h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

One for the chemical engineers this.

I currently teach a course in Industrial Biocatalysis to masters level students.

One issue we push is the 'greenness' of this type of chemistry.


This year I have one student who keeps repeatedly claiming that their chosen topic of process is 'green' because it has to be done at 10 °C or the organism (which is a 'psychrophile') deactivates above this temp.

They claim its greener than a different process making the same chemical that requires 100 °C temps.


Now I maintain that cooling requires more energy than heating since to cool something, in order to not violate the second law of thermodynamics, you need to heat something elsewhere much more than you cool.


So is a process at 10 °C really 'green' through low energy useage?

Secondly I guess an industrial plant at 100°C may well use more than a 10 °C (could be wrong though) - is there a hard and fast rule for at what point they balance out temperature-wise or does it depend in a complicated fashion on reactor/heat exchanger designs...?

Many thanks.

Certainly the energy required for setup costs, maintenance and insulation will be much higher.

As for fuelling, there's a lot of difference between cooling a system to 10 degrees below the ambient temperature and 80 degrees above. If we ignore trifles like relative heat capacities then you are, in effect, assuming the heat exchange system is going to be less than 12.5% efficient. That's pretty poor for heat exchange - most industrial systems aim for around 40%.

Whether your student can justify the merits of factor two over factor one should be an interesting point.

B

Mmm, I had a nasty feeling this would get tricky.

>If we ignore trifles like relative heat capacities then you are, in effect, assuming the heat exchange system is going to be less than 12.5% efficient. <


How am I assuming this?

.. too many assumptions.

I'd suggest your students have a challenge.. They claim a green process, can they actually prove that? model it, make a business case ..

as for a few practical issues .. a reactor at 10 deg .. open air? inside a building? would the reactor need cooling or heating as well? (consider open air reactors in a harsh environment .. winter temperatures down to -20?)

In general it's easier to heat a reactor than to cool it.. cheaper as well.. (thumb-rules).

Yep, as said, heating is generally more cost effective than cooling. Of course, out in the big wide world, being at 10 C is alot more common than being at 100 C, so the siting of the hypothetical factory would make a lot of difference. Though of course if it was somewhere where 10 C temps were regular you've got a whole bunch of other problems re building, staff accomodation, transport etc. Tell your student to look beyond the immediate cost of the process.

You should assume that you can neglect relative heat capacities is that a water-based system, which you will obviously be dealing with in a Bioengineering situation, has such a high heat capacity relevant to any other practicable coolant medium that it may as well be neglected for the purposes of comparison.

Heating and cooling water is very often the over-riding energetic factor in any system, from producing electricity to large-scale ethanol production, any many more.

B

So why base the theoretical system on water? Just because it's generally freely available (I'm assuming you're not building this processing plant in a desert), doesn't mean it's the best for the job.

Water's almost always the best heat transfer medium for the job.

It's cheap, it's ubiquitous, it's pretty safe (safer than dihydrogen monoxide, for instance ), everyone knows how to use it, the gear for handling it is cheap, it has a low viscosity, and it's conveniently liquid at standard temperature and pressure.

The critical question is the heat load. How much heat are you having to shift, and in which direction, in or out?

Depending on the reaction, you may have to heat something to keep it at 10 C and cool something else to keep it down to 100 C. We're not talking here about simply maintaining the temperature of an inert litre of water, are we? There are reactions going on, and they will either require heat input or will generate heat that needs to be taken away. Which applies is critical.

How critical is the ten degrees? Would five be OK? How about twenty?

If you just put the tank outdoors in the UK the environment would do most of the work for you. If you need five, you'd need some additional cooling (but not much) most of the time. If you need 20, you'd need some additional heating (but not much) most of the time. Either way, the heat load vs. ambient is low, compared to getting something to boiling point and keeping it there.

Here's a few numbers for you:

I just installed a plant that has an *optimum* operating temperature of 40 C.

The (mostly) water that goes into it comes off the process upstream at about 25 C. I therefore designed in a heat exchanger, steam supply and control gear.

However, the heat exchanger blocked terribly with a contaminant. Solution? We took out the heat exchanger. That's it.

Over the last six months the process has received water via an outdoor, insulated tank, and it's seen it vary between 12 C in winter and about 18 C now. And for the process in question, it doesn't matter much. It's not optimum, but crucially it's not far off.

Your student seems to have it right, but if I were you I'd need some clear justifications:

- calculated heat load
- estimated variance of heat load with variances in ambient temperature
- proposals for controlling temp when ambient is above/below process set point
- proposals for protecting against frost
- cost/benefit comparison with 100 C process in terms of cost of control gear/services supply/consumption and heat load.

Orcus


Depends how quickly the reaction at 10DegC takes...does it take a lot longer than the 100DegC reaction?

A 10DegC required temperature would require a chiller nto a cooling tower. In Clitheroe North Westish UK minimum is about 28DegC for a cooling Tower. It does depend on the wet bulb temperature of course.

http://www.engineeringtoolbox.com/cooling-loads-d_665.html

(generally good site - lots of stuff on it)

Another excellent site:-

http://www.eng-tips.com/

Get you student to join. They'll be sad people like me and Twiggs who'll be happy to help out...

I know Twiggy-wiggs has said he can get 10DegC but I've not seen a cooling tower that can give that. Not sure where the supply of water to his insulated tank is coming from.

Hence, you then have to decide on what type of chiller to use to get the temperature which will depend on your size of process the temperature from which the cooling medium will go down from. Your greeness here will be how much it is costing you to compress your cooling medium and whether you are then using the cooling medium to cool water to cool your process or are you just using the cooling medium to cooling process? The how geen is your cooling medium?

So is a process at 10 °C really 'green' through low energy useage?



Depends on the process! How complicated is a piece of string? *cough* Mixed metaphors.



Not really.




Don't quite follow you here..tho not firing on all cylinders...you still need to compress something elsewhere to cool...depends whether the compressing energy and efficieny is better or worse than the heating energy required and it's efficiency.

Get you student to work out what it takes to remove 100kW of energy from a cooling system and what it takes to put 100kW of energy into a heating system. That might help...then heating medium...gas, coal, electric? At 100DegC, you are on the edge of steam/water.

Thanks all, I suspected it was going to be a 'how long is a piece of string?' thing.

Careful though, I'm not teaching a course in engineering so none of this is critical to the mark (which has already been given incidentally).
We are more interested in the actual chemistry/biochemistry going on in the organism than in heat flows and such I'm afraid. I just really want them to have an appreciation that translating what we do in a round bottomed flask or 500 ml baffled flask culture is a whole different ball game to doing it as a large scale process. We teach them rudimentary stuff about reactor designs depending upon inhibition types etc. It's safe to say I'm teaching them nothing about heat exchange and heat capacities as I know precious little about this stuff myself from an engineering point of view at least

This was more out of my own curiosity than anything else and I can see it's a can of worms!

The process that she is writing about, if you're interested is biocatalytic production of acrylamide using a Rhodococchus species that as I mentioned is psychrophilic (cold loving).

Upon rereading her report I've noticed that the organism actually works ok at 20-25 °C. The reason it's kept cold (<15 °C) is that the acrylamide polymerises under the culture conditions otherwise.

< I just really want them to have an appreciation that translating what we do in a round bottomed flask or 500 ml baffled flask culture is a whole different ball game to doing it as a large scale process.>

Ah, now that's a *totally* different question about 'scale-up' to do with mixing efficiency, volume/surface area ratios, filtering rates etc etc etc.

Heating is the least of your worries.

"Twiggy-wiggs has said he can get 10DegC"

Eh?

No I didn't. I said what we've actually observed - as in, measured with a calibrated temperature instrument - is between 12 and 18, depending on the season, so far.

The stream is originating process at about 25 C -> open top tank indoors -> pump -> long uninsulated pipe -> insulated tank being recirculated with a centrif pump via an insulated line -> processing vessel with temp probe.

No cooling tower. The temperature drops from 25 C(ish) in the originating process to wherever it gets to, crucially WITHOUT any heat-adjustment equipment other than pipe insulation intended mainly to protect against frost. We originally intended to operate at 40 C, so we installed a heater. If we wanted to *guarantee* we could operate at a fixed 12 C, we'd obviously need to install a chiller.

Intuition suggests operating at 100 C is more energy intensive than operating at 10 C, but intuition doesn't, in this case, include reaction kinetics. If polymerisation happens above 15, what happens at 100? Is there really a choice here? As in, "it's greener" could be meaningless if they're not achieving the same thing.

Oops, sorry old bean...mis-rememebered your temp range.

Hence you essentially have a single pipe heat exchanger being air cooled by air at what ever wind speed is around...or is it inside a buidling?

It's inside a building, although said building has a large roller shutter door so it can get pretty brisk in winter.

The comparison is a combination of overall energy consumption to get reaction mixture to 100DegC and time at 100DegC compared to overall energy consumption to get reaction mixture to 10DegC and time at 10DegC.

Intuition says a reaction at 100DegC will take less time to react than a reaction a 10DegC to gie the same overall yield of the same product.

Hence, *possibly* less energy at 10DegC but over a longer period of time upping the overall kWhr.

That's assuming batch rather than continuous process.

And if a bio-catalyst is being used, I doubt the reaction rate against temperature is an over-riding factor.

B

Not really I would have thought...if a reaction requires a certain amount of energy whether it's batch or continuous it would be the same. You are just applying it slightly differently.

Here's my two cents'...

First of all, cell cultures produce a huge amount of metabolic heat when they grow, so even if the culture were grown at room temperature, some cooling would be required. How much cooling is needed would depend on the type of organism, the growth rate, what growth phase it's in, what sort of genetic engineering you have to do to make it produce acrylamide, and all sorts of things. But industrial growth of cell cultures always requires cooling power. I've grown E. coli in culture volumes of about 6 liters (i.e., a really really really small amount), and if you leave them to grow in an incubator at 20 C and leave the lid of the incubator closed, they produce so much heat just from their own metabolic processes that they'll be dead in a matter of hours.

However, I feel like the "greenness" of a process is determined by lots and lots of other factors other than energy usage. What are the starting materials you need? What potentially toxic waste products are made? Do you have to use harsh chemicals to clean out the bioreactor between batches (if you're using a batch culture system)? Do you have to use a large amount of antibiotics to keep out contaminating organisms? What materials are required to purify your end product, and what by-products do you have to dispose of?

Even if it takes a large amount of energy to heat or cool the industrial production process, one process can still be more green than another based on the starting materials and the products. I don't know the details of the alternative acrylamide production methods of course, but I'd want to look at a variety of factors before deciding between them.

That exactly what I meant by reaction kinetics not being part of my intuitive feel for it. Even if the ambient temperature was exactly 10 degrees, you'd still need to apply cooling, is what you're saying? As I thought.

It's an interesting question with a far from simple answer.

Lower temperature = better for the environment is unforgivably simplistic for a masters level student.

Well it would be if they were an engineering student- although to be fair, I've not really been very impressed with it as a comment - hence my question here.


The comparison is with the* traditional, and still much more ubiquitous industrial process for acrylamide production. What I imagine is the case (since they clearly do use 100 °C temps in the 'traditional' procees) is that there is either a polymerisation inhibitor used in the hot process or there is an oxidant produced by the cells (or even the oxygen they use to grow) that catalyses [polymerisation. We do use persulphate salts to make polyacrylamide gels in the lab so it wouldn't surprise me...






*although I hesitate to say *the* to engineers here