DE-BOTTLENECKING A De-Iso-BUTANIZER WITH … three streams are accumulated in a feed drum. The...

Total Refinery Antwerp 1 AIChE Meeting, 10-14 April 05 Sulzer Chemtech Debottlenecking a DIB with VGPlus

DE-BOTTLENECKING A De-Iso-BUTANIZER

WITH “VGPlus” HIGH PERFORMCE TRAYS

By

Giuseppe Mosca, Gitta Van Hemel, Roberto Tocco, Sulzer Chemtech.

And

Bruno Lestrade, Total Refinery Antwerp, Belgium.

Presented in the Distillation Symposium of the 2005 Spring AIChE Meeting

Atlanta, Georgia, 10 – 14 April 2005

Unpublished

Copyright 2005: Total Refinery Antwerp, and Sulzer Chemtech Winterthur.


Abstract

Increasing throughput in major existing equipment, such as distillation columns, is a key

factor to economically meet production targets.

To successfully maximize plant’s performances, a deep knowledge of the process and its

characteristics is a must, but certainly not enough.

It is also crucial to fully comprehend the equipment being used, their mechanisms of

operation, limitations and capabilities. This includes, among others, furnaces, exchangers,

pumps, and mass transfer devices for distillation columns: fractionation trays, random

and structured packing and associated liquid and vapor distributors.

The purpose of this paper is to provide an overview of the key factors to successfully

implement revamping projects with the goal to maximize capacity without compromising

efficiency.

A commercial experience will be discussed in detail, where the use of one of the most

advanced mass transfer components, i.e. VGPlus tray, yields to the best cost effective

solution of revamping for a De-IsoButanizer at Total Refinery Antwerp, Belgium.

The paper will cover the description of the unit, the scope of the revamping, its

implementation, and the achieved performances.

Process simulation details will be also presented for a better understanding of column

capacity versus mass transfer separation efficiency, and their optimization while dealing

with existing columns.

1) De-IsoButanizer Plant: Process description

The Unit (see figure 1.1) may receive feed from three different sources; the compositions

of the streams differ substantially from each other, one being the richest in Iso-C4,

another one the poorest, and the third in the middle. The richest stream is not fed in a

continuous mode, actually is used only in case of high demand of Iso-butane for the

downstream Alkylation plant. The three streams are accumulated in a feed drum.

The homogenized feed is pumped, preheated by mean of a stream coming from the Iso-

Stripper, and finally enters the distillation column above tray 29. The feed flow rate is in

cascade with level of the accumulator, while its temperature is not automatically

controlled. The fractionator separates Iso-Butane from Normal-Butane and heavier.


Figure 1.1: De-IsoButanizer Plant

The De-IsoButanizer overhead vapor is cooled, condensed by mean of water coolers, and

accumulated at the top receiver. The reflux flow rate set point is in cascade with the

temperature control of the top pilot tray. The flow rate of the Iso-Butane product is

cascaded with the top receiver ‘s level control. A common pump serves both the Iso-

Butane product and the top reflux. The overhead product is one of the feedstocks for the

Alkylation Unit. The duty at the kettle reboiler is provided with a hot fluid coming from

another Unit with which it’s integrated.

The bottom product is pumped and sent to stock after cooling at ambient temperature, its

flow rate is in remote set point with the level control of the kettle reboiler.

Before the revamping, the Unit was able to produce at most 150 tons/day of Iso-Butane.

2) Scope of the Revamping

A feasibility study was commissioned with the scope of identifying the maximum

achievable production of Iso-Butane. The results of the study showed that the Iso-Butane

throughput could have been doubled versus the current production, after modifying and

or adding a certain number of equipment; while keeping unchanged the existing vessel of

the distillation tower, the feed accumulator and the top receiver.

56

1

28

n-C4

29

From Iso-Stripper

Reboiling Fluid

FC

FC

FCLC

PC

LCFC

10FC

TC

Fuel Gas

Iso-C4Feed-1

Feed-2

Feed-3

LC


The study included the evaluation of all the equipment based on two cases with different

compositions: “Design Feed” and “Alternate Feed”, being the first one the main case (see

Table 2.1). The ultimate capacity of the “Alternate Feed” was determined by fully

utilizing the reboiler and condenser duties of the first case; only the bottom product pump

and cooler were designed based on the controlling Alternate Feed case.

Table 2.1: Feed compositions for the Design and Alternate case

Component / Case Design Feed (%wt) Alternate Feed (%wt)

Propane

Iso-Butane

Normal-Butane

Unsaturated C4s

C5s

2.70

31.20

65.72

0.35

0.03

2.78

23.70

73.18

0.21

0.13

Total 100 100

Since the De-IsoButanizer is integrated with another Unit, which provides the reboiling

duty, the study was performed setting a maximum available duty of 13.5 MMKcal/h.

The condenser duty and the reflux ratio were calculated from the required Iso-Butane

recovery and purity, as well as from the achievable mass transfer efficiency at the

existing tower.

A process simulation study was performed to identify the optimum feed inlet location,

reflux ratio and number of trays with the goal of maximizing the capacity of the column.

For a given existing column, only reducing the tray spacing can increase the number of

theoretical stages, provided that the fractionation trays are properly designed.

For a required separation target, the reflux decreases by increasing the theoretical stages,

until a minimum value is approached. At reduced reflux, the vapor & liquid traffic

through the column decreases, thus the hydraulic capacity of the tower should increase.

However while decreasing the tray spacing to achieve more separation stages, the

hydraulic capacity of the column decreases, approximately with the square root of the

tray spacing. Therefore, for a given column (internal diameter and height), and separation

goals, there is an optimum working point, that can be determined from a proper analysis

of the number of stages, reflux ratio and tray spacing. (See Figure 2.1, 2.2, 2.3)


Figure 2.1: Reflux Ratio VS Theoretical Stages

Figure 2.2: Reb. & Cond. Duty VS Theoretical Stages

Figure 2.3: Capacity of the existing Column VS Theoretical Stages

9

11

13

15

17

19

21

35 40 45 50 55 60 65 70 75

N° of Theoretical Stages

Du

ty (M

MK

cal/h

)

Reboiler

Condenser

Duty available

9

11

13

15

17

19

35 40 45 50 55 60 65 70 75

N° of Theorethical Stages

Ref

lux

Rat

io

75

80

85

90

95

100

105

35 40 45 50 55 60 65 70 75

N° of Theoretical Stages

Flo

od

ing

(%)

Optimum Point


For this De-IsoButanizer, the minimum flooding factor (maximum capacity) was found

for a number of actual trays, very close to the existing one, providing with approximately

48 theoretical stages i.e. 86 % tray efficiency, for a total of 50 stages (assuming 1 stage

each for reboiler & condenser). Therefore it was decided to retrofit the tower with a one

by one tray replacement. The hydraulic results of the VGPlus trays are here after shown

in Table 2.2.

Table 2.2: Hydraulic Performances at Revamp conditions

Tray Type: VGPlus

The selected option allowed a big saving for the project implementation; since additional

actual trays were not needed, the job could have been done without adding new tower

attachments, nor welding to the vessel, resulting in a very fast and cost effective solution

of revamping.

However, there are several other revamp projects where a higher number of actual trays

could be required, therefore new support rings are a must. Even for such cases, Sulzer

Chemtech has tailored mechanical solutions to minimize the site work activities, which

will not be discussed since they are out of the scope of this paper.

Case

Section above-feed below-feed above-feed below-feedVaporFlow rate kg/h 208570 182290 201910 180800Density Kg/m3 19.563 19.670 19.497 19.586C-factor m/sec 0.086 0.074 0.083 0.074

LiquidFlow rate kg/h 195080 210030 190910 212010Density Kg/m3 509.4 511.8 509.9 512.3Surf.Tension mN/m 6.95 6.97 6.98 6.99Viscosity cp 0.1 0.1 0.1 0.1L-Loading m3/h/m2 55.50 59.48 54.26 59.98

HydraulicJet flood % 80 70 78 70DC flood % 73 78 72 80Weir loading gpm/inch 9.50 10.18 9.29 10.27Dry Drop mmH20 29.45 22.37 27.69 22.11Delta P mBar 6.15 5.70 5.99 5.69

Column ID: 2964 mm Tray Spacing: 610 mm

Tray Eff. 86 % Tray Eff. 86 %

System Factor: 1

Design Feed Alternate FeedCase




Column ID: 2964 mm Tray Spacing: 610 mm

Tray Eff. 86 % Tray Eff. 86 %

System Factor: 1

Design Feed Alternate Feed


3) Unit modifications

Since the production of the Iso-Butane was significantly increased, several other

equipments had to be modified and or replaced, with the exceptions of the distillation

tower, the feed accumulator and the top receiver (see Figure 3.1).

Figure 3.1: Unit Modifications

Required changes include installation of new condensers, reboiler, feed pumps, overhead

pumps, bottom product pumps, two additional preheat exchangers, and several control

valves. The fractionation trays of the distillation tower have been replaced with VGPlus

high performance trays to handle the big increasing of the vapor/liquid loadings without

compromising the mass transfer efficiency (see Figure 3.2). The VGPlus trays have been

tested at FRI providing excellent performances: very high capacity and efficiency, at

vapor & liquid loadings below the system limit (see Figure 3.4).

While retrofitting existing columns, direct welding to the tower wall should be

minimized, since it would require the post weld heat treatment, a very costly and time-

consuming site activity, which shall be avoided during the Plant turnaround. Tailored

designed adaptors were used to fit the new trays in the existing supports (see Figure 3.3).

A new liquid distributor was installed at the top reflux, as well as a new advanced

degassing box was needed for the flashing feed between tray 28 and tray 29.

56

1

28

n-C4

29

Feed-1

Feed-2

Feed-3

From Iso-Stripper

Reboiling Fluid

FC

FC

LC

FCLC

LCFC

10FC

TC

Fuel Gas

Iso-C4

Major Unit modifications- new feed pumps;- new top pumps;- new bottom pumps;- new feed pre-heater;- new reboiler;- new condensers.

PC

New VGPlusHPTraysNew VGPlusHPTrays


Figure 3.2: VGPlus High Performance Trays

Featuring Enhanced Downcomer, MVG valves, and Push Valves

Figure 3.3: Adaptor for Side & Central downcomer

MVG valve

Existing DC bolting bar

New DC bolting bar

Portion of Existing DC bolting bar to be removed


Figure 3.4: VGPlus tray: FRI test I-C4 / N-C4, 11 Bara, total reflux

4) Unit performances after Revamping

The Unit is running for more than 4 years without any problems either at the equipment

being modified or at the distillation column. To check the actual Unit performances, in

particular the capacity & the efficiency of the fractionator, a test run was done on

November 2004.

4.1) Plant Data

Hourly averaged operating data have been recorded from the control room for more than

two days: flow rates, temperatures, pressures, level ‘s controls, on line analyzers,

openings of control valves. The Unit was running at a capacity lower than design, and

product purities much better than design, therefore the reflux ratio was higher than

design, and consequently the vapor & liquid loadings through the column were

consistently higher than the ones at design conditions.

The pressure profile was slightly lower than design: 6.4 versus 6.7 Barg at top of tower;

as well as the top temperature at the top receiver was approx. 10 °C lower than design.

A temperature survey was done to cross check some critical temperatures: feed inlet to

the tower, in & out of the reboiling fluid, top reflux, and bottom of the tower, by mean of

Effi

cien

cy (%

)

Cs (ft/s)

Competitor Tray VGPlusCs (m/sec)


a laser pyrometer; the measured values were in line with the ones recorded at the

Distributed Control System (DCS).

A plant survey was done also for some critical pressures: the gathered data matched very

well with the DCS at the bottom of the tower; a discrepancy was observed for the top

tower pressure: 6.55 Barg versus 6.46, however this delta did not affect the recalculated

process performances such as duties at reboiler & condenser, reflux flow rate, or product

purities, because the bottom pressure was practically the same and the top receiver

temperature was 20 °C below the bubble point.

Pressure was taken at the feed inlet to the tower after the control valve and the preheat

exchanger; the location was practically the same of the temperature indicator of the DCS,

for the most reliable definition of the specific enthalpy of the feed entering the tower.

Pressure was also taken at the suction and discharge sections of the pumps, together with

the absorbed ampere, providing another alternate of crosschecking the material balance

and top reflux flow rate. Per each day of the test, a complete set of samples was taken and

sent to the lab for the gas chromatography analysis.

4.2) Selection of Plant Data

For a given plant, same size and hold up of the TRA De-IsoButanizer, 24 hours test run at

steady operating conditions should be enough for a reliable evaluation of the process

performances, of course the longer the time frame the better the reliability, provided that

the stability can be maintained. It is also well understood that a so big and complex

Refinery such as TRA is not a pilot plant, therefore as longer is the time frame of a test

run, as higher is the possible variation of the operating conditions. This is even more

valid for this DIB because the feed can come from three different sources, and the

reboiling duty is fully integrated with another Unit.

Among the gathered data, it was possible to select 24 hours in which the Unit was

running quite steady, here after called Test-1:

- Only two types of feed were on stream, the third was stopped since more than two

hours, enough to make sure that the feed accumulator was completely renewed

and the content representative of the two streams only.


- The top & bottom tower pressures were also constant, as well as the top & bottom

tower temperatures, resulting in quite uniform product purities.

- The feed flow rate was within a fluctuation of 4%.

- The level of the feed ‘s accumulator constant, as well as the level of the kettle

reboiler, some fluctuation was detected at the level of the top receiver.

- The top reflux and the reboiling fluid flow rates were also quite steady: variation

within 3.9 and 1.4 % respectively.

The averaged values of these 24 hours have been set as reference for building the

simulation model and for the evaluation of the Unit performances.

4.3) Material Balance Reconciliation

Compensation was done for all the flow meters, as per the actual density at standard and

running temperature versus the design values of the ring orifices. An overall - 3.7% delta

material balance was found (total out higher than total in). It is believed that the outlet

could be more reliable because the control valves of the two feed streams were fully

open: 105%, while the relevant flow meters were away from the full scale.

On the basis of the gas chromatography analysis of the combined feed and the

composition of the top & bottom products, a components material balance was done to

check the expected yields. Since more than 98% of the streams is constituted by Iso-C4

and Nor-C4, the balance has been limited only to these two components. It came out that

the yield of top product should have been a bit higher than measured: 1.3 %, well within

the acceptable deviation. The reconciliated material balance is shown in Table 4.3.1.

Table 4.3.1: Reconciliated Material Balance for Test-1

Component / Stream Feed Top Bottom

Propane

Iso-Butane

Nor-Butane

Unsaturated C4s

C5s

Total

% Wt

% Wt

% Wt

% Wt

% Wt

%Wt

0.38

31.42

67.79

0.27

0.14

100

1.21

97.65

0.82

0.32

100

0.78

98.77

0.25

0.20

100

Flow rate Kg/h 31550 9980 21570


4.4) Process Simulation Model

This is a fundamental path while checking the performances of an existing distillation

column. Deep process know-how, and simulation skills are mandatory to build a true

reliable computer model. The following steps have been taken:

1) The feed composition was set as per the lab gas chromatography analysis of

the total stream coming from the feed ‘s accumulator.

2) The feed flow rate was set as per the reconciliated material balance.

3) The specific enthalpy of the feed at the tower inlet was set from the given

composition, the recorded temperature, and the pressure gathered at the field.

4) The pressure profile of the tower (top & btm) was set as per the data recorded

from the DCS, for the top receiver the data taken at plant was used.

5) The specific enthalpy of the top reflux was set as per the plant data by setting

its temperature as the data recorded from the DCS.

6) Product purities: Normal-Butane at the top and Iso-Butane at the bottom have

been set as per the plant data.

7) The number of theoretical stages in the tower has been changing till matching

the condenser duty (actually the top reflux flow rate) and / or the reboiler

duties (actually the reboiling fluid flow rate).

8) Simulation program: Provision version 7.0 from SIMSCI, with SRK

thermodynamic package.

The model showed a good match for the temperature profile: only half degree C higher

temperature at the top of the tower: 52.5 VS 51.9 C; similar temperature at the top pilot

tray: 53.7 VS 53.8 C; and practically same temperature at the bottom of the tower: 68 VS

67.9 C. The top reflux flow rate was matched for 93 % tray efficiency: 309 m3/h from the

plant versus the simulated 315 m3/h. The only discrepancy was found at the reboiler

duty, being the plant data (measured from reboiling fluid flow rate and delta temperature)

consistently higher than simulated: 24.1 versus 16.1 MMKcal/h. The measured value

from the plant looked unrealistic as it was 79% higher than the reboiler design duty;

while a well designed Kettle may provide not more than 30 % of spare capacity.


To match a so high reboiler duty the efficiency of the trays should have been reduced

down to 84%; the top reflux flow rate would have been extremely high: 420 m3/h, even

higher than the full scale of the instrument; also this data looked unrealistic since the

opening of the relevant control valve never exceeded the 55%.

To crosscheck this data, the hydraulic rating of the tray was performed: at such high top

reflux flow rate, the trays would have been far above the flooding point: 120%, and the

column could not operate at all; while for the other set of data (matching the top reflux

flow rate), the trays were still very loaded, however within the flooding point: 89 %, see

here after table 4.4.1.

A further crosschecking was done with the power absorbed by the electrical motor of the

overhead pump versus the hydraulic one calculated from the flow rate, the head and the

efficiency. Also this method confirmed the validity of the measured top reflux flow rate.

In summary whichever parameter was used for the crosschecking of the condenser and

the reboiler duties, the measured top reflux flow rate was definitely the right one.

Table 4.4.1: Hydraulic Performances for the Test-1

Tray type: VGPlus

Case




Column ID: 2964 mm Tray Spacing: 610 mm System Factor: 1

Matching Top Reflux Matching Reboiler DutyTray Eff. 93% Tray Eff. 84%

Case




Column ID: 2964 mm Tray Spacing: 610 mm System Factor: 1

Matching Top Reflux Matching Reboiler DutyTray Eff. 93% Tray Eff. 84%


4.5) Additional sets of Plant data

In order to check the effect of the delta material balance as well as the fluctuation of the

top receiver level, two more sets of data were evaluated, at which the Unit was running

practically constant without any fluctuation, here after called Test-2 and Test-3.

Test-2 included 11 hours data, all the three feed sources were on stream, therefore the

composition of the combined feed was different from Test-1, material balance deviation

within 2.4 %.

Test-3 included 7 hours data, characterized by a sharper separation Iso-C4 / Nor-C4,

material balance deviation within 3.5%, two only feed sources were on stream, therefore

its composition was the same as per the Test-1. The relevant reconciliated material

balances are here after shown in Table 4.5.1 and 4.5.2.



Propane

Iso-Butane

Nor-Butane

Unsaturated C4s

C5s

Total

% Wt

% Wt

% Wt

% Wt

% Wt

% Wt

0.46

34.05

65.05

0.32

0.12

100

1.33

97.43

0.83

0.41

100

0.84

98.70

0.27

0.19

100

Flow rate Kg/h 29300 10070 19230



Propane

Iso-Butane

Nor-Butane

Unsaturated C4s

C5s

Total

% Wt

% Wt

% Wt

% Wt

% Wt

%Wt

0.38

31.42

67.79

0.27

0.14

100

1.20

97.57

0.87

0.36

100

0.68

98.89

0.23

0.20

100

Flow rate Kg/h 29470 9350 20120


The simulation results confirmed the conclusions highlighted for Test-1, basically a very

good match for all the operating data and product purities, except for the reboiler Duty,

which was still far away from the expected one.

Also these two additional cases showed very high tray efficiency: over 91%. The

hydraulic calculations of the trays, for these two tests, have been omitted since the vapor

and liquid traffic through the column were very close to the ones of Test-1, practically the

same for Test-3 and just 4.8% lower for Test-2.

4.6) Summary of the test run

The Unit was working very well, providing performances higher than expected, without

any significant plant ‘s limitations.

- Within the approximation of the reconciliated data, the duty at the condenser

system was quite high (10% over design), and no limitation was observed,

therefore a higher spare capacity is expected.

- Within the approximation of the reconciliated data, the duty at the reboiler was

also quite high (20% over design), and no limitation was observed, therefore a

higher spare capacity is expected.

- Even if the fractionation trays were highly loaded, the mass transfer efficiency

was very high: over 91 %; the tower was running very smoothly; and 10%

additional capacity is expected.

5) Conclusions

1) While maximizing existing plant’s capacity without compromising efficiency, one

of the most critical equipment to look at is the distillation tower and associated

mass transfer components.

2) The VGPlus High Performance Trays provide the petrochemical industry with a

very cost effective tool to push the towers up to their ultimate capacity set by

vessel diameter.

3) Determining the optimum working point while retrofitting a distillation tower is

not an easy task. A combination of process know-how and mass transfer

components design skill is crucial to successfully achieve challenging targets.


Acknowledgments

Sulzer Chemtech gratefully acknowledge Total Refinery Antwerp for the permission of

publishing this data.

References

1) G. Mosca, E. Tacchini, G. Scribano, “High Performance Trays for Distillation

Columns” presented at the 1st CHEM ARAB Conf., Beirut, Lebanon (January 2001).

2) Dale Nutter, David Perry “Sieve Upgrade 2.0 – The MVG™ Tray”, presented at the

AIChE Spring National Meeting, Houston, Texas, (March 1995).

3) Kister Z.H, Brown E., Sorensen K., “Sensitivity analysis is key to successful DC5

simulation”, Hydrocarbon Processing (October 1998).

4) J.L. Bravo, J. Sikkenk, G. Mosca, L. Tonon, M. Roza, “Design and revamp of modern

C2 Splitters with High Capacity MTC and fast installation techniques” presented at the

ARTC Petrochemical conference, Bangkok, Thailand (March, 2002).

5) Sloley, A.W., “High Capacity Distillation”, Hydrocarbon Processing (August 1998).

6) Kister, H. Z., “Distillation design”, McGraw-Hill, New York (1992).

7) Simulation Sciences Inc. “PRO/II process simulation program” Version 7.0.

8) Sulzer Chemtech “Sultray version 7.2”

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