Contro l of Parallel Overhead Condensers Methanol Distillation To: [email protected]Subject: Parallel condensors in Methanol Distillation column Date: Tue, 27 Aug 2002 14:04:30 +0600 From: D. B. S. Asian Chemical CompanyDear Mr. Sloley: We are carrying out revamp of our Methanol distillation column with new trays. The existing condenser is limiting. In the column we intend to install a new condenser in parallel with the existing condenser. The new condenser is for and additional 25% capacity. We would like to know the precautions to be taken for designing/installing a new condenser to take care of hydraulics and pressure control of the column at various loads. D. B. S. To: DBSSubject: Parallel condensors in Methanol Distillation column Date: Wed, 28 Aug 2002 From: [email protected]Do you mean that you are: A. Installing a second stab-in condenser in parallel with an existing stab-in bundle? B. Installing a second bundle in parallel in a conventional overhead system? A. Sloley To: Andrew Sloley <[email protected]> Subject: Re: Parallel condensors in Methanol Distillation column Date: Thu, 29 Aug 2002 11:33:38 +0500 From: DBSThanks for the prompt reply. We are installing a second condenser (condenser already available from another plant) in parallel with the existing condenser. Hence, vapour from the column will be distributed into the two condensers. We are worried about following:
Pressure drop calculated across 1st condenser is 0.15 bar, condensing capacity 15000 kg/hrvapour. Pressure drop across parallel condenser is 0.1 bar, condensing capacity 5000 kg/hr.
Now when the plant operates at 20000 kg/hr of vapour from the tower, ideally the vapour to the
existing condenser should be 15000 kg/hr & new condenser should be 5000 kg/hr. But aspressure drop across the new condenser is low, more vapour will be pushed through this
condenser and due to limitation of condensing capacity of only 5000 kg/hr, we envisageuncondensed vapour from new exchanger.
To control the distribution, we are thinking to install a low pressure drop control valve on thevapour line to the new condenser.
What is your experience with parallel condensors ?
If we use this valve & during operation this valve is throttled very much, is there any chance of
vacuum creation in the condenser.
I am attaching the schematic of the system for better understanding.
To: DBS
Subject: Parallel condensors in Methanol Distillation column
Overall, my experience with parallel condensers has been very satisfactory. In any unit revamp
you have to balance between adding condensers in parallel and adding them in series. Both
choices have different circumstances that favor them.
The best situation for parallel exchangers is to have identical exchangers with a symmetrical
piping network. This solves most major flow distribution problems. However, non-identicalexchangers in parallel can also work. It is common practice to put a low-pressure drop (often
butterfly) valve to prevent excessive flow through an exchanger that has a lower pressure drop at
desired operating conditions. I have done this many times myself. The real question in this
configuration is what you control the butterfly valve with. We will come back to this point later.
Your other question was could the system in the new exchanger run in vacuum if the flow to the
new exchanger went to a very low rate. Based on your simplified piping diagram (Figure 1), the
answer is no. If you have low flow to the second exchanger and it acts as a total condenser andstarts to create a pressure in the shell lower than the existing exchanger, you will get vapor flow
from the drum to the new exchanger. If your existing pressure control system is adequate toprevent vacuum in the drum it will continue to operate adequately.
Control of your two exchanger system is the real question. Figure 2 shows adding a butterflyvalve to the feed to the new exchanger. Figure 3 shows the alternate location for this valve, in the
outlet line to the drum. While the line is larger, I recommend the location in the line to the drum(Figure 2). In your configuration, putting the control valve in the outlet line (Figure 3) has the
potential to create two problems. First, you could hold a liquid leg on the valve that partially
floods the new condenser and reduces its capacity below your desired level. This could be
avoided by careful place of the exchanger and valve relative to the reflux drum. Second, you use
a nitrogen injection upstream of the condensers to prevent drum pressure from going too low.The nitrogen may serve two purposes: A-it partially blankets the heat-transfer surface of the
exchangers dropping heat transfer and B-adding vapor to the drum to increase its pressure. The
nitrogen guarantees that the flow in the outlet lines from the condenser to the reflux drum is two-phase. Control valves in two-phase flow have erratic performance and many maintenance
problems. This configuration should be avoided. Of course, you could always move the nitrogen
Figure 3 Potential problems with flow control in outlet line
Staying with the good practice of having the control valve in the feed line to the new exchanger,the simplest system would be to use a flow orifice in the lines to the condensers to measure flow
and use a flow ratio control on the valve to maintain a constant split between the exchangers(Figure 4). You need to be careful in this case to prevent condensation that forms upstream of the
orifice plates from accumulating. Liquid pools upstream of the orifice make the flow readings
inaccurate. Additionally, in some piping layouts a liquid leg upstream of an orifice can
accumulate and significantly raised the tower pressure. On one memorable troubleshooting
occasion, pressure field pressure readings revealed a 7 meter tall liquid stack created bycombining a poor piping layout with a readily condensable overhead and an orifice plate.
The tempting alternative of using a DP cell across the exchangers with a differential pressureratio control (Figure 5) should be resisted in your case. You have only a 0.15 bar (2.2 psi)
pressure drop across one exchanger and a 0.10 bar (1.5 psi) pressure drop across the other. Youwould be trying to control on a small number derived from the difference between two larger
numbers. This is a classic error that is always warned against but often made. The differential
pressures would also decrease at low flow rates, making the situation worse.
Figure 5 Differential pressure control to parallel exchangers
(Only recommended if differential pressures are high)
If using direct flow control is unattractive, you can control to have a differential temperature of zero between two thermocouples placed downstream of the condensers (Figure 6). I have usedthis system with great success in several installations.
Pertanyaan Anda yang lain yang bisa sistem dalam jangka penukar baru dalam ruang hampa jika aliran
ke penukar baru pergi ke tingkat yang sangat rendah. Berdasarkan diagram perpipaan sederhana Anda
(Gambar 1), jawabannya tidak. Jika Anda memiliki aliran rendah untuk penukar kedua dan ia bertindak
sebagai kondensor total dan mulai menciptakan tekanan di shell lebih rendah dari exchanger yang ada,
Anda akan mendapatkan aliran uap dari drum ke penukar baru. Jika ada tekanan Anda sistem kontrol
adalah cukup untuk mencegah vakum dalam drum itu akan terus beroperasi secara memadai.
Gambar 1
Usulan overhead sistem
Pengendalian dua sistem penukar Anda adalah pertanyaan sesungguhnya. Gambar 2 menunjukkan
menambahkan katup kupu-kupu dengan umpan ke penukar baru. Gambar 3 menunjukkan lokasi
alternatif untuk katup ini, di baris outlet ke drum. Sementara garis lebih besar, saya sarankan lokasi di
baris ke drum (Gambar 2). Dalam konfigurasi Anda, menempatkan katup kontrol di baris stopkontak
(Gambar 3) memiliki potensi untuk menciptakan dua masalah. Pertama, Anda bisa mengadakan kaki
cairan pada katup yang sebagian banjir kondensor baru dan mengurangi kapasitasnya di bawah tingkat
yang diinginkan. Hal ini bisa dihindari dengan tempat-hati dari penukar dan katup relatif terhadap drum
refluks. Kedua, Anda menggunakan injeksi nitrogen hulu dari kondensor untuk mencegah tekanan
gendang dari pergi terlalu rendah. Nitrogen dapat melayani dua tujuan: A-itu sebagian selimut
permukaan transfer panas dari penukar menjatuhkan perpindahan panas dan B-menambahkan uap ke
drum untuk meningkatkan tekanannya. Nitrogen menjamin bahwa aliran di garis keluar dari kondensor
ke drum refluks adalah dua fase. Kontrol katup dalam dua fase aliran memiliki kinerja yang tidak
menentu dan banyak masalah pemeliharaan. Konfigurasi ini harus dihindari. Tentu saja, Anda selalu bisa
bergerak nitrogen make-up untuk drum langsung.
Gambar 2
Direkomendasikan lokasi untuk katup kontrol aliran
Gambar 3
Potensi masalah dengan kontrol aliran sejalan stopkontak
Tetap dengan praktek yang baik memiliki katup kontrol di garis umpan ke penukar baru, sistem yangpaling sederhana adalah menggunakan lubang aliran dalam baris ke kondensor untuk mengukur aliran
dan menggunakan kontrol aliran rasio pada valve untuk mempertahankan konstan perpecahan antara
penukar (Gambar 4). Anda perlu berhati-hati dalam hal ini untuk mencegah kondensasi yang
membentuk hulu dari pelat orifice dari terakumulasi. Kolam Cair hulu lubang membuat angka aliran
tidak akurat. Selain itu, dalam beberapa layout pipa kaki cair hulu dari suatu lubang dapat terakumulasi
dan secara signifikan meningkatkan tekanan menara. Pada satu kesempatan pemecahan masalah
