Hydrocarbon Processing - April 2021 - 37
Special Focus
Clean Fuels
E. GRIFFITHS, KBR, Leatherhead, UK; and
M. MUKHERJEE, Exelus, Fairfield, New Jersey
Meeting the Tier 3 challenge with ultra-clean alkylate
more than 24-hr cycle times. It also offers robust resistance
to typical poisons (such as mercaptans, diolefins and oxygenates), along with the ability to handle a variety of feedstocks.
The proprietary catalyst forms the core of a safe and efficient
proprietary solid-acid alkylation processb that generates highoctane alkylate without the hazards and costs associated with
liquid acid technology. Additionally, it features a simple fixedbed reactor design and regeneration using hydrogen.
The integration of catalyst science and reaction engineering
allows the proprietary catalyst cycle times that are an order of
magnitude longer than most solid-acid catalysts and produces
high-octane alkylate from isobutane and light olefins (ethylene, propylene, butylenes and amylenes) from any source.
The stable catalyst performance greatly simplifies the overall
process design, which reduces the capital cost of the alkylation
plant, while lowering energy consumption. Innovations in the
proprietary solid-acid catalyst system are shown in FIG. 2.
Catalyst performance with various feedstocks. The pro-
prietary solid-acid catalyst has been tested with various feedstocks and produces alkylate with a high octane rating over a
wide range of operating temperatures, olefin space velocities
and feed compositions. Results from the bench-scale testing
are summarized in TABLE 1. The octane values and Reid vapor
4.5
Alkylate: The ideal blendstock. Accordingly, alkylate has
A proprietary solid-acid alkylation process. Typical
solid-acid catalysts deactivate in minutes. After years of development, a new solid-acid catalyst technologya has reached the
point of outperforming these liquid acids. The engineered solid-acid catalyst has been designed at multiple levels to provide
140
120
3.5
Retail prices, US$ per gallon
emerged as a preferred gasoline blending component because
it contains no sulfur, no olefins and no benzene, and has a
low vapor pressure and a high octane number. U.S. refineries
produce 1.3 MMbpd of alkylate, which is produced by reacting isobutane with light olefins, using liquid acids [either hydrofluoric (HF) or sulfuric acid]. The use of these corrosive
materials raises maintenance costs. Adding significant costs to
the operation are the storage, transport and regeneration of
the acid. Solid-acid-catalyzed alkylation eliminates the EHS
issues and costs associated with using and regenerating corrosive liquid acids.
Regular gasoline price
Premium gasoline price
Premium - regular differential
4
3
100
2.5
80
2
60
1.5
40
1
20
0.5
0
2010
Premium -regular retail price differential, cents per gallon
The U.S. Environmental Protection Agency (EPA) introduced Tier 3 gasoline sulfur standards in 2017, requiring all
U.S. gasoline producers to adhere to an annual 10-ppm average
sulfur limit. A 3-yr extension was provided for about 30 small
refineries, which expired on January 1, 2020. The program
includes a 6-yr provision allowing refineries that cannot produce qualifying gasoline to buy credits from other refineries to
comply with the Tier 3 requirement. The credits are generated
by refineries producing gasoline with an annual sulfur content
below 10 ppm. At the end of October 2019, the price of these
credits increased by 250%, indicating that demand for the credits were possibly greater than their supply.
Refiners have a limited number of options to reduce sulfur levels in gasoline to meet the new 10-ppm requirement.
For Tier 3, removing the remaining, more difficult sulfur molecules may lead to more significant octane loss. Most refiners
are meeting the regulations by increasing hydrotreating severity either with pre-treating fluid catalytic cracking (FCC)
feed or post-treating FCC naphtha. This option increases the
refinery hydrogen consumption and reduces the run length of
these hydrotreaters. While post-treating is effective for reducing the sulfur content, it saturates olefins, resulting in octane
loss in the FCC naphtha. This can be exacerbated at refineries
that consume an increased diet of shale‐derived crudes, which
are naturally light and produce low sulfur but also low-octane
gasoline. The increasing value of gasoline octane in recent
years is illustrated in FIG. 1.
0
2012
2014
2016
2018
2020
FIG. 1. Gasoline prices and premium-regular retail price differential.
Source: U.S. Energy Information Administration (EIA).
Hydrocarbon Processing | APRIL 2021
37
Hydrocarbon Processing - April 2021
Table of Contents for the Digital Edition of Hydrocarbon Processing - April 2021
Contents
Hydrocarbon Processing - April 2021 - Cover1
Hydrocarbon Processing - April 2021 - Cover2
Hydrocarbon Processing - April 2021 - Contents
Hydrocarbon Processing - April 2021 - 4
Hydrocarbon Processing - April 2021 - 5
Hydrocarbon Processing - April 2021 - 6
Hydrocarbon Processing - April 2021 - 7
Hydrocarbon Processing - April 2021 - 8
Hydrocarbon Processing - April 2021 - 9
Hydrocarbon Processing - April 2021 - 10
Hydrocarbon Processing - April 2021 - 11
Hydrocarbon Processing - April 2021 - 12
Hydrocarbon Processing - April 2021 - 13
Hydrocarbon Processing - April 2021 - 14
Hydrocarbon Processing - April 2021 - 15
Hydrocarbon Processing - April 2021 - 16
Hydrocarbon Processing - April 2021 - 17
Hydrocarbon Processing - April 2021 - 18
Hydrocarbon Processing - April 2021 - 19
Hydrocarbon Processing - April 2021 - 20
Hydrocarbon Processing - April 2021 - 21
Hydrocarbon Processing - April 2021 - 22
Hydrocarbon Processing - April 2021 - 23
Hydrocarbon Processing - April 2021 - 24
Hydrocarbon Processing - April 2021 - 25
Hydrocarbon Processing - April 2021 - 26
Hydrocarbon Processing - April 2021 - 27
Hydrocarbon Processing - April 2021 - 28
Hydrocarbon Processing - April 2021 - 29
Hydrocarbon Processing - April 2021 - 30
Hydrocarbon Processing - April 2021 - 31
Hydrocarbon Processing - April 2021 - 32
Hydrocarbon Processing - April 2021 - 33
Hydrocarbon Processing - April 2021 - 34
Hydrocarbon Processing - April 2021 - 35
Hydrocarbon Processing - April 2021 - 36
Hydrocarbon Processing - April 2021 - 37
Hydrocarbon Processing - April 2021 - 38
Hydrocarbon Processing - April 2021 - 39
Hydrocarbon Processing - April 2021 - 40
Hydrocarbon Processing - April 2021 - 41
Hydrocarbon Processing - April 2021 - 42
Hydrocarbon Processing - April 2021 - 43
Hydrocarbon Processing - April 2021 - 44
Hydrocarbon Processing - April 2021 - 45
Hydrocarbon Processing - April 2021 - 46
Hydrocarbon Processing - April 2021 - 47
Hydrocarbon Processing - April 2021 - 48
Hydrocarbon Processing - April 2021 - 49
Hydrocarbon Processing - April 2021 - 50
Hydrocarbon Processing - April 2021 - 51
Hydrocarbon Processing - April 2021 - 52
Hydrocarbon Processing - April 2021 - 53
Hydrocarbon Processing - April 2021 - 54
Hydrocarbon Processing - April 2021 - 55
Hydrocarbon Processing - April 2021 - 56
Hydrocarbon Processing - April 2021 - 57
Hydrocarbon Processing - April 2021 - 58
Hydrocarbon Processing - April 2021 - 59
Hydrocarbon Processing - April 2021 - 60
Hydrocarbon Processing - April 2021 - 61
Hydrocarbon Processing - April 2021 - 62
Hydrocarbon Processing - April 2021 - 63
Hydrocarbon Processing - April 2021 - 64
Hydrocarbon Processing - April 2021 - 65
Hydrocarbon Processing - April 2021 - 66
Hydrocarbon Processing - April 2021 - 67
Hydrocarbon Processing - April 2021 - 68
Hydrocarbon Processing - April 2021 - 69
Hydrocarbon Processing - April 2021 - 70
Hydrocarbon Processing - April 2021 - 71
Hydrocarbon Processing - April 2021 - 72
Hydrocarbon Processing - April 2021 - 73
Hydrocarbon Processing - April 2021 - 74
Hydrocarbon Processing - April 2021 - 75
Hydrocarbon Processing - April 2021 - 76
Hydrocarbon Processing - April 2021 - 77
Hydrocarbon Processing - April 2021 - 78
Hydrocarbon Processing - April 2021 - 79
Hydrocarbon Processing - April 2021 - 80
Hydrocarbon Processing - April 2021 - 81
Hydrocarbon Processing - April 2021 - 82
Hydrocarbon Processing - April 2021 - 83
Hydrocarbon Processing - April 2021 - 84
Hydrocarbon Processing - April 2021 - 85
Hydrocarbon Processing - April 2021 - 86
Hydrocarbon Processing - April 2021 - 87
Hydrocarbon Processing - April 2021 - 88
Hydrocarbon Processing - April 2021 - 89
Hydrocarbon Processing - April 2021 - 90
Hydrocarbon Processing - April 2021 - Cover3
Hydrocarbon Processing - April 2021 - Cover4
Hydrocarbon Processing - April 2021 - GP-1
Hydrocarbon Processing - April 2021 - GP-2
Hydrocarbon Processing - April 2021 - GP-3
Hydrocarbon Processing - April 2021 - GP-4
Hydrocarbon Processing - April 2021 - GP-5
Hydrocarbon Processing - April 2021 - GP-6
Hydrocarbon Processing - April 2021 - GP-7
Hydrocarbon Processing - April 2021 - GP-8
Hydrocarbon Processing - April 2021 - GP-9
Hydrocarbon Processing - April 2021 - GP-10
Hydrocarbon Processing - April 2021 - GP-11
Hydrocarbon Processing - April 2021 - GP-12
Hydrocarbon Processing - April 2021 - GP-13
Hydrocarbon Processing - April 2021 - GP-14
Hydrocarbon Processing - April 2021 - GP-15
Hydrocarbon Processing - April 2021 - GP-16
Hydrocarbon Processing - April 2021 - GP-17
Hydrocarbon Processing - April 2021 - GP-18
Hydrocarbon Processing - April 2021 - GP-19
Hydrocarbon Processing - April 2021 - GP-20
Hydrocarbon Processing - April 2021 - GP-21
Hydrocarbon Processing - April 2021 - GP-22
Hydrocarbon Processing - April 2021 - GP-23
Hydrocarbon Processing - April 2021 - GP-24
Hydrocarbon Processing - April 2021 - GP-25
Hydrocarbon Processing - April 2021 - GP-26
Hydrocarbon Processing - April 2021 - GP-27
Hydrocarbon Processing - April 2021 - GP-28
Hydrocarbon Processing - April 2021 - GP-29
Hydrocarbon Processing - April 2021 - GP-30
Hydrocarbon Processing - April 2021 - GP-31
Hydrocarbon Processing - April 2021 - GP-32
Hydrocarbon Processing - April 2021 - GP-33
Hydrocarbon Processing - April 2021 - GP-34
Hydrocarbon Processing - April 2021 - GP-35
Hydrocarbon Processing - April 2021 - GP-36
Hydrocarbon Processing - April 2021 - GP-37
Hydrocarbon Processing - April 2021 - GP-38
Hydrocarbon Processing - April 2021 - GP-39
Hydrocarbon Processing - April 2021 - GP-40
Hydrocarbon Processing - April 2021 - GP-41
Hydrocarbon Processing - April 2021 - GP-42
Hydrocarbon Processing - April 2021 - GP-43
Hydrocarbon Processing - April 2021 - GP-44
https://www.nxtbook.com/gulfenergyinfo/gulfpub/HPI-Market-Data-2023-v3
https://www.nxtbook.com/gulfenergyinfo/gulfpub/hpi-market-data-2024-v2
https://www.nxtbook.com/gulfenergyinfo/gulfpub/hpi-market-data-2024
https://www.nxtbook.com/gulfenergyinfo/gulfpub/gas-processes-handbook-2022_v2
https://www.nxtbook.com/gulfenergyinfo/gulfpub/gas-processes-handbook-2022
https://www.nxtbook.com/gulfenergyinfo/gulfpub/hp_202212
https://www.nxtbook.com/gulfenergyinfo/gulfpub/hpi-market-data-2023-v2
https://www.nxtbook.com/gulfenergyinfo/gulfpub/hpi-market-data-2023
https://www.nxtbook.com/gulfenergyinfo/gulfpub/hp_202211
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202210
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202209
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202208
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202207_v2
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202207
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202206_v2
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202206
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202205_v2
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202205
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202204_v2
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202204
https://www.nxtbook.com/nxtbooks/gulfpub/hp_032022
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202203
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202202
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202201
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202112
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202111
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202110
https://www.nxtbook.com/nxtbooks/gulfpub/hpimarket_2022_v2
https://www.nxtbook.com/nxtbooks/gulfpub/hpimarket_2022
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202109
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202108
https://www.nxtbook.com/nxtbooks/gulfpub/petrochemical_processes_handbook_2021_v2
https://www.nxtbook.com/nxtbooks/gulfpub/petrochemical_processes_handbook_2021
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202107
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202106
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202105
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202104
https://www.nxtbook.com/nxtbooks/gulfpub/hp_ebooks
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202103
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202102
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202101
https://www.nxtbook.com/nxtbooks/gulfpub/catalyst_handbook_2020_v2
https://www.nxtbook.com/nxtbooks/gulfpub/catalyst_handbook_2020
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202012
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202011
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202010
https://www.nxtbook.com/nxtbooks/gulfpub/hpimarket_2021
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202009
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202008
https://www.nxtbook.com/nxtbooks/gulfpub/refining_processes_handbook_2020
https://www.nxtbook.com/nxtbooks/gulfpub/refining_processes_handbook_2020_v2
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202007
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202006
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202005
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202004
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202003
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202002
https://www.nxtbook.com/nxtbooks/gulfpub/hp_202001
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201912
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201911
https://www.nxtbook.com/nxtbooks/gulfpub/hpimarket_2020_v2
https://www.nxtbook.com/nxtbooks/gulfpub/hpimarket_2020
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201910
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201909
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201908
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201907
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201906
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201905
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201904
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201903
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201902
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201901
https://www.nxtbook.com/nxtbooks/gulfpub/petrochemical_2018_v2
https://www.nxtbook.com/nxtbooks/gulfpub/petrochemical_2018
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201812
https://www.nxtbook.com/nxtbooks/gulfpub/hpimarket_2019_v2
https://www.nxtbook.com/nxtbooks/gulfpub/hpimarket_2019
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201811
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201810
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201809
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201808
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201807
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201806
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201805
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201804
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201803
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201802
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201801
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201712
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201711
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201710
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201709
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201708
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201707
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201706
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201705
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201704
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201703
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201702
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201701
https://www.nxtbook.com/nxtbooks/gulfpub/hp_2017mediaplanner
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201612
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201611
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201610
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201609
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201608
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201607
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201606
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201605
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201604
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201603
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201602
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201601
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201512
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201511
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201510
https://www.nxtbook.com/nxtbooks/gulfpub/hp_2016mediaplanner
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201509
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201508
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201507
https://www.nxtbook.com/nxtbooks/gulfpub/hp_2015tmsd
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201506
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201505
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201504
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201503
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201502
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201501
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201412
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201411
https://www.nxtbook.com/nxtbooks/gulfpub/hp_2015mediaplanner
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201410
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201409
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201408
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201407
https://www.nxtbook.com/nxtbooks/gulfpub/hp_2014petrochemicalprocesses_v2
https://www.nxtbook.com/nxtbooks/gulfpub/hp_2014petrochemicalprocesses
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201406
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201405
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201404
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201403
https://www.nxtbook.com/nxtbooks/gulfpub/hp_2014calendar
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201402
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201401
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201312
https://www.nxtbook.com/nxtbooks/gulfpub/hp_2014mediaplanner
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201311
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201310
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201309
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201308
https://www.nxtbook.com/nxtbooks/gulfpub/hp_srsguide2013
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201307
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201306
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201305
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201304
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201303
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201302
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201301
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201212
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201211
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201210
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201209
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201208
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201207
https://www.nxtbook.com/nxtbooks/gulfpub/hp_srsguide2012
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201206
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201205
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201204
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201203
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201202
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201201
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201112
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201111
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201110
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201109
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201108
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201107_srsguide
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201107
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201106
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201105
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201104
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201103
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201102
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201101
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201012
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201011
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201010
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201009
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201008
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201007
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201006
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201005
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201004
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201003
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201002
https://www.nxtbook.com/nxtbooks/gulfpub/hp_201001
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200912
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200911
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200910
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200909
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200908
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200907
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200906
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200905
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200904
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200903
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200902
https://www.nxtbook.com/nxtbooks/gulfpub/hp_200901
https://www.nxtbookmedia.com