Is it reasonable to use superheated steam to heat the reboiler?

Do large temperature differences and high temperatures have an impact on the process medium?

The principle of a reboiler is to provide heat to the distillation column, thereby further evaporating the light components in the feed at the bottom of the column. A large temperature difference leads to an increased evaporation of the heavier components, which hinders distillation control. Additionally, if the feed contains components prone to coking, excessive temperatures can cause coking to occur

Some media can degrade or coking due to local overheating; it is only in such cases that it is necessary to reduce pressure and temperature to achieve saturation, otherwise this is not required.

I think indirect heating should work for this

If the medium being heated can withstand high temperatures, then there should be no problem (superheated steam at 0.55 MPA generally does not exceed 300 degrees), but in practice, heat exchangers are prone to leaking.

Find out whether saturated steam provides better heat transfer effects or superheated steam

This post was last edited by arpcd on 2016-11-24 06:10. It seems that the original poster is still a beginner (there are reasons given at the end of this post). The problems faced by beginners can be described as follows: The xx branch factory, which is part of the xx head office, has a reboiler named E101 that uses saturated steam to heat materials. It is a vertical shell-and-tube type heat exchanger (the simplest type of heat exchanger), and it is operating normally with good condition. Now, as part of the reform of the steam pipeline network at the head office, superheated steam will be used for transportation in order to minimize condensation during long-distance steam delivery and thus improve steam efficiency. The technical reform department at the head office has required each branch factory to assess its own steam-using equipment. To support the renovation of the entire plant, the Technical Department of XX Branch Factory needs to conduct verification calculations for E101, namely to determine whether it is feasible to use superheated steam under the same pressure conditions as a substitute for the saturated steam currently in use An assessment is made of factors such as energy consumption, process parameters, and equipment safety, so that the technical improvement team can arrange subsequent renovation work. To get the answer, you can create a virtual version of the case mentioned above and carry out actual calculations to draw a conclusion: how did experienced professionals acquire their knowledge? Calculated. . . . The skit begins: The parameters of the E101 heat exchanger and the steam used are as follows. Since it is an operating device, all of the values listed below are actual values (this is very important): the heat exchange area is F=40 m2, the overall heat transfer coefficient is Kmax=500 W/m2·℃, the average temperature difference for heat transfer is T=50℃ (the vaporization temperature of the medium is 112℃), and the maximum heat load that can be achieved during actual operation is Qmax=1000 kW ； The steam used is saturated steam at 0.55 MPaG (gauge pressure); it condenses at its saturation temperature, with P=0.55 MPaG and T=162°C. The specific enthalpy of the steam is H=2760 kJ/kg, the specific heat at constant pressure is Cp=2.5129 kJ/kg·°C, and the specific enthalpy of saturated water is H0=685 kJ/kg. Thus, the latent heat of vaporization is r=2760–685=2075 kJ/kg. Under a given heat load of 1000 kW, the steam consumption is W=1735 kg/h. And as the main character in this play – the technician at XX Plant – your task is to determine whether this damned E101 unit can be used with superheated steam, and to assess energy consumption, process parameters, and equipment safety. All of these are part of your responsibilities. If the work isn’t completed or done well, and the manager is not satisfied, my performance rating at the end of this year will definitely be ruined! ! The parameters for the superheated steam provided by those guys from the Technical Reform Office are as follows: P=0.55 MPaG, T=262°C, superheat of 100°C, specific enthalpy H=2981 kJ/kg, specific heat at constant pressure Cp=2.1047 kJ/kg·°C, and specific enthalpy of saturated water H0=685 kJ/kg. Thus, when this superheated steam condenses into saturated water, the total energy released is equal to the sum of the sensible heat and latent heat, which is r=2981–685=2297 kJ/kg. Now let’s consider the three key issues: energy consumption, process parameters, and equipment safety. 1. Energy consumption: This is easy to calculate. Assuming that all of the superheated steam can be condensed in the heat exchanger, thereby meeting the requirements of maintaining a constant process heat load, the amount of steam required is W=Q/r=1000*3600/2297=1567 kg/h. This is less than the previous value of 1735 kg/h; the reduction is 1735–1567=168 kg/h, representing a reduction of 9.7%, which is almost 10%. The existing steam pipelines and valves can be reused. . When the leader heard this, he said, \"Oh, consumption has decreased, and the pipeline valves can still be reused – that’s great.\" (Sorry, leader, but I usually mention the bad things first.) . 2. Process parameters: First, you need to verify whether the heat exchange area of E101 is sufficient; in other words, it is necessary to ensure that the heat exchange area of E101 when using superheated steam meets the process requirements. Otherwise, the consumption figures calculated in step 1 will be meaningless. Due to the use of superheated steam, E101 necessarily has a sensible heat stage and a latent heat stage; that is, the steam can condense only after the 100°C superheat has been dissipated. This process occurs in two steps: 262°C steam → 162°C steam → 162°C water. The first step involves sensible heat, while the second involves latent heat. If the heat load remains constant at 1000 kW, the heat loads for each stage can be calculated as follows: For the sensible heat stage, the average specific heat at constant pressure between 262°C and 162°C is Cp = (2.5129 + 2.1047)/2 = 2.3088 kJ/kg·°C. Thus, the heat load Q1 = WCpΔT = 1567 × 2.3088 × 100 = 361,842 kJ/h, which is equivalent to 101 kW – roughly 10% of the total heat load ； Latent heat stage: Simple – 1000 – 101 = 899 kW. This value can be obtained by subtracting the sensible heat portion; it accounts for 90% of the total heat load. For the area calculation: It’s simple – the latent heat stage makes up 90% of the total, and since this stage is identical to the original operating conditions, a 10% discount can be applied. Thus, F = 40 x 90% = 36 m2. In other words, out of 40 m2 of total heat exchange area, 36 m2 should be allocated to the latent heat stage, with the remaining 4 m2 going to the sensible heat stage. Will this be sufficient to handle the process of converting 262 units of steam to 162 units of steam? The calculation for the sensible heat stage is not difficult either; determining the value of the overall heat transfer coefficient requires some experience. Generally speaking, the heat transfer coefficient on the gas phase side is quite low. The reason why the original E101 system had an overall heat transfer coefficient as high as 500 is simple: on one side, steam condenses, and on the other side, a saturated solution vaporizes – in both cases, phase changes occur, which results in high heat transfer coefficients. When superheated steam is used, the heat transfer coefficient on that side is typically in the tens range, and it’s hard for it to exceed 100. The overall heat transfer coefficient is usually determined by the side with the lower value; no matter how high the coefficient is on the side where the fluid vaporizes, it doesn’t make a difference. You can use K=100 for calculations; after all, this is just an estimated value for verification purposes. It’s okay if the simulation results are not perfect – some error is acceptable. The heat load is known; you have estimated the overall heat transfer coefficient to be 100. The remaining temperature differences can then be calculated. Here, the average logarithmic temperature difference should be used. The temperature difference at the cold end is 162–112 = 50°C, while the temperature difference at the hot end is 262–112 = 150°C. The logarithmic temperature difference T is calculated as (150–50)/ln(150/50) = 91°C. Note that temperature differences are very important, and this will be discussed further later on. F=Q/(TxK)=101x1000/(91x100)=11 m2; in other words, 11 m2 of heat exchange area is required to meet the demands of the sensible heat stage. Therefore, 11+36=47 m2 of heat exchange area is needed to satisfy the requirements for using superheated steam. Yet, E101 has only 40 m2, which poses a problem for the technical improvement team and management. What is the maximum heat load that this current E101 can handle? 11 m2 is used to cool the superheated steam, leaving 29 m2 for heat load handling. The heat load that can be handled in this area is Q2 = FxKxT = 29 x 500 x 50 = 725,000 W = 725 kW. In other words, after using superheated steam, the existing E101 unit can handle a maximum heat load of Qmax = 101 + 725 = 826 kW (a value of 100 for K represents the maximum). Compared to the required heat load of 1000 kW, the shortfall is 1 – 826/1000 = 17.4%. In other words, the vaporization capacity on the medium side is only 82% of the theoretical value; and this is still under the assumption that K equals 100. Generally, it’s difficult for K to exceed 100. . The leader subconsciously asks you, \"Can we turn up the steam?\" Who said you can only use a flow rate of 1567 kg/h? Is it okay to use 2000 kg/h? Our branch factory can afford two tons of steam per hour; money isn’t an issue. . Leaders are really overbearing; they may scold you, but they still have to account for everything. In such situations, it’s possible to set limits, that is, to determine the extreme operating conditions for the equipment. For E101, the condition of 262 steam → 162 steam → 162 water is normal, while another abnormal condition (and the leader is willing to pay for it, so it’s an abnormal condition) is one in which the superheated steam does not condense at all, and only sensible heat is used for heating – how much steam is required in that case? W=Q/(Cp.t); here, Q is 1000 kW as given, the specific heat capacity is 2.3088 kJ/kg·℃, so what is the temperature difference? I don’t know. If we’re not sure, we can assign a value or make an estimate. The superheat of your superheated steam is 100°C; this represents the maximum temperature difference. Since the steam does not condense, we need to calculate how much superheated steam is required W=1000x3600/(2.3088x100)=15592 kg/h, which is 15.6 tons/h of steam consumption. . . Is this value correct? ? Q=WCpt is your heat balance equation; meanwhile, Q=FKT represents the limit imposed by your heat exchanger on the value of Q. There is a maximum value for this Q related to sensible heat (the maximum temperature difference that E101 can achieve is 262–112=150°C). Thus, Qmax=40x150x100=600kW, which is only 60% of E101’s original heat load of 1000kW. That’s not good – it means that the 15.6 tons/h of steam available cannot be fully utilized. With such a limited heat exchanger capacity, even though the area available is 40m2, it’s like a highway where the maximum traffic capacity is determined by the narrowest part of the road; the rest of the route has four lanes (meaning the steam supply is sufficient), but at the narrowest section there are only two lanes (the 40m2 area), and the maximum speed is fixed at 120 km/h. The throughput capacity is thus limited – no matter how many lanes there are, the flow rate of steam cannot exceed this limit (the steam flow has a maximum value); even with eight lanes, it’s still impossible to exceed this capacity... the steam flow can only reach 60% of the maximum possible level. . . A proper calculation should actually be carried out using the trial-and-error method after writing WCpt = FKT, because there are two variables here: the steam consumption W and the temperature difference t (where t represents the temperature drop of the superheated steam, and T is the logarithmic mean temperature difference of the heat exchanger; once t is determined, T is also determined). Trial-and-error calculation is necessary (how can an equation with two unknowns be solved?) How is your math skills? ), but the cumbersome trial-and-error method is no longer necessary. Regardless of the operating conditions of your steam system, or whether it condenses or not, this E101 unit cannot meet the original requirement of 1000 kW. The maximum heat load it can handle is 82% of the original value when the superheated steam can condense, and 60% when it does not condense. These are the limit values in the process calculations, and they are also the answers that the management expects from you. You need to tell your boss, who doesn’t care about money, that even if we provided 100 tons/h of steam, it would be useless; the area designated for E101 is already there, and we’ve only built two lanes on this highway – which is not enough! Even if your boss gives you a BMW X5, on county roads full of cameras with a speed limit of 40 km/h and speed checks at various points, you can still only drive at 40 km/h, right? (The leader felt a shock in his heart: I just bought an X5 for my mistress who lives in the suburban villa – did this guy see that?) Go back and deal with him! ! The plot intensifies! ) Now you know why one of the given conditions is a maximum heat load of Qmax=1000kW, right? ? In practice, it’s difficult for the data related to the heat exchangers in use to reach Qmax, as there is a design margin. In other words, your task now is to identify this margin – understand? Oh my God, we’re heating thermosensitive materials here; they’re sensitive to temperature, so the temperature difference can’t be that large. . . . . . The plot can be enhanced. . . . . . 3. Equipment safety: The last issue is that the actual temperature difference in E101 is only 50°C. In typical shell-and-tube heat exchangers, expansion joints are required when the temperature difference between the hot and cold sides exceeds 60°C in order to address the problem of equipment expansion. However, the existing E101 model does not have such expansion joints. Even if superheated steam is used – and the material of E101 can withstand temperatures up to 262°C – the lack of expansion joints means that E101 cannot handle a temperature difference of up to 100°C between the hot and cold sides (150°C at the hot side and 50°C at the cold side); the resulting temperature stress is extremely high! The equipment poses a huge risk! ! If you are not familiar with thermal stress, you can consult someone who specializes in equipment; if you are at the factory, you should ask the equipment technician to see what serious consequences a 100°C temperature difference could have on E101. . Oh, our heat exchanger is a high-end, modern type – it doesn’t require things like expansion joints or old-type sleeves. . . . . . The device safety is fine; we can omit item 3, and we can also reduce the amount of storyline. . . . . . Well, it’s been calculated and written about; a lot has been explained in the process – so you should still provide conclusions and recommendations, right? ? The calculation process is sufficient to lead you to the conclusion that using superheated steam cannot meet the original process load; in other words, operation at a reduced load will be necessary, and there are significant risks associated with the equipment itself, as the stresses resulting from the excessive temperature difference compared to the original design will certainly cause damage to the E101. . . The leader looks furious? ? ? ? Don’t worry, a suggestion still needs to be given. . . . . If superheated steam has been used previously, the existing E101 unit must be replaced due to insufficient heat exchange area; it needs to be redesigned and manufactured, which is extremely costly. The management will surely say that it’s a time-consuming and labor-intensive task – is there a simpler way? Yes, a temperature and humidity reducer can be added before entering E101 to saturate the superheated steam. There are many established manufacturers of such devices, so it’s simple to purchase one. As for the water required for the temperature reducer, it can be obtained by taking some from the condensate water from E101. . . . . The leader’s eyes began to shine; this kid made a good point. We’ll do as he suggests. By the end of the year, the factory will be recognized as advanced, and his performance bonus will double. In two years, he can be promoted to manager. His technical skills are good, and his arguments are well-founded. He’s not one of those unreliable young people who just talk without any basis, who have no data, no suggestions, and no solutions. If he proves himself capable in the future, he could become my assistant~~~ That’s the simulated scenario. . . . . . The most crucial thing in this scenario is that, as the protagonist, you need to do the calculations and provide the relevant data in order to have a basic understanding of all possible scenarios. Without doing these calculations, how can you know that the heat exchange area is insufficient, or that the temperature difference is as high as 91°C? The experience that veterans can pass on to you is this: superheated steam has a sensible heat phase in E101, which reduces the area available for the latent heat phase; as a result, the area of E101 might not be sufficient. Moreover, superheated steam increases the temperature difference, posing significant risks to the equipment. Those words like “maybe” and “possibly” are used by veterans to subtly indicate that they themselves don’t know anything without conducting calculations – it’s up to you to carry out those calculations; they won’t do it for you. . Why isn’t the area enough? ? Where exactly lies the high risk? ? You don’t count; in three days you’ll have forgotten what the veteran said. Even if you remember it for three days, you’ll surely forget it after 300 days. Moreover, when managers or people from the technical improvement department ask you questions, you’ll be confused (don’t think that those managers and staff from the technical improvement department are idiots; if you’re not sure about the answers, you’ll naturally get flustered and expose yourself!). If you can’t provide the data, all you can say is, “It was my master who said it; why don’t you ask him?” ? The boss will definitely tell you to get out within three seconds…… As for why the original poster is a novice, it’s quite simple: aside from writing down the saturated steam pressure without specifying whether it’s gauge pressure or absolute pressure, what’s the superheat level? Never mind, what are the parameters of the heat exchanger? What’s the area? What is the operating heat load? Don’t write it – what form is the heat exchanger in? God only knows whether the materials can withstand large temperature differences I don’t know. . . . . Since it’s a question, you first need to learn how to phrase it clearly. Be more thorough – how would a beginner approach this problem? What are the obstacles you face in the process of thinking about it? Have they never studied the principles of chemical engineering, or don’t they know how to do the calculations, or are they too lazy to do them? All they can say is that they need some advice from their teachers. . . . . If the problems aren’t clear and the thoughts are not organized, how can the work be done properly? The food is terrible. . . . Or rather, lazy, extremely lazy. . . . Don’t be lazy – do the calculations; once you do that, you’ll become an expert. You won’t be able to forget it for 300 days. The calculation process is difficult, and understanding the parameters of operating heat exchangers might be an even more challenging task. . . . . . . Of course, such calculations require experience, and the process of acquiring this experience, as well as the process of performing these calculations, is the process by which one goes from being a novice to an expert. No one can become an expert right from birth. . I happen to have here some reading notes that a language teacher prepared a few years ago for a girl in the seventh grade; I’m giving them to the original poster. The original classical text – from \"Kong Congzi•Ju Wei\": It is said that Yu, Tang, Wen, Wu, and the Duke of Zhou were diligent in their thoughts and hardworking in their efforts; some of them even lost arms while striving to achieve their goals, or became bald and hunched over, yet they remained saints. ” The teacher’s comments: “Thoughtful effort” certainly involves “physical labor”; it’s very hard work – otherwise, how can one become a “sage”?