Process and system for cleaning acetylene sulfuric acid

Abstract

A process and a system for cleaning acetylene and sulfuric acid relate to the technical field of acetylene cleaning processes, and the system comprises an acetylene pretreatment device, an acetylene water removal device, an acetylene impurity removal device, an acid mist trapping device, a sulfuric acid circulating device, a sulfuric acid storage device, an acetylene cooler of the acetylene pretreatment device and an acetylene demister, the acetylene demister is communicated with a sulfuric acid washing tower of an acetylene water removal device, the sulfuric acid washing tower is connected with a bubble cap combination tower of an acetylene impurity removal device, the bubble cap combination tower is connected with an acid mist trapping device, and the acid mist trapping device is an acid mist trap; the sulfuric acid circulating device comprises a sulfuric acid washing tower circulating device and a bubble cap combined tower circulating device. According to the invention, the problem of sewage discharge is solved; waste acid can be recycled and mixed with limestone powder to serve as a cement raw material, sulfur and phosphorus in acetylene gas are removed more thoroughly, and meanwhile, the water mass fraction of acetylene gas is reduced, so that the service time of a synthetic converter catalyst is prolonged, the unit consumption of the catalyst is reduced, and the safety coefficient is improved.

Description

Technical Field

[0001] This invention relates to the technical field of acetylene cleaning processes, and in particular to a process and system for acetylene sulfuric acid cleaning. Background Technology

[0002] Acetylene VCR conversion is one of the important methods for producing vinyl chloride, a crucial monomer in the synthesis of polyvinyl chloride (PVC) and other polymeric materials. Through acetylene VCR conversion, two important chemical raw materials, acetylene and hydrogen chloride, can be converted into high-value-added vinyl chloride products. The catalyst plays a vital role in acetylene VCR conversion. However, the catalyst is subject to degradation during use; impurities in the reaction system may adsorb onto the catalyst surface, reducing its activity. Prolonged high-temperature reactions may also alter the catalyst's structure, thereby reducing its catalytic performance.

[0003] Sulfur and phosphorus impurities in acetylene can affect catalyst activity, shorten catalyst life, and impact subsequent polymerization reactions and product quality. The main purification process for crude acetylene involves sodium hypochlorite purification. Sodium hypochlorite purification primarily utilizes its oxidizing properties to oxidize hydrogen sulfide or phosphine, achieving a relatively ideal purification effect. After removing hydrogen sulfide and phosphine gases from acetylene gas with sodium hypochlorite solution, hypochlorite ions are reduced to chloride ions and enter the acetylene generation system with the wastewater. This results in a high chloride ion content in the calcium carbide slag slurry of the acetylene generation system, rendering the calcium carbide slag unusable for cement production. The waste sodium hypochlorite solution, which cannot be recycled, has a high chloride ion content, leading to high treatment costs. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a process and system for acetylene purification using sulfuric acid. It utilizes the strong oxidizing properties of concentrated sulfuric acid to remove impurities such as hydrogen sulfide and phosphine from crude acetylene, achieving purification. The waste acid generated during the purification of crude acetylene using sulfuric acid can be recycled in fertilizer plants for the production of potassium sulfate. Alternatively, the waste acid can be directly neutralized with calcium carbide slag slurry to produce calcium sulfate, which can be used as a raw material for cement production in dry-process cement plants. The waste sulfuric acid can also be recycled by producing concentrated sulfuric acid through pyrolysis. This addresses the aforementioned technical problems.

[0005] This invention provides a system for acetylene-sulfuric acid purification, comprising: an acetylene pretreatment device, an acetylene dehydration device, an acetylene impurity removal device, an acid mist collection device, a sulfuric acid circulation device, and a sulfuric acid storage device. The acetylene pretreatment device includes an acetylene cooler and an acetylene demister. The acetylene demister is connected to the acetylene dehydration device, which is a sulfuric acid scrubbing tower. The sulfuric acid scrubbing tower is connected to the acetylene impurity removal device, which is a bubble cap combined tower. The bubble cap combined tower is connected to the acid mist collection device, which is an acid mist collector. The sulfuric acid circulation device includes a sulfuric acid scrubbing tower circulation device and a bubble cap combined tower circulation device. The sulfuric acid storage device includes a concentrated sulfuric acid storage tank and a waste acid tank. The acetylene impurity removal device is connected to the concentrated sulfuric acid storage tank, and the acetylene dehydration device is connected to the waste acid tank.

[0006] The sulfuric acid circulation device includes a sulfuric acid washing tower circulation device and a bubble cap combined tower circulation device. The washing tower circulation device includes a washing acid cooler, a washing acid circulation pump, a sulfuric acid circulation cooler, and a sulfuric acid circulation cooling pump. The bubble cap combined tower circulation device includes a sulfuric acid intermediate tank and a sulfuric acid pump. The sulfuric acid storage device includes a concentrated sulfuric acid storage tank and a waste acid tank. The acetylene impurity removal device is connected to the concentrated sulfuric acid storage tank, and the acetylene dehydration device is connected to the waste acid tank.

[0007] The concentrated sulfuric acid storage tank is connected to the concentrated sulfuric acid cooler via a concentrated sulfuric acid pump. The concentrated sulfuric acid cooler is connected to the top of the bubble cap tower via a pipeline. The fourth tray of the bubble cap tower is connected to the intermediate sulfuric acid tank via a pipeline. The intermediate sulfuric acid tank is connected to the sulfuric acid cooler via a sulfuric acid pump for cooling. The sulfuric acid cooler is connected to the fifth tray of the bubble cap tower via a pipeline.

[0008] The bottom of the bubble cap tower is connected to the sulfuric acid circulating cooler via a pipe and a sulfuric acid circulating pump. The sulfuric acid circulating cooler is connected to the sulfuric acid washing tower via a pipe. The bottom of the sulfuric acid washing tower is connected to the washing acid cooler via a pipe and a washing acid circulating pump. The washing acid cooler is connected back to the sulfuric acid washing tower via a pipe. The bottom of the sulfuric acid washing tower is connected to the waste acid tank via a pipe.

[0009] The concentrated sulfuric acid in the concentrated sulfuric acid storage tank is pumped to the concentrated sulfuric acid cooler. After being cooled by the concentrated sulfuric acid cooler, the concentrated sulfuric acid is sent to the top of the bubble cap combined tower. The sulfuric acid coming out of the fourth tray of the bubble cap combined tower enters the sulfuric acid intermediate tank. The concentrated acid in the sulfuric acid intermediate tank is pumped to the sulfuric acid cooler for cooling. The concentrated acid cooled by the sulfuric acid cooler is sent to the fifth tray of the bubble cap combined tower for self-circulation.

[0010] The sulfuric acid at the bottom of the bubble cap tower is pumped to the sulfuric acid circulating cooler by a sulfuric acid circulating pump. The sulfuric acid cooled by the sulfuric acid circulating cooler is then sent to the sulfuric acid washing tower. Part of the sulfuric acid diluted at the bottom of the sulfuric acid washing tower is pumped to the washing acid cooler by a washing acid circulating pump. The sulfuric acid cooled by the washing acid cooler is then sent to the sulfuric acid washing tower for self-circulation, and part of it is sent to the waste acid tank.

[0011] The lower part of the bubble cap tower is a packed tower, and the upper part is a bubble cap tower.

[0012] The sulfuric acid in the waste acid tank is recovered by mixing it with limestone powder as a raw material for cement production or by cracking it to produce concentrated sulfuric acid.

[0013] The process for acetylene sulfuric acid purification includes the following steps:

[0014] Step 1: Cool the compressed acetylene gas to 10-12℃ using an acetylene cooler;

[0015] Step 2: The cooled acetylene gas is passed through an acetylene demister to remove the mist-like substances;

[0016] Step 3: Maintain the pressure of the acetylene gas after demisting at 0.045-0.05 MPa, and pass it through a sulfuric acid scrubbing tower to ensure full contact with the sulfuric acid in the packing layer, thereby removing the water vapor entrained in the acetylene gas;

[0017] Step 4: The acetylene gas, after being dehydrated, passes through a bubble cap combined tower and comes into full contact with concentrated sulfuric acid on the trays to remove impurities such as hydrogen sulfide and phosphine from the acetylene gas.

[0018] Step 5: Pass the purified acetylene gas through an acid mist collection device to remove sulfuric acid mist entrained in the gas flow;

[0019] Step 6: Control the moisture content of acetylene to 50-200 ppm before feeding it into the conversion section.

[0020] The concentration of sulfuric acid in step three is 80-90%.

[0021] The concentration of concentrated sulfuric acid in step four is 95-98%.

[0022] The beneficial effects of this invention are as follows: concentrated sulfuric acid first passes through a bubble cap combined tower to deeply dehydrate and remove impurities from acetylene. After dilution, the sulfuric acid is then passed into a sulfuric acid scrubbing tower to further dehydrate and remove impurities from the acetylene. The sulfuric acid route is exactly the opposite of the acetylene gas process route, thereby reducing sulfuric acid consumption and achieving more thorough removal of sulfur and phosphorus from the acetylene gas. At the same time, the water content of the acetylene gas is reduced, thus extending the catalyst usage time in the synthesis converter, reducing catalyst consumption, increasing the safety factor, and preventing the explosion problem caused by the mixing of chlorine and acetylene gas. The waste acid can be reused and mixed with limestone powder as a cement raw material or recovered through pyrolysis, solving the wastewater discharge problem. Attached Figure Description

[0023] Figure 1 This is a process route diagram for the present invention;

[0024] Figure 2 This is a process diagram of the acetylene dehydration section in a specific embodiment of the present invention;

[0025] Figure 3 This is a process diagram of the acetylene impurity removal section in a specific embodiment of the present invention;

[0026] Figure 4 This is a process diagram illustrating the recycling of waste acid in a specific embodiment of the present invention;

[0027] In the diagram: 1. Acetylene cooler; 2. Acetylene demister A; 3. Acetylene demister B; 4. Sulfuric acid scrubbing tower; 5. Scrubbing acid cooler A; 6. Scrubbing acid cooler B; 7. Scrubbing acid circulating pump A; 8. Scrubbing acid circulating pump B; 9. Sulfuric acid circulating pump A; 10. Sulfuric acid circulating pump B; 11. Sulfuric acid circulating cooler A; 12. Sulfuric acid circulating cooler B; 13. Bubble cap combined tower; 14. Sulfuric acid intermediate tank; 15. Sulfuric acid pump A; 16. Sulfuric acid pump B; 17. Sulfuric acid cooler A; 18. Sulfuric acid cooler B; 19. Concentrated sulfuric acid cooler A; 20. Concentrated sulfuric acid cooler B; 21. Acid mist collector A; 22. Acid mist collector B; 23. Concentrated sulfuric acid pump A; 24. Concentrated sulfuric acid pump B; 25. Concentrated sulfuric acid storage tank; 26. Waste acid tank. Detailed Implementation

[0028] Example 1

[0029] Referring to the accompanying drawings and descriptions, this invention provides a system for an acetylene-sulfuric acid purification process. The system includes: an acetylene pretreatment device, an acetylene dehydration device, an acetylene impurity removal device, an acid mist collection device, a sulfuric acid circulation device, and a sulfuric acid storage device. The acetylene pretreatment device includes an acetylene cooler and an acetylene demister. The acetylene cooler is connected to the acetylene demister, and the acetylene demister is connected to the acetylene dehydration device. The acetylene dehydration device is a sulfuric acid scrubbing tower, which is connected to the acetylene impurity removal device. The acetylene impurity removal device is a bubble cap combination tower, which is connected to the acid mist collection device, which is an acid mist collector.

[0030] The sulfuric acid circulation system includes a sulfuric acid washing tower circulation system and a bubble cap combined tower circulation system. The washing tower circulation system includes a washing acid cooler, a washing acid circulation pump, a sulfuric acid circulation cooler, and a sulfuric acid circulation cooling pump. The bubble cap combined tower circulation system includes a sulfuric acid intermediate tank and a sulfuric acid pump. The sulfuric acid storage system includes a concentrated sulfuric acid storage tank and a waste acid tank. The acetylene impurity removal system is connected to the concentrated sulfuric acid storage tank, and the acetylene dehydration system is connected to the waste acid tank.

[0031] The concentrated sulfuric acid storage tank is connected to the concentrated sulfuric acid cooler via a concentrated sulfuric acid pump. The concentrated sulfuric acid cooler is connected to the top of the bubble cap tower via a pipeline. The fourth tray of the bubble cap tower is connected to the intermediate sulfuric acid tank via a pipeline. The intermediate sulfuric acid tank is connected to the sulfuric acid cooler via a sulfuric acid pump for cooling. The sulfuric acid cooler is connected to the fifth tray of the bubble cap tower via a pipeline.

[0032] The bottom of the bubble cap tower is connected to the sulfuric acid circulating cooler via a pipe and a sulfuric acid circulating pump. The sulfuric acid circulating cooler is connected to the sulfuric acid washing tower via a pipe. The bottom of the sulfuric acid washing tower is connected to the washing acid cooling tower via a pipe and a washing acid circulating pump. The washing acid cooling tower is connected back to the sulfuric acid washing tower via a pipe. The bottom of the sulfuric acid washing tower is connected to the waste acid tank via a pipe.

[0033] Example 2

[0034] Referring to the accompanying drawings and descriptions, the present invention provides a process flow for acetylene sulfuric acid purification, comprising the following steps:

[0035] Step 1: Cool the compressed acetylene gas to 10-12℃ using an acetylene cooler;

[0036] Step 2: The cooled acetylene gas is passed through an acetylene demister to remove the mist-like substances;

[0037] Step 3: Maintain the pressure of the acetylene gas after demisting at 0.045-0.05 MPa, and pass it through a sulfuric acid packed tower to ensure full contact with the sulfuric acid in the packing layer, thereby removing the water vapor entrained in the acetylene gas;

[0038] Step 4: The acetylene gas, after being dehydrated, passes through a bubble cap combined tower and comes into full contact with concentrated sulfuric acid on the trays to remove impurities such as hydrogen sulfide and phosphine from the acetylene gas.

[0039] Step 5: Pass the purified acetylene gas through an acid mist collection device to remove sulfuric acid mist entrained in the gas flow;

[0040] Step 6: Control the moisture content of acetylene to 50-200 ppm before feeding it into the conversion section;

[0041] Preferably, the concentration of sulfuric acid in step three is 70-85%;

[0042] Preferably, the concentration of concentrated sulfuric acid in step four is 95-98%.

[0043] Example 3

[0044] Referring to the accompanying drawings and descriptions, this invention provides a system for an acetylene-sulfuric acid purification process, the system being designed to support a PVC plant with an annual production capacity of 300,000 tons.

[0045] Specific process flow: Acetylene is compressed by the existing compressor and then enters the existing alkaline washing tower for preliminary washing with alkaline solution. It is then transported through an external pipe to the sulfuric acid purification section. The acetylene gas, after alkaline washing at a pressure of 0.05 MPa and a temperature of 45°C, is indirectly cooled to 15°C by acetylene coolers using 7°C chilled water. It then enters parallel acetylene demisters A and B to remove droplets, reducing the water content in the acetylene and decreasing concentrated sulfuric acid consumption. The 15°C acetylene enters the sulfuric acid washing tower and comes into countercurrent contact with 80-90% sulfuric acid. Some water is absorbed by the circulating sulfuric acid, and some impurities such as hydrogen sulfide and phosphine are oxidized and removed by the sulfuric acid. A portion of the sulfuric acid at the bottom of the washing tower is pumped through series-connected washing acid circulation pumps A and B to parallel washing acid coolers A and B for cooling and removal of the sulfuric acid dilution heat and reaction heat, which reduces the water vapor partial pressure of the sulfuric acid and improves the drying and purification efficiency. A portion of the sulfuric acid absorbs water, and after the reaction, its concentration decreases; this portion is then discharged to the waste acid tank through a level regulating valve.

[0046] Acetylene gas exiting the sulfuric acid scrubbing tower enters the bottom of the bubble cap combined tower, where it comes into countercurrent contact with 95-98% concentrated sulfuric acid. This further reaction removes sulfur and phosphorus, and absorbs moisture, reducing the acetylene's moisture content to below 100 ppm. The purified and dried acetylene is then demisted by parallel acid mist collectors A and B before being sent to the existing VCM conversion unit.

[0047] The lower part of the bubble cap combined tower is a packed tower, and the upper part is a bubble cap tower. The concentrated sulfuric acid in the concentrated sulfuric acid storage tank is pumped to the parallel concentrated sulfuric acid cooler A and concentrated sulfuric acid cooler B by concentrated sulfuric acid pump A and concentrated sulfuric acid pump B connected in parallel. After being cooled by concentrated sulfuric acid cooler A and concentrated sulfuric acid cooler B, the concentrated sulfuric acid is sent to the top of the bubble cap combined tower. The sulfuric acid coming out of the fourth tray of the bubble cap combined tower enters the sulfuric acid intermediate tank. The concentrated acid in the sulfuric acid intermediate tank is pumped to the parallel sulfuric acid cooler A and sulfuric acid cooler B by sulfuric acid pump A and sulfuric acid pump B connected in parallel to cool it. The temperature of the circulating sulfuric acid is controlled at 12-15℃. The cooled concentrated acid is sent to the fifth tray of the bubble cap combined tower for self-circulation.

[0048] The sulfuric acid at the bottom of the bubble cap combined tower is pumped to the sulfuric acid circulating cooler A and sulfuric acid circulating cooler B in parallel through sulfuric acid circulating pump A and sulfuric acid circulating pump B in parallel to cool it, and the circulating sulfuric acid temperature is controlled at 12-15℃. The cooled sulfuric acid is then sent to the sulfuric acid washing tower.

[0049] Waste acid is neutralized with limestone powder. The neutralized slurry is pumped to a filter press for filtration until the water content is 40%. The filter cake is sold as a by-product. The neutralization waste gas is absorbed by alkaline solution before being discharged.

[0050] Based on an acetylene flux of 13440 Nm 3 The initial calculation was based on a PVC production rate of 900 t / d. However, during the design phase, the production rate was scaled up to 1000 t / d.

[0051] Based on the fact that 20-25 kg of waste sulfuric acid is generated per ton of PVC, and the concentration of dilute acid in the concentrated sulfuric acid purification tower is controlled at 80-85%, the calculation is based on a dilute acid concentration of 85%.

[0052] Calculate the actual demand:

[0053] The average annual atmospheric pressure at Qinghai Yihua is 76.5 kPa, and the acetylene flux is 13440 Nm³. 3 The output is calculated at 900 t / d, with an acetylene gas gauge pressure of 45 kPa and a temperature of 5℃.

[0054] According to the calculation, the acetylene gas was cooled to 10°C, the pipeline was drained cleanly, and no liquid water was carried to the sulfuric acid for cleaning; the partial pressure of vapor at 10°C is 1.23 kPa.

[0055] The corresponding total water content in acetylene gas:

[0056] 1.23÷(45+76.5)×13440÷22.4×18kg=109.3kg / h;

[0057] Calculate how much water (x) is needed to dilute 98% concentrated sulfuric acid to 85% sulfuric acid per ton: According to the law of conservation of sulfuric acid mass:

[0058] 0.98 ÷ (1 + x) = 0.85

[0059] Calculate x = 0.153t

[0060] Calculate the amount of sulfuric acid required per hour to cool acetylene gas by 10°C without the presence of liquid water: 109.3 ÷ 1000 ÷ 0.153 = 0.715 t / h

[0061] That is, the daily requirement of concentrated sulfuric acid is: 0.715 t / h × 24 = 17.15 t.

[0062] Sulfuric acid consumption per ton of PVC: 17.15 ÷ 900 × 1000 = 19 kg

[0063] Reduced catalyst consumption and increased efficiency: PVC catalyst consumption is reduced from 2.0 kg / t to 1.0 kg / t. Based on an annual PVC production of 310,000 tons and a catalyst cost of 51,000 yuan / ton (excluding tax), the annual efficiency increase is: (2.0-1.0) / 1000×310000×5.1=15.81 million yuan / year.

[0064] Reduce alkali consumption for PVC conversion and purification: The 32% alkali consumption for PVC conversion and purification is reduced from 28 kg / t to 20 kg / t. Based on an annual PVC production of 310,000 tons and a 32% alkali price of 1,150 yuan / ton excluding tax, the annual profit can be: (28-20) / 1000*310000*1150 / 10000=2.852 million yuan / year.

[0065] Increased limestone powder consumption: The project uses limestone powder to react with waste sulfuric acid; the sulfuric acid usage is 20 tons / day, and the limestone usage is 4 times that, requiring 80 tons of limestone powder per day. The cost of limestone powder is calculated at 50 yuan / ton. Limestone consumption: 50 yuan / ton * 80 tons / day * 330 days = 1.32 million yuan / year.

[0066] The project generates a total annual benefit of 17.342 million yuan (i.e., 15.81 million + 2.852 million - 1.32 million), and after deducting financial expenses, the annual economic benefit is 11.73 million yuan.

Claims

1. A system for an acetylene-sulfuric acid purification process, characterized in that: The system includes an acetylene pretreatment unit, an acetylene dehydration unit, an acetylene impurity removal unit, an acid mist collection unit, a sulfuric acid circulation unit, and a sulfuric acid storage unit. The acetylene pretreatment unit includes an acetylene cooler and an acetylene demister. The acetylene demister is connected to the acetylene dehydration unit, which is a sulfuric acid scrubbing tower. The sulfuric acid scrubbing tower is connected to the acetylene impurity removal unit, which is a bubble cap combined tower. The bubble cap combined tower is connected to the acid mist collection unit, which is an acid mist collector. The sulfuric acid circulation unit includes a sulfuric acid scrubbing tower circulation unit and a bubble cap combined tower circulation unit. The sulfuric acid storage unit includes a concentrated sulfuric acid storage tank and a waste acid tank. The acetylene impurity removal unit is connected to the concentrated sulfuric acid storage tank, and the acetylene dehydration unit is connected to the waste acid tank.

2. The system for an acetylene-sulfuric acid purification process according to claim 1, characterized in that: The acid washing tower circulation device includes a washing acid cooler, a washing acid circulation pump, a sulfuric acid circulation cooler, and a sulfuric acid circulation cooling pump; the bubble cap combined tower circulation device includes a sulfuric acid intermediate tank and a sulfuric acid pump.

3. The system for an acetylene-sulfuric acid purification process according to claim 2, characterized in that: The concentrated sulfuric acid storage tank is connected to the concentrated sulfuric acid cooler via a concentrated sulfuric acid pump. The concentrated sulfuric acid cooler is connected to the top of the bubble cap tower via a pipeline. The fourth tray of the bubble cap tower is connected to the intermediate sulfuric acid tank via a pipeline. The intermediate sulfuric acid tank is connected to the sulfuric acid cooler via a sulfuric acid pump for cooling. The sulfuric acid cooler is connected to the fifth tray of the bubble cap tower via a pipeline. The bottom of the bubble cap tower is connected to the sulfuric acid circulating cooler via a pipe and a sulfuric acid circulating pump. The sulfuric acid circulating cooler is connected to the sulfuric acid washing tower via a pipe. The bottom of the sulfuric acid washing tower is connected to the washing acid cooling cooler via a pipe and a washing acid circulating pump. The washing acid cooling cooler is connected back to the sulfuric acid washing tower via a pipe. The bottom of the sulfuric acid washing tower is connected to the waste acid tank via a pipe.

4. The system for an acetylene-sulfuric acid purification process according to claim 3, characterized in that: The sulfuric acid in the waste acid tank is recovered by mixing it with limestone powder as a raw material for cement production or by cracking it to produce concentrated sulfuric acid.

5. A process for acetylene sulfuric acid purification, characterized in that: The process employs the acetylene sulfuric acid purification process as described in claim 4, and the process flow includes the following steps: Step 1: Cool the compressed acetylene gas to 10-12℃ using an acetylene cooler; Step 2: The cooled acetylene gas is passed through an acetylene demister to remove the mist-like substances; Step 3: Maintain the pressure of the acetylene gas after demisting at 0.045-0.05 MPa, and pass it through a sulfuric acid packed tower to ensure full contact with the sulfuric acid in the packing layer, thereby removing the water vapor entrained in the acetylene gas; Step 4: The acetylene gas, after being dehydrated, passes through a bubble cap combined tower and comes into full contact with concentrated sulfuric acid on the trays to remove impurities such as hydrogen sulfide and phosphine from the acetylene gas. Step 5: Pass the purified acetylene gas through an acid mist collection device to remove sulfuric acid mist entrained in the gas flow; Step 6: Control the moisture content of acetylene to 50-200 ppm before feeding it into the conversion section.

6. The process for acetylene sulfuric acid purification according to claim 5, characterized in that: The concentration of sulfuric acid in step three is 80-90%.

7. The process for acetylene sulfuric acid purification according to claim 6, characterized in that: The concentration of concentrated sulfuric acid in step four is 95-98%.

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