GB2122107A - Process for gaseous reactions - Google Patents

Abstract

In a process for reacting two or more gaseous reactants to form at least one product with the evolution or absorption of heat, the improvement comprises adding at least one compound to the reactants in an amount sufficient to reduce the temperature change of the overall process by at least 15% of the temperature change that would have occurred had the at least one compound not been added, the at least one compound having a molar specific heat of at least 50% greater than that of the reactants evaluated at the same reaction temperature. The process is used particularly for the synthesis of methanol, ammonia, SNG or ethylene oxide. A preferred added compound is cyclopentane. For processes in which unreacted gases are recycled, the condensing out of the added compound prior to condensing out of the product is also described.

Description

SPECIFICATION Process for gaseous reactions Background of the Art Field of the Invention The present invention relates to reducing temperature changes occurring in gaseous reactions by the addition of relatively high molar specific heat substances.

In this specification I have not sought to differentiate between the gaseous and vapour states, but have used the general term 'gas' to cover both states.

Description of the PriorArt There are a number of commercial processes in which gases react exothermally to form a product, such as those for ammonia, methanol and SNG synthesis. For these products their reactions are carried out under pressure and there are significant changes in temperature as the gases pass through the catalyst bed.

Because of the exothermic nature of the methanol, SNG and ammonia forming reactions there is an increase in temperature of the gases as the reactions proceed. This makes the product more difficult to synthesise because the equilibrium amount of product in the reacting gases decreases as their temperature increases.

In the cases of methanol, Synthetic Natural Gas and ammonia, as the reactions proceed the number of molecules present decrease.

Particularly with the cases of ammonia and methanol, in order to raise the equilibrium level of product the reactions are carried out under pressure; but to obtain this pressure in a commercial plant is expensive.

The prior art shows the need at any given total pressure to maximise the partial pressure of the reactants. A considerable amount of work has been carried out to obtain the overall economic level of inert gases present in a loop. These are there because the cost of removing them entirely from the feed gas would be uneconomic; e.g. in the case of ammonia the cost of reforming the feed to significantly lower methane levels than obtained by the current state of the art processes, and removing oxides of carbon to very low levels, so that on methanation the methane formed may be neglected thereafter, would be very expensive.

However, this invention, for any given total pressure in the loop, teaches that by using the invention, a lowering of the partial pressures of the reactants may be economic.

Temperature is a very important function and this invention affects it in a way which has many advantages both during product synthesis and during the processes carried out to effect removal of the product from other compounds.

Some prior art teaches that one may use the latent heat of substances in order to cool the reaction. However, the introduction of a liquid phase additive into the reactor introduces problems of mechanically arranging the liquid to enter at the right place and the problem of overquenching the reaction. Also, thermodynamically, quenching with a cold liquid is undesirable. By introducing a high molar specific heat material prior to the reactor such problems are avoided.

Some processes introduce steam into the mixture of gases to be reacted, mainly as a diluant to quench the reaction. Such addition has the advantage of pushing the shift equilibrium towards the CO2-side of the equation thus minimising the residual CO content, but this also pushes the reforming equilibrium in the undesired direction.

The addition of water, although sometimes necessary because of a high concentration of CO in an SNG reactor feed stream, is thermodynamically undesirable because of its very high latent heat, and residual water, and has to be condensed out with the consequent loss of energy.

An object of the invention is to reduce the capital and/or running costs of the gaseous reaction processes, particularly the production of SNG.

A further object of the invention is the reduction or even elimination, of conventional product recycle in the case of SNG production.

In accordance with the present invention there is provided in a process for reacting two or more gaseous reactants to form at least one product with the evolution or absorption of heat, the improvement comprising adding at least one compound to the reactants in an amount sufficient to reduce the temperature change of the overall process by at least 1 5% of the temperature change that would have occurred had the at least one compound not been added, the at least one compound having molar specific heat of at least 50% greater than that of the reactants evaluated at the same reaction temperature.

The present invention includes within its scope all products when formed using the process of the present invention.

Description of Preferred Embodiments The essence of this invention is to add to the reactants a compound or compounds having: A. a high molar specific heat in comparison with the reactants or weighted molar specific heat of the reactants, all being taken at the same reaction temperature.

It should desirably also: B. have chemical inertness with respect to the compounds normally present in the reacting gases, C. have a liquid phase which 5 immiscible with the product, D. have a fugacity such that it will condense at a higher temperature than the product, E. be thermally stable at temperatures likely to be encountered in the synthesis process, F. have a low latent heat, G. dissolve gases present with the exception of the product.

Considering each of these in detail, the need for high molar specific heat, the essence of this invention, arises because it ensures that any heat evolved or taken in results in a reduced temperature change than would otherwise be the case. The reduced temperature change leads to a larger amount of product present because of equilibrium effects, and thus makes it easier to achieve a greater amount of product in the gases leaving a catalyst bed than would be the case at a higher or lower temperature and at the same pressure.

The ratio, when expressed as a percentage, of the molar specific heat of the added compound, or weighted molar specific heat of added compounds to the molar specific heat of the reactant or weighted molar specific heat of reactants is preferably greater than 150% and more preferably greater than 170%, all molar specific heats being taken at the same reaction temperature. It is possible to obtain ratios of greater than 180%, e.g.

200% and even greater than 400%. 700% is achievable in certain processes, e.g. SNG production.

The quantity of compound added to gain the reduction in temperature change is such as to reduce the magnitude of the change in temperature by at least 15%, generally at least 30%, preferably at least 60% and more preferably at least 65%, of the temperature change that would have occurred had the compound not been added to the reactant.

For a single bed reaction, this temperature change reduction is easily obtained by comparing the temperature difference between the reactants and products using the added compound with the temperature difference between the reactants and products in the absence of the added compound.

For a multi-bed operation, the sum must be taken of all of the temperature changes across each of the beds. For example, a process using two beds having temperature profiles of 2500C to 3500C and 2800C to 3300C with inter-bed cooling gives a total temperature change of 1 5O0C.

It will therefore be appreciated that the reduction in temperature change required by the present invention may involve a reduction in the number of reaction stages or of catalyst bed, if the process be catalyzed, but this is not essential. In multi-stage or multi-bed operations temperature change reductions of at least 50% and preferably at least 75% can be achieved.

In terms of mole percentage of the overall gaseous mixture being reacted, amounts of added compound range from 2, through 7 and 15, to 25, 40, 60 and more.

The desirability of chemical inertness and thermal stability is self-evident -- byproducts can cause a waste of feedstock and may introduce a need to purify the product.

The desirability for immiscibility of liquid phase arises because if such immiscibility exists it is much easier to separate the product from the added compound. Preferably the mutual solubility should be as low as possible.

The fugacity criterion arises from the desire to maximise the concentration of the product in the gaseous phase in those processed where the product is condensed out from the gaseous phase.

By condensing the added compound prior to condensing the product, the concentration of the product in the remaining gaseous phase is increased. This also has the advantage that the condensed, added compound may be recycled by means of a pump rather than an energy-expensive compressor. A recycle of at least 30% is desirable.

In the case of ammonia product an objective is to reduce the amount of refrigeration used in cooling the gases in order to condense out the product ammonia.

The advantage of low latent heat is that it reduces the heat load when removing the latent heat of the added compound as it condenses and replacing it after pumping in order to re-evaporate the added compound back into the stream flowing towards the reactor.

For those products normally removed from their reactants by condensation, if significant quantities of reacting gases, excluding the product, dissolve into the condensing added compound, the concentration of the product in the gaseous phase is increased with the result that it need not be cooled to such a low temperature in order to remove a given amount from the gaseous phase.

In addition the gases which dissolve may be recompressed by pumping the liquid phase as opposed to the more energy-intensive gaseous phase compression.

Compounds meeting some or all of the above criteria include compounds having carbon-tocarbon bonds, and members of the paraffin series.

It may be desirable to use more than one compound.

Particular examples of suitable compounds are C3-C12 paraffins, C3-C12 olefins, mononuclear and polynuclear aromatic compounds, including those with fused rings, and any compound or mixture of compounds which, under the conditions of the process reactions, undergo partial or complete transformation into such suitable compounds. The compounds may be cyclic and/or substituted with at least one carbon, oxygen or nitrogen group.

Examples of suitable compounds are propane, butane, pentane, pentene (which is hydrogenated to pentane), hexane, heptane, ethylene glycol, ethylamine, methyl cyclohexane, benzene, propyl carbonate, toluene, ethyl benzene, anthracene, naphthalene and diphenyl. In the case of SNG, Liquid Petroleum Gas (LPG) or Natural Gas Liquids (NGL) may be added as their degradation would not to be a disadvantage.

Most preferably cyclic C4-C7 hydrocarbons, especially cyclopentane are used particularly for SNG production. Depending on the selection of the catalyst and the temperature profile, a small amount of the added compound may reform but the products of reforming would be mainly methane and thus would not be a problem, except possibly in the case of phthallic anhydride.

In order to reduce the amount of reforming occurring a three reactor system- could be used which would very significantly reduce the maximum temperature.

This would also allow at least partial catalysis by using the high surface area catalysts supports such as gamma alumina, silica, chromia, bearing in mind that such catalysts have to be kept below 500"C.

In such processes, and in the case of ammonia in particular, the supply of refrigeration to cool the loop to condense out ammonia is a considerable charge on the process. One advantage of this invention is that, bearing in mind that capital and energy used by the refrigeration unit, it is possible, in a manner more economical than hitherto, to condense out liquid ammonia by cooling water alone without added, refrigerated cooling of the gases circulating in the synthesis loop and without exceeding the combined power or capital cost levels that are normally required if refrigeration is used. Expressing this another way, although it may be necessary to raise the pressure of the ammonia synthesis loop, the energy for doing this is less than the energy normally needed to run the refrigeration system.Likewise, the capital cost of a solely water-cooled loop, although greater than a refrigerated loop, will not be greater than the capital cost of the refrigerated loop and the refrigeration unit.

In a modern ammonia plant the process elements are normally: primary, reformer, secondary reformer, high temperature shift, low temperature shift, carbon dioxide removal, methanation, and synthesis loop. If a physical solvent is used to remove the carbon dioxide it is a known art to utilise the heat given up by condensing water between LT shift and CO2-removal to drive an absorption/refrigeration unit which is used to cool the ammonia synthesis loop.

A further advantage of the present process is that, in loops which are cooled much more by cooling water than are current, state-of-the-art, loops, the heat given up by this water may be used to help re-evaporate the compound added to the loop.

Another advantage of the present process is that it is possible to purify synthesis loops by taking a portion of the condensed, added compound, separating it, and then separating the dissolved gases and/or byproducts. For example, because of the higher solubility of methane in cyclopentane in an ammonia loop than nitrogen or hydrogen, the loop may be purged of methane by this means.

The invention is particularly applicable to the formation of methane, e.g. for synthetic natural gas, but may be applied to the formation of carbon dioxide by means of the water-gas shift reaction, the formation of ethylene oxide by air or oxygen oxidation of ethylene, and the formation of phthallic anhydride. The addition of cyclopentane and its substituted homologues is particularly advantageous here because of their thermal stability.

In the case of processes wherein unreacted reactant gases are recycled to the reactor, using this invention the added compound need not be condensed but just recycled with the gas. In cases where the added compound is such that it does not condense, its molecular weight is usually lower than in the case where it does condense.

Thus in the case of ammonia production, the addition of propane, or butane, either alone or in addition to the other compounds mentioned herein, is advantageous. The addition of such compounds is also advantageous in the production of SNG and methanol.

Another advantage of the process is that as the gases, to which the compound is added, react there is a decrease in volume. This effectively raises the partial pressure of the added compound and so it will condense out at a temperature higher than that at which it evaporates into the gases (at equal pressure). This facilitates the transfer of heat energy from the added compound as it condenses to the added compound as it is re-evaporated. Usually, this transfer of heat may also be facilitated by what may be described as an inversion of ideal gas behaviour. With paraffintype compounds, at the temperatures and pressures normally encountered in the processes mentioned (SNG excepted) at increased pressure, e.g. in order to recycle the unreacted gases and condensed the added compound, the molecular fraction of the added compound present at equilibrium in the gaseous phase is increased.The fact that the fresh gases, or make-up gases, and, in the case of ammonia, hydrogen and nitrogen recovered from loop purge gas, do not contain any added compound also has a beneficial effect on the transfer of heat energy as described above.

A further advantage of the addition of compounds to reacting gases is that a source of cold (refrigeration) may be arranged by contacting the fresh make up gases with the added compound in the liquid phase and causing some of the added compound to evaporate into the make up gases. Such may be used to cool the loop, the purge gas from a loop, to cool a physical solvent used to remove carbon dioxide, or to help meet the dew point specifications of the SNG; cooling to below 250C is desirable.

In the processes mentioned above, but particularly in the case of SNG, a further advantage is that the design temperature of equipment (for SNG, principally the first reactor and its downstream equipment) is significantly reduced giving substantial capital cost savings.

Typically, in the case of SNG, a reduction from 7500C to below 6000C, e.g. 5500C, can be achieved; together with a big reduction in the number of between-reactor cooling stages required for any given conversion effected. The need to recycle gases can be eliminated.

After the formation of SNG from hydrogen and oxides of carbon, any water present has to be condensed out. Sometimes the cooling of SNG to reach a typical dew point specification may cause trace quantities of carbon-containing compounds also to condense. These trace quantities have generally been regarded as a nuisance to the process, or have been ignored. We have found that adding such compounds to the reacting gases is advantageous in the removal of such compounds.

In all the processes mentioned herein, the compound may all be added prior to the first reactor, or some may be added, cold, to the reacting gases to gain additional cooling effect. It may be added alone, or as a mixture together with fresh or recycled gases. In some cases it could be added in liquid form.

In processes, say for ammonia and methanol, where there may be significant mutual solubility in the liquid phases between the added compound and the product, the processes can be designed in one of two ways.

Firstly an added compound may be selected such that the mutual solubility is high so as to maximize the solution of the product. in the liquid phase of the added compound. The product-rich liquid phase (if such exists) is removed from the synthesis loop together with all or a significant part of the added compound-rich liquid phase. If desired the added compound is then separated from the product by such means as distillation, molecular sieves, osmosis, or solvent extraction (e.g. by water-washing ammonia from the added compound).

Secondly an added compound in which the mutual solubility is low may be selected. In this case only the product-rich liquid phase may be removed from the loop and if necessary is separated from the much smaller amount of added compound by the means described above, or below.

In both cases a final purification may be required depending on the desired purity of the product. Such purification may be effected by ion exchange resin beds, activated carbon, or diatomaceous earth.

In the case of methanol, the distillation may be effected as part of the normal, water-removal distillation unit because the bottom stream from the distillation column would usually separate into two phases.

Examples of endothermic processes wherein the invention may be employed are the cracking of methanol and ammonia for, e.g. hydrogen production.

In this specification temperature changes are adiabatic temperature changes, i.e. no heat is exchanged with the surroundings. Of course the invention may be used in conjunction with heat exchange devices.

EXAMPLE As an example of the application of the invention to methane production, the following (in kg mols/hr) are fed to a (uncooled) methanation reactor: CO 20 H2 60 Cyclohexane 40 (hereafter referred to as C6) at a temperature of 280"C and a pressure of 40 ata.

The outlet from the reactor at about 6000C contains: CO 0.456 H2 9.748 CO2 2.095 H20 15.354 CH4 17.449 C6 39.996 + trace amount of degradation product of C6 In the first reactor the shift and the methanation represent approximately a 200C and a 700C approach to equilibrium (GirdlerTables).

The outlet from the first reactor is then fed to a second reactor at the same pressure but at a temperature of 2500C and the outlet from this second reactor at about 3000C contains: CO trace amount H2 1.034 CO2 0.258 H20 19.438 CH4 19.741 C6 39.996 + trace amount of degradation product of C6 The total adiabatic temperature rise to get to 94% CH4 (dry basis) would be ca. 201 OOC spread over 7 reactors. The total adiabatic temperature rise in the example using the invention is 3700C over 2 reactors. Thus the reduction in temperature rise is 2010--370 x 100 = 82% 2010 The need for the multiplicity of adiabatic reactors arises because the temperature rise eventually stops the reaction at chemical equilibrium.

While certain embodiments and details have been given to illustrate the present invention, it will be apparent to those skilled in the art that various changes and modifications could be made therein without departing from the spirit or scope of the invention as claimed in the appended

Claims (17)

claims. If desirable the CO in an SNG product could be shifted at low temperature. For the example, the molar specific heats of the reactants and added compound are given below together with their ratio to illustrate the very large ratios obtainable, they are given for the inlet and outlet of the first bed, with CO2 regarded as a reactant at the outlet. 2800C 6000C CO 7.1986 7.7445 CO2 (11.0241) 12.5731 H2 7.0007 7.1211 WTD.SP.HT 7.050 8.0729 C6 50.0292 70.3895 Ratio 7.096 8.719 % (709.6) (871.9) The invention may also be applied in the following manner. The gas stream to be methanated may be divided into two or more parts, to the first of which sufficient added compound is mixed such as to keep the temperature rise through the first methanation bed to a comparatively low amount, thereby reducing the small amount of reaction of the added compound by, e.g. reforming, and also easing the mechanical problems associated with higher temperatures. To the effluent from the first methanation reactor is added the remainder or a further part of the remainder of the gases to be methanated which in turn may contain some added compound. This mixed stream is then further methanated.The sequence of methanation and addition to the methanator effluent of further gases to be methanated may be continued to an economic limit. The addition of the gases to the methanated stream may be used as a quench or temperatures may be adjusted by means of indirect heat exchange. SECOND EXAMPLE A typical clean-up gas from a British Gas Slagging Gasifier has an analysis (in mol %) of H2 31, CO 58, CO2 5, and CH4 6. Considering these to be kg mols per hour of gas at 25 ata to be methanated to SNG, saturate the stream at c. 1 36CC introducing 1 5 kg mols of steam into this stream. Split the stream into two equal parts and raise the temperature of one to 1 550C and again using a saturator saturate with cyclopentane (hereafter called C5) to give a 65 mol % (dry basis) C5 stream. The steam is added so as to allow the excess CO present to shift to CO2 and this produces the hydrogen necessary for methanation and also to give a small amount of residual steam to reduce the residual CO concentration.This stream containing C5, at a temperature of 250do to avoid carbonyl formation, is then methaned in an adiabatic reactor. The outlet from the first reactor is at a temperature of 4650C and contains (kg mols/h) H20 1.28, H2 0.79, CO 1.85, CO2 19.19,CH4 CH413.46, Cs 75- The second part of the feed is then, without further addition, added to the effluent of the first reactor and fed at a temperature of 2800C to a second methanation reactor. The stream leaves the second methanation reactor at a temperature of 4530C and contains H20 2.26, H2 1.05, CO 2.56, CO2 39.1, CH4 27.3, C5 92.86. The effluent from the second reactor is cooled to 2500C and then fed to a third reactor.The outlet from the third reactor at a temperature of 2680C comprises H20 1.48, H2 .06, CO .025, CO2 40.75, CH4 28.23, C5 92.86. After CO2 removal the residual CO in this case is 0.1%, but of course this can be reduced if necessary either by increasing the pressure, increasing the water, or passing the outlet gas through an LT shift bed at low temperature. As can be seen the water in the outlet of the third reactor is very low and could even be reduced if the pressure were higher. It would be possible to use a saturator containing a mixed (2-phase) liquid, i.e. the added compound and water, in the case of SNG. In this case it would be desirable to split the feed to the SNG reactor and saturate only one part thereof, add another part, and then methanate one or more times as described herein. For SNG production the higher the CP of the material, the less that has to be used, thus for any given pressure the lower mol fraction present in the outlet of the final reactor. This means that the partial pressure of the hydrogen can be increased which is far more beneficial than increasing the partial pressure of water. Thus, for instance, cyclohexane in this respect is better than cyclopentane. CLAIMS

1. In a process for reacting two or more gaseous reactants to form at least one product with the evolution or absorption of heat, the improvement comprising adding at least one compound to the reactants in an amount sufficient to reduce the temperature change of the overall process by at least 1 5% of the temperature change that would have occurred had the at least one compound not been added, the at least one compound having a molar specific heat of at least 50% greater than that of the reactants evaluated at the same reaction temperature.

2. A process as claimed in claim 1 wherein the reduction in temperature change is at least 30%.

3. A process as claimed in claim 1 wherein the reduction in temperature change is at least 60%.

4. A process as claimed in claim 1 wherein the reduction in temperature change is at least 75%.

5. A process as claimed in any preceding claim wherein the molar specific heat of the added compound is more than twice that of the gases reacting to form the products at the same reaction temperature.

6. A process as claimed in any preceding claim wherein the molar specific heat of the added compound is more than four times that of the gases reacting to form the products at the same reaction temperature.

7. A process as claimed in claim 1 wherein the at least one compound contains at least three carbon atoms.

8. A process as claimed in claim 1 wherein the reactants are nitrogen and hydrogen and the product is ammonia, and wherein the at least one compound is selected from the group consisting of a C3-C12 paraffin, a cycloparaffin, a substituted paraffin, a substituted cycloparaffin, a compound containing at least one aromatic ring, and a mixture thereof.

9. A process as claimed in claim 1 wherein the reactants are hydrogen and an oxide of carbon and the product is methane, and wherein the at least one compound is selected from the group consisting of a C3-C12 paraffin, a cycloparaffin, a substituted paraffin, a substituted cycloparaffin, a compound containing at least one aromatic ring, and a mixture thereof.

10. A process as claimed in claim 1 wherein the reactants are hydrogen and an oxide of carbon and the product is methanol, and wherein the at least one compound is selected from the group consisting of a C3-C12 paraffin, a cycloparaffin, a substituted paraffin, a substituted cycloparaffin, a compound containing at least one aromatic ring, and a mixture thereof.

11. A process as claimed in claim 1 wherein the reactants are ethylene and oxygen and the product is ethylene oxide, and wherein the at least one compound is selected from the group consisting of a C3C12 paraffin, a cycloparaffin, a substituted paraffin, a substituted cycloparaffin, a compound containing at least one aromatic ring, and a mixture thereof.

12. A process as claimed in any one of the preceding claims wherein, after reaction, the gaseous reactants are cooled, and at least 30% of the at least one compound is condensed out, thereby increasing the concentration of the product remaining in the gaseous phase.

13. A process as claimed in claim 9 for a multistage process wherein only a portion of the reactants are reacted in the first stage with at least a portion of the at least one compound being added, the remaining reactants with any remaining at least one compound being reacted in at least one subsequent stage.

14. A process as claimed in claim 13 wherein a saturator containing a two phase liquid mixture is used to saturate the reactants.

1 5. A process as claimed in claim 9 substantially as hereinbefore described.

1 6. A process as claimed in claim 13 substantially as hereinbefore described.

17. A process as claimed in claim 14 substantially as hereinbefore described.