PLANT ADAPTATION complexity of interrelationships with other industries, is actually giving us a vastly important degree of flexibility for any future requirement for rapid national preparedness.
Summary The major advantages for plant adaptation are lower cost and shorter time to produce new products in sizable quantities for extensive commercial evaluation. Also earning power may be regainedon otherwise obsolete facilities. Plant adaptation provides an important alternate course of action for varying technical and economic circumstances. Practical adaptability requires flexibility of original design which is also advantageous from the viewpoint of making process improvements after initial plant operation. Several examples illustrate that adaptation usuallyinvolves some similarity of basic processing conditions but not necessarily any similarity of raw materials, intermediates, or final product.
Very abnormal processing conditions may impose limitations on the applicability of plant adaptation. The attitude and resourcefulness of people is a particularly important factor in plant adaptation. Safety considerations must always be given careful attention in making adaptations. The principle of plant adaptation can be used more extensively in any future national emergency to save time in producing vital materials.
Literature Cited (1) Manufacturing Chemists’ Association, Inc., Washington, D. C., “The Chemical Industry-Facts Book,” 1st ed., 1953. (2) Thompson, W. D., Paper Makers Chemical Research Division,
HerculesPowder Co., private correspondence, W. D. Thompson, February 26, 1954. RECEIVED for review March 26, 1954.
ACCEPTED -4ugust 26, 1954
Conversion from Coke to Natural Gas as Raw Material in Ammonia Production R. 6. BURT Tennessee Volley Aufhorify, Wilson Dam, A l a
W h e n the TVA ammonia plant was built semiwater gas produced from coke was the most practical source of hydrogen. However, when natural gas became available the plant was converted to use this cheaper raw material. The conversion from the use o f coke to natural gas was a major problem in plant adaptation and required numerous engineering and economic evaluations both of processes and of equipment. Studies were made concerning the choice of reformers, the retention of existing heat-recovery equipment, the use of by-product gases from other processes, the need for sulfur removal equipment, and the limiting capacities o f other units in the original plant. Two separate trains of two-step reformers and additional gas compression facilities were installed. Minor changes were made to other parts o f the orginal plant to ensure their efficient performance at the expanded rate of production with natural gas. These changes increased the capacity of the plant from 160 to 250 tons o f ammonia per day. The change-over from use of coke to natural gas was made one train at a time; the first was out of production only 29 days, the second only 26 days, and both simultaneously less than one day.
W
HEN TVB was created in 1933 it inherited the nitrogen fixation facilities of Nitrate Plant No. 2 a t Muscle Shoals, Ala., which had been built during World War I. However, in its initial fertilizer development program, TVA did not utilize these facilities to produce nitrate fertilizers. They depended on the calcium cyanamide process for the production of ammonia and t h a t process had become obsolete, primarily because many new plants had been built during the 1920’s which utilized the pressure-synthesis ammonia process (1). With the advent of World War 11, rapid expansion of the national production of nitrates for munitions was necessary. Therefore, TVA investigated the possibilities of utilizing its facilities to augment the national program. As a result of its studies on the best way of contributing to that program, TVA concluded that a new pressure-synthesis ammonia plant should be built to supplant the old cyanamide facilities. It recommended t,his step t o the War Department in July 1940 and coordinated the TVA program with that of the War Department. Initially, the plan was t o get the necessary hydrogen from semiwater gas produced from coke. The fact that a national emergency existed, and that the nitrate plant December 1954
could be made operable in a relatively short time, led to the choice of the semiwater gas process. Greater efficiency and economy could be obtained through the use of natural gas as a source of hydrogen, but the natural gas was not available a t Muscle Shoals. The TVA plant was among the first of the plants sponsored by the Ordnance Department to start operation and has remained in continuous operation since 1942 while supplying ammonium nitrate for munitions and fertilizers ( 2 ) . The demand for nitrate fertilizers increased steadily during the years immediately after World War 11,and the operation of the TVA plant was continued in order t o help meet this demand. However, maximum economy in nitrate fertilizer production could not be obtained with the continued use of the semiwater gas process. This situation grew worse because of the steady rise in the cost of coke and the increased operating costs associated with the use of the variable-quality coke available for plant operation. Therefore, u-hen a supply of natural gas became available t o the Rluscle Shoals plant through development by private companies, TVA re-exnmined its position and decided t o utilize this more economical raw material. TVA’s aid to the private interests who developrd
INDUSTRIAL AND ENGINEERING CHEMISTRY
2479
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
A B
5 9 9 213 722 244
0 7 ' 2 4
01
01
Figure 1.
PRESSURE, L B / W IN GAGE
Gas Compression and Ammonia Synthesis Section
this service was instrumental in obtaining the approval of tlie Federal Power Commission and helpful in the successful financing of the installation. This paper covers the technical and economic studies made for the plant adaptation and describes the principal design changes. It also presents plant operating data t o show the effect of the changes on the perfoimance of major process units in the plant.
Plans for Conversion Although reformed natural gas is an inherently cheaper sourre of hydrogen than gas made from coke, an additional important economic advantage-as applied to t8heTV;I plant-arose from the fact that reformed natural gas is a much more concentrated source of hydrogen than semiwater gas. Therefore, the USA of natural gas would result in a larger plant capacity per unit of gas supplied to the purification and compression equipment and would mean savings in the cost of power for compression, water, and other factors. I n recognition of this inherent advantage, it, was necessary to operate the remodeled plant a t maximum capacity in order t o achieve greatest economy. The first planning studies for t,he plant conversion involved a rigid analysis of the operation of each major unit in terms of its performance and capacity if the plant were operated with reformed natural gas. During t,hese studies visits were made to other ammonia pla,nts using natural gas; their operations were observed, and the performance data n-ere reviewed. This information was applied to the known performance characteristics of the individual process units in the TVA plant to determine pot,ential equipment limitations and how t o modify the limitations by reasonable changes in the design or operation of the equipment. The flexibility of the TVA plant had been demonstrat,ed by its satisfact.ory operation Tvith semiwater gas a t rates substantially above the rated capacity of 160 tons of ammonia per day (180 tons per day). Ample capacity for greater production in the synthesis and gas purification sections was indicated by the long catalyst life and by the record of free from trouble operation ntt>ributabIe t o impure gas (3,4). These preliminary studies 2480
/50/
1681 1 9 1 0 1 ' 0 0
indicat,ed that the optimum capacity of the TVAIplant would be approximately 250 tons of ammonia per day if natural gas were used. The major requirements for reaching this capacity would be the installation of natural gas reforming facilities and the provision of additional compressor capacity. The six stages of existing compressors would have t o be changed in order to accommodate the new voluniet,ric ratio bet,wveen convert,ed gas and synthesis gas. With the selection of a capacit,y of 2\50 tons of ammonia per day as the goal for the converted plant, the problem resolved itself into several principal objectives: 1. Selection and purchase of new equipment 2. Adaptat,ion of existing equipment to the convert,ed plant 3 . Integration of the new operations in those of the over-all plant Although the plant conversion was planned on the basis of these Objectives, the factors that governed t,lie mannrr in v,-lii?li the conversion JTas effected were: 1. Maximum economy in the use of natural gas Maximum use of existing equipment 3. Utilization of unchanged equipment a t optimum capacity 4. Maximum pract,ical production during the period of plant ehange-over 5 . The possibilitv of eventual reconversion of the plant to the iise of semiivater g a i 2.
Description of Plant Tlie original TVA eyn thetic ammonia plant may be described as a 350-atmosphere plant, in which the hydrogen was produrid from semiwater gas derived from coke, most of the carbon dioxidr was removed from the synthesis gas by water scrubbing, and the carbon monoxide and residual carbon dioxide were removed by scrubbing the synthesis gas pith ammoniacal copper formate solution. The converted plant may be described in the same terms except that in it the hydrogen is produced by the use of steam and a nickel catalyst to reform natural gas. Figure 1 shows Q flow diagram of the gas purification and ammonia-syntheeiq sections of the plant. The semiwater gas plant for production of hydrogen is shown in Figure 2. Figure 3 shows a flow diagram
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46, No. 12
PLANT ADAPTATION of the nitrogen required and, although the flue gas increases the carbon dioxide content of the final reformed gas mixture, the net result is that less carbon monoxide is produced in the two-step process than in the one-step process. The selection of the reforming equipment required an analysis of the ammonia process under conditions that would exist with each of these two types of equipment. Studies of some of the problems that would be involved in adapting the new equipment t o the existing ammonia pIant equipment are discussed below. Semiwater gas contains 33 to 36% carbon monoxide, and the original plant had been provided 15 ith equipment to recover from the gases the large quantities of residual heat liberated during the conversion of this carbon monoxide to hydrogen by catalytic reaction with steam. Therefore, the possibility had to be considered of utilizing the saturator-water heater-heat exchanger arrangement already installed, as well as the cost of removing this equipment if it ~ e r enot used. In the original plant this equipment had been used to preheat the steam-semiwater gas mixture and to produce part of the steam required for the conversion reaction. However, such equipment is not ordinarily installed in plants utilizing reformed natural gas, because its carbon monoxide content is IOU er, and the converted gas contains much less recoverable heat. An evaluation of the advantages to be derived from the usc of the existing heat recovery equipment with the reformed gas posed t11 o questions: 1. Whether the return from the heat recovered and the steam produced outweighed the cost of maintaining- arid operating - the equipmen t . 2. Whether the carbon monoxide conversion system would remain autothermal, as it had been during operation with semiwater gas.
of the natural gas-reforming unit, which replaced the semiwater gas plant for the production of hydrogen. Typical gas compositions, pressures, and temperatures a t critical points in the process are shown in each diagram. Details of the design and const,ruction of the ammonia plant have been presented in earlier publications by others of the TVAstaff ( d , 3, 4 ) .
Selection of Natural Gas Reforming Equipment
In determining the requirements for the reforming equipment, it was conrluded that, for reasons of economy, gas should be used only for furnace raw material and for fuel to maintain the proper furnace temperature. Any supplemental steam requirement was to be furnished by existing facilities. Calculations indicated, however, that with careful design to utilize waste heat, little or no auxiliary steam would be required by the reformers. The major portion of the gas supplied to the reformers is converted to hydrogen accoi ding to the over-all reaction 2CH4
+ 3Hz0 = CO + COz + 7Hn
The air used to burn the gaseous fuel furnishes nitrogen (as flue gas) to the reformer product gas mixture. Reformers are of two general types-one-step and tyo-step. I n one-step reformers, the flue gas is added to the gas stream ahead of the reformer so that any nitrogen oxides present may be reduced over the reformer catalyst, to avoid possible damage to the compressors later in the process. This early addition of flue gas and the relatively high temperature at which the onestep unit operates shift the equilibrium of the reformer reaction and results in higher concentrations of carbon monoxide (16 t o 17y0)in the reformed gas. In the two-step reformer the flue gas is added after the primary reformer unit and ahead of the secondary reformer unit. IXowever, only about halE as much flue gas is added as is used in the one-step process, because supplemental air is also added ahead of the secondary reformer in order to maintain the temperature in that unit by the combustion of hydrogen produced in the primary unit. This air supplies some
Calculations indicated that it w a ~worth while t o recover the heat liberated during operation when the carbon monoxide level was 16 to 17%, but the advantage was small when the level was 10 to 11%. However, in eithcr case use of the equipment still seemed worth while if consideration was given to the cost of removal, and t o the possible complications removal might introduce in the final change-over from operation with coke to gas.
E Q U I P M E N T BELOW DIVIDED INTO TWO T R A I N S HOT WATER
P
WATER SECONDARY
a i '
121 X l l X
-
GAS
PROCESS REOUIREMENTS PER TON OF AMMONIA COKE, T O N S -1.27 STEAM, L E .12.400 POWER, K WH 1,280 WATER GAL .. 197,000 OPERA~ING LABOR, MAN-HR 2 99 M A I N T LABOR, M A N - H R - - I 37
---------- - - ---- - - - - - -__ - -- -- -- - _ -_ -
SEWER
a S/G
TEMPERATURE, 'F PRESSURE, P S I G VOL RATIO, STEAM GAS
~
Figure 2.
December 1954
NUMBER IN PARENTHESIS A D J A C E N T TO EQUIPMENT INDICATE THE NUMBEROF I T E M S IN P L A N T - ( T W O T R A I N S A F T E R ELECTROSTATIC PREClPITATOR.1
Hydrogen from Semiwater Gas
INDUSTRIAL AND ENGINEERING CHEMISTRY
248 1
ENGINEERING. DESIGN. AND PROCESS DEVELOPMENT
TYPICAL GAS ANALYSES
1
H2 A 8
C 0
- 1 743
Np
08 570/225 1863
-
GO)
- % BY
VOL
I o? 1 CH. - - 1960 1 27 105 GO
--
06 117 100 1 0 4 4 0 95
-
01
CMg 4 4
-
- -
PROCESS REOUIRfMEWS PER TON OF AMMONIA NATURAL GAS PROCESS GAS,HtF - 21 3 FUEL GAS,MCF. - .9 9 STEAM,LB 6100 POWER,KWH - - . - 1050 WPTER,GAL - .176,500 OPERATING LABC47,MArl-HR -183 M I N T LABOR MAN-HR -071
--- - - ---
E W I P M E N T SHOWN FOR ONE TRAIN ONLY NUMBERS IN PARENTHESES ADJACENT TO EQUIPMENT NAMES INDIMTE THE NUM0ER OF ITEMS PER TRAIN
@ SIG
Figure 3.
Hydrogen from Natural G a s
Calula tions also indicated that the carbon monoxide conversion system would rcmain autothermal even at. the 10 to 11% carbon monoxide level; sufficient heat would be released to offset the radiation losses and raise the temperature of the incoming reformed gas t o the required conversion temperat,ure. However, the temperature differential would be decidedly less with reformed gas t,han with semiwater gas. Since the heat t,ransfer driving force would be lower, this raised a question whether there was ample surface in the exchangers to transfer the heat involved. Heat transfer calculations indicated t,hat, \vith gas containing 16 to 17y0carbon monoxide, there would be sufficient.hea,t,transfer surface for the process; but, with gas containing 10 to 11% carbon monoxide there would not be enough surface. Thus, a final decision on utilization of the original heat recovery facilities in the layout of the converted plant would have t,o be governed by other factors in t,he design of the reformer equipment. Therefore, the operating characteristics and the possible iimitations of the existing equipment were called to the attention of the bidders in the specifications for the reformer equipment, and they were informed that a considerat,ion of the plans for utilizing or abandoning the existing units would constitute an important. part, of the bid evaluation. In planning the reformer installation, several other points were considered that involved the integrat,ion of the remodeled plant Kith existing operations in other parts of TVA's chcmical plant. The use of by-product carbon monoxide from the electric phosphorus furnace operations was considered as a stand-by fuel in the reformer operat,ion. However, t,his was not done because residual phosphorus unless removed a t addit,ional expense before the gas was used in the reformers would have presented a corrosion problem. Instead, oil was used as a stand-by fuel for reformer operation during periods of short gas supply. High nitrogen tail gas from the nitric acid plant was considered as a source of nitrogen, thereby eliminating the coniplication encountered when carbon dioxide is added with the flue gas. High nitrogen tail gas has been used successfully in other plants where residual nitrogen oxides were removed by reducing them with hydrogen over a platinum cat,alyst. Because of the added cost of purifying the gas and the inherent disadvantage of having the ainnionia plant dependent on the nitric acid plant, as well as other fact>ors,the tail gas was not used. 2482
TEMPERATURE ,.F PRESSURE, PS I G VOL RATIO,STE&U. CAS
Another decision involved the possible installat,ion of sulfur removal equipment to prevent sulfur poisoning of the reformer catalyst. Usually natural gas is free from sulfur, but the distributors were not. prepared to guarantee a sulfur-free gas. Therefore, the cost of providing sullur-removal equipment in the initial installat,ion wa.s compared wit'li the possible cost of providing it later if such became neceesary. This latter cost includd an estimate of the cost of the loss in production that would be encountered while the installation was being made and the the possible loss of catalyst a8 a result, of poisoning. It was decided to provide the sulfur-removal equipment in the initial installa6ion as an insurance measure; t'his st8cphas been followed in several other ammonia plants in t h k country under similar conditione. -4fter the major process questions had been resolved, spccificat,ions were prepared and bids were solicited from the several manufacturers of gas reforming equipment. This required additional economic evaluations in order to decide such point,s 8,s what reforming efficiency would be required, what degree of heat recovery should be specified, and what maximum residual methane and nitrogen oxide contents should be allowed. Considerable flcxibility was allowed the bidders in that either one- or two-step reformers might be considered. The catalyst type was not epecified, although evidence to support, a satisfactory life expectancy and a guarantee of efficiency were required. As indicated earlier, consideration of the use of existing heat exchange equipment was requested, and the costs of fuel and steam in the TVA plant were furnished for the bidders' use in making their estimates. In t'hese considerations, it was only specified that the carbon monoxide converters be operat,ed under t'he temperature conditions that viere optimum for TVA carbon monoxide conversion catalyst in the previous plant operations. Finally, a demonstration run was specified in order to ensure TVA that furnished equipment would meet all specified requirements. In evaluating the bids submibted by the manufacturers, hcsides the net cost of the installation proposed, consideration w a r given to other factors which affected the cost of operating the different units, such as gas consumption, fuel requiremcnt, and steam production. Other items, such as patent licenses, securit~y restrictions, exceptions t o contract terms, and guaranteed r o m pletion date, were also considered.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46, No. 12
PLANT ADAPTATION The contract for the natural gas reforming installation provided for two separate trains of two-step reforming units and was awarded on the basis of competitive bids. Each train was t o contain two primary reformer units and a secondary reformer unit. Each primary unit contained 20 tubes of catalyst, and the secondary units were packed with catalyst in bulk. Since equipment was included to ensure reformed gas at the desired temperature for carbon monoxide conversion, the contract stipulated abandonnient of the original saturator-water heater-heat exchanger setup. Six waste heat boilers were to be installed for the two reformer-carbon monoxide conversion trains. Nitrogen was t o be supplied from a mixture of air and flue gas introduced after the primary reformer units. Flue gas was t o come from the primary reformers. During periods of gas shortage, oil was t o be used as a stand-by fuel and, a t such times, air instead of air and flue gas would be used as a source of nitrogen t o avoid possible catalyst poisoning by sulfur in the flue gas derived from the combustion of the oil. Temperatures in the secondary reformers were to be controlled by varying the air-flue gas ratio and when oil was used as fuel, steam would be used to control the temperature, Figure 3 shows a flow diagram of the natural gas reforming and carbon monoxide convcrsion installations. Figure 4 shows a diagram of the plant layout indicating the relative locations of the old and new equipment and the changes in the locations of some of the original equipment. The available space lent itself t o a n orderly arrangement of the equipment, and gas ducts and process lines of reasonable length were possible. A permanent building was available t o house the control room and much of the equipment. Figure 5 shows the saturator-water heater-heat exchanger equipment installed in the carbon monoxide conversion section of the original plant. Since it was not to be used in the operation with natural gas, this equipment was removed and the removal constituted one of the major jobs performed during the changeover period. The electrostatic precipitator used in the original plant t o remove dust from the semiwater gas is also shown in Figure 5 . This precipitator was retained in the plant for operation with natural gas and was used to remove dust from the air supplied to the reformer units. This constituted another beneficial adaptation of existing equipment t o the new plant, although -
__
it performed a function different from the one it had served in the original plant. Figure 6 shows a general view of the natural gas reformer installation, and Figure 7 shows the semiwater gas generation facility used in the original plant with the incoming coke conveyor and screening house on the right and the sulfur removal tower on the left. This facility was placed in stand-by condition when conversion of the plant t o operation with natural gas was completed.
Adaptation of Existing Equipment Simultaneously with studies that led t o the selection of t h e reformer equipment, the limiting capacities of other parts of the ammonia plant were determined. Studies were also conducted t o determine what changes would be required t o adapt other equipment for operation a t the ammonia production rate of 250 tons per day. The carbon monoxide converters were the first units of t h e existing equipment t o be studied. Preliminary studies had indicated that these units might limit the production capacity of the plant. There are eight converter vessels-four in each train, arranged in two parallel lines of two vessels each. This makes two primary and two secondary converter units for each train. The catalyst bed in each converter is supported by a grate on which the mass of catalyst is spread. I n the original plant a catalyst bed depth of about 30 inches was used. Gas entered each converter vessel through a horizontal connection in one end near the top, passed downward through the catalyst bed, and out through a horizontal connection in the same end near the bottom. I n previous operation when the converters were opened for catalyst recharging, the catalyst bed had shifted as a result of the high velocity of the inlet gas stream. This left a depression in the bed immediately in front of the inlet, piled the catalyst t o a greater depth further along the vessel, and caused a gas passage of vaxiable depth in the catalyst bed. This had permitted channeling of the gas as it passed through the bed and caused a decrease in the capacity of the converters. Confirmation t h a t channeling had occurred was obtained by the results of temperature traverses made throughout the beds while the units were in operation and by analyses of gas samples that were collected from different points beneath the catalyst beds. The inlet gas connections t o
7
UNCHANGED
-
NEW EQUIPMENT
-----
REMOVED OR I D L E
AMMONIA STORAGE
cr2Oo
COOLING U TOWERS
4 ,,'
.
3 3
I
,:- '
.
r-7
I
LOW TEMPERATURE CONDENSERS
0 00 0
COOLERS
3 0
AMMONIA VAPORIZERS
0
S E%"n#gfbikS
JACKET WATER TANK
uu
STRIPPER
COMPRESSOR
0
----
REFRIGERATION
PUMPS
S O L U l-ION PUMPS
ORIGINAL GAS COMPRESSORS CIRCULATORS GAS COMPRESSION AND CIRCULATION S Y S T E M
Figure 4.
December 1954
S E M I - W A T E R GAS P L A N T
ili BLOWERS
Y O
GOMPRESSORS
N A T U R A L GAS R E F O R M I N G S Y S T E M
Ammonia Plant Layout
INDUSTRIAL AND ENGINEERING CHEMISTRY
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ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT v-as indicated by the low concentration of carbon monoxide in the converted gas (2.2% in the operation with natural gas, as compared with 1.0% in the operat,ion Tvith semiwat,er gas). The performance of the converters a t the expanded production rate witchnatural gas was facilit,ated by the increase in the quanbity of cat'alyst charged as \\-ell as by the changes made in the units t o eliminate gas channeling through the catalyst beds. This performance x a s also affected by other changes that were made in the operating conditions when natural gas was used. Although a slightly higher space velocity \vas used in the converters during the operation with natural gas-which would favor a decrease in conversion efficiency-a higher steamcarbon monoxide ratio was also used and this favored an increase in the conversion efficiency. The data also shorn that satisfact,ory performance a-it,h reformed natural gas was obtained with a lower catalyst temperature than that used in the operation with semiwater gas. (With tlie T V A catalyst the higher temperature 17-ould favor a Fiaure 5 . Saturator-Water Heater-Heat Exchanaer Equipment higher carbon monoxide conversion.) Appar__ Installed in Carbon Monoxide Conversion Section of t h e Original ently an adequate ma.rgin of safety remained ~r-hrii Plant t,he carbon monoxide converters were o p c r n t d with reformed natural gas. Electrostatic precipitator for removal of dust from remiwater gas is shown in right It was feared that the water scrubbers used t o foreground remove carbon dioxide from the gas after t h e third stage of compression might also prove to lie R the convert,ers x-ere changed to avoid catalyst shifting and gas limitation in the operation of the ammonia plant a t the PIchanneling. panded rate with natural gas. Alt,hough a lower carbon diStudies were also conducted with test converters installed in the oxide concentration v a s expected in the reformed gas operacontrol laboratory to establish the optimum temperature-space tion than in the semiwater gas operation, a net increase in velocity relationships for the TVA converter catalyst when using the volume of gas to be scrubbed n-as anticipated. Calcula tioiia gas of approximately the same composition as that expected from ba.sed on absorption coefficients from the scrubber performanee the natural gas reformers. The results of these studies indicated data for semiwater gas operation indicated that t'he water and gas rates involved might approach flooding conditions in tlie that the quant,ity of catalyst charged t o each converter vessel should be increased from the 16 t,ons used in the operation with scrubbers with the reformed gas operation. This left a choice semiwater gas, t o about 20 tons for the operation with natural l>et\veen either modifying the scrubber in order to avoid flooding :md tolerating the higher carbon dioxide concentrations in the gas. Besides the increase in the quantity of cat,alyst charged, when the plant change-over was made the inlet pipes a t t,he carbon outlet gas, thereby t,hro\ving the extra load on the copper liquor monoxide converter vessels mere nioved to the top center of system; or providing supplemcntal equipment for carbon dioxide each vessel, and a baffle was installed under t'he inlet opening t o removal. Thus it became important to know the performance distribute the gas over the entire catalyst bed in an attempt to characteristics of t,he water scrubbers for the natural gas operaavoid channeling. Table I shows that these changes result'ed in tion. Therefore, a plant test mas made in which a part of the satisfactory performance of the carbon monoxide conversion gas from the sixth stage of the compressor was returned to the equipment a t the expanded rate. The adequacy of the conversion scrubber inlet to increase the gas throughput of the water scruhof carbon monoxide achieved in the operation with nat,ural gat: ber. The addit,ion of the purified gas from the sixth stage t o
.,
d
Table 1.
Carbon Monoxide Conversion-One-Train
Ammonia Ram Gas T e m p . a t Produced, 3Iaterial Hottest Point Of Steam-Gas Tons/Day Psed catalyst, F. Ratio 90 Coke 1,000 2 7 850 1.0 125 Natural gas a Toiume of d r y gas per hour per roiuine of catalyst. b Includes inert gases.
Table II.
Inlet Dry Gas Volume, C u . Ft./AIin. R ,560 10,320
Apacen Velocity
CO?
191
6.5 10.5
241
ODeration
Typical Gas Compositions. Q By Volume ___ Leaving converter Entering converter CO H2 CH4 Xzb COi: CO H2 CHr 362 36.3 0.2 21.8 29.0 1.9 52.2 0.2 9.9 57.4 0.2 22.0 16.0 2.2 60.1 0.2
Carbon Dioxide Removal in Water Scrubber"-One-Train watcr - Flow ~
~
~.
Ulb 17.7 20.';
Operation
Typical Gas Composition ___. ~ Released from mater leaving scrubber, "0 ~ Leaving ~ , scrubber. c& ___
Gas Entering _ _ scrubber _ ~ R ~ Typical composition, Yo Ainnionia Ram Produced, AIaterial Volume, CO T e m p . , Gal./ CO co A Con CHa H2 ?iib .'1 l l i n . COi: CHa K? N2b C0nC CHr I*2 S?b Tons/Day Used Cu. Ft./lIin. 28.7 2.3 50.7 18.3 50 4,0600.50 3.1 Coke 10.100 71.025.489.8 0.4 7.12.7 100 2.3 50.8 18.3 9,000 28.6 76 6,0500.58 3.1 70.925.485.8 0.5 10.33.3 90 Coke 16.5 2 3 60.0 21.2 54 6,650 0.70 Natural gas 11,000 2.9 71.0 25.4 76.7 0.8 17.2 5 . 3 130 16.4 2.3 60.1 21.0 76 8,4600.90 S a t u r a l gas 10,500 2.9 72.224.075.2 1.0 19.04.8 120 a T h e scrubber was 10 feet in diameter and had a 20-inch water pipe up t h e center. T h e scrubber r a s packed with two sections of 3-inch ceramic Raschip rings and one section of 2-inoh ceramic Raschig rings; each section \%-as17 t o 18 feet in depth. b Includes inert gas. c A small part of COz remains dissoiyed in the water.
+
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+
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Vol. 46, No. 12
PLANT ADAPTATION the gas from the third stage gave a gds mixture a t the scrubber inlet with a carbon dioxide content comparable to that expected from the operation with reformed natural gas. This trst showed that the water scrubbers could be operated without flooding a t a rate above that required for operation with natural gas. Therefore, no significant change was made in the water scrubbers. When operation was started with reformed natural gas, the adequacy of the water scrubbers was deinonstrated further, as indicated by the data in Table 11. Even during the summertime, when the water temperatures were highest and the water rates used were necessarily a t a maximum, satisfactory carbon dioxide removal was Figure 6. Natural Gas Reformer Installation Showing Stack, Heat Exchanger, achieved without encountering flooding Primary Reformer, and Secondary Reformer conditions. I n this case, however, a slight compensation resulted from the was provided by two new, six-stage compressors. Thus each lower ammonia production and gas scrubbing rates t h a t are train in the ammonia-synthesis sectjon was provided with two chziracteristic of summertime operation. modified original compressors and one new compressor. Another plant modification adapting original equipment t o use natural gas involved changes in the gas compression section. The limiting capacity of this equipment had been reached in the Change-over of ~~~~~i~ piant fro,,, useof coke to operation with semiwater gas when the daily production capacity Use of Natural Gas had been increased from the design rate of 160 tons per day to The final change-over of the ammonia plant operation from the about 180 tons per day. Therefore, early in the study it was known that additional compressor capacity would be required, use of semiwater gas t o operation with reformed natural gas involved the careful coordination of numerous details of construcbut the question of how much was related to other changes to bc made. The percentage of carbon dioxide expected in the option, maintenance, and operation. The starting dates were eration with reformed natur:tl gas was quite different from that based on the contractors’ completion date for the reformer inpresent in operation with semiwater gas, and carbon dioxide is stallation and the effective date of the natural gas contract. Once these were set, plans were laid and equipment made ready removed after the third stage of compression. Therefore, the for the performance of the related work to be done by TVA ratio of the volume required in the different stages of the compresforces during the shutdown period. In this work the principal sors was governed by the process steps finally adopted for the objective was t o complete the change-over in a minimum of time hydrogen production section of the plant. After selection of the so that the plant would be out of production no longer than reformer process, new liners or cylinders were purchased for necessary. Termination of coke purchases, preparatory t o shutthe first three stages of the existing compressors and the last ting down the operation of the semim-ater gas generators, was three stages were bored to provide the proper volume ratio for coordinated with the change-over. Completion of the work on the new gas composition This change provided only a part of expanding the capacity of the nitric acid and ammonium nitrate the additional compressor capacity required, and the remainder production facilities to handle the increased ammonia production also called for close coordination. The final step in the change-over was made one train a t a time, so that loss of ammonia production was held t o a minimum Train No. 1 was shut down while train No. 2 continued operation on semiwatei gas. When the work on train IYo. 1 was completed, its operation with natural gas WBS started. Then train No. 2 was shut down and the final change-over steps were made on this train. Most of the work on the equipment other than the reformer proper was done by TS’A pcrsonnel, and the stepwise procedure permitted the maximum use of the maintenance crew without calling in extra help. During the change-over period all operating equipment was thoroughly inspected and minor maintenance work was aerFigure 7. Semiwater Gas Generator Facilities Showing Coke Conveyor and formed. This group also charged the Screen, Generator Building, and Sulfur Removal Towers additional catalyst, repacked the water
December 1954
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ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT scrubbers, and coordinated all phases of the work. The efficiency with which the final change-over operation was carried out is reflected in the fact that train No. 1 was out of production only 29 days, train No. 2 only 26 days, and both trains were out of production simultaneously less than one day. After a short period of preliminary operation, during which flows and temperatures were adjusted and the operators obtained the “feel” of the new plant, operating rates were stepped-up t o those anticipated in design and a test run was carried out to demonstrate adequacy of the performance. Operating rates have remained at satisfactory levels for the three years subsequent t o the plant conversion. A4daily peak production capacity of over 277 tons of ammonia has been reached, and the peak monthly average has been approximately 271 tons per day. Annual production has averaged 252 tons per day. Anticipated savings in the operating and maintenance labor have been realized. The performance of all units in the plant has justified the decisions that n-ere made st various stages of the project.
Acknowledgment The author wishes to acknowledge the contribution of the staff of TVA’s Office of Chemical Engineering t o the work covered in this paper. E. J. O’Brien and J. L. Snyder were responsible for operational phases of the work. IF. H. Haynie, A. V. Slack, and L. B. Hein conducted design studies, and H. Y. Allgood waq responsible for plant tests carried out in connection with the work
Literature Cited (1) Curtis, H. A., “Flxed Sitrogen,” The Chemical Catalog Co., h e . ,
New York, 1932. (2) Miller, A. IM., and Junkins, J. N.,C h e m dl- M e t . Eng., 50, KO. 11 119 (1943). (3) Hein, L. B., Chem. Eng. P T O ~ T 48, . . No. 8, 412 (1952). (4) Slack, A. V., Allgood, H. Y . ,and Llaune, Harold E., Ibad.. 49, No. 8, 393 (I 953). RECEIVED for review May 24, 1954.
ACCEPTEDSeptember 3 lq5.4
Process Research in Plant and Expansion FENTON
H. SWEZEY
E. 1. du Pont d e Nemours & Co., Inc., Waynesboro, Va.
Plans to expand manufacturing facilities should include a detailed study o f the manufacturing process; this should begin well before the expansion i s planned. This research should indicate how plant capacity can b e increased without duplicating existing equipment. Increased output of present equipment follows decreased reaction times, increased solution concentrations, and combined process steps. After achieving maximum output from existing equipment, the possibilities o f adding low cost supplementary equipment are determined. Thus, process research can show where a reaction can be divided and the reacting materials transferred from the starting equipment to an inexpensive vessel for reaction completion. Savings can be realized b y taking advantage o f special process features and adapting techniques and equipment from other industries. Alternative processes offer possibilities and in t h e event that they do not fit present plant conditions, useful information about the existing process may result from their investigation.
P
LAKT adaptation and expansion present real opportunities for adding to presentfacilities at attractive investment figures,
as well as for achieving lower operating cost, increasing flexibility, developing new process features, and improving product properties. Skillful process research, defined for our industry as a specialized combination of chemistry, engineering, intuition, inventiveness, and experience, has a key role in arriving at these desirable objectives. Process research starts with equipment speed-ups, step simplification, and changes from batch to continuous processes, and progresses to the development of llew procedures and the application of unique of old and new knowledge. The continued expansions of the chenlical industrj. haxre repeatedly t h a t larger output w-ill be needed from the present on at a later date. I n order to avoid the method of adding only additional equipment to duplicate present facilities, process research studies directed at adaptation should be initiated. 1. Make a n intensive study of all features of the present process t o establish t h e chemical principles and t o find variation effects: Much additional information remains to be developed
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besides the current process data. One fertile line of investigation is t o determine exactly what takes place and what the chemical composition is a t the various time intervals throughout the cycles and reactions used. 2. Set up a pilot plant program t o determine t h e effect of operating the equipment a t increased rates. This will define to be overcome. the 3. Initiate a chemical equipment and engineering survey to locate opportunities for simplification and combination, as well as unique new arrangements. These studies will provide the groundl\-ork and data for showink what can be done. As soon as the possibility of expansion becomes quite definite, a process study group ehould be set up which includes chemical engineers, process men, and specialists, the number depending on the magnitude of the problem. I n v i e r O f the time required, it is essential to initiate this part of the program as far as possible before the expansion date is set. -% year before the decision has been reached to proceed is often insufficient. This group will have the responsibility for selecting the main lines of attack, visualizing t h e possibilities for conductillg them, and then effecting the necessary program. New and unusual techniques will be sought, and the help of experts along
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46, No. 12
