas produced by mixing the main flow of gas and that from the PSA unit is suitable for ammonia synthesis in the existing equipment or a modification of it. <IMAGE>

Description

SPECIFICATION Ammonia plant re-vamp process The invention relates to a method for expanding the output of existing ammonia plants and achieving a reduction in both specific light hydrocarbon consumption and in total energy per ton of ammonia product. A key feature is the way the invention causes minimum interference with the existing plant equipment. The invention uses part of an earlier H & development known as BYAS where part of the light hydrocarbon feedstock is fed directly into the secondary reformer. By using an excess of air over that to produce a stoichiometric mixture the heat load of the existing primary reformer can be maintained close to the original figure for increases in plant throughput of over 50%.After operating the heat recovery and shift system in a conventional manner the extra gas is treated in a PSA unit to remove unwanted impurities and sufficient nitrogen so that the gas produced by mixing the main flow of gas and that from the PSA unit is suitable for ammonia synthesis in the existing equipment or a modification of it.

The principal features are best understood by referring to Fig. I. The stream of light hydrocarbon 1 is heated and desulphurised quite conventionally. Part is separated 3 while the main stream is mixed with steam 2 before being further heated in the reformer feed heater 8 after which it enters the tubes of the primary reformer 7 where most of the hydrocarbon is converted to hydrogen and a mixture of oxides of carbon. The reformed gas 13 leaves at conventional conditions of 15-50 kg/cm2a and 700 to 890 C. It is then mixed with the stream of hydrocarbon 3 bypassing the primary reformer. In cases where only a small expansion is required then it is unnecessary either to heat the stream further or to add steam to it.

At larger increases it may be advantageous for both thermodynamic and economic reasons to either heat the feed further or to add some steam to it or to do both. Thermodynamically for the prevention of carbon formation more steam is required per mol of hydrocarbon in a primary reformer than in a secondary reformer.

In the cases where the envisaged expansion is small then there is no need to bypass any of the new feedstock around the Primary Reformer thus reducing the number of modifications to the existing unit.

In the secondary reformer 10 it is necessary to increase the quantity of hydrocarbon reformed as the duty of the primary reformer 7 is kept less than or about equal to the original design value. This extra reforming is achieved autothermically by adding an excess of air 5 through the mixer 9 in the top of this reactor. Due to the much increased flow of air even for relatively small expansions the mixer 9 is likely to need modification. In most cases the secondary reformer and its WH boiler 11 would not be modified as the gas flow is generally increased at less than the ratio of the expansion. However, in some cases it may be advantageous to either replace the existing secondary reformer or to add another in parallel. Extra compression facilities will of course be needed for the extra air flow.

The reformer gas 12 from the secondary reformer may contain more unreacted methane than in the original gas. After cooling to about 350-380 C it enters the CO shift system 14. Extra capacity 14A may be necessary and its size and nature will vary from case to case. It may also be advantageous to change to a more modern catalyst system with greater conversion when less steam is present.

After the lower temperature stage of the CO shift system 14 the gas stream 15 is then split into two streams. A stream 16 up to the capacity of the existing CO2 absorber 22 is treated as previously. The remainder 17 preferably together with the purge gas from the ammonia loop 29 is then sent to a PSA unit 20 which can be operated in two modes. The more conventional mode is to completely remove all impurities and all the nitrogen leaving a very pure hydrogen stream containing about 70 to about 92% of the hydrogen in the original scheme. The other mode is to operate the PSA unit so that some nitrogen passes through along with the hydrogen. With the latter mode some carbon monoxide will also pass through if too much nitrogen is allowed to slip through the PSA unit.In this case then the product stream 27 will require to be treated in the methanator 21 to convert the carbon monoxide to methane.

The gas stream 27 from the PSA system 20 is mixed with the main stream 26 coming from the existing methanator 24. The mixed stream 28 has a composition capable of being synthesised into ammonia in the existing or modified ammonia loop 29. In most cases this will mean a hydrogen nitrogen ratio of between 2.8:1 to 3.2:1. However, in certain circumstances this may be as wide as 2.0:1 to 3.2:1.

The ammonia loop may be expanded in any number of ways and may or may not be modified to operate more energy efficient. However, only minor modifications may be needed to recover hydrogen from the purge and flash gases 19 as these can also be reprocessed in the PSA unit 20.

In certain circumstances it may be required to increase the capacity of an associated urea plant at the same time. In these circumstances a CO2 removal unit may be installed in stream 17. Since the PSA units performance is satisfactory with no CO2 removal then it is not necessary to design a unit capable of reducing the residual CO2 to below 1% which will reduce both the cost and the energy requirements of the unit.

The PSA unit purge gas 31 (by difference) is used as fuel within the plant.

In this scheme it is seen that an existing ammonia plant can be extended by using its reforming section virtually unmodified even for expansions of over 50%. In most cases there is no need to add extra CO2 removal facilities as the PSA unit will remove all the extra CO2 and the excess nitrogen, the only energy penalty being the loss of hydrogen. However, in most cases the loss of hydrogen from the process gas stream is under 5% which is better than conventional plants and roughly comparable to modern low energy flowsheet. Since the PSA unit can also remove all methane from the extra stream the ammonia loop can either be fed with less inerts or the residual methane from the secondary reformer can be allowed to rise thus reducing the energy consumption of the whole plant.

A simplified mass balance is given below of the dry gas components when the plant is operated at about 35% expansion and with no nitrogen leak through the PSA unit. Flows are in molslhr.

1 3 5 12 15 H2 81.7 96.7 N2 47.2 47.2 47.2 CO 15.6 0.6 CO2 15.1 30.1 CH4 32.3 10.3 1.6 1.6 A 0.6 0.6 0.6 O2 12.7 17 19 26 27 28 30 H2 29.1 8.4 66 33 99 N2 14.2 2.8 33 33 CO 0.2 CO2 9.0 CH4 0.5 1.6 1.6 1.6 A 0.2 0.4 0.4 0.4 NH2 60.4 This invention can be used to design a new plant using either all the features or only two being those where excess air is used in the secondary reformer 10 and the PSA unit 20 is used to adjust the gas composition to a stoichiometric or close to stoichiometric hydrogen:nitrogen ratio.

An extension to the invention is where the feedstock is liquid at room temperature and some bypassing of the Primary Reformer is necessary for the additional capacity. Steam would be mixed with the additional vaporised liquid feed stream 3 before reacting it adiabatically in a commercially available Rich Gas Process to produce a gas containing methane as the only major hydrocarbon component. This gas would then be mixed with the outlet gas from the Primary Reformer.

PSA means a unit or device utilizing molecular sieves used in the Pressure Swing Absorption mode.

Thermal swing devices, or a combination of the two may also be used.

The nitrogen required to react with hydrogen to form ammonia is termed the stoichiometric amount of nitrogen. Any amount above the stoichiometric amount is termed excess nitrogen. Air containing excess nitrogen is termed excess air.

An ammonia process characterized in that air and excess air are used in a secondary reformer to produce a stream of synthesis gas, which stream, prior to entry into the synthesis loop, is split and one of the streams of the split is passed to a molecular sieve containing device wherein at least part of the excess nitrogen is removed.

When the effluent from the PSA unit contains more than 5 ppm oxides of carbon such effluent is sent to the methanation unit.

A process wherein the PSA unit is additionally operated in parallel with the low temperature carbon monoxide shift unit.

A process wherein the purge gas from the synthesis loop is also passed to the molecular sieve unit.

The process may also be used to design a completely new plant.

Claims

1. An ammonia process characterized in that air and excess air are used in a secondary reformer to produce a stream of synthesis gas, which stream, prior to entry into the synthesis loop, is split and one of the streams of the split is passed to a molecular sieve containing device wherein at least part of the excess nitrogen is removed.

2. A process as Claim 1 wherein part of the hydrocarbon feedstock is bypassed around the Primary Reformer and mixed with the Primary Reformer Outlet gas.

3. A process as Claim 1 or 2 where such process is employed to revamp an existing ammonia plant causing an expansion without the need to physically alter the Primary Reformer.

4. A process in 1, 2 or 3 wherein the molecular sieve unit is operated in parallel with the methanation unit; the molecular sieve unit producing either part hydrogen or a mixture of hydrogen and nitrogen.

5. A process in 1, 2 or 3 wherein the molecular sieve unit is operated in parallel with the Co2 removal and methanation units; the molecular sieve unit producing either pure hydrogen or a mixture of hydrogen and nitrogen.

6. A process in 1, 2 or 3 where the molecular sieve unit is operated in parallel with the CO2 removal unit and where oxides of carbon greater 5ppm are present in the effluent, such effluent is sent to the methanation unit.

7. A process as claim 5 or 6 wherein the molecular sieve unit is additionally operated in parallel with the low temperature carbon monoxide reaction unit.

8. A process as Claim 1 where the feedstock is normally liquid at ambient temperature and the bypassed feedstock around the Primary Reformer is reformed with steam in an adiabatic reactor into a gas where heaviest major hydrocarbon component is methane.

9. A process as Claims 1 to 8 where the washed purge gas from the ammonia loop is also sent to the molecular sieve unit.

10. A process in 1 -9 where the waste gas from the molecular sieve unit is used as fuel in the Primary Reformer or for heaters and boilers on the site.

11. A process as claimed in 1,3-10 which is a completely new unit.

12. Ammonia produced by any of the above processes.