PROCESS FOR INCREASING THE CAPACITY OF AN EXISTING UREA PROCESS

The invention relates to a process for increasing the capacity of an existing urea process comprising, in the high-pressure section of the process, a reactor in which carbon dioxide and ammonia react to form urea, a thermal stripper in which the process stream from the reactor is stripped by supplying heat or an ammonia stripper in which the process stream from the reactor is stripped by supplying heat with the aid of ammonia as stripping gas and a condenser in which the stripping gases are condensed, whereupon the condensate formed is returned to the reactor, in which process the N/C ratio in the reactor is between 2.8 and 3.3 mol/mol, the pressure in the high-pressure section of the process is between 13.5 and 16.0 Mpa, at least a portion of the process stream from the reactor is stripped in a CO₂ stripper in which the process stream from the reactor is stripped by supplying heat and with the aid of carbon dioxide as stripping gas and the condensing capacity in the high-pressure section of the process is increased.
PROCESS FOR INCREASING THE CAPACITY OF AN EXISTING UREA PROCESS
The invention relates to a process for increasing the capacity of an existing urea process comprising, in the high-pressure section of the process, a reactor in which carbon dioxide and ammonia react to form urea, a thermal stripper in which the process stream from the reactor is stripped by supplying heat or an ammonia stripper in which the process stream from the reactor is stripped by supplying heat with the aid of ammonia as stripping gas and a condenser in which the stripping gases are condensed, whereupon the condensate formed is returned to the reactor. Such an existing process is described in for example Ullmann's
Encyclopedia of Industrial Chemistry, Vol. A27, 1996, p. 344-350 as the Snamprogetti Self-Stripping Process.
In such a process ammonia and carbon dioxide are contacted in a reactor at a pressure of 15.0-16.5 MPa and at an N/C ratio of 3.0-4.0 mol/mol. The process stream that forms in the reactor is passed to a high-pressure stripper in which this process stream is heated in order to decompose the ammonium carbamate and to discharge the excess ammonia, along with the ammonia and carbon dioxide from the decomposed ammonium carbamate, as a gas stream from the high-pressure stripper. Ammonia, too, can be used here as a stripping gas. The gas stream from the high- pressure stripper is partly condensed in the high-pressure condenser, to which a carbamate stream from the medium-pressure recovery section is also added. Subsequently, the gas/liquid stream from the high-pressure condenser is supplied to a high-pressure separator, the liquid fraction being returned to the reactor via an ejector. The gas from the high-pressure separator is passed to the medium-pressure recovery section.
A process known to one skilled in the art is to increase the capacity of existing processes by replacing those process items that form a bottleneck in the process with larger equipment items. An example of an equipment item which would ^• need to be replaced by a larger unit is for example the urea reactor. Such a process is described in for example EP-0751121-A1. This patent publication discloses that the capacity of a Snamprogetti Self-Stripping Process can be increased by adding a second reactor or by replacing the existing reactor with a larger reactor.
A drawback of expanding the reactor in this manner is that the costly high-pressure ammonia pumps also need to be replaced by larger pumps when the original process conditions are maintained. The condenser and the stripper, too, will
probably need to be replaced by units having a higher capacity.
The drawback of replacing the reactor and high-pressure ammonia pumps is that high costs are involved.
The aim of the invention is to develop a process for increasing the capacity of a urea process whereby replacement of costly equipment is avoided as much as possible. This is achieved by
• the N/C ratio in the reactor being between 2.8 and 3.3 mol/mol,
• the pressure in the high-pressure section of the process being between 13.5 and 16.0 MPa,
• at least a portion of the process stream from the reactor being stripped in a CO ₂ stripper in which the process stream from the reactor is stripped by supplying heat and with the aid of carbon dioxide as stripping gas and
• the condensing capacity in the high-pressure section of the process being increased.
The N/C ratio is the molar ratio between ammonia (N) and carbon dioxide (C) in the reactor. In the existing urea process, this ratio was between 3.0 and 4.0 mol/mol. One measure taken to increase the capacity of the existing process is to lower the N/C ratio to a value between 2.8 and 3.3 mol/mol. In the existing urea process the pressure in the high-pressure section of the process was between 15.0 and 16.5 MPa. In increasing the capacity of the existing urea process, this pressure is reduced to a pressure of between 13.5 and 16.0 MPa.
In increasing the capacity, a third requirement is that the process stream from the reactor, comprising urea, ammonia, carbon dioxide, water and ammonium carbamate, be at least partly stripped in a CO ₂ stripper, in which the process stream from the reactor is stripped by supplying heat and with the aid of carbon dioxide as stripping gas. In the existing process this implies that a CO ₂ stripper is added. The increased-capacity process then comprises a thermal stripper or an ammonia stripper as well as a CO ₂ stripper in which a portion of the process stream from the reactor is stripped. One skilled in the art can readily control the optimum distribution of the process stream among the two types of stripper.
It is also possible to convert the existing thermal stripper or ammonia stripper into a CO ₂ stripper whereby the whole process stream from the reactor is
stripped in a CO ₂ stripper. The existing thermal stripper or ammonia stripper can, of course, also be replaced with a new CO ₂ stripper. The option to be chosen by one skilled in the art is dictated by the condition of the existing thermal stripper or ammonia stripper, bearing in mind that, in increasing the capacity, replacing high-cost equipment is avoided wherever the equipment is in good physical condition. However, on account of the technical simplicity of the process it is preferable to strip the whole gas stream from the reactor in a CO ₂ stripper.
A fourth requirement for increasing the capacity of an existing urea process is to increase the condensing capacity in the high-pressure section of the process. This can be accomplished in various ways. For example, it is possible to add a high-pressure scrubber or a second high-pressure condenser. Alternatively, it is possible to increase the condensing capacity of the existing condenser.
The off-gases from the condenser are at least partially condensed in the high-pressure scrubber. The high-pressure scrubber can be designed in two ways:
1 : Substantially complete scrubbing of ammonia and carbon dioxide from the off-gas to be achieved by cooling with the aid of a heat exchanger followed by scrubbing with a medium-pressure carbamate solution.
2: Partial scrubbing of ammonia and carbon dioxide from the off-gas, with the ammonia and carbon dioxide only being condensed in a heat exchanger. In this design, the carbamate solution originating from a medium-pressure recovery section is supplied to the high-pressure scrubber and/or the high-pressure condenser.
For increasing the condensing capacity it is also possible to add a high-pressure condenser in which the off-gases from the existing high-pressure condenser are condensed in a carbamate stream supplied from the medium-pressure recovery section to the additional high-pressure condenser.
The high-pressure condenser to be added can be designed as a falling-film condenser or as a kettle type condenser. , The added high-pressure condenser may be arranged in parallel with or in series with the existing high-pressure condenser. Steam or hot water may be generated in the additional high-pressure condenser. When the added high-pressure condenser is arranged in parallel the off-gas stream from the stripper and the carbamate stream from the medium-pressure recovery section are split and directed to both high-pressure condensers. The carbamate stream that is formed in the high-
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pressure condensers is returned to the reactor and the off-gases from the high- pressure condensers are directed to the medium-pressure recovery section.
In the series arrangement the off-gas from the existing high-pressure condenser is condensed in the added high-pressure condenser, with at least a portion of the carbamate stream from the medium-pressure recovery section being supplied to the added high-pressure condenser. The carbamate stream from the added high- pressure condenser can be supplied to the existing high-pressure condenser either separately or together with a portion of the carbamate stream from the medium- pressure recovery section. The carbamate stream from the existing high-pressure condenser is returned to the reactor and the off-gases from the high-pressure condensers are discharged to the medium-pressure recovery section. It is also possible to combine the carbamate streams from both high-pressure condensers and to return them, optionally via a separator, to the reactor.
Preferably, the condensers are installed at a low elevation (near the ground). Such installation requires the use of ammonia-driven ejectors.
For increasing the capacity of the existing urea process still further it is preferred to increase the reaction capacity of the existing process also. This can be accomplished by, for example, by increasing the reaction volume of the existing reactor. It is known to those skilled in the art that in a urea process the condensing capacity and the reaction capacity can be increased at the same time by adding equipment to the high-pressure section of the process in which condensation and reaction can be carried out simultaneously.
Examples of such equipment are a pool condenser, a pool reactor and a combi-reactor.
The pool condenser is disclosed in for example EP-0155735-A1. The pool condenser can be installed horizontally or vertically. In the pool condenser, the off-gas from the stripper(s) is condensed and, additionally, a portion of the quantity of urea to be produced is formed in the pool condenser. The liquid stream that is passed from the pool condenser to the existing reactor thus comprises both carbamate and urea.
The pool reactor is disclosed in for example US-A-5767313. The pool reactor comprises a condenser section and a reactor section in an apparatus placed in horizontal position. The combi-reactor is disclosed in for example US-B1 -6392096, in
US-B2-6680407 and in US-A-5936122. The combi-reactor comprises a condenser section and one or two reactor sections in an apparatus placed in vertical position. The condenser section may be placed above or below the reactor section. If two reactor sections are present, the condenser section is located between the two reactor sections.
In the pool reactor or the combi-reactor the off-gas from the stripper(s) is condensed in the condenser section, whereupon urea is formed in the reactor section or the reactor sections. At least a portion of the carbamate stream from the medium-pressure recovery section is supplied to the condenser section of the pool reactor or combi-reactor. The process stream from the reactor section is passed to the CO ₂ stripper and optionally the thermal or ammonia stripper.
A pool reactor and a combi-reactor may also be used for replacing the existing reactor and condenser.
The invention also relates to a urea plant comprising, in the high- pressure section of the process, a reactor, a thermal stripper or an NH ₃ stripper and a condenser, in which, besides the thermal stripper or NH ₃ stripper, a CO ₂ stripper is also present in the high-pressure section of the process.
The urea plant may also comprise a high-pressure scrubber or a second condenser if the condensing capacity in the high-pressure section of the process has been increased
If both the condensing capacity and the reaction capacity in the high- pressure section of the process have been increased, the urea plant may comprise a pool condenser, a pool reactor or a combi-reactor.
The invention also comprises a urea plant comprising, in the high- pressure section of the process, a pool reactor or a combi-reactor, a thermal stripper or an NH ₃ stripper and a CO ₂ stripper.
The invention is elucidated with reference to the following examples without being limited thereto.
Figure 1 represents the Snamprogetti Self-Stripping Process according to the state of the art. In a reactor (R) ammonia and carbon dioxide are contacted at a pressure of 15 MPa at an N/C ratio of 3.5 mol/mol. The process stream from the reactor is directed to a stripper (S) in which the process stream from the reactor is stripped with the aid of heat. Subsequently, the urea-containing process stream from the stripper is passed to the medium-pressure recovery section (MP) in which this process stream is recovered further and in which process a carbamate
stream is formed. In addition, a gaseous stream is separated in the medium-pressure recovery section, which stream is directed to a section (N) in which ammonia gas is recovered. This ammonia gas is returned to the reactor (R) via the ejector (E). The urea-containing stream passes from the medium-pressure recovery section to a low- pressure recovery section (LP). On leaving the low-pressure recovery section, the urea stream (U) is concentrated and recovered further. The carbamate stream from the low-pressure recovery section is returned to the medium-pressure recovery section.
The stripping gases from the stripper are mixed in mixer (M), together with the carbamate stream from the medium-pressure recovery section and are directed to the condenser (C). Here, the stripping gases are partly condensed. The gas/liquid stream from the condenser is supplied to a separator (A). The liquid fraction is returned from the separator to the reactor by means of the ejector (E) which is driven by the ammonia feed. The gas stream from the separator passes to the medium-pressure recovery section.
The capacity of a process according to figure 1 is 1550 tonnes/day. Figure 2 represents a Snamprogetti Self-Stripping Process with increased capacity according to the invention. In a reactor (R), whose reaction volume has been increased, ammonia and carbon dioxide are contacted at a pressure of 14.0 MPa at an N/C ratio of 3.0 mol/mol. The process stream from the reactor is directed to the strippers (Sn and Sb). In the stripper (Sb) the process stream from the reactor is stripped with the aid of heat and in the stripper (Sn) with the aid of heat and with carbon dioxide as stripping gas. Subsequently, the urea-containing process stream from the stripper (Sb) passes to the medium-pressure recovery section (MP) in which this process stream is recovered further, whereby a carbamate stream is formed.
The urea-containing process stream from the stripper (Sn) passes to a newly installed low-pressure recovery section (LPn), in which this process stream is recovered further, whereby a low-pressure carbamate stream is formed. Additionally, in the medium-pressure recovery section a gaseous stream is separated, which is directed to a section (N) in which ammonia gas is recovered. This ammonia gas is returned to the reactor (R) via the ejector (E). The urea-containing stream is also directed from the medium-pressure recovery section to the low-pressure recovery section (LPb). On leaving the low-pressure recovery sections (LPb and LPn), the urea streams (U) are concentrated and recovered further. The carbamate streams from the low-pressure recovery sections are returned to the medium-pressure recovery section.
The stripping gas from the stripper (Sb) passes to the condenser (C). A portion of the carbamate stream from the medium-pressure recovery section may optionally be added to the condenser. The stripping gases are partially condensed in the condenser. The non-condensed gases are directed from the condenser to the scrubber (SC). The stripping gas from stripper (Sn) and the off-gas from the reactor are also directed to scrubber (SC). In the scrubber practically all gases are condensed in the carbamate stream from the medium-pressure recovery section, which stream is also supplied to the scrubber. The condensate returns to the reactor via ejector (E). Waste gases (a), containing traces of ammonia and carbon dioxide, are discharged from the scrubber to an absorber.
The capacity of a process according to figure 2 is 2400 tonnes/day. Figure 3 represents a Snamprogetti Self-Stripping Process with increased capacity according to the invention. In a reactor (R), whose reaction volume has been increased, ammonia and carbon dioxide are contacted at a pressure of 14.0 MPa at an N/C ratio of 3.0 mol/mol. The process stream from the reactor is directed to the strippers (Sn and Sb). In the stripper (Sb) the process stream from the reactor is stripped with the aid of heat and in the stripper (Sn) with the aid of heat and with carbon dioxide as stripping gas. Subsequently, the urea-containing process stream from the stripper (Sb) passes to the medium-pressure recovery section (MP) in which this process stream is recovered further, whereby a carbamate stream is formed.
The urea-containing process stream from the stripper (Sn) passes to a newly installed low-pressure recovery section (LPn), in which this process stream is recovered further, whereby a low-pressure carbamate stream is formed. Additionally, in the medium-pressure recovery section a gaseous stream is separated, which is directed to a section (N) in which ammonia gas is recovered. This ammonia gas is returned to the reactor (R) via the ejector (E). The urea-containing stream is also directed from the medium-pressure recovery section to the low-pressure recovery section (LPb). On leaving the low-pressure recovery sections (LPb and LPn), the urea streams (U) are concentrated and recovered further. The carbamate streams from the low-pressure recovery sections are returned to the medium-pressure recovery section.
The stripping gas from the strippers (Sn and Sb) passes to the condensers (Cn and Cb). A portion of the carbamate stream from the medium- pressure recovery section may optionally be added to the condenser (Cb). The stripping gases are partially condensed in the condensers. The non-condensed gases are directed from the condensers (Cn and Cb) to the scrubber (SC). The off-gas from
the reactor is also directed to scrubber (SC). In the scrubber practically all gases are condensed in the carbamate stream from the medium-pressure recovery section, which stream is also supplied to the scrubber. The condensate returns to the reactor via ejector (E). Waste gases (a), containing traces of ammonia and carbon dioxide, are discharged from the scrubber to an absorber.
The capacity of a process according to figure 3 is 2400 tonnes/day. Figure 4 represents a Snamprogetti Self-Stripping Process with increased capacity according to the invention. In a reactor (R), whose reaction volume has been increased, ammonia and carbon dioxide are contacted at a pressure of 14.0 MPa at an N/C ratio of 3.0 mol/mol. The process stream from the reactor is passed to the stripper (Sn). In the newly added stripper (Sn), which replaces the existing stripper, the process stream from the reactor is stripped with the aid of heat and with carbon dioxide as stripping gas. The urea-containing process stream from the stripper is then directed to the medium-pressure recovery section (MP) in which this process stream is recovered further, whereby a carbamate stream is formed.
In the medium-pressure recovery section a gaseous stream is also separated, which stream passes to a section (N) in which ammonia gas is recovered. This ammonia gas returns to the reactor (R) via ejector (E). The urea-containing stream is directed from the medium-pressure recovery section to the low-pressure recovery section (LP). On leaving the low-pressure recovery section, the urea stream (U) is concentrated and recovered further. The carbamate stream from the low- pressure recovery section is returned to the medium-pressure recovery section. The stripping gas from the stripper (Sn) is supplied to the newly installed pool condenser (PC), which replaces the existing condenser. A portion of the carbamate stream from the medium-pressure recovery section may optionally be added to the pool condenser. The stripping gases are partially condensed in the pool condenser. The non-condensed gases are directed from the pool condenser to the scrubber (SC). The reactor off-gas is also directed to scrubber (SC). In the scrubber, practically all gases are condensed in the carbamate stream from the medium- pressure recovery section, which stream is also supplied to the scrubber. Waste gases (a), containing traces of ammonia and carbon dioxide, are discharged from the scrubber to an absorber. The condensate returns to the pool condenser. The condensate that forms in the pool condenser is returned to the reactor via the ejector (E). The capacity of a process according to figure 4 is 2610 tonnes/day.
Figure 5 represents a Snamprogetti Self-Stripping Process with increased capacity according to the invention. In a reactor (R), whose reaction volume has been increased, ammonia and carbon dioxide are contacted at a pressure of 14.0 MPa at an N/C ratio of 3.0 mol/mol. The process stream from the reactor is passed to the stripper (Sn). In the newly installed stripper (Sn), which replaces the existing stripper, the process stream from the reactor is stripped with the aid of heat and with carbon dioxide as stripping gas. The urea-containing process stream from the stripper is then directed to the medium-pressure recovery section (MP) in which this process stream is recovered further, whereby a carbamate stream is formed. In the medium-pressure recovery section a gaseous stream is also separated, which stream passes to a section (N) in which ammonia gas is recovered. This ammonia gas returns to the reactor (R) via ejectors (E1 and E2). The ammonia gas can be heated before it enters ejector (E1) and/or (E2). The urea containing stream is directed from the medium-pressure recovery section to the low-pressure recovery section (LP). On leaving the low-pressure recovery section, the urea stream (U) is concentrated and recovered further. The carbamate stream from the low- pressure recovery section is returned to the medium-pressure recovery section.
A portion of the stripping gas from the stripper (Sn) is directed to the newly installed pool condenser (PC), which replaces the existing condenser. A portion of the carbamate stream from the medium-pressure recovery section may optionally be added to the pool condenser. The stripping gases are partially condensed in the pool condenser. The non-condensed gases are passed from the pool condenser to the scrubber (SC). The reactor off-gas is also directed to scrubber (SC). In the scrubber, practically all gases are condensed in the carbamate stream from the medium- pressure recovery section, which stream is also added to the scrubber. Waste gases (a), containing traces of ammonia and carbon dioxide, are discharged from the scrubber to an absorber. The condensate is returned to the pool condenser. The condensate that forms in the pool condenser is returned to the reactor via the ejector (E1). Another portion of the stripping gas from the stripper (Sn) is returned directly from the stripper to the reactor via an ejector (E2).
This design allows the carbon dioxide to be added as much via the stripper as possible, as a result of which a lower steam consumption is achieved.
The capacity of a process according to figure 5 is 2610 tonnes/day.