USING DIRECT STEAM INTRODUCTION AS A MEANS OF HEAT-UP OF A MOIST WEB

[SASC]

Hello All,

Process Description:
I have a problem with a system where a moist, but porous, continuously fed "web" of cellulose, at a consistent/steady basis weight, is transported on a porous industrial belt through a process that is designed to engage the drying of the cellulose web(web goes from higher moisture level at the start of the process to lower moisture level at the end of the process as solid content increases continuously). As the web is taken off of the process it is transported onto the next distinct "section" of the final process. Prior to the web leaving the process it is sent over a "suction" zone where vacuum under the porous belt is introduced to facilitate the drying of the web laying on top of the belt through the differential pressure gradient. Directly above the "suction" zone is a steaming box that impinges saturated steam directly onto the surface of the web. The idea of this set-up is to get the steam to migrate into the moist web, with the vacuum motivating this migration, where the steam will completely, or nearly completely, condense with the moisture in the moist web and become "condensate" and then be in whole or partially extracted by the vacuum in the "suction" zone. The net gain to be had is that: 1) the condensate is more easily removed because of its elevated temperature/ruduced viscosity than is the cooler moisture prior to the steam introduction, and 2) the the web gains a thermal enhancement as it is heated by the steam and the because of this it helps to facilitate greater solids gain/moisture removal in the rest of the remaining/overall process.
Problem Description:
I am having a hard time resolving what the resulting reactions will be with steam introduction onto the web, like: 1) How do you figure condensing rates with directr steam introduction into something like a moist porous web? 2) What will be the temperature of the porous web once it leaves the steam introduction if the steam is at a steady pressure and flow rate and the moisture level in and out of the suction/steaming zone is known?, and 3) How would you go about predicting how much steam is needed to be applied onto the web in order to drive the web temperature to a desired level?

Here I give what is a typical problem that I hope somebody can help me with here. Eventually, I want to get enough handle on this thing to put together a little spreadsheet that will enable me to simulate this steam application onto web process so web temperature can be predicted and this will help in developing a means to optimize the system:
1) STREAM 1: Web coming into the suction zone @ 112762 #/hr. H2O and 6198 #/Hr. cellulose, is @ 140 degrees F
2) STREAM 3: Web leaving suction zone is @ 27358 #/Hr. H2O and 6198 #/Hr. cellulose, is @ 185 degree F
3) STREAM 4: Water flowing out of "suction" zone is at 85404 #/Hr. and @ 185 degree F
4) STREAM 2: Steaming steam is supplied @ 15 psig.

Question #1: How do you calculate the saturated steam usage, in terms of #'Hr. at the given pressure, that needs to be applied in order to attain the above system characteristics and condense all the moisture in the web, given that the process is steady-state and that no to very little steam is escaping to atmosphere at the steam impingment point unpon the moist porous web?
Question #2: How do you figure the condensed volume of the steam, in terms of #/Hr., in the contribution to the removed condensate and residual moisture leaving the suction/steaming zone?
Question #3: What effect would the use of superheated steam instead of saturated steam have?
Question #4: Besides evacuating the moisture and condensate, what effect would various vacuum levels have in the "suction" zone upon steam consumption?

Your help will be greatly appreciated in solving this worrisome problem!
SASC

 

[quark]

SASC!

I spent one hour yeaterday at home understanding your problem but couldn't convince myself about the condensation part. Infact the process you apply(heating and reduction of pressure by vacuum application) helps in evaporation. What my idea is that, steam is introduced to avoid case hardening of the web. Otherwise this could have been done with dried air more efficiently. If at all there is condensation, it will be of impinged steam. (but when we apply vacuum, evaporation takes place again)

Secondly, your mass balance shows no indication of impinged steam.

If you know the moisture holding capacity of cellulose at various temperatures, the process can be estimated.

Check the latent heat of steam at the applied vacuum from the steam tables and multiply this with quantity(rate) of moisture to be removed.

Multiply specific heat of cellulose with its mass rate and temperature difference. (generally it will be 0.4 btu/lb deg.f)

Adding these two values will give you total amount of heat input. Devide this value with latent heat of steam to get total steam flow rate.

I strongly feel superheated steam would assist better drying.

Reducing suction will also assist better drying.

It is better if you would let us know about the details of the product.

Best Regards,

 

[SASC]

Quark,

Thanks for your input! Its hard to describe the problem, but I'm trying. What other details would you like/need to know? Remember, this is a wet-laid process. There is no relationship between the moisture holding capacity of the cellulose at various temperatures at temperatures below the evaporation point. In fact we are only trying to evacuate free water and heat-up the web body. I see how you are approaching the problem, however, there is the one point I want to make you acutely aware of: The vacuum does indeed assist in the process because of two distinct qualities: 1) it motivates the impinged steam to migrate through the body of the cellulose mass and 2) it evacuates both moisture and condensate to reduce overall mass of the cellulose body and this gives way to A) less moisture to "heat-up" so more percentile of the remaining moisture becomes condensed/condensate, and B) the body of cellulose becomes more "porous" as the moisture/condensate is evacuated because less moisture therein is present to impede the air flow/suction provided by the vacuum/suction zone.

Thanks Quark.....hope this helps...let me know what else you may need.

SASC

 

[quark]

It seems more difficult as am thinking about it. What is your product actually? What is the boiler capacity you have now? It seems some back calculations are better now
Don't know where all the experts have gone?

I sincerely say, I don't want to leave this matter here.

Awaiting your reply

Regards,

 

[SASC]

Quark,

The problem is a real "bugger" that I can't shake off. Thanks for the life-line, I need some help....The product is actually pulp. The steam is simply saturated steam that would otherwise be vented, but we are using the BTUs here. What else do you think you need? I've comptemplated Humid Volume of the steam...along those lines. There really is no "evaporation" but rather physically forced extraction(from vacuum forces under the web) and web heat-up that eases work to be done on the web later in the process. I wish I could e-mail or fax a small Excel drawing to you. Tell me what you think might be needed here.

Thanks Quark!

SASC

 

[quark]

builblock1@yahoo.com. Was this dryer made inhouse or bought out? Who is the manufacturer.

Regards,

 

[SASC]

Quark,

I'll send you an Excel drawing. It really is not a "dryer" because no evaporation is taking place....not a "dyer" in the conventional sense at all. Thanks for your help!

SASC

 

[hacksaw]

direct contact web heating has been used or tried in the past. Increasing the humidity air adjacent to the sheet does not help water removal and in fact increase the water content of the sheet.

You'd be better off, upgrading the syphons in the drier drums.

you might try TAPPI. The have documented a lot of development work along those lines.

 

[SASC]

Hacksaw,

I think I confused you: I'm not talking about direct steam in the Dryer Section....I'm talking prePress Section over the flatboxes. I bet this makes a lot more sense, right? Help me if you can Hacksaw!

SASC

 

[hacksaw]

in that case, all you'll accomplish is adding heat to the sheet and load to the suction boxes.

TAPPI is your best source. There are all sorts of improvements to that part of the machine that have to do with the wire, sheet formation, etc. that cannot be made without a comprehensive review of what your machine is or is not doing. A lot has to do with the rush/drag setup, etc, etc,...

Some have spent life times optimizing the wet end of paper/pulp machines. The TAPPI organizaton is a good starting point.

 

[quark]

SASC!

All these days I was searching on web to compensate my wrong initial concept. There are very few links which describe your process under "Pulp Forming" and "Impingement Drying". However, I din't come across with anything constructive.

The links hint at decreasing surface tension (and also decreasing vapor pressure in the vicinity of web) and making it easy for a pressure differential to pull water out of the web.

The only idea I got at present is to check the differential pressure required to overcome the surface tension. If you increase the temperature, the required pressure differential decreases. But I think there is no direct relation between steam flow rate and moisture removal rate from the web. It is just a trial and error.

I went through your spread sheet and the calculations are correct. Your idea of steam increasing the sensible heat(only) of the moist web is right.

TAPPI as suggested by hacksaw may be of some help.
The guy in this link seems to have done some research on pulp drying method. Try contacting him.

http://www.kat.lth.se/staff/Johan_N/Johan.htm

I am very sorry for, I cannot offer any more help at this moment. But I would like to share if you come up with anything in future.

By the way, do you have any idea about the pore size of the web?

Good Luck and Regards,