SEAL GAS OPTIMIZATION SYSTEMS AND METHODS FOR A DIRECT REDUCTION PROCESS

Abstract

A method and system for operating a seal gas compressor utilized in a direct reduction process including: monitoring a pH level of a water stream used in the seal gas compressor, wherein the pH level of the water stream is affected by a reformer flue gas stream that comes into contact with the water stream, wherein the monitoring step is carried out one or more of upstream of the seal gas compressor and downstream of the compressor; and adjusting the pH level of the water stream to maintain the pH level of the water stream within a predetermined range based on feedback from the monitoring step. The method includes maintaining the pH level of the water stream upstream of the seal gas compressor in a range between 7.5 and 10 and maintaining the pH level of the water stream downstream of the seal gas compressor in a range between 7.8 and 9.5.

Claims

1. A method for operating a seal gas compressor utilized in a direct reduction process, the method comprising: monitoring a pH level of a water stream used in the seal gas compressor, wherein the pH level of the water stream is affected by a reformer flue gas stream that comes into contact with the water stream, wherein the monitoring step is carried out one or more of upstream of the seal gas compressor and downstream of the compressor; and adjusting the pH level of the water stream to maintain the pH level of the water stream within a predetermined range based on feedback from the monitoring step.

2. The method of claim 1, further comprising maintaining the pH level of the water stream upstream of the seal gas compressor in a range between 7.5 and 10.

3. The method of claim 1, further comprising maintaining the pH level of the water stream downstream of the seal gas compressor in a range between 7.8 and 9.5.

4. The method of claim 1, wherein adjusting the pH level of the water stream comprises injecting a pH adjusting chemical into the water stream based on the feedback from the monitoring step.

5. The method of claim 1, further comprising: monitoring the corrosiveness of the water stream used in the seal gas compressor, wherein the corrosiveness of the water stream is affected by the reformer flue gas stream that comes into contact with the water stream, and adjusting the corrosiveness of the water stream to maintain the corrosiveness of the water stream within a predetermined range based on feedback from the monitoring step.

6. The method of claim 5, further comprising maintaining the corrosiveness of the water stream in a range under 2.0 mpy.

7.-20. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The present disclosure is illustrated and described herein with reference to the various drawings, in which like reference numbers are used to denote like system components/methods steps, as appropriate, and in which:

[0030] FIG. 1 is a schematic diagram illustrating one exemplary embodiment of the seal gas optimization system of the present disclosure;

[0031] FIG. 2 is a schematic diagram illustrating another exemplary embodiment of the seal gas optimization system of the present disclosure; and

[0032] FIG. 3 is a schematic diagram illustrating a further exemplary embodiment of the seal gas optimization system of the present disclosure.

DESCRIPTION OF EMBODIMENTS

[0033] Central to a DRI process is the seal gas system. The seal gas system protects both personnel and equipment from toxic gases and potential fires and explosions. The seal gas is used to exclude the flammable reducing gases (H2 and CO) from the atmosphere, thereby enabling the reaction of iron ore to be converted to metallic iron and protecting the environment outside the associated furnace. Additionally, the seal gas prevents the atmosphere from entering the furnace, which would inhibit the metallic reduction process.

[0034] The source of the seal gas is primarily flue gas from the reformer—which includes the byproducts of combustion. The make-up of the flue gas is typically:

CO.sub.2−15-18%  (6)

H.sub.2O−20-22%  (7)

N.sub.2−60-65%  (8)

O.sub.2−<2%  (9)

[0035] Other inert gases could be substituted—for example, a greater percentage of nitrogen, but flue gas is normally vented to atmosphere and is therefore an available and inexpensive source for the seal process.

[0036] Referring now specifically to FIG. 1, in one illustrative embodiment of the seal gas system 10 of the present disclosure, spent reformer flue gas and, optionally, seal gas conditioner are cooled from ˜1000 C to 40 C in a seal gas cooler 12 and then enter the seal gas compressor 14, followed by a pulsation dampener 16. Following the pulsation dampener 16, the seal gas goes through a seal gas aftercooler 18 before being segmented into three streams—wet seal gas, dry seal gas, and bottom seal gas. The dry seal gas is distinguished from the wet seal gas in that the dry seal gas is passed through a seal gas dryer 20 (optionally with a bypass), while the bottom seal gas is further passed through a bottom seal gas compressor 22 and a bottom seal gas aftercooler 24. The water delivered to the seal gas compressor 14 includes cold, clean process water and make-up water (which may be spray injected subsequent to the seal gas cooler 12).

[0037] The general problem for seal gas compressor management is that the incoming feed water stream is impacted by the composition of the reformer flue gas. Generally the feed water used in the seal gas compressor is softened, clean industrial or RO water. The typical make-up analysis for this water is provided in Table 2 below.

TABLE-US-00002 TABLE 2 Full Water Chemistries Parameter Unit Raw Water Soft Water RO Water pH — 7.0-9.0 8.0-9.0 5.0-7.0 Conductivity μS/m 462 — — Total suspended mg/l 1 <1 <1 solids (TSS) Total dissolved mg/l 320 — <25 solids (TDS) Oil and grease mg/l 1 ND ND Total hardness mg/l CaCO.sub.3 160 <20 <10 Calcium hardness mg/l CaCO.sub.3 <35 <10 <5 Magnesium hardness mg/l CaCO.sub.3 <50 <10 <5 M alkalinity (TAC) mg/l CaCO.sub.3 35 Set by <5 vendor based upon treatment Chlorides mg/l Cl.sup.− 60 <60 <5 Sulphates mg/l SO.sub.4 130 <100 <5 Nitrates mg/l NO.sub.3 5 <5 <5 Sodium and mg/l Na 43 113/<3 <5/<2 potassium Total iron mg/l Fe 0.1 <0.1 <0.1 Manganese mg/l Fe 0.05 — — Silica mg/l SiO.sub.2 10 ND ND Turbidity, NTU <2 <2 <1 TOC ppm 2 — <5

[0038] Regardless of the make-up water source, the CO.sub.2 in the flue gas mixes with the make-up water stream and forms carbonic acid as follows:

CO.sub.2(aq)+H.sub.2O←.fwdarw.H.sub.2 CO.sub.3(aq)  (10)

[0039] This is a weak acid, so some of it dissociates to produce H+ ions (see below), hence the incoming water stream to the seal gas compressor 14 is acidic. The pH could be as low as 4.5.

H.sub.2CO.sub.3(aq).fwdarw.H.sup.+(aq)+HCO.sub.3.sup.−(aq)  (11)

HCO.sub.3.sup.−(aq)H.sup.30(aq)+CO.sub.3.sup.−2(aq)  (12)

[0040] At this depressed water pH, the corrosion rate for the carbon steel and cast iron components is great. Without chemical adjustment to the water, the life of the equipment is greatly reduced.

[0041] A secondary concern if clean industrial water is used (and to a lesser extent if softened water is used) is the potential scaling that occurs due to the following reaction:

CaO+H.sub.2O.fwdarw.Ca(OH).sub.2  (13)

Ca(OH).sub.2+CO.sub.2.fwdarw.CaCO.sub.3↓+H.sub.2O  (14)

[0042] In this reaction, as the heat of compression occurs, the precipitation of calcium carbonate increases because the solubility of calcium carbonate is inverse to temperature. Based upon the water chemistry (i.e., the calcium ions) and the pH adjustment chemical, scaling potential varies, but can be significant.

[0043] Both of these issues are ongoing concerns within DRI plants that use lobe compressors because the existing treatments and monitoring systems have not fully eliminated the aforementioned problems. Within the design of conventional DRI plants, no comprehensive solution to these problems has been developed. Typically, licensees and water treatment suppliers have offered some degree of problem avoidance, discussed below, but there is not a comprehensive resolution of the above reactions with the current state of equipment and process control. Due to these limitations, corrosion and scaling can result in the following: [0044] 1) Reduced production, [0045] 2) Excessive maintenance and repair requirements of the compressor or lobe components, the wet seal gas system, and the seal gas dryer 20, [0046] 3) Downstream deposition that hinders production rates, [0047] 4) Loss of product quality: re-oxidation of the metalized product, and/or [0048] 5) Indirectly, the occurrence of severe clustering in the furnace, increasing downtime for removing it.

[0049] Buffering feed solutions have the desired outcome of adjusting the pH out of the corrosion range between 7.5 and 9.0, but result in excessive feed scaling and under-feed corrosion, which is controlled by a dosing pump and optional timer. Excessive feeding can cause too high a pH (>10.0) and result in scaling of the lobes and caustic buildup downstream of the seal gas compressor 14, with under-feeding (pH<7.0) simply allowing corrosion problems to continue. No real measurement method is available in the applicable environment and the practical outcome is inconsistent performance and consistent field checks to ensure that the system is working properly. It is not currently possible to monitor discharge water pH, resulting in inconsistent performance, no real time data to make decisions, and the requirement for frequent manual system checks.

[0050] Corrosion inhibitor solutions have the desired outcome of reducing seal gas compressor corrosion by providing electrochemical protection of the compressor parts and lobes with water treatment chemicals, but provide inconsistent chemical feed or effectiveness because the pH is often incorrect, which is again controlled by a dosing pump and optional timer. If the pH control is not working properly, the chemical control is less effective because the chemicals used have effective and ineffective pH ranges. Performance is based upon levels of active chemistry in the water, and the chemicals do not protect downstream, where gaseous vapors carry moisture and film formers and vapor phase inhibitors are needed. Coupons provide lagging indicators and a corrater provides a real time indicator and the practical outcome is a lack or real time decision making and potential corrater failure if scaling in the system occurs.

[0051] Alternative metallurgical lobe material (stainless steel and nickel) solutions have the desired outcome of providing improved lobe performance and longer equipment life, but are expensive. Lobe life is improved, but corrosive water still affects the pipework utilized.

[0052] Referring now specifically to FIG. 2, in another illustrative embodiment of the seal gas system 10 of the present disclosure, spent reformer flue gas from a reformer 26 and afterburner 28 and, optionally, seal gas conditioner are cooled from ˜1000 C to 40 C in the seal gas cooler 12 and then enter the seal gas compressor 14, followed by the pulsation dampeners 16. Following the pulsation dampeners 16, the seal gas goes through the seal gas aftercooler 18 before being segmented into the three streams—wet seal gas, dry seal gas, and bottom seal gas. The dry seal gas is distinguished from the wet seal gas in that the dry seal gas is passed through the seal gas dryer 20 (optionally with a bypass), while the bottom seal gas is further passed through the bottom seal gas compressor 22 and the bottom seal gas aftercooler 24. Here, the water delivered to the seal gas compressor 14 includes soft water delivered from one or more water softeners 30, and monitored by a calcium meter 32. A corrosion inhibitor injector 34 is fluidly coupled to the soft water stream and controlled by the DCS 36, which is in communication with the corrosion inhibitor injector 34.

[0053] Functionally, the critical variable which monitors the corrosive nature of the CO.sub.2 gas laden water—pH—is reliable, and effective in the environment which heretofore did not exist. A pH probe 38 monitors incoming water pH used in the SGC 14. With DCS control, the proper level of corrosion/scale inhibitor injection into the water stream can be accomplished. The incoming pH range is shown in the Table 3 below, which also references the range which the process is maintained.

TABLE-US-00003 TABLE 3 Incoming pH Water Stream Incoming pH Incoming pH Lower Upper without treatment with treatment acceptable limit acceptable limit 4.5-6.0 7.8-9.0 7.5 10.0

[0054] In order to ensure incoming flue gas CO.sub.2 levels are adjusted throughout the process, a secondary pH monitor 40 on the SGC drain is utilized. The discharge pH is in the ranges shown in Table 4.

TABLE-US-00004 TABLE 4 Discharge pH Water Stream Discharge pH Discharge pH Lower Upper without treatment with treatment acceptable limit acceptable limit 5.5-6.0 8.2-9.0 7.8 9.5

[0055] This second probe 40 is incorporated into a compressor discharge drain sufficient in capacity to enable the probe 40 to have a continuous stream of discharge water moving throughout the process. The pH probes 38, 40 are integrated into the DCS system 36 so that upper and lower acceptable ranges pH can be maintained. The control of the pH is enabled by a DCS signal to the chemical feed pump 42 based upon continuous monitoring.

[0056] The corrosion control methodology of the present disclosure differs from current practices due to the use of multiple functionality corrosion inhibitors. As illustrated in FIG. 3, the corrater probe 45 is disposed adjacent to the pH probe 39 of one of the pH monitors 40. The corresponding treatment 34 includes film forming, vapor phase, and surface-active chemistries fed into the entry water stream of the SGC 14 along with the pH adjustment 42. The chemical dosing is predicated on the make-up water source, and the system 10 uses a real time corrater 44 at the discharge of the SGC 14 to determine the corrosion rate in mpy. Additionally, a corrosion rack 46 with standard 1018 steel coupons or the like is evaluated every 45 days (using the ASTM D4778—15 test method), for example, as a redundant test. The corrosion rate limits applied are provided in Table 5.

TABLE-US-00005 TABLE 5 Corrosion Rates Corrosion Corrosion rate without Rate with Lower Upper chemical chemical acceptable acceptable treatment treatment limit limit >20 mpy 0.1-1.0 mpy NA 2.0 mpy

[0057] The corrater probe 45 is located adjacent to the pH probe 39 and likewise wired to the DCS 36 (FIG. 2). The corrater 44 (FIG. 2) has an upper corrosion rate established for optimal control. It is not required to have a lower limit, as the process objective is to minimize corrosion. At any point when the corrosion rate exceeds 2.0 mpy, the chemical feed pump 34 (FIG. 2) with a signal from the DCS 36 increases feed until the rate drops down below 1.0 mpy, for example.

[0058] Thus, the use of a reliable pH probe to control such a process has not been done before. The present disclosure incorporates the following unique features into a probe that can now be used in this environment with high reliability and accuracy. These probe features are listed below: [0059] Glass formulation is environmentally appropriate, [0060] Chemically designed to withstand the environment, resulting in chemical and wear resistance. [0061] Resistant to aggressive gases, [0062] Resistant to abrasion and high velocity solids, [0063] Electronic transmission function, [0064] Solid state construction is completely sealed and customized to the environment, [0065] No internal O-rings are used as these degrade and are prone to failure, [0066] No gels or electrolytes as these can easily contaminate and increase maintenance, [0067] Extremely large surface area reduces fouling and improves reliability, [0068] Glass characteristics result in very low drift and reliable readings over prolonged periods—2-4 weeks, with no daily adjustment required, [0069] Glass construction is highly durable, and [0070] Double and triple junction construction provides long life.

[0071] Thus, reliable pH monitoring enables reformer flue gas cooling water to be correctly buffered to prevent corrosion and scaling conditions from occurring. Real time monitoring via the DCS enables pH buffering to be adjusted as needed. The redundant monitoring feature on the discharge of the compressor will ensures pH control and that buffering is not under or over fed. Chemical corrosion control is effectively maintained as the correct pH of the water enables the corrosion control products to be utilized at the proper pH—efficacy of treatment is enhanced. Corrosion rates (and thereby the efficacy) are monitored and controlled through the DCS and ensure that desired results are achieved within the desired ranges. Corrosion control uses vapor phase, film forming, and surface acting inhibitors so the compressor and downstream processes (prior to the dryers) are enhanced and a protective layer is formed and maintained. Vapor phase control ensures that water droplets in the seal gas do not cause further corrosion prior to the dryer. All the aerosol water droplets that leave the gas phase (i.e., condense) deposit with a neutral pH, thereby reducing the corrosive nature of the water. As a result of all this, the seal gas compressor and sundry equipment operate more reliably and result in optimized production because maintenance and repairs caused by corrosion or scale build up are eliminated.

[0072] Although the present disclosure is illustrated and described herein with reference to preferred embodiments and specific examples thereof, it will be readily apparent to those of ordinary skill in the art that other embodiments and examples may perform similar functions and/or achieve like results. All such equivalent embodiments and examples are within the spirit and scope of the present disclosure, are contemplated thereby, and are intended to be covered by the following non-limiting claims for all purposes.