264 replies to this topic

[MurtazaHakim]

Adding the above condensate enthalpies to their vapor counterparts yields a profile which goes from positive total (V+L) enthalpy to negative total (V+L) enthalpy. How does one interpret this profile physically ? Is it possible to have a condensation profile including both positive and negative total (V+L) enthalpy. What does negative (V+L) enthalpy indicate ?

 

The attached thumbnail depicts the total vapor enthalpy (V),condensate enthalpy (L) and the their sum (V+L) at temperatures ranging from 152.8 C to 45 C .

Attached Files

•  final (V+L).PNG   16.93KB   7 downloads

[PingPong]

That seems about right.

 

How does one interpret this profile physically ? Is it possible to have a condensation profile including both positive and negative total (V+L) enthalpy. What does negative (V+L) enthalpy indicate ?

Negative enthalpy means nothing.

Enthalpy is always relative to a reference level. We used IG@25C as a zero reference because that is most convenient in case of chemical reactions. Obviously the enthalpy of a liquid is lower than that of its ideal gas due to condensation enthalpy, hence the negative numbers.

See what I wrote in this other topic: https://www.cheresou...lpy-calulation/

 

Absolute enthalpy values do not matter because it is about the enthalpy change when heating, cooling or reacting any stream.

 

Cooling the effluent from 420 to 152.8 ^oC means a cooling duty of 423 - 132 = 291 kW

Cooling the effluent from 152.8 to 45 ^oC means a cooling duty of 132 - (-350) = 482 kW

Total cooling duty from 420 to 45 ^oC is 423 - (-350) = 773 kW

[MurtazaHakim]

It seems quite strange to me that cooling the WGSR effluent from 420 ⁰C to 152.8 ⁰C requires 291 kW of cooling duty whereas cooling the same effluent further from 152.8 ⁰C to 45 ⁰C requires a much higher cooling duty of 482 kW. Is this behavior observed because of the fact that the WGSR effluent is in gas phase only until it reaches 152.8 ⁰C after which it gets split into vapor and condensate and the quantity of condensate increases as the effluent further cools down to 45 ⁰C ?

 

We have tried generating the curve for this hot stream as having non-linear enthalpy in HINT but it says “The temperature-enthalpy data entered is incorrect. The enthalpy should be a monotonous function of temperature”. Can we enter this hot stream as having linear m.C_p with enthalpy as 773 kW in case we are unable to generate the temperature-enthalpy profile for this stream ?

[PingPong]

Until 152.8 ^oC the cooling only needs to remove sensible heat.

From 152.8 ^oC onwards water starts condensing so the cooling not only needs to remove the sensible heat from the vapor (and liquid), but also the latent heat of the condensing water vapor.

 

I am not familiar with HINT so I don't know what can or cannot be done with it, but surely you can enter multiple streams for the WGSR effluent.

For example: one from 420 to 152.8 ^oC with a certain m.C_p , one from 152.8 to say 110 ^oC with another m.Cp and one from 110 to 45 ^oC with again another m.Cp. Or even more streams if you like.

 

Note however that the dimension of m.C_p is not kW but kW/^oC.

[MurtazaHakim]

The streams have now been corrected and the composite curve generated. The DT_min is 118 K and the minimum number of heat exchanger is calculated as 12. The thumbnails are attached herewith. The WGSR effluent has been divided into different hot streams as suggested. Kindly have a look at them and correct them so that we move further into the process.

Attached Files

•  final cascade.PNG   18.09KB   1 downloads
•  Final CC.PNG   7.94KB   3 downloads
•  final CC list.PNG   15.32KB   5 downloads
•  final grid diagram.PNG   15.61KB   1 downloads

Edited by MurtazaHakim, 02 April 2018 - 07:27 AM.

[PingPong]

I doubt the H(kW) data in the table for streams 4 , 6 , 7 and 8.

 

In any case your Grid Diagram is not useful as it does not show the heat exchangers between the streams. Look again at the example that I posted earlier.

[MurtazaHakim]

Earlier in the process the amount of process HPS calculated was 65.244 kmol/hr. The required heat for raising this quantity of BFW from 30 C to 242 C (prior to sending it to the WHB downstream of reformer) is

 

(1047.2-125.7) kJ/kg * 65.244 kmol/hr * 18.015 kg/kmol/3600  = 300.862239 kW ....................................(stream 4)

 

Similarly streams 6,7,8 constitute the export HPS, the quantity of BFW from which it is produced was calculated as 59.63997 kmol/hr so firstly 59.63997 kmol/hr is raised from 30 C to 257 C using the amount of heat calculated as follows 

 

(1120.1-125.7) kJ/kg * 59.63997 kmol/hr * 18.015 kg/kmol/3600 = 296.777039 kW ................................. (stream 6)

 

Vaporising the same stream excluding the CBD requires 

 

(2797.85-1120.1) kJ/kg * 56.65797 kmol/hr * 18.015 kg/kmol/3600 = 475.6856204 kW ............................(stream 7)

 

Further superheating this 56.65797 kmol/hr from 257 C to 385 C requires 

 

(3169.15-2797.85) kJ/kg * 56.65797 kmol/hr * 18.015 kg/kmol/3600 = 105.273175 kW ............................(stream 8)

 

The heat required has changed a bit now that I have calculated these values again. Prior to generating the condensation profile we never anticipated the cooling duty of 773 kW but now that we have calculated 773 kW of cooling duty, will the quantity of BFW increase since taking only 65.244 kmol/hr will provide only 300.86 kW of cooling duty. If not, how is the remaining 473 kW of cooling duty to be supplemented ?

[PingPong]

Stream 4 is still wrong.

 

Streams 6 , 7 and 8 are now correct, but were wrong in the table.

 

Cooling of excess heat in WGSR effluent can be done by air cooling and water cooling.

[MurtazaHakim]

Stream 4 is raised from 30 to 242 degrees Celsius before routing it to the (SD+WHB) for process steam generation. The temperature of SD+WHB is 257 degrees Celsius and it was mentioned previously that the BFW is to be raised to a temperature 15 degrees lower than that at SD+WHB (257 degrees C) and SD+WHB constitutes a black box . We are unable to trace any mistake in stream 4 calculations. Is the quantity of BFW taken correct ? Please inform us of the mistake we are making in calculating the stream 4 enthalpy. What is still wrong in stream 4 ?

[PingPong]

Is the quantity of BFW taken correct ?

No.

[MurtazaHakim]

How should then the correct quantity of BFW be determined ? The 65.244 kmol/hr was calculated previously assuming that this much amount would be completely converted from saturated liquid to saturated vapor by absorbing the WHB duty of 573 kW. We had not considered the BFW preheat then, we had directly assumed that the BFW reaching SD+WHB  is at 242 degrees C, but now how should the process of evaluating the quantity of BFW be carried out since 65.244 kmol/hr BFW is incorrect ?

[PingPong]

It is becoming increasingly more difficult for me to keep track of all your numbers, but I seem to remember

that that 65.244 kmol/h is the HPS product flow rate from the WHB, not the BFW feed rate to it.

[MurtazaHakim]

The HPS product flowrate of 65.244 kmol/hr is 95% of the BFW feed rate into the SD+WHB system, remaining 5% being the CBD. Hence the quantity of BFW to the SD+WHB system is 65.244/0.95 = 68.67789 kmol/hr.

 

Enthalpy of BFW at the entrance of SD (242 degrees C) = 68.67789*18.015*1047.2/3600 = 359.89709 kW ............... (1)

 

Enthalpy of BFW at 257 degrees C = 68.67789*18.015*1120.1/3600 = 384.95104 kW ...............................................................(2)

 

Enthalpy of vaporisation of 95% of BFW = 65.244*18.015*1677.75/3600  = 547.7717 kW ......................................................... (3)

 

Total heat required for the above changes = (2) - (1) + (3) = 25.05395 + 547.7717 = 572.82565 kW which is the available heat in the waste heat boiler.

 

Now the (Steam Drum + Waste Heat Boiler) constitutes a black box which makes it difficult to comprehend how the above changes take place actually inside the system, but I think the BFW rate to the SD+WHB is 68.67789 kmol/hr. Is the BFW rate correct now ?

 

I know it must have been difficult for you to remember the numbers lately but kindly look into the numbers above. We have complete faith in your technical prowess.

Edited by MurtazaHakim, 07 April 2018 - 03:59 AM.

[PingPong]

OK, but you still need to correct the H(kW) and mcp(kW/K) data for Stream 4 in the table.

 

And then the proof of the pudding: design the heat integration between the hot and the cold streams to actually achieve the targets..

[MurtazaHakim]

We have attached the thumbnails of final list of hot and cold streams along with the composite curve obtained from them. We have made the necessary changes in the list of streams. Please verify for correction. However we are unable to generate the HEN Grid Diagram in HINT (perhaps due to the software being open source and zero cost). How do we develop the HEN diagram then ?

 

We have a few queries regarding the process.

1. The tail gas contains CO₂ which absorbs the heat released by the combustion of the rest of the components in the tail gas in the reformer fired heater. Is this effect of CO₂ interference already accounted for in the calculations done so far involving the amount of heat obtained by burning the tail gas in the reformer furnace ?

 

2. All the calculations done so far are on the basis of equilibrium conversion but the actual conversion would be far from the equilibrium conversion and the kinetic data would be required to calculate the actual conversion and hence the amount of catalyst required to achieve that actual conversion. Is it possible to achieve the equilibrium conversion completely ?

 

Our supervisor has asked us to begin designing the reforming tubes (amount of catalyst, GHSV, no.of tubes etc..) , WGSR reactor and other equipments involved (WHB, PSA, Heat exchangers etc..) using actual conversion and not the equilibrium conversion. He insisted that the equilibrium conversion would require infinite amount of catalyst and so is impossible to achieve in real time. In case of conversion changing, the entire mass and heat balance would require to be repeated. Please advise on the query.

Attached Files

•  modified composite curve.PNG   8.38KB   1 downloads
•  modified list of streams.PNG   15.12KB   1 downloads

Edited by MurtazaHakim, 10 April 2018 - 06:45 AM.

[PingPong]

We have attached the thumbnails of final list of hot and cold streams along with the composite curve obtained from them. We have made the necessary changes in the list of streams. Please verify for correction.

Looks OK to me.

How do we develop the HEN diagram then ?

You can do it by hand, using the Pinch rules, or using common sense.

 

1. The effect of inerts or excess steam, et cetera, is automatically taken into account in your enthalpy calculations as those components are present in the feed as well as the product enthalpy of each reaction. CO₂ in tailgas is no different from N₂ and excess O₂ in combustion air, excess steam in Reformer effluent and excess steam in WGSR effluent.

 

2. It is already a couple of months ago, but you should still remember that we used ATE to account for the fact that in practice true equilibrium can never be reached.

 

 

I seem to remember that long time ago you stated that you wanted to do the H&M balances first by hand but afterwards you would also use a process simulator. Is that still the intention, or not? If it is, it seems premature to already start designing equipment, unless you are running out of time.

[MurtazaHakim]

We have tried making the HEN Grid Diagram in HINT. Please see the attached image. There are 2 hot streams namely the flue gas stream and the WGSR effluent stream. The hot stream 1 (flue gas) is used for heating 6 streams in the convection section. The order of heating is as follows:

1. NG+ rec. H₂ preheat = 89 kW

2. Export HPS superheat = 105 kW

3. Process HPS superheat = 121.2 kW

4. Mixed Feed preheat = 194.5 kW

5. Export HPS preheat = 296.7 kW

6. Export HPS generation = 475.6 kW

Stream 1 is heated first followed by stream 2 and so on. Is the sequence of heat exchange correct ?

 

However the WGSR effluent requires a cooling duty of 773 kW whereas the BFW (process) preheat would only consume 317 kW of heat which results in requirement of 456 kW of duty to be fulfilled by cooling utility. This cooling duty can be fulfilled either by taking higher quantity of BFW (process) or by using other means of cooling. Taking only the calculated BFW (process) reduces the temperature of WGSR effluent from 420 degrees C to 247 degrees C.    

Attached Files

•  HEN GD.PNG   19.12KB   0 downloads

[PingPong]

When designing an HEN you should always exchange cold with cold, warm with warm and hot with hot. It seems to me that that is not quite what you did.

 

To match the composite curves you posted before, you could do as follows:

 

Split BFW into two parallel streams, about 53 % is preheated up to 242 ^oC against WGSR effluent which is thereby cooled from 420 to about 150 ^oC.

 

The other 47 % of BFW is preheated to 242 ^oC against flue gas at the cold end (just before stack) which is thereby cooled from about 349 to about 150 ^oC.

 

HPS is generated at 257 ^oC against flue gas which is thereby cooled from about 679 to about 349 ^oC.

 

Natgas+H₂ Feed is preheated from 42 to 370 ^oC against flue gas which is thereby cooled from about 736 to about 679 ^oC.

 

All HPS is superheated from 257 to 385 ^oC against flue gas which is thereby cooled from about 877 to about 736 ^oC.

 

Mixed Feed is preheated to 550 ^oC against flue gas which is thereby cooled from 1000 to about 877 ^oC.

 

Note that in reality there is only one HPS stream. Both the WHB and the steam generating coil in the convection section take water from the steam drum and send a steam/water mixture back to the steam drum. The saturated HPS from the steam drum is then superheated in the convection coil to 385 oC, part of it is mixed with the feed gas, the excess HPS is exported.

 

Above mentioned temperatures are only approximate.

 

 

Additional remarks:

 

You use a B.L. temperature of 30 ^oC for the BFW, however normally BFW is availabale at 105 - 110 ^oC as it comes from a steam stripping deaerator (degassifier) operating slightly above atmospheric pressure.

 

Demin water and condensates can be at 30 ^oC before preheated and sent to the deaerator to remove any dissolved gases.

 

Most often an SMR has its own degassifier, stripping its own condensate and heat integrated with the WGSR effluent.

[MurtazaHakim]

The HEN Grid Diagram is developed in ASPEN ENERGY ANALYZER. The temperature of flue gas reduces almost in the range you mentioned. The flue gas reduces 

(i) From 1000 °C to 871 °C by exchange with mixed feed

(ii) From 871°C to 790.8 °C by exchange with process HPS superheat

(iii) From 790.8 °C to 721.2°C by exchange with export HPS superheat

(iv) From 721.2 °C to 662.2°C by exchange with NG+H₂ preheat

However we are finding it difficult to add HEx to the hot stream of export HPS generation without which we cannot proceed in the flue gas hot stream section.

 

The WGSR effluent temperature is reduced from 420 °C to 266.5 °C only by exchange with process BFW (53% of split) when the BFW supply temperature is taken as 30 °C.

 

The queries are :

1. How to have flue gas and export HPS generation streams connected with an HEx in ASPEN ENERGY ANALYZER ?

2. Should the BFW supply temperature be 30 °C, since taking 110 °C would result in even higher WGSR effluent temperature than 266.5 °C at the target side  ? 

Kindly see the attached thumbnail.

Attached Files

•  HEN Synthesis.PNG   145.1KB   5 downloads

Edited by MurtazaHakim, 16 April 2018 - 09:04 AM.

[PingPong]

Temperature differences between you and me are caused by fact that I use actual enthalpy data at each intermediate temperature while you use constant m.Cp over the whole temperature range which is less accurate.

 

Once again: there is only one HPS stream of about 122 kmol/h coming from the steam drum, that is superheated in one coil and after that split into process steam and the excess steam is then export HPS.

 

Same for BFW: there is only one stream of about 128 kmol/h that is split into two parallel streams, about 53 % is preheated  against WGSR effluent and about 47 % against Flue Gas and then combined and fed to the steam drum. The 53/47 split is only for optimal heat exchange.

 

1. I am not familiar with ASPEN ENERGY ANALYZER but I suppose that for steam generation you could use an inlet T of 256 ^oC and an outlet temperature of 257 ^oC and then use a C_p = 1678 kJ/kg.K (equal to the numerical value of the latent heat at 257 ^oC).

m.Cp is then .....*1678 kW/K

 

2. I was merely trying to explain:

BFW of 30 ^oC does not exist in the real world,

either you get demin water at 30 ^oC from OSBL utility system which, together with your condensate, has to be degassed inside the SMR unit,

or you get BFW at say 110 ^oC from OSBL utility system and you send your condensate to OSBL to be degassed together with demin water and turned into BFW. That requires however availability of steam at OSBL.

Normally such matters are discussed and agreed with the client at the beginning of a project, and specified in the BOD.

[MurtazaHakim]

The steam generation (in fired heater convective coil) stream does not converge when added through HEx with flue gas stream after the flue gas stream exchanges heat with (1) mixed feed (2) total HPS superheat (3) NG+H₂ preheat respectively. The export HPS generation is stream (4) in sequence after the the above three streams. The error shows that there is a temperature cross over.

 

about 53 % is preheated up to 242 ^oC against WGSR effluent which is thereby cooled from 420 to about 150 ^oC

How does the temperature drop from 420 ^oC to 150 ^oC ? We are getting WGSR effluent dropped to 266.5 ^oC only (not 150 ^oC) .

 

You mentioned splitting the BFW stream into 53/47. Should we have two separate cold streams then having their corresponding enthalpies  ?

Attached Files

•  non convergence.PNG   41.25KB   2 downloads

Edited by MurtazaHakim, 18 April 2018 - 08:08 AM.

[PingPong]

Your steam generation duty 4.061e+006 kJ/h (1128 kW) is MUCH too high. Should be about 490 kW.

 

The total BFW stream is about 128 kmol/h. Use about 53 % (68 kmol/h) of that and preheat it from 30 to 242 ^oC against WGSR Effluent which therby cools from 420 to 150 ^oC (duty is about 310 kW), and preheat the remaining 47 % (60 kmol/h) to 242 ^oC against the Flue Gas (duty is about 270 kW).

[MurtazaHakim]

We have regenerated the HEN Grid diagram again. Please have a look at the attached thumbnails. The heat exchanger area has been calculated by the software. The attached thumbnail shows the heat exchanger area required. However the flue gas at last reaches around 163 degrees Celsius at stack. The WGSR effluent too reaches 266 degrees Celsius only thereby requiring additional cooling utility for the WGSR effluent to reach 45 degrees Celsius. There is an option of having the heat exchanger area changed in order to achieve the expected target temperature for the hot/cold stream. Are the results obtained in the thumbnails correct ?

 

How do we manually calculate the heat exchanger area required for achieving the target temperature(s) of hot/cold stream(s) ?

 

The entire BFW is gathered into the steam drum and from there gets split into two streams one going to the WHB section and another to the convective steam generating coil inside the fired heater after which both the streams again combine for getting superheated in the convective section of the furnace. Why is the stream first preheated inside the convective section and then sent to the steam drum and again routed back to the same convective section for the superheat ? I mean does it not increase the piping costs ? Is it not possible to have the hot stream directly entering the steam generating coil after preheat (bypassing the steam drum) in the same convective section and the HPS from the WHB section be combined later with the HPS of convective section and superheated ?

Attached Files

•  HEx area for acheiving energy targets.PNG   13.88KB   0 downloads
•  final HEN Grid diagram.PNG   37.28KB   1 downloads

Edited by MurtazaHakim, 19 April 2018 - 09:05 AM.

[PingPong]

You must be doing something very wrong if you keep finding results that differ soo much from my message #193 (15 april 2018).

 

I can't look over your shoulder to see what you are doing, so I suggest you go back to the messages that you posted a few weeks ago where you posted the correct composite curves and correctly calculated how much additional BFW and HPS generation you could do in the convection section of the furnace so as to reach a target of 150 ^oC for flue gas and consequently also for WGSR effluent.

 

Preheated BFW is sent to the steam drum bacuase the WHB and the vaporizing coil in the convection section operate on partial vaporization, not once through as that would result in scale deposits. Design is usually for a vaporization of 20 % or so.

For example: you calculated weeks ago that the WHB produces about 65 kmol/h saturated HPS. The WHB and associated piping is then designed for 65/0.20 = 325 kmol/h hot water from steam drum to WHB, and 65 kmol/h HPS plus 260 kmol/h hot water back to the steam drum. So there is 260 kmol/h water circulating over the WHB without being vaporized. Same story for the vaporizing convection coil: net vaporization is 57 kmol/h HPS but total circulation is 5 times as high.

 

Note also that the heat exchange between Flue Gas and cold streams is not in conventional heat exchangers but in a furnace convection section.

[PingPong]

Still looking for the mistake(s)?

 

Let's first recap the whole BFW/HPS/CBD balance:

 

In message #114 you calculated that the WHB duty is 572.76 kW and assuming 5 % CBD and 242 ^oC BFW preheat temperature that results in 65.24 kmol/h HPS production from 68.51 kmol/h BFW. Delta is the CBD.

In messages #160 thru 166 you calculated that, based on a target of 150 ^oC for the Flue Gas (and WGSR Effluent), it is possible to generate an additional 56.66 kmol HPS from 59.64 kmol/h BFW. Delta is the CBD.

 

Generating that 56.66 kmol/h HPS from 242 ^oC BFW (and taking into account 5 % CBD at 257 ^oC) requires a duty of about 497 kW.

A total amount of 65.24 + 56.66 = 121.9 kmol/h HPS is to be superheated from 257 to 385 ^oC against Flue Gas.

That is a duty of about 226 kW (use steam table).

A total amount of 68.51 + 59.64 = 128.15 kmol/h BFW is to be preheated from 30 to 242 ^oC.

That is a total absorbed duty of about 591 kW (use steam table).

 

Preheating 53.5 % of total BFW against WGSR Effluent  requires a duty of 0.535 * 591 = 316 kW.

WGSR Effluent is thereby cooled from 420 to 151 ^oC.

 

Preheating remaining 46.5 % of total BFW against WGSR Effluent requires a duty of 0.465 * 591 = 275 kW.


Now let's check the Flue Gas heat balance:

 

Absorbed duty for mixed feed preheat up to 550 ^oC is about 195 kW (your message #160)
Absorbed duty for HPS superheat up to 385 ^oC is about 226 kW (see above)
Absorbed duty for NG + H₂ preheat up to 370 ^oC is about 89 kW (your message #160)
Absorbed duty for additional HPS generation is about 497 kW (see above)
Absorbed duty for preheat of 46.5 % of BFW is about 275 kW (see above)

 

So total absorbed duty in furnace convection section is 195+226+89+497+275 = 1282 kW.

 

To cool the Flue Gas from 1000 to 150 ^oC delivers about 1282 kW (your message #155) so Flue Gas will indeed be cooled to 150 ^oC by above streams.