use tempdb;
set ansi_nulls, quoted_identifier, xact_abort on;
go

create table dbo.Replacements (s varchar(100), s_replacement varchar(100));
insert into dbo.Replacements
values
 ('a', 'b'), ('c', 'd');
go

create function dbo.fnBatchReplace
(
 @s varchar(max)
)
returns varchar(max)
as
begin
 with r as
 (
  select top (cast(0x7fffffff as int))
   s, s_replacement
  from
   dbo.Replacements
  order by
   len(s) desc
 )
 select @s = replace(@s, s, s_replacement) from r;

 return @s;
end;
go

declare @s varchar(max) = 'acac';
select @s, dbo.fnBatchReplace(@s);
go

drop function dbo.fnBatchReplace;
drop table dbo.Replacements;
go