Patent Application
20090191113
The subject matter of this invention relates to reducing the concentration of ammonia in a methanol containing stream.
The Rectisol® process was developed in 1951 by Linde and Lurgi. For purposes of this invention, the phase “Rectisol process” means a process that is capable of removing sulfur and sulfur containing compounds such as hydrogen sulfide from industrial gas process streams such as process streams generated by coal gasification, among other industrial processes. Rectisol processes typically operate at temperatures less than 32 F and employ an organic solvent such as methanol to solubilize and remove sulfur containing compounds from the industrial gas process stream. The Rectisol process can also remove carbon dioxide, ammonia, among other compounds from the industrial process stream. The Rectisol process is described in greater detail in Advances in Cryogenic Engineering, Vol. 15, Proceedings of the 1969 Cryogenic Engineering Conference, Jun. 16-18, 1969. The Rectisol process can produce a stream that is enriched in sulfur containing compounds. This stream can be sent to a Claus process wherein the sulfur compounds are recovered for use or disposal. A typical Claus process is described in greater detail in Kirk-Othmer, fourth edition, volume 23, pages 440-443.
In a Rectisol process H2S and COS are removed by absorption with cold methanol and concentrated; the resultant concentrated or sulfur enriched stream is then sent to other processes, most commonly the Claus process, for sulfur recovery or disposal. The concentrated sulfur stream is cooled and condensed methanol solvent is produced in order to limit loss of methanol from the overall process, and contamination of the sulfur stream with methanol, which can interfere with downstream sulfur recovery processes. During this cooling and condensation process, trace impurities such as ammonia and hydrogen cyanide can accumulate. This accumulation can result in process problems and/or corrosion. One important problem is the reaction of ammonia with carbon dioxide (normally present at substantial concentrations in the concentrated sulfur stream). This reaction can result in deposition of solid ammonium carbamate in the cooling heat exchanger, which can require shutdown of the entire Rectisol plant to remove this deposit.
Conventional processes for stripping or removing ammonia are disclosed in U.S. Pat. Nos. 5,929,126; 5,948,378; 3,824,185; 3,985,859 and 4,689,156. The disclosure of these patents is hereby incorporated by reference.
The instant invention solves problems with conventional methods by reducing, if not eliminating, fouling of heat exchangers and other equipment (e.g., equipment used in the Rectisol® process), that can be caused by the accumulation of ammonium carbamate which can occur when the concentration of ammonia is sufficient to permit a reaction between ammonia and carbon dioxide. The known solutions to this problem involve periodic plant shutdowns to defrost and remove the ammonium carbamate (which are very costly), or the discharge of ammonia-contaminated methanol. Since the discharged methanol may also contain hydrogen cyanide and hydrogen sulfide, among other toxic compounds, the disposal of this methanol involves the permitting, handling, transportation and disposal of toxic and flammable materials. As a result, the invention can also eliminate these disposal and handling issues.
The invention provides simple and cost-effective methods of removing a sufficient amount of ammonia from the system to prevent heat exchanger fouling by ammonium carbamate. A stream (some times referred to as a slipstream) of methanol in which ammonia, hydrogen cyanide, among other compounds that have accumulated is sent to the top of a stripping column or other device, in which an inert gas, such as nitrogen, is contacted with the methanol stream in a countercurrent flow. The ammonia is at least partially stripped or removed by the nitrogen, and the overhead nitrogen stream containing the ammonia is removed from the Rectisol process in order to prevent the ammonia from building up or accumulating in the process (and in turn reacting to form ammonia carbamate). If desired, this ammonia-containing nitrogen stream can be added to the concentrated sulfur stream which is produced by the Rectisol process. The stripped liquid methanol exiting the bottom of the stripper column can be returned to the Rectisol process.
In some cases, accumulation of hydrogen cyanide may corrode equipment employed in the Rectisol process. In one aspect of the invention, hydrogen cyanide can be removed along with ammonia.
The instant invention relates to improving, for example, an industrial process (e.g., a Rectisol process) wherein sulfur species (e.g., H2S, COS, among other compounds), are removed from an industrial gas feed stream by methanol absorption, and then concentrated for sulfur recovery or disposal in another unit, typically in a Claus plant wherein the sulfur species are recovered as liquid sulfur (for purposes of this invention the “Claus process”). The concentrated sulfur stream is normally generated in a steam-heated desorption column within equipment designed to conduct the Rectisol process. This concentrated stream can be cooled to condense excess methanol vapor before being sent to disposal or sulfur recovery, otherwise methanol losses from the system can be relatively large. The concentrated sulfur stream typically also contains relatively large amounts of carbon dioxide, often about 50% or more on a molar basis.
The cooling and methanol condensation of the Rectisol process are typically done in a cooling loop using some source of relatively cold temperatures, typically below zero degrees F, such as a cold vent gas or a refrigerant. The temperatures required to condense methanol from the concentrated sulfur stream can also condense ammonia, hydrogen cyanide, among other compounds or components. These trace components are typically present at relatively low concentrations in the feed gas to the Rectisol unit (e.g., effluent from a gasifier). Typically the Rectisol process is operated in a manner to route all ammonia and hydrogen cyanide into the concentrated sulfur stream. However, condensation of ammonia and hydrogen cyanide in connection with the methanol condensation can result in a build-up of these species. If the ammonia reaches a critical concentration, which may vary depending upon the temperatures and other species present, ammonia can react with carbon dioxide to form solid ammonium carbamate in the following reaction:
2 NH3 (g)+CO2 (g)→NH4COONH2 (solid)
The solid ammonium carbamate can foul cold heat transfer surfaces and cause excessive pressure drop in the cooling exchanger of the Rectisol process. Typically, the only practical solution once the pressure drop becomes too high is to shut down the heat exchanger (and necessarily the entire Rectisol process) and warm up the fouled surfaces. At temperatures above 130-140 F the ammonium carbamate sublimates from the surfaces and can be purged.
Whether or not fouling occurs, depends upon the mass balance of the ammonia in the system and on the temperatures employed during cooling. If all ammonia absorbed from the Rectisol feed stream is disposed in the concentrated sulfur stream so that the resulting ammonia concentration in the cooling loop remains below the critical concentration, no significant fouling occurs. If the ammonia in the feed stream rises, or other process changes occur such that the critical ammonia concentration is reached, then fouling occurs and the plant must be shut down (with the attendant loss of production and revenue). While hydrogen cyanide does not cause fouling, it can build up in a similar way and potentially cause corrosion within the system. Fouling and/or corrosion are difficult to predict since prior to starting the Rectisol process, the ammonia and hydrogen cyanide levels in the Rectisol feed stream may be unknown or vary during operation of the Recitsol process.
In one aspect of the instant invention, a sidestream comprising methanol, ammonia and/or hydrogen cyanide is removed from the cooling train for the concentrated sulfur stream in the Rectisol system. For example, this stream can be taken from the process location where ammonia and/or hydrogen cyanide are at their maximum concentrations. This sidestream is fed to a stripping column (e.g., the top of a stripping column), which may use random or structured packing or trays, depending upon the design and size and other known variables. Desirable results have been obtained by using Random packing.
A gas (also known as a stripping gas) is fed to the bottom of the column and flows counter-current upward to the down-flowing methanol. This gas can be nitrogen or any other gas stream that is compatible with the species present and with the utility of the column's overhead vapor. In one aspect of the invention, the overhead vapor containing ammonia and/or hydrogen cyanide and the stripping gas are combined with the previously described concentrated sulfur stream (produced by the Rectisol process) and further processed (e.g., in a Claus process). While any suitable gas can be employed in the inventive process, examples of suitable gases comprises at least one member from the group consisting of nitrogen, argon, hydrogen, methane or natural gas are suitable. Desirable results have been obtained by using nitrogen.
The methanol containing stream exiting the bottom of the column, with reduced concentrations of ammonia and/or hydrogen cyanide, is typically returned to the Rectisol process. It should be noted that complete removal of ammonia and/or hydrogen cyanide may not be required or practical in the stripping column; it is only necessary to remove sufficient amounts to eliminate or substantially reduce fouling or corrosion. In general the methanol flow to the stripping device should be as low as practical, as this minimizes the stripping gas employed. The relative flows of liquid and stripping as to the column can be optimized depending upon the desired amount of ammonia to be removed. In some cases it will be useful to provide relatively large amounts of methanol to the stripping column and remove a lower percentage of ammonia and/or hydrogen cyanide; this may allow lower flows of stripping gas to be used.
The amount of stripping gas supplied to the column may vary depending upon the intended usage of the overhead vapor from the column. If the overhead vapor is provided to a Claus process, there may be a concentration limits on stripping gas, methanol or other components in the overhead vapor. The overall system can be optimized to meet all concentration parameters on streams exiting the stripping column (including, for example, the elimination of ammonia and/or hydrogen cyanide) while minimizing the required flows of stripping gas and methanol fed to the column.
Referring now to
While any suitable temperature can be employed for operating the column, the temperature will normally range from about 40 to about 110 F. The process pressure will normally range from 50 to about 100 psig.
While this description emphasizes a process for treating a methanol stream, the instant invention can be used to remove a wide range of compounds from a wide range of organic streams. Similarly, the instant invention can be used to treat a wide range of process streams other than those produced by a Rectisol process.
The following Examples are provided to illustrate certain aspects of the invention and do not limit the scope of the claims appended hereto.
The following example is based upon a gas stream that was produced in a commercial industrial process and which was modeled in ASPEN using a proprietary thermodynamics package in accordance with conventional methods. The ammonia removal rate was adequate to reduce fouling of the Rectisol process equipment.
The following example represents the same ammonia mass removal rate as in Example 1: 0.28 lbmoles/hr. But in this Example the liquid methanol feed rate to the stripper is doubled, and the nitrogen stripping gas flow is adjusted to maintain that same mass removal rate. While the liquid feed rate doubles, the required N2 stripping flow declines by 27%. The per cent ammonia removal (as opposed to the mass removal) drops from 40.4% in Example 1, to 20.8% in Example 2. This example illustrates that it is possible to optimize the stripping column in different ways to achieve a predetermined removal rate, depending on which variables are most important in a given facility.
The present invention is not limited in scope by the specific aspects disclosed in the examples which are intended as illustrations of a few aspects of the invention and any embodiments that are functionally equivalent are within the scope of this invention. Indeed, various modifications of the invention in addition to those shown and described herein will become apparent to those skilled in the art and are intended to fall within the scope of the appended claims.
