Modularized system and method for urea production using stranded natural gas

Abstract

A modular system and method for producing urea from stranded natural gas includes removal of foreign particulate matter to obtain a substantially homogeneous gas. The gas is processed by controlling the quality of the stranded natural gas to maintain a substantially homogenous mixture The resultant gas stream is further cleaned and compressed to a high pressure of about 3,000 psi. The resultant ammonia stream is processed in a bypass recycling loop system at 30% conversion rate at a high pressure of about 6,000 to 7,000 psi. The equipment associated with each of the process steps may be skid mounted for portability and/or contained within the footprint of a standard 48-foot flatbed trailer.

Claims

1. A method for providing a consistent feedstock to a fertilizer production process that makes use of stranded natural gas as the feedstock, the method comprising the steps of: i. capturing a natural gas feedstock that includes at least two stranded natural gas feedstocks from different stranded natural gas source; ii. blending the at least two captured natural gas feedstocks with one another to form a blended natural gas feedstock having a BTU content less variable than a BTU content of the at least two captured stranded natural gas feedstocks.

2. A method according to claim 1 wherein the at least two captured natural gas feedstock each have at least one different processing characteristic than the other.

3. A method according to claim 1 further comprising the blending step resulting in a consistent sulfur content for the homogeneous blend.

4. A method according to claim 1 further comprising a moisture removal step, wherein the moisture removal step removes moisture to a predetermined moisture content.

5. A method according to claim 1 further comprising a reformulating step, a portion of the reformulating step occurring in a temperature range of about 500 to 800 C.

6. A method according to claim 5 further comprising the reformulating step including the sub-step of compressing a resultant CO.sub.2 stream to a pressure of at least about 3,000 psi.

7. A method according to claim 1 further comprising the step of processing a NH.sub.3 stream in a bypass recycling loop.

8. A method according to claim 7 wherein the bypass recycling loop operates at a high pressure range of between about 6,000 to 7,000 psi and results in about a 30% conversion rate.

9. A method according to claim 1 wherein equipment embodying the method is placed in series with at least one other set of equipment used in the fertilizer production process.

10. A method according to claim 1 wherein equipment embodying the method is placed in parallel with at least one other set of equipment used in the fertilizer production process.

11. A method according to claim 1 wherein equipment associated with at least one of the steps (i) and (ii) is substantially immediately portable between a first and second stranded natural gas source site.

12. A method according to claim 11 wherein all equipment associated with at least one of steps (i) and (ii) is temporarily mounted within a footprint of a standard flatbed truck trailer.

13. A method according to claim 11 wherein all equipment associated with at least one of the steps (i) and (ii) is skid mounted.

14. A method according to claim 1 wherein all equipment associated with at least one of the steps (i) and (ii) is temporarily positioned at a location in fluid communication with at least one of the two stranded natural gas sources.

15. A system for use in a fertilizer production process that uses stranded natural gas as a feedstock, the system comprising: a purification module arranged to blend together at least two stranded natural gas feedstocks each being from a different stranded natural gas source, the blended together stranded natural gas feedstock having a less variable BTU content than a BTU content of the at least two stranded natural gas feedstocks prior to being blended together.

16. A system for producing fertilizer from a stranded natural gas source, the system comprising a set of portable modules arranged to convert a stranded natural gas feedstock into the fertilizer, the set including a purification module, the purification module including means for blending together stranded natural gas streams from different sources, the blended together stranded natural gas streams having a less variable BTU content than that of the stranded natural gas streams.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a process flow diagram of a urea production process that converts Stranded/Flared Natural Gas into urea. The gas is cleaned prior to being fed into a gasification unit.

(2) FIG. 2 is a block layout of the gasification system module arranged on a standard 48-foot flatbed trailer.

(3) FIG. 3 is a block layout of the urea conversion module arranged on a standard 48-foot flatbed trailer.

(4) FIG. 4 is a schematic representation of the interrelationship between the carbon dioxide compression component, condenser/reactor, and water extraction and drying components of the urea conversion module and the gas stream.

(5) FIG. 5 is a schematic representation of the carbon dioxide compression component of the urea conversion module.

(6) FIG. 6 is a schematic representation of the pool condenser/reactor component of the urea conversion module.

(7) FIG. 7 is a schematic representation of the water extraction and drying component of the urea conversion module.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(8) The following disclosure provides many different embodiments, or examples, for implementing different features of the invention. These are, of course, merely examples and are not intended to limit the invention from that described in the claims. Well known elements are presented without detailed description in order not to obscure the present invention in unnecessary detail. For the most part, details unnecessary to obtain a complete understanding of the present invention have been omitted inasmuch as such details are within the skills of persons of ordinary skill in the relevant art.

(9) Current plants for the production of high nitrogen fertilizers are large-scale, permanent facilities that take several years to build. To be economically viable, these plants, and their associated processing methods and equipment, require commercial grade natural gas at sufficient volume and pressure. Because the sources of stranded natural gas are geographically scattered, the quality of the gas is poor, and the volumes and pressures of the gas produced are relatively low, the gas is not a viable feedstock for these plants.

(10) Unlike conventional plants, a production plant made according to the present invention can be built in about half the time. Because the cost of the plant is relatively low, and because the plant makes use of different ways to treat the gas feedstock, the plant is economically viable to produce high nitrogen fertilizers such as urea and other mixed fuels. Further, because the plant is modularized, the plant may be sited on a mobile pad (such as a flatbed trailer) or temporally sited on a concrete pad and then deconstructed, moved, and reconstructed in a matter of a few months. The modularized design of the plant allows the plant to go to the sources of stranded natural gas rather than require those sources come to it. This makes the design ideal for use in remote rural areas that have geographically scattered or low producing well sites, or areas that produce low quality gas or lack the infrastructure necessary to move large quantities of gas over long distances to a central location. Last, because of the design's modularity, the plants are easily maintained, self-sufficient and highly automated. This lends itself well to operating in remote well head locations.

(11) The use of stranded natural gas for the production of urea is based on utilizing approximately 30,000 cubic feet of stranded natural gas having an average BTU content of 1,000 BTU per cubic foot to produce one ton of nitrogen fertilizer. Based on this relationship, the following production estimates are derived:

(12) TABLE-US-00001 Stranded Gas Feedstock Yield Urea in CF/hr (000) (ton/hr or TPH) 40.5 1.35 82.5 2.75 124.8 4.16
Achieving yields using stranded gas feedstock that are comparable to those using higher quality natural gas is a result of the unique and inventive characteristics of the method disclosed and claimed herein. Preferably, embodiments of the present invention are available in 1.35, 2.75 and 4.16 TPH sizes.

(13) Regardless of TPH size, a plant made according to this invention may be paralleled or placed in series with other like-made plants to produce electrical power or bio-liquids (e.g., gasoline, diesel, jet fuel, fertilizers and other chemicals) in larger quantities. When compared to conventional plants, the smaller TPH size, provides many advantages, including: improved reliability; customizability; efficiency; portability; economy; compact units; environmentally friendly (meeting, for example, Environmental Protection Agency regulations and Texas Commission on Environmental Quality regulations) and operational ease.

(14) The modular construction of the present invention also allows a user to optimize production based on the availability of stranded natural gas in a particular field. The modular construction also allows for the movement of the plant when a field or well becomes nonproductive.

I. Urea Production

(15) Purification Module 100

(16) The system and process of the present invention will now be described in the following paragraphs referring to FIG. 1.

(17) The purification module 100 starts with filtering step 101 to reduce the moisture content of the stranded natural gas stream and obtain a substantially water-free fuel mixture of nitrogen and hydrogen in the stoichiometric ratio of 1:3. Once the moisture content is reduced to a predetermined level, high pressure steam is introduced to heat the fuel mixture to approximately 400 C. The heated fuel mixture is passed over a catalyst to remove potential disruptive inorganics and organics from the mixture. The catalyst converts nonreactive organic sulfur compounds to hydrogen sulfide. Hydrogen sulfide is removed by passing the mixture over a bed of zinc oxide particles in the desulferizing step 102. The zinc oxide particles absorb the hydrogen sulfide. The purified gas stream is then ready for the reforming module 200.

(18) Filtering step 101 or desulferizing step 102 may be proceeded by a blending step (not shown) in which two of more different stranded natural gas streams are blended together to form a single substantially homogeneous stream. The importance of creating a homogenous feedstock when using biomass to produce urea is discussed in our earlier international application PCT/US2009.053537, titled Modularized System and Method for Urea Production Using a Biomass Feedstock, published as WO/2010/019662 on Feb. 18, 2010, the content of which is hereby incorporated by reference.

(19) Blending the streams to produce a single stream is important when using stranded natural gas because the gas produced by different well sites may have different processing characteristics, such as the amount of moisture, sulfur or BTU content. Failing to provide downstream modules with a consistent quality of gas (regardless of whether that quality is relatively high or low) makes it difficult to control the processes associated with those downstream modules and produce an end product having consistent quality. Unlike prior art processes, which require a certain quality of natural gas, the process described herein makes use of whatever quality of gas is available. For this reason, stranded natural gas is acceptable as a feedstock and could, if desired, be blended together with a higher quality, commercial-grade natural gas stream and processed.

(20) Reforming Module 200

(21) The reforming module 200 starts with a primary reforming step 201 in which the purified gas stream from Module 100 flows into indirectly heated tubes filled with nickel containing a reforming catalyst. The indirectly heated tubes raise the temperature of the gas stream to about 500 to 800 C. In primary reforming step 201 the reaction is controlled to achieve only a partial conversion of approximately 65% based on the methane feed from module 100. In a subsequent secondary reforming step 202 the partially converted gas stream is passed through a refractory lined reaction vessel with nickel catalyst and mixed with a controlled amount of combustion air. The combustion of the partially converted gas stream further raises the temperature to approximately 1,200 C. The combusted gas stream then flows through another catalyst layer where the outlet temperature is lowered to approximately 1,000 C. and the residual methane is less than 0.5%. The outgoing reformed gas stream, which is compressed to at least 206 bar (about 3,000 psi), is then ready for shift conversion.

(22) Shift Conversion Module 300

(23) Shift conversion module 300 uses a water-gas shift reaction. The carbon monoxide (CO) serves as a reducing agent for water to yield hydrogen (H) and carbon dioxide (CO.sub.2). Module 300 not only produces more H for ammonia module 400 but also converts the CO to CO.sub.2 which will be used as a chemical component in the urea production module 500.

(24) Shift conversion module 300 begins with step 301, high temperature shift conversion, which utilizes an iron-based catalyst with an additional 5 to 10% chromic oxide. Steam is introduced to the incoming reformed gas stream and the temperature of the reaction is held to a range of about 300 to 500 C. This is a controlled process and is dependent on the ratio of CO/CO.sub.2.

(25) Low temperature shift conversion step 302 utilizes an iron-chromium and copper-zinc catalyst that is active at a temperature range of about 320 to 360 C. Step 302 furthers the reaction and also works to absorb residual sulfur (<0.1 ppm) to prevent poisoning of the catalyst. CO.sub.2 is stripped 303, 303a, compressed at approximately 206 bar (about 3,000 psi) and flowed to the urea conversion module 500.

(26) Ammonia Module 400

(27) Ammonia module 400 involves a purification process using a simple reversal of the primary reforming step 201 to reduce carbon oxides to less than 10 ppm. A nickel catalyst, at a pressure of about 25 to 35 bar (about 360 to 510 psi), controlled at temperature between about 250 to 350 C. is utilized in the methanation process step 401. The processed gas exiting step 401 is then compressed in syngas compression step 402 at approximately 150 to 175 bar (about 2,175 to 2,550 psi) and flowed to the ammonia convertor loop 403. The ammonia convertor loop 403 is used to continuously recycle the gas over an iron catalyst using a H.sub.2 recovery feed 404, 404a. A refrigeration loop 405 is utilized to cool the gas after passing over the catalyst which allows for the pure ammonia (NH.sub.3) to condense out. Ammonia converter loop 403 is a bypass recycling loop at a high pressure range of between about 410 to 485 bar (about 6,000 to 7,000 psi) and results in about a 30% conversion rate.

(28) Urea Conversion Module 500

(29) Urea production module 500 is described in our previously mentioned international application. Urea conversion module 500 receives the compressed CO.sub.2 from step 303a and the NH.sub.3 from step 403 and flows the compressed CO.sub.2 and NH.sub.3 to a pool condenser step 501 (see FIGS. 4, 5 & 6). NH.sub.3 and CO.sub.2 are introduced into the pool condenser 501a by a high-pressure ammonia pump and a carbon dioxide compressor (see FIG. 6). The CO.sub.2 and NH.sub.3 gas streams are flowed counter-current to one another in order to improve the overall reaction within the pool condenser 501a. About two-thirds of the urea conversion takes place in the pool condenser 501a. After the pool condenser 501a the remaining gases and urea-carbamate liquid enter the vertical pool reactor 501b in which the final urea formation takes place. Any un-reacted carbamate may be routed to a scrubber/recycler 501c for reintroduction to vertical pool reactor 501b.

(30) The resulting urea slurry or solution is sent to a drying step 502 where water is removed (see FIG. 7). Water extraction occurs by way of vacuum extraction 502a. The remaining urea melt is then sent to a drying unit or film dryer (granulation) 502b where it is further dried using a film drying process to result in a final product to be stored. The water removed from vacuum extraction 502a, and from film dryer 502b, is preferably recycled through the system but may be treated and discharged.

II. Modular Arrangement

(31) Referring now to FIGS. 2 and 3 (and referring back to FIG. 1), the urea production process may be a modularized process, with process steps 101 to 102 comprising purification module 100, steps 201 to 202 comprising reforming module 200, steps 301 to 303 comprising shift conversion module 300, steps 401 to 405 comprising the ammonia module 400, and steps 501 to 502 comprising a urea conversion module 500. In FIGS. 2 and 3, the various pieces of process equipment associated with each module have been mapped to the corresponding process steps of FIG. 1.

(32) The purification module 100, reforming module 200, shift conversion module 300, ammonia module 400, and urea conversion module 500 may be arranged for turn-key operation preferably on a concrete pad (if a semi-permanent installation is required) or on standard 48-foot flatbed trailers T, respectively. If a smaller size flatbed trailer is used, it may be necessary to divide the individual component parts of the module 100, 200, 300, 400, or 500 into two or more flatbed trailers with appropriate connections being provided.

(33) Each module 100, 200, 300, 400, and 500 is preferably skid-mounted for ease of offloading to a remote site. A portable power plant P may be provided to power one or more of the modules 100, 200, 300, 400, 500. Although the process flow and interconnections between various components are not shown in FIGS. 2 and 3 (as well as in FIGS. 4 to 7), a person of ordinary skill in the art would recognize the flow pattern and the types of connections required for various process components.

(34) Referring now to FIGS. 6 to 7, an alternate preferred embodiment of urea conversion module 500 is shown which may be arranged so as to fit within the footprint of a standard 48-foot flatbed trailer T (or concrete pad). Similar to FIGS. 2 to 5 above, the various pieces of equipment associated with the urea conversion module 500 have been mapped to the corresponding process steps of FIG. 1.

(35) While a modular system and method for urea production has been described with a certain degree of particularity, many changes may be made in the details of construction and the arrangement of components and steps without departing from the spirit and scope of this disclosure. A system and method according to this disclosure, therefore, is limited only by the scope of the attached claims, including the full range of equivalency to which each element thereof is entitled.